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Biogeochemistry and global elemental cycles 7. Microbe-mineral interactions: metal binding, etc.

Biomarkers Molecular ecology and phylogenetics Coral Reefs: Can they control ocean composition? Global warming: What will be the evolutionary effects?

Global carbon cycle: What if there were no green plants, would CO2 increase leading to greater global warming?

There would be less O2 in the atmosphere, but still quite a bit Venus: It has no plate tectonics now, but may have had intermittent in the past?

Only modest topological variation. Venus may have had plate tectonics 3 Ga 3 billion years ago. Investigators from numerous fields are involved in geobiologic research, including, but not limited to, such disciplines as: paleontology, paleobiology, microbiology, mineralogy, biochemistry, sedimentology, genetics, physiology, geochemistry organic and inorganic , and atmospheric science.

One major subdiscipline of geobiology is geomicrobiology, an area of study that focuses on investigating the interactions between microbes and minerals.

Another related area of research is astrobiology, an interdisciplinary field that uses a combination of geobiological and planetary science data to establish a context for the search for life on other planets Why not later?

May have been due to dinosaurs eating foliage and compacting the soils, changing forest dynamics? What is Life? Some of the relevant issues and questions: 1.

Viruses, Prions, Rickettsia, etc. Rickettsiae are obligate intracellular parasites, depending on entry into and growth and replication within the cytoplasm of eukaryotic host cells.

Are these entities truly alive? Complex Molecules: We are finding ever more complex organic molecules on Earth. When does a complex collection of molecules become alive?

There is much extracellular DNA in the oceans6. RNA and ribozymes can catalyze reactions. Large Systems: Ever more complex systems are being postulated as being alive.

Does a living thing need to have a boundary? Is life dependent on a particular chemical composition? Ethical Dilemmas: When does life begin and end.

How much of a living system organism must remain alive in order for the system organism to be still considered alive?

Searching for Extraterrestrial Life: How will we recognize it? NASA has retooled all solar system exploration to search for existing and past extraterrestrial life.

Is this a good use of tax dollars? Yes, if you are an astrobiologist. Past Times and Time Intervals Ga, b.

Our global estimates indicate that up to 0. We demonstrate that extracellular DNA accounts for about one fifth of the total organic phosphorus regeneration and provides almost half of the prokaryotic demand for organic phosphorus.

It therefore plays a key role in deep-sea ecosystem functioning on a global scale. Science 30 September Organic: Life is based on complex polymers of Carbon, etc.

Only minimal amounts of inorganic noncarbon minerals are used, primarily for rigidity, protection, and some ions for electron transport.

There are 64 43 possible codons. Coding for the 20 standard amino acids8 is in most cases redundant e. Some organisms code for and incorporate two additional amino acids.

RNA is found as a. Proteins: Found in all living things. Include enzymes, which are the main catalytic molecules, allowing life to function out of thermodynamic equilibrium with respect to its environment.

Selenocysteine is now viewed as the 21st amino acid, and pyrrolysine is viewed as the 22nd The SECIS element is defined by characteristic nucleotide sequences and secondary structure base-pairing patterns.

It has a 3' terminal site for amino acid attachment. This covalent linkage is catalyzed by an aminoacyl tRNA synthetase. It also contains a three base region called the anticodon that can base pair to the corresponding three base codon region on mRNA.

Each type of tRNA molecule can be attached to only one type of amino acid, but because the genetic code contains multiple codons that specify the same amino acid, tRNA molecules bearing different anticodons may also carry the same amino acid.

The function of the rRNA is to provide a mechanism for decoding mRNA into amino acids and to interact with the tRNAs during translation by providing peptidyl transferase activity.

Mutations changing the amino acid sequence coded by DNA may be silent, or result in major disorders such as in Sickle Cell Disease, in which Glutamic Acid is replaced with Valine at the 6th position of the beta chain of Hemoglobin.

Lipids: Fatty substances used for energy storage, cell membranes, etc. Chirality: All key biomolecules are constructed of only one form of enantiomer.

Enantiomers are two stereoisomers that are related to each other by a reflection: they are mirror images of each other, which are non-superimposable.

Water is used as a solvent, to aid catalysis, to facilitate hormonal signaling, etc. What are the essential characteristics of life?

Silicon16 can form chains, but polymerizes well only at very low temperatures c. It only forms single bonds easily. It bonds well with O but not well with other atoms.

