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Total solar output is now measured to vary over the last three year sunspot cycles by approximately 0. There are no direct measurements of the longer-term variation and interpretations of proxy measures of variations differ; recent results suggest about 0.

The combination of solar variation and volcanic effects has very likely been the cause of some climate change, for example during the Maunder Minimum.

A study and review of existing literature, published in Nature, determined that there has been no net increase in solar brightness since the mid s, and that changes in solar output within the past years are unlikely to have played a major part in global warming.

It should be stressed, the same report cautions that "Apart from solar brightness, more subtle influences on climate from cosmic rays or the Sun's ultraviolet radiation cannot be excluded, say the authors.

However, these influences cannot be confirmed, they add, because physical models for such effects are still too poorly developed.

G and F types extend this range to 1. The Sun is a G2 star. Each spectral class denotes a typical range of mass and luminosity. For classes O through M, each spectral class represents a successively more numerous population of stars -- in other words, stellar populations vary inversely according to mass.

The spectra of K stars are dominated by the H and K lines of calcium and lines of neutral iron and titanium, with molecular bands due to cyanogen CN and titanium dioxide TiO becoming increasingly prominent at the cooler end of the range.

K-type main 56 GeobiologyESS If a star is more than 2 MSol, the time available for formation of liquid water is less than 1 billion years before the star becomes a red giant.

Humans have a narrow temperature tolerance range, but less complex organisms are not as picky. Stars like the Sun tend to increase in luminosity60,61 with time as continuing fusion of H to He reduces internal pressure.

The appropriate spectral range for [stars suitable for planetary habitability] is presently considered to be "early F" or "G", to "mid-K".

This corresponds to temperatures of a little more than 7, K down to a little more than 4, K; the Sun, a G2 star, is well within these bounds.

They live at least a few billion years, allowing life a chance to evolve. More luminous main-sequence stars of the "O," "B," and "A" classes [which are larger] usually live less than a billion years and in exceptional cases less than 10 million years.

They emit enough high-frequency ultraviolet radiation to trigger important atmospheric dynamics such as ozone formation, but not so much that ionisation destroys incipient life.

Liquid water may exist on the surface of planets orbiting them at a distance that does not induce tidal lock. Red giants are stars with radii hundreds of times larger than that of the Sun which have exhausted the supply of hydrogen in their cores and switched to fusing hydrogen in a shell outside the core.

Since the inert helium core has no source of energy of its own, it contracts and heats up, and its gravity compresses the hydrogen in the layer immediately above it, thus causing it to fuse faster.

This in turn causes the star to become more luminous from 1, to 10, times brighter and expand; the degree of expansion outstrips the increase in luminosity, thus causing the effective temperature to decrease.

Main sequence stars of spectral types A through K are believed to become red giants The Sun is expected to become a red giant in about 4.

It is calculated that the Sun will become almost sufficiently large to engulf the current orbits of some of the solar system's inner planets, including Earth.

However, the gravitational pull of the Sun will have weakened by then due to its loss of mass, and all planets but Mercury will escape to a wider orbit.

That said, Earth's biosphere will be destroyed as the Sun gets brighter while its hydrogen supply becomes depleted.

The extra solar energy will cause the oceans to evaporate to space, causing Earth's atmosphere to become temporarily similar to that of Venus, before its atmosphere also gets driven off into space.

Venus will become a burnt out planet; its atmosphere having long been driven off, and its rock will melt.

This is a line calculated by computer models of where a star will be when it begins hydrogen fusion; its brightness and surface temperature typically increase from this point with age.

Stars usually enter and leave the main sequence from about when they are born or when they are starting to GeobiologyESS The X-axis was time in Ga going from c.

Te is the earth temperature considered as a "black body" i. The diagram also shows a graph of relative solar luminosity variation over this same time period, rising from c.

This diagram shows a line for the freezing point of water, above which the Ts line eventually rises, but not until about 1.

Stars like the Sun increase in luminosity as fusion of H to He reduces internal pressure. Its gravity compresses the hydrogen in the layer immediately above it [the helium core], thus causing it to fuse faster than hydrogen would fuse in a main-sequence star of the same mass.

See also pp. Subdivided into apparent measuring visible light only and bolometric measuring all forms of radiant energy.

It is equal to the luminosity of the Sun, which is 3. The standard solar model describes the history and evolution of stars.

An aspect of this model is that stars similar to the Sun should gradually brighten over their life time excluding a very bright phase just after formation.

This prediction is supported by the observation of lower brightness in young stars of solar type. However, with the predicted brightness 4 billion years ago and with greenhouse gas concentrations the same as are current for the modern Earth, any liquid water exposed to the surface would quickly freeze solid.

This contradicts geological observations of sedimentary rocks, which required the presence of flowing liquid water to form.

The tension between the two hypotheses stems from the incorrect assumption that atmospheric gas concentrations in the past were the same as today.

First, before the advent of abundant life, the atmospheric oxygen concentrations were orders of magnitude lower than today.

In the presence of oxygen, methane breaks down to carbon dioxide, so in the absence of oxygen the methane concentration could be much larger than currently observed.

Methane is a more potent GeobiologyESS If temperature nears the boiling point of water, can get positive feedback since water vapor is a strong greenhouse gas , causing more vaporization, thus more heat [infrared] trapping, thus more boiling, etc.

In the Venusian atmosphere, the only zone potentially habitable with liquid water is possible in a band in the atmosphere [at about 50 to 70 km] where temperature drops from the c.

The lowest temperature is at about km, and temperatures increase above this altitude in the higher atmosphere up to km.

Radiation Levels: The region near the center of the galaxy has high ionizing radiation levels, lessening the likelihood of life, especially if the galaxy has a central black hole.

These emissions arise from gamma ray bursters and supernovae, etc. Thus carbon concentration declines as one goes outward, decreasing the likelihood of carbon-based life.

The Sun is estimated at 28, light years from the center, or 8. Further, the inorganic version of the carbon cycle can be expected to provide negative feedback towards an Earth with liquid water.

Carbon dissolved in liquid water can form carbonic acids, which can then interact with calcium to produce calcium carbonate. If rainfall were to cease and the oceans froze over, then this part of the inorganic carbon cycle would shut down.

Periodic explosions from volcanoes would then cause a net increase in the atmospheric carbon dioxide and methane levels with no liquid water to absorb these emitted gases.

Eventually the concentrations would become large enough that the surface temperature would rise due to the greenhouse effect.

When the surface temperature became large enough for the oceans to melt and rainfall to resume the other half of the inorganic carbon cycle would turn on and moderate the greenhouse gas concentrations.

It is also noteworthy, that even though evidence of flowing water exists even from very early in Earth's history, there may still have been a number of examples of periods when the Earth's oceans froze over completely.

Guillermo Gonzalez, one of the initial proponents for GHZ, was denied tenure at Iowa State University due to his support for intelligent design.

Was the circumstellar habitable zone and the galactic habitable zone divinely created? Earth 2. It was warmer and wetter before 3.

Are there possible biosignatures in a 4. See further discussion on these topics below. Europa moon of Jupiter Europa has liquid salty ocean under ice cap [MCM: frozen upper crust or surface], so could harbor life.

The icy crust exhibits magnesium sulfate line, sulfites, etc Might conceivably get energy from deep ocean hydrothermal vents resembling black smokers.

Scientists aren't sure why. It could be that the Sun formed in a part of the Galaxy that had an abundance of metals, and then migrated to its present position Besides requiring a metal-rich star, a Galactic Habitable Zone excludes stars too close to the galactic center.

Our Sun is a nice distance away from the galactic center, about 28, light years Staying away from the galactic center has an additional advantage.

The center of the Galaxy is awash in harmful radiation. Solar systems near the center would experience increased exposure to gamma rays, X-rays, and cosmic rays, which would destroy any life trying to evolve on a planet Keeping out of the way of the Galaxy's spiral arms is another requirement of the Galactic Habitable Zone The intense radiation and gravitation of a spiral arm would cause disruptions in our Solar System just as surely as if we were closer to the center of the Galaxy.

Luckily, our Sun revolves at the same rate as the Galaxy's spiral-arm rotation. This synchronization prevents our Solar System from crossing a spiral arm too often The unusually circular orbit of our Sun around the galactic center also tends to keep it clear of the spiral arms.

Most stars the same age as our Sun have more elliptical orbits Thus, thanks to a lot of unusual characteristics of our Sun, our Solar System is lucky enough to lie in a Galactic Habitable Zone.

Gonzalez argues that these characteristics made it possible for complex life to emerge on Earth.

More than 95 percent of stars in the Galaxy, says Gonzalez, wouldn't be able to support habitable planets simply because their rotation is not synchronized with the rotation of the galaxy's spiral arms.

On discovery, its mass was 1. On August 6, ALH became newsworthy when it was announced that it was believed that the meteorite contained traces of life from Mars, as published in an article in Science by Dr.

