3.1+Origin+of+the+solar+system

__Origin of the Solar System__: The basic premise in the understanding of our origins, and the properties of all the planets we have studied this term, is that natural forces created and shaped the Solar System. And that there is a continuity to that process, i.e. it is not a sequence of random events.

Any model or theory for the formation of the Solar System must have a set of explanations for large-scale and small-scale properties. Large-Scale: Small-Scale: Also note that the overall architecture of our Solar System is orderly and the ages of its members uniform. All indicators point to a single formation event about 4.6 billion years ago. The above is not to ignore the fact that a great deal of evolution occurred in the Solar System after it formed (see below). For example, the origin secondary atmospheres of the terrestrial worlds underwent a large amount of chemical processing (Venus was baked, Mars was frozen, Earth developed life). There was also orbital evolution as well, rings were formed, moons captured, tidal locking between worlds (e.g. Pluto and Charon). So the Solar System is not a static system, it is dynamic.
 * 1) the planets are isolated in orderly intervals
 * 2) orbits are nearly circular
 * 3) orbits are in the same plane
 * 4) all planets revolve prograde
 * 1) most planets rotate prograde
 * 2) the systems of moons can be divided into regular objects (spherical) with direct orbits versus irregular objects with eccentric orbits
 * 3) terrestrial planets have
 * 4) high densities
 * 5) thin or no atmospheres
 * 6) rotate slowly
 * 7) rocky, poor in ices and H/He
 * 8) jovian worlds have
 * 9) low densities
 * 10) thick atmospheres
 * 11) rotate rapidly
 * 12) many moons
 * 13) fluid interiors, rich in ices, H/He
 * 14) most of outer SS objects (not just jovian worlds) are ice-rich

__How does one test a hypothesis?__ To answer scientific questions requires the formulation of a hypothesis. The hypothesis is tested against the facts to look for contradictions that rule out or require modification to the hypothesis. Note that the process of hypothesis formulation and then theory building is a lengthy, career dependent operation. So the sociology of science requires that a hypothesis be tested and confirmed by many scientists since the creator of the hypothesis has a strong psychological attachment to his work.

__Encounter Hypothesis__: One of the earliest theories for the formation of the planets was called the encounter hypothesis. In this scenario, a rogue star passes close to the Sun about 5 billion years ago. Material, in the form of hot gas, is tidally stripped from the Sun and the rogue star. This material fragments into smaller lumps which form the planets. This hypothesis has the advantage of explaining why the planets all revolve in the same direction (from the encounter geometry) and also provides an explanation for why the inner worlds are denser than the outer worlds.

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However, there are two major problems for a theory of this type. One is that hot gas expands, not contracts. So lumps of hot gas would not form planets. The second is that encounters between stars are extremely rare, so rare as to be improbable in the lifetime of the Universe (15 billion years).

__Nebular Hypothesis__: A second theory is called the nebular hypothesis. In this theory, the whole Solar System starts as a large cloud of gas that contracts under self-gravity. Conservation of [|angular momentum] requires that a rotating disk form with a large concentration at the center (the proto-Sun). Within the disk, planets form.

While this theory incorporates more basic physics, there are several unsolved problems. For example, a majority of the angular momentum in the Solar System is held by the outer planets. For comparison, 99% of the Solar System's mass is in the Sun, but 99% of its angular momentum is in the planets. Another flaw is the mechanism from which the disk turns into individual planets.

__Protoplanet Hypothesis__: The current working model for the formation of the Solar System is called the [|protoplanet hypothesis]. It incorporates many of the components of the nebular hypothesis, but adds some new aspects from modern knowledge of fluids and states of matter.

Meanwhile in the inner Solar System: Note that as the planet's began to form they grew in mass by accreting planetesimals. Since force of gravity is proportional to mass, the largest planetesimals are accreted first. The early proto-planets are able to sweep the early Solar System clean of large bodies. Notice also that the lighter compounds are vaporized in the inner Solar System. So where did all the outgassing material come from? The answer is comets that fall from the outer Solar System after the planets form. Meanwhile in the outer Solar System:

The Jovian worlds, having an early edge on gathering mass in the colder outer solar disk, were the most efficient at capturing planetesimals, which only served to increase their already large masses. As the planetesimals shrink in average size, collisions with proto-planets lead to fragmentation. So quickly the Solar System divided into large proto-planets and smaller and smaller planetesimals which eventually became the numerous meteors we see today. Any leftover large bodies were captured as moons or ejected by gravity assist into the Oort cloud. The start of thermonuclear fusion in the Sun's core created enough luminosity so that the remaining hydrogen and helium gas in the solar disk was removed by radiation pressure. The only remaining problem is the distribution of angular momentum. The current explanation for the fact that most of the angular momentum is in the outer planets is that, by some mechanism, the Sun has lost angular momentum. The mechanism of choice is //magnetic braking//.