Complex long-chain silicone molecules are more unstable than their carbon counterparts. It dissolves polar molecules well but also can organize and interact with non-polar molecules.

Has high specific heat allowing energy storage and transport as in the oceans. It has a high heat of vaporization, allowing dissipation of heat through evaporation.

High surface tension allows molecules to accumulate. High dielectric constant allows proton pumping across cell membranes. Other potential solvents might include HCN hydrogen cyanide , NH3 ammonia, is unstable over long periods.

All living things have greater internal order lower entropy than exists external to them. Interestingly, in biological systems most of these compounds are of the same chirality: most amino acids are L and sugars are D.

Typical naturally occurring proteins, made of L amino acids, are known as left-handed proteins, while D amino acids produce right-handed proteins.

These processes are the basis of life, allowing cells to grow and reproduce, maintain their structures, and respond to their environments.

Metabolism is usually divided into two categories. Catabolism yields 12 13 GeobiologyESS We cannot conceive of life that would have higher entropy than its surroundings, so therefore all life must metabolize.

Almost certainly must have a barrier between a life form and the external disordered environment, to maintain lower entropy and prevent energy escaping.

Living things have bilayer lipid membranes viruses have protein walls. Might conceivably have a steep environmental gradient instead.

Could be a chemical barrier rather than a physical barrier. Hard to envision life without some kind of physical barrier, though possibly not a distinct membrane.

Having senses and movement capability seems fundamental to concepts of life. Most obvious feature of life is potential to replicate itself.

Viruses reproduce, but are they alive? Not every individual of a species has this ability all the time e. Reproduction should not occur at random, but should be controlled and regulated by an information system.

To control all life processes, life needs to maintain and transmit information. Evolution is probably essential to any definition of life.

Homeostatic organisms interacting with their environments will undergo natural selection with respect to these variations in information quality, and evolution will necessarily occur in such a system.

However, it may not be Darwinian but could follow some other pattern. Even organisms which reproduce usually with parthenogenesis or cloning sometimes revert to sexual reproduction using information transfer.

If there are alien life forms, they are most likely microorganisms [speaking probabilistically]. Spontaneous Generation: Arose in dirty underwear or dirt, slime, etc.

Speculations by ancients including Anaximander, Aristotle. The 17C Belgian Jan Baptista van Helmont 19 contended that mice arise from grains mixed with dirty underwear.

Anabolism, on the other hand, uses this energy to construct components of cells such as proteins and nucleic acids.

But what is more remarkable is that mice of both sexes emerge from the wheat and these mice successfully reproduce with mice born naturally from parents?

But GeobiologyESS His experiments with swannecked flasks holding [sterile] nutrient broths showed that no [bacterial] growth occurs if the neck is intact [and therefore microorganisms and their spores cannot enter].

Hooker20 proposed specific conditions, which no longer exist, in which life might have formed in a warm pond. Diversity of energy sources b.

Diversity of simple chemical compounds as reactants c. Restricted environment capable of concentrating reactants d. Polymerization of simple compounds adding order to the system e.

Conditions that do not exist now. This would allow the formation of organic chemicals from light, lightning, and volcanic heat, which could accumulate by raining down on the oceans to form a hot dilute primordial soup.

S Haldane. But if and oh! He was the scientist who originally coined the term "primordial soup" to describe the organic molecules that were dissolved in water billions of years ago.

Many people credit Haldane with the major contributions to defining "neo-Darwinism". He also anticipated the Miller-Urey experiments in abiogenesis.

They concluded from various chemical grounds that the Earth had originally had hydrogen, ammonia, carbon dioxide, methane, and other GeobiologyESS These concepts presuppose: a.

An anoxic primordial atmospheric environment b. Photochemical reactions caused by UV causing organic synthesis c.

Chemical evolution follows the fundamental laws of physics and chemistry. Flaw: The theory is flawed because the oceans would be so dilute that needed chemical reactions are improbable.

These experiments have been extensively replicated and expanded, including successful organic syntheses using other energy sources such as UV, heat, sonic shock, x-rays, etc.

The yields of relevant organic compounds in any of these experiments were substantial only in reducing conditions24 in which the gaseous medium reacted on contains H2, NH3, simple compounds in its atmosphere -- but no oxygen molecules.

And larger and larger molecules formed by various chemical processes until some of them succeeded in making copies of themselves, forming the first living things.