At just over kilometers in diameter, it is slightly smaller than the Earth's moon and the sixth largest moon in the solar system.

The satellite has a very tenuous oxygen atmosphere and one of the smoothest surfaces in the solar system. The young surface of the moon is striated by cracks and streaks, while craters are relatively infrequent.

Due to a hypothesized water ocean beneath its icy surface, and an energy source provided by tidal heating, Europa has been cited as a possible host of extraterrestrial life.

The heat energy ensures the ocean remains liquid and also drives geological activity The intriguing character of Europa has led to a number of ambitious exploration proposals; to date, only flyby missions have visited the moon.

The Galileo mission provided the bulk of current data on the satellite, while the abortive Jupiter Icy Moons Orbiter, cancelled in , was the most ambitious planned spacecraft.

Conjecture on extraterrestrial life has ensured a high profile for the moon and led to continued lobbying for future missions Titan moon of Saturn It has suspected hydrocarbon lakes and rains including methane, ethane, etc.

Enceladus moon of Saturn Its South Pole exhibits water geysers. It is ice-covered but also thought to have a subsurface ocean warmed by tidal heating as a result of its proximity to Saturn , so perhaps could harbor life there.

Europa's most striking surface feature is a series of dark streaks criss-crossing the entire globe. Close examination shows that the edges of Europa's crust on either side of the cracks have moved relative to each other.

The larger bands are roughly 20 km 12 mi across commonly with dark diffuse outer edges, regular striations, and a central band of lighter material Among the controversial hypotheses put forward to explain these features [Lineae], one states that they may have been produced by a series of volcanic water eruptions or geysers as the Europan crust spread open to expose warmer layers beneath.

The effect would have been similar to that seen in the Earth's oceanic ridges. These various fractures are thought to have been caused in large part by the tidal stresses exerted by Jupiter; since Europa is tidally locked to Jupiter, and therefore always maintains the same approximate orientation towards the planet, the stress patterns should form a distinctive and predictable pattern.

However, only the youngest of Europa's fractures conform to the predicted pattern; other fractures appear to have occurred at increasingly different orientations the older they are.

This could be explained if Europa's surface rotates slightly faster than its interior, an effect which is possible due to the subsurface ocean mechanically decoupling the moon's surface from its rocky mantle and to the effects of Jupiter's gravity tugging on the moon's outer ice crust.

Comparisons of Voyager and Galileo spacecraft photos serve to put an upper limit on this hypothetical slippage of no faster than once every 10, years for the surface relative to its interior The Galileo orbiter has also found that Europa has a weak magnetic field about one quarter the strength of Ganymede's field and similar to Callisto's which varies periodically as Europa passes through Jupiter's massive magnetic field.

A likely explanation of this is that there is a large, subsurface ocean of liquid salt water. Spectrographic evidence suggests that the dark reddish streaks and features on Europa's surface may be rich in salts such as magnesium sulfate, deposited by evaporating water that emerged from within.

Sulfuric acid hydrate is another possible explanation for the contaminant observed spectroscopically. In either case, since these materials are colorless or white when pure, some other material must also be present to account for the reddish color.

Sulfur compounds are suspected. It has been suggested that life may exist in this under-ice ocean, perhaps subsisting in an environment similar to Earth's deep-ocean hydrothermal vents or the Antarctic Lake Vostok.

Life in such an ocean could possibly be similar to life on earth in the deep ocean. So far, there is no evidence that life exists on Europa but due to the likely presence of liquid water, there are proposals to send a probe there.

Europa today, probably, is a habitable environment. These are the only large, stable bodies of surface liquid known to exist anywhere other than Earth.

The surface is geologically young; although mountains and several possible cryovolcanoes have been discovered, it is relatively smooth and few impact craters have been discovered.

The atmosphere of Titan is largely composed of nitrogen and its climate includes methane and ethane clouds. With its liquids both surface and subsurface and robust nitrogen atmosphere, Titan is viewed as analogous to the early Earth, although at much lower temperature.

The satellite has thus been cited as a possible host for microbial extraterrestrial life or, at least, as a prebiotic environment rich in complex organic chemistry.

Researchers have suggested a possible underground liquid ocean might serve as a biotic environment. Cassini performed several close flybys of Enceladus in , revealing the moon's surface GeobiologyESS Venus: Perhaps had life before runaway greenhouse took over.

Phoenix is a Mars lander [scheduled to land on Mars on May 25, ] at edge of polar ice cap. Evidence for liquid water includes: a.

In particular, the probe discovered a water-rich plume venting from the moon's south polar region. This discovery, along with the presence of escaping internal heat and very few if any impact craters in the south polar region, shows that Enceladus is geologically active today.

Moons in the extensive satellite systems of gas giants often become trapped in orbital resonances that lead to forced libration or orbital eccentricity; proximity to the planet can then lead to tidal heating of the satellite's interior, offering a possible explanation for the activity.

Enceladus is one of only three outer solar system bodies along with Jupiter's moon Io and Neptune's moon Triton where active eruptions have been observed.

Analysis of the outgassing suggests that it originates from a body of sub-surface liquid water, which along with the unique chemistry found in the plume, has fueled speculations that Enceladus may be important in the study of astrobiology.

The discovery of the plume has added further weight to the argument that material released from Enceladus is the source of the E-ring Thanks to data from a number of instruments on the Cassini spacecraft in , cryovolcanism, where water and other volatiles are the materials erupted instead of silicate rock, has been discovered on Enceladus.

But in the Martian arctic, water ice lurks just below ground level. Discoveries made by the Mars Odyssey Orbiter in show large amounts of subsurface water ice in the northern arctic plain.

The Phoenix lander targets this circumpolar region using a robotic arm to dig through the protective top soil layer to the water ice below and ultimately, to bring both soil and water ice to the lander platform for sophisticated scientific analysis.

The mineral can precipitate out of water and collect in layers at the bottom of a lake, spring, or other standing water.

But hematite can also occur without water, as the result of volcanic activity. Scientists have wanted to find out which of these processes created grey hematite on Mars since , when Mars Global Surveyor spotted large concentrations of the mineral near the planet's equator Pseudomorphs: Many concretions show crystals pseudomorphs of sulfate minerals.

These contain water and form by precipitation from evaporating water evaporites. Some minerals contain bound water, e. Polar ice caps: Beneath carbon dioxide surface at polar ice caps water ice is found The polar caps expand and contract.

Geologic features: River channels, Elongate islands, Ripple cross-bedding Liquid water from comets etc. May have had a thicker atmosphere, providing greenhouse effect to keep planet warmer.

It was hit by large meteorites temporarily warming local areas. The meteorite ALH was found in Antarctica see earlier notes and footnote.

It is thought to have come from Mars due to composition of trapped gases. Does it show evidence of fossil life? Lines of investigation included: a.

These are smaller than known cellular life which is typically nm, probably too small for DNA processes, etc. The smallest theoretical living cell is c.

Pyrite crystals or sulfide minerals consistent with sulfide metabolism by bacteria. But the sulfur isotopes are ambiguous in origin, and may be nonbiological.

Other investigators thought these were probably contaminants from snowmobile exhaust, based on unexpected high 14C suggesting recent origin.

Several other sites also showed hematite, e. Because terrestrial hematite is typically a mineral formed in aqueous environments, or by aqueous alteration, this detection was scientifically interesting enough that the second of the two Mars Exploration Rovers was targeted to a site in the Terrra Meridiani region designated Meridiani Planum.

In-situ investigations by the Opportunity rover showed a significant amount of hematite, much of it in the form of small spherules that were informally tagged by the science team "blueberries" a term which is somewhat confusing, since in spectrally-correct color images they are, in fact, silver-grey in color.

Analysis indicates that these spherules are apparently concretions formed from a water solution. The spirochetes contain very thin bacteria with some species having cell diameters of about 0.

Apart from this group, the author is not aware that any of other phylogenetic groups produce cells or buds that are less than 0.

Likewise, buds, baeocytes, resting, and dispersal stages such as spores and cysts are not known to be less than 0.

Magnetite crystals resembling inclusions found in magnetotactic bacteria used by them for orientation.

This is the only evidence that has not been readily refuted as nonbiological, and these may be biogenic. Magnetite tends to form octahedral crystals.

The crystals found in magnetotactic bacteria are distinctive. But similar nonbiogenic magnetite crystals have been synthesized.

The solar system solid bodies formed c. Thus the evidence for life presented by this meteorite has been largely discredited. However, life on Mars has not been ruled out.

Methane in atmosphere has been detected in certain areas, but may be nonbiogenic. But the methane raises the question of methanogenic microbes such as Archaea found on earth living subsurface or under polar ice-caps.

This ratio should be studied further81, including by Phoenix lander. Such study can be done with robotic craft and does not require humans to be present.

However, there is at present very little laboratory verification of the isotopic fractionation factors involved in abiogenic hydrocarbon formation.

Therefore, if Martian methane has an excess of carbon relative to carbon, it would suggest a Red Planet teeming with organisms.

In March the Mars Express Orbiter also suggested the presence of methane in the Martian atmosphere, with a concentration of about 10 ppb by volume.

The presence of methane on Mars is very intriguing, since as an unstable gas it indicates that there must be or have been within the last few hundred years a source of the gas on the planet.

Volcanic activity, comet impacts, and the existence of life in the form of microorganisms such as methanogens are among possible but as yet unproven sources.

The methane appears to occur in patches, which suggests that it is being rapidly broken down before it has time to become uniformly distributed in the atmosphere, and so it is presumably also continually being released to the atmosphere.

Plans are now being made to look for other companion gases that may suggest which sources are most likely; in the Earth's oceans biological methane production tends to be accompanied by ethane, while volcanic methane is accompanied by sulfur dioxide.

It was also recently shown that methane could be produced by a non-biological process involving water, carbon dioxide, and the mineral olivine, which is known to be common on Mars.

The required conditions for this reaction i. To prove this process is occurring, one could try detecting serpentine, a mineral by-product of the process.

The ESA found that the concentrations of methane in the martian atmosphere was not even, but rather that it coincided with the presence of water vapour.

In the upper atmosphere these two gasses are uniformly distributed, but near the surface they concentrate in three equatorial regions, namely Arabia Terra, Elysium Planitia, and Arcadia Memnonia.

Planetary scientist David H. Grinspoon Southwest Research Institute feels that the coincidence of water vapour and methane increases the chance of a biological source, but cautions that it is uncertain how life could have survived so long on a planet as inhospitable as Mars.

Ultimately, to prove an organic nature for the methane, a future probe or lander hosting a mass spectrometer will be needed, since the isotopic proportions of C12 to C14 [MCM: this is wrong isotope, should be C13] can clearly distinguish between GeobiologyESS Depicted in movie Contact.

Not funded by US Government, private. Nothing found yet in 11 years of listening. The Drake Equation83 of astronomer Dr.

Frank Drake estimates the number of extraterrestrial civilizations in our Milky Way galaxy with which we might be able to communicate.

RB: Drake equation etc. But evolution to our current state of development took 4. Currently, we have no evidence for life outside the Earth.

Panspermia [MCM: This was in slides but not discussed in class fully] Microbial life might have originated often in the Universe.

Microbial life is easier to evolve than technological organisms, and is longer lived because it can survive more extreme conditions than higher organisms.

It can travel through space microbes survive space exposure for months. There is the possibility of transfer between planets by meteorites or comets.

Early Earth History per RB and supplemented an inorganic and an organic biologic - bacterial decay origin of the methane.

Exogenesis is a more limited hypothesis that proposes life on Earth was transferred from elsewhere in the Universe but makes no prediction about how widespread it is.

Because the term "panspermia" is more well-known, it tends to be used in reference to what would properly be called exogenesis.

Nothing survives from the original formation of the Earth, due to effects of plate tectonics. The primordial nearly spherical solar nebula of gas and dust condensed into a central Sun and a protoplanetary disk with myriad planetesimals including oblique orbits.

For example, the meteorites known as chondrites87 are undifferentiated since their formation, have chondrules see footnote. These chondrules include refractory Ca-Al inclusions CAls , which are the oldest known solid structures in the universe at 4.

They formed when various types of dust and small grains that were present in the early solar system accreted to form primitive asteroids.

Prominent among the components present in chondrites are the enigmatic chondrules, millimeter-sized objects that originated as freely floating, molten or partially molten droplets in space; most chondrules are rich in the silicate minerals olivine and pyroxene.

Chondrites also contain refractory inclusions including Ca-Al Inclusions , which are among the oldest objects to form in the solar system, particles rich in metallic Fe-Ni and sulfides, and isolated grains of silicate minerals.

The remainder of chondrites consists of fine-grained micrometer-sized or smaller dust, which may either be present as the matrix of the rock or may form rims or mantles around individual chondrules and refractory inclusions.

Embedded in this dust are presolar grains, which predate the formation of our solar system and originated elsewhere in the galaxy. There are currently over 27, chondrites in the world's collections.

The largest individual stone ever recovered, weighing kg, was part of the Jilin meteorite shower of Chondrite falls range from single stones to extraordinary showers consisting of thousands of individual stones, as occurred in the Holbrook fall of , where an estimated 14, stones rained down on northern Arizona.

The method relies on the coupled chronometer provided by the decay of U to Pb, with a half-life of 4. One of the advantages of uranium-lead dating is the two separate, chemically identical chronometers.

Loss leakage of lead within the sample will result in a discrepancy in the two decay schemes, resulting in a different age determined by each decay scheme.

This effect is referred to as discordance, and provides a check on the reliability of the age Uranium-lead dating is usually performed on the mineral zircon ZrSiO4 , though it can be used on other minerals such as monazite, titanite, and baddeleyite.

Zircon incorporates uranium and thorium atoms into its crystalline structure, but strongly rejects lead.

Zircon is very chemically inert and resistant to mechanical weathering -- a mixed blessing for geochronologists, as zones or even whole crystals can survive melting of their parent rock with their original uranium-lead age intact The chondrites are only slightly younger than the chondrules they contain.

The coagulation of the solar nebula by cold accretion happened rapidly, perhaps over a million years or less.

IDPs are identifiable at South Pole ice. We are also searching for IDPs in Archean sediments and in upper stratospheric studies.

But what is the age of the Earth? Moon Forming Impact and Attainment of Final Earth Mass : It is perhaps best to regard the age of the Earth as based on the time when it gained the current mass and composition.

The Earth was knocked off its normal [perpendicular] orbital axis [to the present tilt of The Earth is the densest92 of the 4 inner planets [Mercury 5.

The oldest Moon rocks are 4. The source rocks for these zircon crystals have not yet been found. The ages measured for Earth's oldest rocks and oldest crystals show that the Earth is at least 4.

Measurements of zircons indicate that the Earth is at least 4. When the protoplanet Theia had grown to about the size of Mars, it became too massive to reside stably in a Trojan orbit.

As a result, its angular distance from Earth fluctuated, with the fluctuations growing larger until it hit the Earth.

This is calculated to have occurred 4. Current estimates based on computer simulations of such an event suggest that some two percent of the original mass of Theia ended up as an orbiting ring of debris, about half of which coalesced into the Moon between one and years after the impact The impact has been computer modeled, leading to hypotheses about the relative distributions of mantle and core components.

Before the impact, the Earth was probably more homogeneous with possibly a chondritic composition. Afterwards the Earth was hot and became differentiated [core and mantle formed, etc.

The collision added to the angular momentum of the Earth. This may represent a robust estimate for the primary crystallization age of the earliest lunar crust.

Hartmann : a. This is because Earth's iron had already drained into the core by the time the giant impact happened.

Therefore, the debris blown out of both Earth and the impactor came from their iron-depleted, rocky mantles. The iron core of the impactor melted on impact and merged with the iron core of Earth, according to computer models.

Earth has a mean density of 5. The reason is the same, that the moon lacks iron. The moon has exactly the same oxygen isotope composition as the Earth, whereas Mars rocks and meteorites from other parts of the solar system have different oxygen isotope compositions.

This shows that the moon formed from material formed in Earth's neighborhood. If a theory about lunar origin calls for an evolutionary process, it has a hard time explaining why other planets do not have similar moons.

Only Pluto has a moon that is an appreciable fraction of its own size. Our giant impact hypothesis had the advantage of invoking a stochastic catastrophic event that might happen only to one or two planets out of nine.

Siderophile elements are the high-density transition metals that tend to bond with metallic iron in the solid or molten state. Cameron and William R.

Ward Harvard University, Cambridge MA pointed out that a collision with a body having at least the mass of Mars would be needed to give the Earth the present angular momentum of the Earth-Moon system Measurements show that the Moon is receding from Earth at a rate of about 3.

This is a direct consequence of friction and the dissipation of energy as water moves over the ocean bottom and into or out of bays and estuaries.

Each bulge exerts a small amount of gravitational attraction on the moon, with the bulge closest to the moon pulling in a direction slightly forward along the moon's orbit, because the earth's rotation has carried the bulge forward.

The opposing bulge has the opposite effect, but the closer bulge dominates due to its comparative closer distance to the moon.

As a result, some of the Earth's rotational momentum is gradually being transferred to the Moon's orbital momentum, and this causes the Moon to slowly recede from Earth at the rate of approximately 38 millimetres per year.

Due to conservation of angular momentum, the Earth's rotation is gradually slowing, and the Earth's day thus lengthens by about 17 microseconds every year.

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Neptune's moon Triton is believed to be a captured KBO. Pluto, a dwarf planet, is the largest known member of the Kuiper belt.

Originally considered a planet, it has many physical properties in common with the objects of the Kuiper belt, and has been known since the early s to share its orbit with a number of similarly sized KBOs, now called Plutinos.