Specifically, the experiment tested Oparin and Haldane's hypothesis that conditions on the primitive Earth favored chemical reactions that synthesized organic compounds from inorganic precursors.

Considered to be the classic experiment on the origin of life, it was conducted in by Stanley L. Miller and Harold C.

Urey at the University of Chicago. The chemicals were all sealed inside a sterile array of glass tubes and flasks connected together in a loop, with one flask half-full of liquid water and another flask containing a pair of electrodes.

The liquid water was heated to induce evaporation, sparks were fired between the electrodes to simulate lightning through the atmosphere and water vapor, and then the atmosphere was cooled again so that the water could condense and trickle back into the first flask in a continuous cycle.

Two percent of the carbon had formed amino acids, including 13 of the 22 that are used to make proteins in living cells, with glycine as the most abundant.

Sugars, lipids, and some of the building blocks for nucleic acids were also formed. As observed in all consequent experiments, both left-handed L and right-handed D optical isomers were created in a racemic mixture.

However, the experiment also produced a "toxic carcinogenic" substance, also known as tar, which, in a natural environment, would destroy any amino acids created.

However, no one has ever determined what kind of atmosphere a primitive chondritic planet would generate. We assume that the planets formed out of chondritic material, and we sectioned up the planet into layers, and we used the composition of the mix of meteorites to calculate the gases that would have evolved from each of those layers," said Schaefer.

For the Miller-Urey experiment to work, a reducing atmosphere is a must. An oxidizing atmosphere makes producing organic compounds impossible.

Yet, a major contingent of geologists believe that a hydrogen-poor, carbon dioxide-rich atmosphere existed because they use modern volcanic gases as models for the early atmosphere.

Volcanic gases are rich in water, carbon dioxide, and sulfur dioxide but contain no ammonia or methane. The origin of life can be studied experimentally b.

It is easy to create the building blocks of life in the right conditions. Flaw: The flaw in this approach if that the early atmosphere was possibly not highly reducing at least, volcanic gases are neutral, contain CO2, N2, not CH4 or NH3, and yields are low in these conditions.

But see discussion here Was Miller-Urey experiment correct? One example is hydrogen gas, which acts as an oxidizing agent when it combines with metals and as a reducing agent when it reacts with nonmetals.

Scientists believe that Earth's original atmosphere might contain less of the reducing molecules than was thought at the time of Miller-Urey experiment.

There is abundant evidence of major volcanic eruptions 4 billion years ago, which would have released carbon dioxide, nitrogen, hydrogen sulfide, and sulfur dioxide into the atmosphere.

Experiments using these gases in addition to the ones in the original Miller-Urey experiment have produced more diverse molecules.

Originally it was thought that the primitive secondary atmosphere contained mostly NH3 and CH4. However, it is likely that most of the atmospheric carbon was CO2 with perhaps some CO and the nitrogen mostly N2.

These then attack methane, giving eventually CO2 and releasing H2 which would be lost into space. The H atoms come mostly from water vapor.

In fact, in order to generate aromatic amino acids under primitive earth conditions it is necessary to use less hydrogen-rich gaseous mixtures.

Most of the natural amino acids, hydroxyacids, purines, pyrimidines, and sugars have been produced in variants of the Miller experiment.

More recent results may question these conclusions. The University of Waterloo and University of Colorado conducted simulations in that indicated that the early atmosphere of Earth could have contained up to 40 percent hydrogenimplying a much more hospitable environment for the formation of prebiotic organic molecules.

The escape of hydrogen from Earth's atmosphere into space may have occurred at only one percent of the rate previously believed based on revised estimates of the upper atmosphere's temperature.

One of the authors, Prof. Owen Toon notes: "In this new scenario, organics can be produced efficiently in the early atmosphere, leading us back to the organic-rich soupin-the-ocean concept I think this study makes the experiments by Miller and others relevant again.

Although lightning storms are thought to have been very common in the primordial atmosphere, they are not thought to have been as common as the amount of electricity used by the Miller-Urey experiment implied.

These factors suggest that much lower concentrations of biochemicals would have been produced on Earth than was originally predicted although the time scale would be million years instead of a week.

Similar experiments, both with different sources of energy and with different mixtures of gases, have resulted in amino and hydroxy acids being produced; it is likely that at least some organic compounds would have been generated on the early Earth.