The Kuiper belt should not be confused with the hypothesized Oort cloud, which is a thousand times more distant.

The objects within the Kuiper belt, together with the members of the scattered disc and any potential Hills cloud or Oort cloud objects, are collectively referred to as trans-Neptunian objects TNOs.

This is approximately times the distance from the Sun to Pluto or nearly a light year. The outer extent of the Oort cloud places the boundary of our Solar System at nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun.

Although no confirmed direct observations of the Oort cloud have been made, astronomers believe it to be the source of all long period and Halley-type comets entering the inner solar system some short-period comets, based on their orbits, may come from the Kuiper belt.

According to the hypothesis, the Oort cloud contains trillions of comet nuclei, which are stable because the Sun's radiation is very weak at their distance.

The cloud provides a continual supply of new comets, replacing those that are destroyed. In order for it to supply the necessary volume of comets, the total mass of comets in the Oort cloud must be many times that of Earth.

The Oort cloud is thought to occupy a vast space from the outer boundary of the Kuiper belt at 50 AU to as far as 50, AU from the Sun.

It can be subdivided into spherical outer Oort cloud 20,, AU and doughnut-shaped inner Oort cloud , AU.

The outer cloud is only weakly bound to the Sun and supplies the long period and possibly Halley-type comets to the inner part of the Solar System.

The inner Oort cloud is also known as the Hills cloud, and may be the source of Halley-type comets. Some scientists think that the Hills cloud may contain much more material than the outer cloud.

This hypothesis is employed to explain the continued existence of the Oort cloud over the course of billions of years. It is thought that other stars are likely to possess Oort clouds of their own, and that the outer edges of two nearby stars' Oort clouds may sometimes overlap, causing perturbations in the comets' orbits and thereby increasing the number of comets that enter the inner solar system.

The interactions of the Oort cloud with those of neighboring stars, and its deformation by the galactic tide are thought to be the main triggers which send the long-period comets into the inner Solar System.

Its mass is not known with certainty, but is unlikely to be more than a few Earth masses. Earlier it was thought to be more massive up to Earth masses.

However, the improved knowledge about the size distribution of the long period comets led to much lower values. The mass of the inner Oort cloud is not currently known.

Beyond the ice-line radius, water exists only as water ice. Later, one or both of the following occurs53 in planets that develop liquid water: a.

Migration: Planets with water ice migrate inwards due to a gravitational disturbance of their orbits that place them closer to the host star and thereby allow liquid water to exist.

Mercury lies inside the ice-line, and has no liquid water. It was one of the major factors in the differentiation of the planets into two very distinct groups: terrestrials and giants.

Searching for Water in the Universe. Springer New York. This boundary is also known as the snow line. The location of the ice line depends on the host star's luminosity.

In the Solar System, the ice line is located at a radius of about 3 AU. This marks the boundary between the inner Solar System, which is dominated by rocky planets and asteroids, and the outer Solar System, which is dominated by the four giant planets, their icy moons, and the debris fields of the Kuiper Belt.

For M dwarf stars, the ice line will be correspondingly closer to the host star, at about 0. The location of the ice line is a key variable in planetary system formation, since rocky planets like Earth and Mars must form within the ice line, while gas giants like Jupiter, Saturn, and most known extrasolar planets preferentially form just outside the ice line.

The finding supports a controversial idea that cometary impacts billions of years ago could have provided most of the water in Earth's oceans.

The floors of some deep craters near the poles are never exposed to direct sunlight, and temperatures there remain far lower than the global average.

Water ice strongly reflects radar, and observations reveal that there are patches of very high radar reflection near the poles.

While ice is not the only possible cause of these reflective regions, astronomers believe it is the most likely The Sun was formerly less luminous at visible wavelengths though not in UV wavelengths.

Note that other features listed above and below also play a role in planetary habitability. For life to succeed, the Solar variation: Solar variations are changes in the amount of radiant energy emitted by our Sun.

There are periodic components to these variations, the principal one being the year solar cycle or sunspot cycle , as well as fluctuations which are aperiodic.

Solar activity has been measured via satellites during recent decades and through 'proxy' variables in prior times. Climate scientists are interested in understanding what, if any, effect variations in solar activity have on the Earth.

Any such mechanism is referred to as "solar forcing". The variations in total solar irradiance TSI remained at or below the threshold of detectability until the satellite era, although the small fraction in ultra-violet wavelengths varies by a few percent.

Total solar output is now measured to vary over the last three year sunspot cycles by approximately 0. There are no direct measurements of the longer-term variation and interpretations of proxy measures of variations differ; recent results suggest about 0.

The combination of solar variation and volcanic effects has very likely been the cause of some climate change, for example during the Maunder Minimum.

A study and review of existing literature, published in Nature, determined that there has been no net increase in solar brightness since the mid s, and that changes in solar output within the past years are unlikely to have played a major part in global warming.

It should be stressed, the same report cautions that "Apart from solar brightness, more subtle influences on climate from cosmic rays or the Sun's ultraviolet radiation cannot be excluded, say the authors.

However, these influences cannot be confirmed, they add, because physical models for such effects are still too poorly developed.

G and F types extend this range to 1. The Sun is a G2 star. Each spectral class denotes a typical range of mass and luminosity.

For classes O through M, each spectral class represents a successively more numerous population of stars -- in other words, stellar populations vary inversely according to mass.

The spectra of K stars are dominated by the H and K lines of calcium and lines of neutral iron and titanium, with molecular bands due to cyanogen CN and titanium dioxide TiO becoming increasingly prominent at the cooler end of the range.

K-type main 56 GeobiologyESS If a star is more than 2 MSol, the time available for formation of liquid water is less than 1 billion years before the star becomes a red giant.

Humans have a narrow temperature tolerance range, but less complex organisms are not as picky.

Stars like the Sun tend to increase in luminosity60,61 with time as continuing fusion of H to He reduces internal pressure. The appropriate spectral range for [stars suitable for planetary habitability] is presently considered to be "early F" or "G", to "mid-K".

This corresponds to temperatures of a little more than 7, K down to a little more than 4, K; the Sun, a G2 star, is well within these bounds.

They live at least a few billion years, allowing life a chance to evolve. More luminous main-sequence stars of the "O," "B," and "A" classes [which are larger] usually live less than a billion years and in exceptional cases less than 10 million years.

They emit enough high-frequency ultraviolet radiation to trigger important atmospheric dynamics such as ozone formation, but not so much that ionisation destroys incipient life.

Liquid water may exist on the surface of planets orbiting them at a distance that does not induce tidal lock. Red giants are stars with radii hundreds of times larger than that of the Sun which have exhausted the supply of hydrogen in their cores and switched to fusing hydrogen in a shell outside the core.

Since the inert helium core has no source of energy of its own, it contracts and heats up, and its gravity compresses the hydrogen in the layer immediately above it, thus causing it to fuse faster.

This in turn causes the star to become more luminous from 1, to 10, times brighter and expand; the degree of expansion outstrips the increase in luminosity, thus causing the effective temperature to decrease.

Main sequence stars of spectral types A through K are believed to become red giants The Sun is expected to become a red giant in about 4.

It is calculated that the Sun will become almost sufficiently large to engulf the current orbits of some of the solar system's inner planets, including Earth.

However, the gravitational pull of the Sun will have weakened by then due to its loss of mass, and all planets but Mercury will escape to a wider orbit.

That said, Earth's biosphere will be destroyed as the Sun gets brighter while its hydrogen supply becomes depleted. The extra solar energy will cause the oceans to evaporate to space, causing Earth's atmosphere to become temporarily similar to that of Venus, before its atmosphere also gets driven off into space.

Venus will become a burnt out planet; its atmosphere having long been driven off, and its rock will melt.

This is a line calculated by computer models of where a star will be when it begins hydrogen fusion; its brightness and surface temperature typically increase from this point with age.

Stars usually enter and leave the main sequence from about when they are born or when they are starting to GeobiologyESS The X-axis was time in Ga going from c.

Te is the earth temperature considered as a "black body" i. The diagram also shows a graph of relative solar luminosity variation over this same time period, rising from c.

This diagram shows a line for the freezing point of water, above which the Ts line eventually rises, but not until about 1. Stars like the Sun increase in luminosity as fusion of H to He reduces internal pressure.

Its gravity compresses the hydrogen in the layer immediately above it [the helium core], thus causing it to fuse faster than hydrogen would fuse in a main-sequence star of the same mass.

See also pp. Subdivided into apparent measuring visible light only and bolometric measuring all forms of radiant energy. It is equal to the luminosity of the Sun, which is 3.

The standard solar model describes the history and evolution of stars. An aspect of this model is that stars similar to the Sun should gradually brighten over their life time excluding a very bright phase just after formation.

This prediction is supported by the observation of lower brightness in young stars of solar type. However, with the predicted brightness 4 billion years ago and with greenhouse gas concentrations the same as are current for the modern Earth, any liquid water exposed to the surface would quickly freeze solid.

This contradicts geological observations of sedimentary rocks, which required the presence of flowing liquid water to form.

The tension between the two hypotheses stems from the incorrect assumption that atmospheric gas concentrations in the past were the same as today.

First, before the advent of abundant life, the atmospheric oxygen concentrations were orders of magnitude lower than today.

In the presence of oxygen, methane breaks down to carbon dioxide, so in the absence of oxygen the methane concentration could be much larger than currently observed.

Methane is a more potent GeobiologyESS If temperature nears the boiling point of water, can get positive feedback since water vapor is a strong greenhouse gas , causing more vaporization, thus more heat [infrared] trapping, thus more boiling, etc.

In the Venusian atmosphere, the only zone potentially habitable with liquid water is possible in a band in the atmosphere [at about 50 to 70 km] where temperature drops from the c.

The lowest temperature is at about km, and temperatures increase above this altitude in the higher atmosphere up to km.

Radiation Levels: The region near the center of the galaxy has high ionizing radiation levels, lessening the likelihood of life, especially if the galaxy has a central black hole.

These emissions arise from gamma ray bursters and supernovae, etc. Thus carbon concentration declines as one goes outward, decreasing the likelihood of carbon-based life.

The Sun is estimated at 28, light years from the center, or 8. Further, the inorganic version of the carbon cycle can be expected to provide negative feedback towards an Earth with liquid water.

Carbon dissolved in liquid water can form carbonic acids, which can then interact with calcium to produce calcium carbonate. If rainfall were to cease and the oceans froze over, then this part of the inorganic carbon cycle would shut down.

Periodic explosions from volcanoes would then cause a net increase in the atmospheric carbon dioxide and methane levels with no liquid water to absorb these emitted gases.

Eventually the concentrations would become large enough that the surface temperature would rise due to the greenhouse effect.

When the surface temperature became large enough for the oceans to melt and rainfall to resume the other half of the inorganic carbon cycle would turn on and moderate the greenhouse gas concentrations.

It is also noteworthy, that even though evidence of flowing water exists even from very early in Earth's history, there may still have been a number of examples of periods when the Earth's oceans froze over completely.

Guillermo Gonzalez, one of the initial proponents for GHZ, was denied tenure at Iowa State University due to his support for intelligent design.

Was the circumstellar habitable zone and the galactic habitable zone divinely created? Earth 2. It was warmer and wetter before 3.

Are there possible biosignatures in a 4. See further discussion on these topics below. Europa moon of Jupiter Europa has liquid salty ocean under ice cap [MCM: frozen upper crust or surface], so could harbor life.

The icy crust exhibits magnesium sulfate line, sulfites, etc Might conceivably get energy from deep ocean hydrothermal vents resembling black smokers.

Scientists aren't sure why. It could be that the Sun formed in a part of the Galaxy that had an abundance of metals, and then migrated to its present position Besides requiring a metal-rich star, a Galactic Habitable Zone excludes stars too close to the galactic center.

Our Sun is a nice distance away from the galactic center, about 28, light years Staying away from the galactic center has an additional advantage.

The center of the Galaxy is awash in harmful radiation. Solar systems near the center would experience increased exposure to gamma rays, X-rays, and cosmic rays, which would destroy any life trying to evolve on a planet Keeping out of the way of the Galaxy's spiral arms is another requirement of the Galactic Habitable Zone The intense radiation and gravitation of a spiral arm would cause disruptions in our Solar System just as surely as if we were closer to the center of the Galaxy.

Luckily, our Sun revolves at the same rate as the Galaxy's spiral-arm rotation. This synchronization prevents our Solar System from crossing a spiral arm too often The unusually circular orbit of our Sun around the galactic center also tends to keep it clear of the spiral arms.

Most stars the same age as our Sun have more elliptical orbits Thus, thanks to a lot of unusual characteristics of our Sun, our Solar System is lucky enough to lie in a Galactic Habitable Zone.

Gonzalez argues that these characteristics made it possible for complex life to emerge on Earth.

More than 95 percent of stars in the Galaxy, says Gonzalez, wouldn't be able to support habitable planets simply because their rotation is not synchronized with the rotation of the galaxy's spiral arms.

On discovery, its mass was 1. On August 6, ALH became newsworthy when it was announced that it was believed that the meteorite contained traces of life from Mars, as published in an article in Science by Dr.

At just over kilometers in diameter, it is slightly smaller than the Earth's moon and the sixth largest moon in the solar system.

The satellite has a very tenuous oxygen atmosphere and one of the smoothest surfaces in the solar system.

The young surface of the moon is striated by cracks and streaks, while craters are relatively infrequent.

Due to a hypothesized water ocean beneath its icy surface, and an energy source provided by tidal heating, Europa has been cited as a possible host of extraterrestrial life.

The heat energy ensures the ocean remains liquid and also drives geological activity The intriguing character of Europa has led to a number of ambitious exploration proposals; to date, only flyby missions have visited the moon.

The Galileo mission provided the bulk of current data on the satellite, while the abortive Jupiter Icy Moons Orbiter, cancelled in , was the most ambitious planned spacecraft.

Conjecture on extraterrestrial life has ensured a high profile for the moon and led to continued lobbying for future missions Titan moon of Saturn It has suspected hydrocarbon lakes and rains including methane, ethane, etc.

Enceladus moon of Saturn Its South Pole exhibits water geysers. It is ice-covered but also thought to have a subsurface ocean warmed by tidal heating as a result of its proximity to Saturn , so perhaps could harbor life there.

Europa's most striking surface feature is a series of dark streaks criss-crossing the entire globe. Close examination shows that the edges of Europa's crust on either side of the cracks have moved relative to each other.

The larger bands are roughly 20 km 12 mi across commonly with dark diffuse outer edges, regular striations, and a central band of lighter material Among the controversial hypotheses put forward to explain these features [Lineae], one states that they may have been produced by a series of volcanic water eruptions or geysers as the Europan crust spread open to expose warmer layers beneath.

The effect would have been similar to that seen in the Earth's oceanic ridges. These various fractures are thought to have been caused in large part by the tidal stresses exerted by Jupiter; since Europa is tidally locked to Jupiter, and therefore always maintains the same approximate orientation towards the planet, the stress patterns should form a distinctive and predictable pattern.

However, only the youngest of Europa's fractures conform to the predicted pattern; other fractures appear to have occurred at increasingly different orientations the older they are.

This could be explained if Europa's surface rotates slightly faster than its interior, an effect which is possible due to the subsurface ocean mechanically decoupling the moon's surface from its rocky mantle and to the effects of Jupiter's gravity tugging on the moon's outer ice crust.

Comparisons of Voyager and Galileo spacecraft photos serve to put an upper limit on this hypothetical slippage of no faster than once every 10, years for the surface relative to its interior The Galileo orbiter has also found that Europa has a weak magnetic field about one quarter the strength of Ganymede's field and similar to Callisto's which varies periodically as Europa passes through Jupiter's massive magnetic field.

A likely explanation of this is that there is a large, subsurface ocean of liquid salt water. Spectrographic evidence suggests that the dark reddish streaks and features on Europa's surface may be rich in salts such as magnesium sulfate, deposited by evaporating water that emerged from within.

Sulfuric acid hydrate is another possible explanation for the contaminant observed spectroscopically. In either case, since these materials are colorless or white when pure, some other material must also be present to account for the reddish color.

Sulfur compounds are suspected. It has been suggested that life may exist in this under-ice ocean, perhaps subsisting in an environment similar to Earth's deep-ocean hydrothermal vents or the Antarctic Lake Vostok.

Life in such an ocean could possibly be similar to life on earth in the deep ocean. So far, there is no evidence that life exists on Europa but due to the likely presence of liquid water, there are proposals to send a probe there.

Europa today, probably, is a habitable environment. These are the only large, stable bodies of surface liquid known to exist anywhere other than Earth.

The surface is geologically young; although mountains and several possible cryovolcanoes have been discovered, it is relatively smooth and few impact craters have been discovered.

The atmosphere of Titan is largely composed of nitrogen and its climate includes methane and ethane clouds.

With its liquids both surface and subsurface and robust nitrogen atmosphere, Titan is viewed as analogous to the early Earth, although at much lower temperature.

The satellite has thus been cited as a possible host for microbial extraterrestrial life or, at least, as a prebiotic environment rich in complex organic chemistry.

Researchers have suggested a possible underground liquid ocean might serve as a biotic environment.

Cassini performed several close flybys of Enceladus in , revealing the moon's surface GeobiologyESS Venus: Perhaps had life before runaway greenhouse took over.

Phoenix is a Mars lander [scheduled to land on Mars on May 25, ] at edge of polar ice cap. Evidence for liquid water includes: a.