However, when oxygen gas is added to this mixture, no organic molecules are formed. Others have run experiments and elaborated on it.

He favored clays because they have a specific regularly ordered molecular crystal structure, and some have surface charges which can bind simple organic molecules in an orderly manner.

Thus some clays have catalytic surfaces that can facilitate the formation of polymers by allowing small molecules to be concentrated in an orderly manner and be polymerized.

These opponents argue that this presence of oxygen precludes the formation of prebiotic molecules via a Miller-Urey-like scenario, attempting to invalidate the hypothesis of abiogenesis.

However, the authors of the paper are arguing that this presence of oxygen merely evidences the existence of photosynthetic organisms 3.

Though there is somewhat controversial evidence for very small less than 0. In fact most evidence suggests that oxygenic photosynthesis was present during time periods from which there is evidence for a non-oxygenic atmosphere".

Conditions similar to those of the Miller-Urey experiments are present in other regions of the solar system, often substituting ultraviolet light for lightning as the driving force for chemical reactions.

On September 28, , the Murchison meteorite that fell near Murchison, Victoria, Australia was found to contain over 90 different amino acids, nineteen of which are found in Earth life.

Comets and other icy outer-solar-system bodies are thought to contain large amounts of complex carbon compounds such as tholins formed by these processes, in some cases so much so that the surfaces of these bodies are turned dark red or as black as asphalt.

The early Earth was bombarded heavily by comets, possibly providing a large supply of complex organic molecules along with the water and other volatiles they contributed.

Hodgkin, received the Nobel Prize in chemistry in for X-ray studies of compounds, such as vitamin B12 and penicillin. We have shown experimentally that RNA molecules bind efficiently to clays and that montmorillonite can catalyze the formation of longer molecules oligomers , thus lending support to the RNA world hypothesis.

Goldschmidt independently proposed that clay minerals may have played an important role in prebiotic synthesis Bernal ; Goldschmidt After Miller-Urey, it could be explained how amino acids formed, but this new hypothesis provided a much-needed explanation for the formation of the sugars required by RNA or DNA and also required to provide energy for life forms.

See also G. Key concepts: a. Surface chemistry such as on clays is important for making polymers. Gas and aqueous phases are not sufficient, must also have surface reactions.

Crystalline structure of clays impart order to the chemical forming on them of polymers. Flaws: a. It is very difficult to remove the molecules once they are bonded onto minerals.

There is no evidence of clay-like minerals participating as catalysts in modern extant life. Many enzymes have active centers resembling iron pyrite centers, but there is no evidence of silicate-related centers Analysis of meteorites showed that cold planetary [MCM: or celestial] surfaces could be rich in organic molecules such as amino acids.

Spectroscopy of comets showed that they were organic-rich snowballs. Experiments irradiating ice with UV have yielded amino acids and simple polymers.

It is named after Montmorillon in France. Montmorillonite, a member of the smectite family, is a clay, meaning that it has 2 tetrahedral sheets sandwiching a central octahedral sheet.

The particles are plate-shaped with an average diameter of approximately 1 micrometre. It is the main constituent of the volcanic ash weathering product, bentonite.

Their bulk composition is mainly silicates, oxides and sulfides, while the minerals olivine and serpentine are characteristic.

Carbonaceous chondrites are grouped according to distinctive compositions thought to reflect the type of parent body from which they originated.

These are named after a prominent meteorite often the first to be discovered - in the group The CM meteorite from Murchison, Victoria has over 70 extraterrestrial amino acids and other compounds including carboxylic acids, hydroxy carboxylic acids, sulphonic and phosphonic acids, aliphatic, aromatic and polar hydrocarbons, fullerenes, heterocycles, carbonyl compounds, alcohols, amines and amides.

Here we report a laboratory demonstration that glycine, alanine and serine naturally form from ultraviolet photolysis of the analogues of icy interstellar grains.

Such amino acids would naturally have a deuterium excess similar to that seen in interstellar molecular clouds, and the formation process could also result in enantiomeric excesses if the incident radiation is circularly polarized.

These results suggest that at least some meteoritic amino acids are the result of interstellar photochemistry, rather than formation in liquid water on an early Solar System body Max P.

Bernstein et al. Nature Vol Freezing provides a good concentrating mechanism for forming polymers. Freezing dehydrates things, encouraging polymer formation.