In particular, the probe discovered a water-rich plume venting from the moon's south polar region. This discovery, along with the presence of escaping internal heat and very few if any impact craters in the south polar region, shows that Enceladus is geologically active today.

Moons in the extensive satellite systems of gas giants often become trapped in orbital resonances that lead to forced libration or orbital eccentricity; proximity to the planet can then lead to tidal heating of the satellite's interior, offering a possible explanation for the activity.

Enceladus is one of only three outer solar system bodies along with Jupiter's moon Io and Neptune's moon Triton where active eruptions have been observed.

Analysis of the outgassing suggests that it originates from a body of sub-surface liquid water, which along with the unique chemistry found in the plume, has fueled speculations that Enceladus may be important in the study of astrobiology.

The discovery of the plume has added further weight to the argument that material released from Enceladus is the source of the E-ring Thanks to data from a number of instruments on the Cassini spacecraft in , cryovolcanism, where water and other volatiles are the materials erupted instead of silicate rock, has been discovered on Enceladus.

But in the Martian arctic, water ice lurks just below ground level. Discoveries made by the Mars Odyssey Orbiter in show large amounts of subsurface water ice in the northern arctic plain.

The Phoenix lander targets this circumpolar region using a robotic arm to dig through the protective top soil layer to the water ice below and ultimately, to bring both soil and water ice to the lander platform for sophisticated scientific analysis.

The mineral can precipitate out of water and collect in layers at the bottom of a lake, spring, or other standing water. But hematite can also occur without water, as the result of volcanic activity.

Scientists have wanted to find out which of these processes created grey hematite on Mars since , when Mars Global Surveyor spotted large concentrations of the mineral near the planet's equator Pseudomorphs: Many concretions show crystals pseudomorphs of sulfate minerals.

These contain water and form by precipitation from evaporating water evaporites. Some minerals contain bound water, e.

Polar ice caps: Beneath carbon dioxide surface at polar ice caps water ice is found The polar caps expand and contract.

Geologic features: River channels, Elongate islands, Ripple cross-bedding Liquid water from comets etc. May have had a thicker atmosphere, providing greenhouse effect to keep planet warmer.

It was hit by large meteorites temporarily warming local areas. The meteorite ALH was found in Antarctica see earlier notes and footnote.

It is thought to have come from Mars due to composition of trapped gases. Does it show evidence of fossil life? Lines of investigation included: a.

These are smaller than known cellular life which is typically nm, probably too small for DNA processes, etc.

The smallest theoretical living cell is c. Pyrite crystals or sulfide minerals consistent with sulfide metabolism by bacteria. But the sulfur isotopes are ambiguous in origin, and may be nonbiological.

Other investigators thought these were probably contaminants from snowmobile exhaust, based on unexpected high 14C suggesting recent origin.

Several other sites also showed hematite, e. Because terrestrial hematite is typically a mineral formed in aqueous environments, or by aqueous alteration, this detection was scientifically interesting enough that the second of the two Mars Exploration Rovers was targeted to a site in the Terrra Meridiani region designated Meridiani Planum.

In-situ investigations by the Opportunity rover showed a significant amount of hematite, much of it in the form of small spherules that were informally tagged by the science team "blueberries" a term which is somewhat confusing, since in spectrally-correct color images they are, in fact, silver-grey in color.

Analysis indicates that these spherules are apparently concretions formed from a water solution. The spirochetes contain very thin bacteria with some species having cell diameters of about 0.

Apart from this group, the author is not aware that any of other phylogenetic groups produce cells or buds that are less than 0.

Likewise, buds, baeocytes, resting, and dispersal stages such as spores and cysts are not known to be less than 0. Magnetite crystals resembling inclusions found in magnetotactic bacteria used by them for orientation.

This is the only evidence that has not been readily refuted as nonbiological, and these may be biogenic. Magnetite tends to form octahedral crystals.

The crystals found in magnetotactic bacteria are distinctive. But similar nonbiogenic magnetite crystals have been synthesized.

The solar system solid bodies formed c. Thus the evidence for life presented by this meteorite has been largely discredited. However, life on Mars has not been ruled out.

Methane in atmosphere has been detected in certain areas, but may be nonbiogenic. But the methane raises the question of methanogenic microbes such as Archaea found on earth living subsurface or under polar ice-caps.

This ratio should be studied further81, including by Phoenix lander. Such study can be done with robotic craft and does not require humans to be present.

However, there is at present very little laboratory verification of the isotopic fractionation factors involved in abiogenic hydrocarbon formation.

Therefore, if Martian methane has an excess of carbon relative to carbon, it would suggest a Red Planet teeming with organisms.

In March the Mars Express Orbiter also suggested the presence of methane in the Martian atmosphere, with a concentration of about 10 ppb by volume.

The presence of methane on Mars is very intriguing, since as an unstable gas it indicates that there must be or have been within the last few hundred years a source of the gas on the planet.

Volcanic activity, comet impacts, and the existence of life in the form of microorganisms such as methanogens are among possible but as yet unproven sources.

The methane appears to occur in patches, which suggests that it is being rapidly broken down before it has time to become uniformly distributed in the atmosphere, and so it is presumably also continually being released to the atmosphere.

Plans are now being made to look for other companion gases that may suggest which sources are most likely; in the Earth's oceans biological methane production tends to be accompanied by ethane, while volcanic methane is accompanied by sulfur dioxide.

It was also recently shown that methane could be produced by a non-biological process involving water, carbon dioxide, and the mineral olivine, which is known to be common on Mars.

The required conditions for this reaction i. To prove this process is occurring, one could try detecting serpentine, a mineral by-product of the process.

The ESA found that the concentrations of methane in the martian atmosphere was not even, but rather that it coincided with the presence of water vapour.

In the upper atmosphere these two gasses are uniformly distributed, but near the surface they concentrate in three equatorial regions, namely Arabia Terra, Elysium Planitia, and Arcadia Memnonia.

Planetary scientist David H. Grinspoon Southwest Research Institute feels that the coincidence of water vapour and methane increases the chance of a biological source, but cautions that it is uncertain how life could have survived so long on a planet as inhospitable as Mars.

Ultimately, to prove an organic nature for the methane, a future probe or lander hosting a mass spectrometer will be needed, since the isotopic proportions of C12 to C14 [MCM: this is wrong isotope, should be C13] can clearly distinguish between GeobiologyESS Depicted in movie Contact.

Not funded by US Government, private. Nothing found yet in 11 years of listening. The Drake Equation83 of astronomer Dr.

Frank Drake estimates the number of extraterrestrial civilizations in our Milky Way galaxy with which we might be able to communicate.

RB: Drake equation etc. But evolution to our current state of development took 4. Currently, we have no evidence for life outside the Earth.

Panspermia [MCM: This was in slides but not discussed in class fully] Microbial life might have originated often in the Universe.

Microbial life is easier to evolve than technological organisms, and is longer lived because it can survive more extreme conditions than higher organisms.

It can travel through space microbes survive space exposure for months. There is the possibility of transfer between planets by meteorites or comets.

Early Earth History per RB and supplemented an inorganic and an organic biologic - bacterial decay origin of the methane. Exogenesis is a more limited hypothesis that proposes life on Earth was transferred from elsewhere in the Universe but makes no prediction about how widespread it is.

Because the term "panspermia" is more well-known, it tends to be used in reference to what would properly be called exogenesis.

Nothing survives from the original formation of the Earth, due to effects of plate tectonics. The primordial nearly spherical solar nebula of gas and dust condensed into a central Sun and a protoplanetary disk with myriad planetesimals including oblique orbits.

For example, the meteorites known as chondrites87 are undifferentiated since their formation, have chondrules see footnote.

These chondrules include refractory Ca-Al inclusions CAls , which are the oldest known solid structures in the universe at 4.

They formed when various types of dust and small grains that were present in the early solar system accreted to form primitive asteroids.

Prominent among the components present in chondrites are the enigmatic chondrules, millimeter-sized objects that originated as freely floating, molten or partially molten droplets in space; most chondrules are rich in the silicate minerals olivine and pyroxene.

Chondrites also contain refractory inclusions including Ca-Al Inclusions , which are among the oldest objects to form in the solar system, particles rich in metallic Fe-Ni and sulfides, and isolated grains of silicate minerals.

The remainder of chondrites consists of fine-grained micrometer-sized or smaller dust, which may either be present as the matrix of the rock or may form rims or mantles around individual chondrules and refractory inclusions.

Embedded in this dust are presolar grains, which predate the formation of our solar system and originated elsewhere in the galaxy.

There are currently over 27, chondrites in the world's collections. The largest individual stone ever recovered, weighing kg, was part of the Jilin meteorite shower of Chondrite falls range from single stones to extraordinary showers consisting of thousands of individual stones, as occurred in the Holbrook fall of , where an estimated 14, stones rained down on northern Arizona.