Proteins are formed by linking amino acids in peptide bonds created by a dehydration synthesis reaction. Flaw: Cold temperature slows chemical reactions and immobilizes reaction products, for instance, in Carbonaceous chondrite meteorite.

Early Earth must have had many similar environments given the high radiogenic heat flow from within. The vents are formed in fields hundreds of meters wide when superheated water from below the Earth's crust comes through the ocean floor.

It can also be known as a Sea Vent. T he superheated water is rich in dissolved minerals from the crust, most notably sulfides, which crystallize to create a chimney-like structure around each vent.

When the superheated water in the vent comes in contact with the cold ocean water, many minerals are precipitated, creating the distinctive black color.

The metal sulfides that are deposited can become massive sulfide ore deposits in time. They were observed using a small submersible vehicle called Alvin.

Today, black smokers are known to exist in the Atlantic and Pacific Oceans, at an average depth of meters. The water is also extremely acidic, often having a pH value as low as 2.

Each year 1. Although life is very sparse at these depths, black smokers are the center of entire ecosystems.

In turn, more complex life forms like clams and tubeworms feed on these organisms. The organisms at the base of the food chain also deposit minerals into the base of the black smoker, thus completing the life cycle.

Metabolism here means a cycle of chemical reactions that produce energy in a form that can be harnessed by other processes. The idea is that once a primitive metabolic cycle was established, it began to produce ever more complex compounds.

A key idea of the theory is that this early chemistry of life occurred not in bulk solution in the oceans, but on mineral surfaces e.

The first 'cells' would have been lipid bubbles on the mineral surfaces. Acetic acid is part of the citric acid cycle that is fundamental to metabolism in cells Cody et al.

It occurs in numerous metabolic pathways, notably the reductive citric acid cycle and the pathways that produce amino acids and sugars.

It has been suggested that pyruvic acid or its anion pyruvate formed primordially by double carbonylation They show that pryuvic acid forms from formic acid in the presence of nonylmercaptane and iron 34 GeobiologyESS John A.

Baross, oceanographer at UW, has made important contributions in this origin of life topic. Presence of S and metal can promote this reaction with energy release.

Resembles metabolism. Excessive energy and reducing power could lead to biological takeover. A non-organic metabolism that would lead to organic living metabolism.

Energy gradient is important for primitive metabolism. Reducing micro-environments were common on the early Earth. Flaw: a. Heat rapidly destroys most organic polymers b.

Hydrothermal systems are very short lived, precipitated minerals plug up the vents within c.

Water is initially absent and forms only by the dehydration of the formic acid. This result poses fascinating thermodynamic and kinetic questions.

Pyruvic acid is an extremely heat-sensitive compound that decomposes at. It appears paradoxical that at the very high temperature required for dehydration of formic acid, the relatively unstable pyruvic acid can form and exist at detectable concentrations.

Moreover, it is astonishing that acetic acid is formed at a lower yield than pyruvic acid. The explanation may well lie in the very high pressure.

It remains to be established whether such conditions are geophysically possible. The new finding, if it holds, fills a critical gap in the experimental picture of the iron-sulfur world see the figure The challenge will now be to overcome the discrepancies in the reaction conditions and to establish the right conditions for autocatalysis reproduction and evolution The reaction scheme in the figure is in substantial agreement with extant metabolism in terms of overall metabolic patterns, reaction pathways, and catalysts.

The newly demonstrated formation of pyruvic acid by double carbonylation, however, has no analog in extant metabolism.

It may have disappeared because of metabolic takeover But more importantly, the reactions shown in the figure can still occur today because the required conditions are in general still available on Earth, albeit at a lesser frequency It is occasionally suggested that experiments within the iron-sulfur world theory demonstrate merely yet another source of organics for the prebiotic broth.

This is a misconception. The new finding drives this point home. Pyruvate is too unstable to ever be considered as a slowly accumulating component in a prebiotic broth.

The prebiotic broth theory and the iron-sulfur world theory are incompatible Science Thermodynamics in Mineral Sciences: An Introduction.

The Chemistry and Technology of Coal. Experiment shows that RNA can be forced to evolve to develop better catalytic properties. RNA can perform many of the major functions of life by itself b.

The Last Common Ancestor Of All Life may have been considerably more complex than the first life, limiting what we can learn from modern biology about first life.

Flaws: No known environment for synthesizing and polymerizing RNA. For example, where do the phosphate groups come from phosphate is rare in geologic environments?

No supply of energy to power RNA world. The Archean Earth shows complex environment and many energy sources, many organic sources, many catalytic surfaces, micro-environments allowing segregation.

Seek a site with maximal environmental interface between atmosphere, hydrosphere, lithosphere, where catalytic minerals accumulate, dehydration is enhanced, energy supply and gradients are high but not too high , conditions are variable but long-lived.

This describes a Beach! Maximize environmental diversity to allow best aspects of all models to contribute to formation of life.

Extraterrestrial ET life is the subject of astrobiology. This is a term that NASA has applied to the search for extant and extinct life, and to justify massive budget expenditures for this research, etc.

The key questions of astrobiology regarding ET life forms are: 1. Are they out there? No ET life has been found so far. Where might they live?

Is there any in our solar system? In the s, Dr. Cech had been studying the splicing of RNA in the unicellular organism Tetrahymena thermophila when he discovered that an unprocessed RNA molecule could splice itself.

In , Dr. Cech became the first to show that RNA molecules are not restricted to being passive carriers of genetic information - they can have catalytic functions and can participate in cellular reactions.

RNA enzymes are known as ribozymes and have provided a new tool for gene technology. They also have the potential to provide new therapeutic agents - for example, they have the ability to destroy and cleave invading, viral RNAs.

What might they be like? Are they humanoid? How can we find them? The planetary characteristics thought to make a planet41 in orbit around a host star potentially habitable include the following [note that additional needed stellar and galactic characteristics are listed further below]: 1.

Planet Not Too Close to a Gas Giant Planet42 : Otherwise gravity prevents the formation of small rocky planets which are the most likely to support life.

This is needed to avoid tidal locking in which one side of the planet is always turned toward the start, e. Tidal locking leads to severe persistent temperature extremes.

Mercury has no atmosphere and Mars has only a trace currently [about millibar or 0. In contrast, Earth recycles some of its gases to the mantle through plate tectonics and subduction.

This provide shielding against influx of high energy charged particles. The surface of Mars has high rate of bombardment by high energy charged particles and ionizing radiation, due both to its thin atmosphere and low magnetic field, and would therefore be hazardous to astronauts TES p.

The only directly imaged example is the brown dwarf 2M and its planetary companion 2Mb. Substellar objects more massive than 13 MJUP are considered to be brown dwarfs rather than planets.

Among the planets of the Solar System, Jupiter seems to be a typical gas giant, while Saturn is just above the dividing line between gas giant planets and the second most massive category, ice giants.

Combined with a thin atmosphere, this permits a significant amount of ionizing radiation to reach the Martian surface. Average doses were about 22 millirads per day micrograys per day or 0.

A three year exposure to such levels would be close to the safety limits currently adopted by NASA.

Levels at the Martian surface would be somewhat lower and might vary significantly at different locations depending on altitude and local magnetic fields.

The Earth has But Mars, currently tilted at It also has significantly greater orbital eccentricity46 47than Earth [causing the seasons to have much more unequal length].

Mars has had more widely fluctuating temperatures and migrating ice caps. Like Earth, the obliquity of Mars undergoes periodic changes which can lead to long-lasting changes in climate.

Once again, the effect is more pronounced on Mars because it lacks the stabilizing influence of a large moon.

Jacques Laskar, of France's National Centre for Scientific Research, argues that the effects of these periodic climate changes can be seen in the layered nature of the ice cap on the planets north pole.

The eccentricity of this conic section, the orbit's eccentricity, is an important parameter of the orbit that defines its absolute shape.

Eccentricity may be interpreted as a measure of how much this shape deviates from a circle The eccentricity of the Earth's orbit today is 0.

Through time, the eccentricity of the Earth's orbit slowly changes from nearly 0 to almost 0. Comet Shoemaker-Levy , which impacted Jupiter, was presumably one such comet.

Absence of High Radiation see below. But while the asteroid belt is composed primarily of rock and metal, the Kuiper belt objects are composed largely of frozen volatiles dubbed "ices" , such as methane, ammonia, and water.

Since the first was discovered in , the number of known Kuiper belt objects KBOs has increased to over a thousand, and more than 70, KBOs over 1 km in diameter are believed to reside there.

The Kuiper belt is believed to be the main repository for periodic comets, those with orbits lasting less than years.

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.

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