The method relies on the coupled chronometer provided by the decay of U to Pb, with a half-life of 4. One of the advantages of uranium-lead dating is the two separate, chemically identical chronometers.

Loss leakage of lead within the sample will result in a discrepancy in the two decay schemes, resulting in a different age determined by each decay scheme.

This effect is referred to as discordance, and provides a check on the reliability of the age Uranium-lead dating is usually performed on the mineral zircon ZrSiO4 , though it can be used on other minerals such as monazite, titanite, and baddeleyite.

Zircon incorporates uranium and thorium atoms into its crystalline structure, but strongly rejects lead. Zircon is very chemically inert and resistant to mechanical weathering -- a mixed blessing for geochronologists, as zones or even whole crystals can survive melting of their parent rock with their original uranium-lead age intact The chondrites are only slightly younger than the chondrules they contain.

The coagulation of the solar nebula by cold accretion happened rapidly, perhaps over a million years or less. IDPs are identifiable at South Pole ice.

We are also searching for IDPs in Archean sediments and in upper stratospheric studies. But what is the age of the Earth?

Moon Forming Impact and Attainment of Final Earth Mass : It is perhaps best to regard the age of the Earth as based on the time when it gained the current mass and composition.

The Earth was knocked off its normal [perpendicular] orbital axis [to the present tilt of The Earth is the densest92 of the 4 inner planets [Mercury 5.

The oldest Moon rocks are 4. The source rocks for these zircon crystals have not yet been found. The ages measured for Earth's oldest rocks and oldest crystals show that the Earth is at least 4.

Measurements of zircons indicate that the Earth is at least 4. When the protoplanet Theia had grown to about the size of Mars, it became too massive to reside stably in a Trojan orbit.

As a result, its angular distance from Earth fluctuated, with the fluctuations growing larger until it hit the Earth. This is calculated to have occurred 4.

Current estimates based on computer simulations of such an event suggest that some two percent of the original mass of Theia ended up as an orbiting ring of debris, about half of which coalesced into the Moon between one and years after the impact The impact has been computer modeled, leading to hypotheses about the relative distributions of mantle and core components.

Before the impact, the Earth was probably more homogeneous with possibly a chondritic composition. Afterwards the Earth was hot and became differentiated [core and mantle formed, etc.

The collision added to the angular momentum of the Earth. This may represent a robust estimate for the primary crystallization age of the earliest lunar crust.

Hartmann : a. This is because Earth's iron had already drained into the core by the time the giant impact happened.

Therefore, the debris blown out of both Earth and the impactor came from their iron-depleted, rocky mantles.

The iron core of the impactor melted on impact and merged with the iron core of Earth, according to computer models.

Earth has a mean density of 5. The reason is the same, that the moon lacks iron. The moon has exactly the same oxygen isotope composition as the Earth, whereas Mars rocks and meteorites from other parts of the solar system have different oxygen isotope compositions.

This shows that the moon formed from material formed in Earth's neighborhood. If a theory about lunar origin calls for an evolutionary process, it has a hard time explaining why other planets do not have similar moons.

Only Pluto has a moon that is an appreciable fraction of its own size. Our giant impact hypothesis had the advantage of invoking a stochastic catastrophic event that might happen only to one or two planets out of nine.

Siderophile elements are the high-density transition metals that tend to bond with metallic iron in the solid or molten state. Cameron and William R.

Ward Harvard University, Cambridge MA pointed out that a collision with a body having at least the mass of Mars would be needed to give the Earth the present angular momentum of the Earth-Moon system Measurements show that the Moon is receding from Earth at a rate of about 3.

This is a direct consequence of friction and the dissipation of energy as water moves over the ocean bottom and into or out of bays and estuaries.

Each bulge exerts a small amount of gravitational attraction on the moon, with the bulge closest to the moon pulling in a direction slightly forward along the moon's orbit, because the earth's rotation has carried the bulge forward.

The opposing bulge has the opposite effect, but the closer bulge dominates due to its comparative closer distance to the moon. As a result, some of the Earth's rotational momentum is gradually being transferred to the Moon's orbital momentum, and this causes the Moon to slowly recede from Earth at the rate of approximately 38 millimetres per year.

Due to conservation of angular momentum, the Earth's rotation is gradually slowing, and the Earth's day thus lengthens by about 17 microseconds every year.

Thus Hf is a very short lived isotope. This led to segregation of metals from silicates. The mantle was probably initially a magma ocean, but quickly cooled because the first crust had formed by the time the Jack Hills zircons appeared in 4.

It must have solidified and then remelted partially to allow the zircons to form. Zircons come from Differentiation: In planetary science, planetary differentiation is a process by which the denser portions of a planet will sink to the center; while less dense materials rise to the surface.

Such a process tends to create a core, crust, and mantle. This was produced by the decay of hafnium Hf , an isotope with a half-life of only 9 million years.

Because tungsten dissolves enthusiastically in metallic iron and hafnium does not, it is possible to use the abundance of W in rocks formed by melting of the silicate mantle as an indicator of the timing of core formation Hafnium and tungsten have distinctive locations in early Earth, says Thorsten Kleine of the University of Munster in Germany.

Undecayed hafnium in Earth's mantle, the region that accumulated around the growing core, would have remained locked in minerals there.

In contrast, tungsten produced in the mantle would have sunk into the molten core during the time that the core was forming. Any tungsten now found in the mantle must have been produced by the decay of hafnium after the core had finished forming and no longer interacted with the mantle.

Therefore, the abundance of tungsten in rocks from Earth's mantle provides a measure of the core's age. Kleine and his colleagues, as well as a team led by Qingzhu Yin of Harvard University, measured the concentration of tungsten in a variety of Earth rocks originally from the mantle.

To date the rocks, the teams had to compare these numbers with the amount of tungsten found in objects in the solar system that had never formed a core.

Meteorites, the fragments of asteroids that have fallen to Earth, fit the bill. Kleine, T. Nature Aug.

Magma ocean must have formed before zircons formed. Granite rocks date back to 4. Granitic rocks require liquid water alteration of the melt source.

Continental crust is thick, rigid, cool, buoyant, and felsic The oldest definitely continental [subaerial] crusts date to 3.

Australia was presented with a slide. Perhaps continental crust formed as early as 3. But see Wilde et al on dating the Jack Hills zircons as early as 4.

It is the oldest known crustal rock outcrop in the world The rock exposed in the outcrop formed just over four billion 4 x years ago; an age based on radiometric dating of zircon crystals The term combines the words "feldspar" and "silica.

This is to be contrasted with submarine events or structures, those located under the sea. Subaerial weathering is weathering by rain, frost, rivers etc.

Subaerial materials are materials at the surface of the planet exposed to the air. Green, Magmatic oxygen isotope ratios in this range point toward the involvement of supracrustal material that has undergone lowtemperature interaction with a liquid hydrosphere.

This zircon thus represents the earliest evidence for continental crust and oceans on the Earth. The hypothesis of a cool early Earth suggests long intervals of relatively temperate surface conditions from 4.

Meteorite impacts during this period may have been less frequent than previously thought. Once formed, continental crust provided a more stable platform for the origin and early evolution of life: shallow water environments, diverse habitats, opportunity for heating and cooling, wetting and drying, dilute solutions, phosphate source, etc.

It is very different from early atmosphere. This bombardment was heavy enough to form craters up to 1, km in diameter on the Moon and up to 2, km on Mars Hellas, Argyre [see also below].

It was nearly neutral in redox capacity, not highly reducing like that required for Miller-Urey experiment.

This value gives the absolute number of atoms of a given isotope in atoms of total element Absolute isotope ratios R are measured for sample and standard, and the relative measure delta is calculated The evidence for this event comes primarily from the dating of lunar samples, which indicates that most impact melt rocks formed in this very narrow interval of time.

While many hypotheses have been put forth to explain a "spike" in the flux of either asteroidal or cometary materials to the inner solar system, no consensus yet exists as to its cause.

One popular theory postulates that the gas giant planets migrated in orbit at this time, causing objects in either the asteroid belt or Kuiper belt to be put onto eccentric orbits that reached the terrestrial planets.

Nevertheless, some argue that the lunar sample data do not require a cataclysmic cratering event near Mya, and that the apparent clustering of impact melt ages near this time is an artifact of sampling material affected by a single large impact basin.

If Earth formed dry, water may have been delivered to the Earth by bombardment by comets and carbonaceous chondrites. Water formed steam atmosphere.

But clouds have high albedo, reflected lots of sunlight, so surface gradually cooled. Liquid water veneer then developed.

It was once thought the oceans condensed late, and were kept hot by high radiogenic heat flow, and high greenhouse effect cf.

But first sedimentary rocks show evidence of liquid water as early as 3. These are sedimentary conglomerates with round pebbles indicating liquid water.

The Acasta gneiss 4. The Jack Hills zircons 4. If so, life could have originated in an aqueous site very early in Earth history.

There may have been liquid oceans as early as 4. But did these oceans survive continuously, even the time of the Late Heavy Bombardment?

Oxygen isotopes are fractionated by ocean vents etc Impact Frustration of Life Heavy meteorite impacts around 10x the current rate occurred, culminating in the Late Heavy Bombardment c.

Earth was also impacted, probably by even larger impactors. Craters up to 2, km formed on Mars Hellas, Argyre. The energy released in the largest of these impacts could have vaporized all oceans and all subsurface ground water down to 0.

But clouds increase albedo and cooling would have rapidly occurred. It consists of five tectonic domains The tree has its basis in a concatenation of 31 orthologs [A gene in two or more species that has evolved from a common ancestor] occurring in species with sequenced genomes Water may have been delivered to the Earth subsequently by bombardment by comets, etc.

Has the basal ratio [? But sedimentary evidence for liquid water, the absence of clear signs of glaciation before 2.

Atmospheric CO2 is transparent to visible light but not to infrared IR. The What were the possible ancient greenhouse gases?

The following summarizes greenhouse and nongreenhouse atmospheric gases]: a. Water Vapor: Water is a strong greenhouse gas, but it complicates the calculation of temperature by forming clouds that raise albedo and reflect light, partly canceling heating effect.

The relative effects of these opposing heating and cooling factors are hard to quantify, and introduce considerable uncertainty into climatic modeling of effects of CO2 on global temperatures.

It is only a weak absorber of infrared, and therefore must have at nearly 1 bar about current Earth atmospheric pressure concentration at 3.

N2: not a greenhouse gas. O2: not a greenhouse gas. It is stable and long-lived but can only exist when there is no atmospheric O2 otherwise gets oxidized to water and CO2.

Modern sources include natural and anthropogenic emissions. Microbes affect the isotopic signature of light carbon by enriching 12C relative to 13C [i.

It was one of the most severe ice ages in geologic history and some geologists believe that it was very similar to the Snowball earth ice age that happened in the neoproterozoic era.

The best studied is the Sturtian-Varangian glaciation, around Ma [MCM: thus in late Precambrian, specifically the late Proterozoic, just prior to the Cambrian which began at Ma], which may have brought glacial conditions all the way to the equator, resulting in a "Snowball Earth".

The earliest methane was released into the atmosphere by volcanic activity. During this time, Earth's earliest life appeared.

These first, ancient bacteria added to the methane concentration by converting hydrogen and carbon dioxide into methane and water.

Oxygen did not become a major part of the atmosphere until photosynthetic organisms evolved later in Earth's history. However, this high temp would have produced clouds Also, these cherts probably are tuff Thus this is an unreliable method for determining ocean temperatures The original PDB sample was a sample of fossilized shells of an extinct organism called a [Cretaceous] belemnite something like a shelled squid collected decades ago from the banks of the Pee Dee River in South Carolina.

The original sample was used up long ago, but other reference standards were calibrated to that original sample. Note that these calculations do take into consideration the stability of the gas in the atmosphere over the year period on which the calculation is based.

For the other gases The scarcity of anhydrite in modern evaporite deposits is predicted by the present results. Wells' counts of daily growth rings on fossil coral from the Middle Devonian period turned up more than per year.

The specimens he studied were about million years old and contained an average of rings annually. This means that as each year goes by, each day of the year lasts 2 hundred-thousandths of a second longer.

Thus the length of the day changes by about 20 seconds every million years. Since the earth's rotation is slowing down, it took less time in the past for the earth to rotate on its axis than it does today.

But the time for one complete rotation is the length of a day. So if the days were shorter in the past, then there were once more days in the year than there are now.

This, of course, assumes that the length of the year has not changed. This is a reasonable assumption, since the year is the time measured in unchanging units it takes for the earth to go once around the sun, and there is no known mechanism to make any measurable changes in this period over a few billion years Suppose we want to know how many days made up a year in the Devonian period, estimated to have been some million years ago.

This means each day was only A similar calculation for the Pennsylvanian period, beginning about million years ago, gives The reason for choosing the Devonian and Pennsylvanian periods is that we can check to see if these calculations correspond to reality.

In certain modern corals and shellfish, we find growth-bands that indicate yearly, monthly, and even daily growth, rather like the annual rings that trees produce.

By counting these bands, we can determine how long a particular coral or shellfish lived just as we can for a tree by counting its rings.

We can also see that there are But careful analysis of the growth-bands of fossil corals and shellfish from the Devonian and Pennsylvanian has confirmed that years in these periods contained more days than years do now, and that the number of days per year for both these periods is remarkably close to the values calculated above.

Over long time periods, leap seconds must be added at an ever increasing rate The name is based on the term "leap years", though in doing so it is a little inconsistent: leap seconds result in an extra second, while leap years result in an extra day, not an extra year The solar day has gradually become 1.

For example, bottom friction, induced by tidal currents flowing across the seabed, various kinds of wave breaking, and scattering of tidal waves into oceanic internal waves are all thought to play a role.

The plane of the Moon's orbit around the Earth lies close to the plane of the Earth's orbit around the Sun the ecliptic , rather than in the plane perpendicular to the axis of rotation of the Earth the equator as is usually the case with planetary satellites.

The mass of the Moon is sufficiently large and it is sufficiently close to raise tides in the Earth: the matter of the Earth, in particular the water of the oceans, bulges out along both ends of an axis passing through the centers of the Earth and Moon.

The average tidal bulge GeobiologyESS However there is no geologic record of such an event, which would have been accompanied by massive tides.

Therefore we must assume that Earth is dissipating tidal energy now faster than in the past. The current higher rate is thought due to the intermediate sea level and moderate-sized polar ice caps we now have.

If there were much more or much less ocean, there would be less tidal friction. RB notes plus supplements Plate tectonics is important for life because it maintains planetary homeostasis: recycles atmospheric gases [particularly CO2] and volatiles by subduction, [provides elevated land through mountain building and sediments through erosion], [?

On Earth, it is a slow and steady process, in contrast to Venus which undergoes catastrophic volcanic resurfacing every few million years.

Mars currently has no plate tectonics but has 6 large volcanoes, including the supergiant Olympus Mons, the largest volcano in the solar system [27 km high km width, compared to the largest Earth volcano Mauna Loa, which is 9 km in height from seafloor].

Intense volcanism is not good for planetary homeostasis. Evidence for earliest known plate tectonics: This is based on the distinctive rocks formed at plate boundaries.

Island arc volcanoes form where both of the converging plates are made of oceanic crust, have distinct chemistries, particularly boninites, named after the closely follows the Moon in its orbit, and the Earth rotates under this tidal bulge in just over a day.

However, the rotation drags the position of the tidal bulge ahead of the position directly under the Moon. As a consequence, there exists a substantial amount of mass in the bulge that is offset from the line through the centers of the Earth and Moon.

Because of this offset, a portion of the gravitational pull between Earth's tidal bulges and the Moon is perpendicular to the Earth-Moon line, i.

This accelerates the Moon in its orbit, and decelerates the rotation of the Earth. Ribbon-terrain distribution The rock is named for its occurrence in the Izu-Bonin arc south of Japan.

Boninite is considered to be a primitive andesite derived from melting of metasomatised mantle These are unusual lavas, combining high Si with high Mg, Ni and Cr.

They are thought to have formed by wet-melting of rather refractory lithosphere. Chichijima, at the centre of the chain, represents the type locality for the high-Mg andesitic lava termed boninite.

They are a part of an island arc known geologically as a fore arc. They lie above a subduction zone between the Pacific Plate and the Philippine Plate.

The Pacific Plate is subducting under the Philippine Plate, which creates an oceanic trench to the east of the islands.

In the lab, the class studied an Archean basalt with pale color from Mg, but most basalts are not Si rich, though boninites are.

They have been dated as early as 2. In the past 6 months, a report from Isua Greenland describes 3.

Blueschists and eclogites which are formed under high pressure metamorphosis at compressive convergent plate boundaries would be the best evidence for ancient convergent tectonic boundaries, but none have been found in the Archean Eon, or before the Mesoproterozoic Era [begins 1.

One can also look for physical evidence of subduction. Seismic tomographic traverses across the Canadian Shield N of Lake Huron show a major crustal suture associated with hanging slabs of dense cold i.

Note: A craton of stable continental crust is underlain by the mantle root zone, which is cooler than surrounding mantle for several hundred km, depleted The Ogasawara Islands may represent the exposed parts of an ophiolite that has not yet been emplaced on oceanic crust.

The rocks of the Volcano Islands are much younger; Iwo Jima is a dormant volcano characterized by rapid uplift and several hot springs.

Finland] greenstone belt, NE part of the Baltic Shield, Russia, reveal the oldest known boninite series occurring at least in two area of the belt.

The first area referred to here as the Khizovaara structure shows apparent evidence of a late Archean ocean-island volcanic arc collage formed during two distinct tectonic episodes nearly 2.

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