The early Sun had a much heavier flow of solar winds particles. Many of the particles in the solar wind are charged, and are effected by the laws of motion as well as electromagnetic forces. As the solar wind leaves the solar surface, they are ``dragged'' by the magnetic field, which in turn slows down the Sun's rotation.

__Migrating Planets__: The protoplanet hypothesis explains most of the features of the Solar System; however, the outer solar system is still strange, especially the properties of Pluto/Charon. One explanation is that the Solar System was not born in the configuration that we see today. That the planets in the outer Solar System migrated to their present positions. Migration requires some interaction between the planet and a fairly large body or the gravitational forces are too weak. Early in the formation of the Solar System, there were lots of Moon-sized to Mars-sized bodies, especially in the outer SS. A large planetesimal that crosses near Neptune will lose some energy, fall down near Jupiter, gain energy to be ejected into the Oort Cloud.

This will have the effect of decreasing the size of Jupiter's orbit, and expanding the size of Saturn, Uranus and Neptunes' orbits. As Neptune moves outward, it will beginning to perturb the orbits of the trans-Neptunian objects (large ice covered astroids of which Pluto/Charon are a member). This pushes Pluto/Charon into a highly eccentric, inclined 3:2 resonant orbit that it occupies today.

All the leftover planetesimals near Neptunes orbit are pushed into a torus shaped region called the Kuiper belt. Smaller planetesimals are thrown farther out into the Oort cloud.



__Extrasolar Systems__: Support for the protoplanet hypothesis has been found by the detection of disk material around of stars, such as Beta Pictoris and by Hubble images of the Orion Nebula.

There are now numerous verified solar type stars that have Jupiter size planets in orbit around them.





__Doppler Detection__: Doppler spectroscopy is used to detect the periodic velocity shift of the stellar spectrum caused by an orbiting giant planet. This method is also referred to as the radial velocity method. From ground-based observatories, spectroscopists can measure Doppler shifts greater than 3 m/sec due to the reflex motion of the star This corresponds to a minimum detectable mass of 33 Earth masses for a planet at 1 AU from a one solar-mass star.

All stars exhibit some Doppler shift because all stars are moving in the gravitational field of our Galaxy. But the stars that move toward us, then away from us, then toward us again wobbling with periodic motion are gravitationally bound to another object. The magnitude of this changing velocity, together with the period of the motion, reveals the mass of an invisible companion to a visible star. Our Sun moves with a speed of about 12 meters/sec due to the most massive planet in our Solar System, Jupiter. The periodicity of the Sun's motion is the same as the orbital period of Jupiter: one cycle takes about twelve years.



__Transit Method__: A more direct measure occurs is a planet transits across the front of a star, as happened with HD 209458, a distant planet passing in front of its star, providing direct and independent confirmation of the existence of extrasolar planets.

Astronomers predicted the planet would cross the face of the star if the planet's orbital plane were lucky enough to carry it between Earth and the star. Until now, none of the 18 other extrasolar planets discovered has had its orbital plane oriented edge-on to Earth so that the planet could be seen to transit the star, nor have any of the other planets discovered by other researchers. However, on Nov. 7, 1999 an automatic telescope observed a 1.7 percent dip in the star's brightness.

With the orbital plane of the planet known, the astronomers for the first time could determine precisely the mass of the planet and, from the size of the planet measured during transit, its density. Interestingly, while the planet's mass is only 63 percent of Jupiter's mass, its radius is 60 percent bigger than that of Jupiter. This fits with theories that predict a bloated planet when, as here, the planet is very close to the star.

__Habitable Zone__: One of the main ingredients for life as we know it is liquid water. Water exists as a liquid between 273K and 373K (unless the pressure is too low, in which case the water sublimates into gaseous water vapor). The region on the solar system (or any planetary system) where the temperature is in this range, is called the habitable zone. Planets are in equilibrium with their surroundings: they are neither getting hotter nor colder. All planets absorb incident radiation from the Sun (this heats them up); to maintain equilibrium, they must radiate away the same amount of energy. The temperature of a planet can be approximated by assuming that it is a black body.

Planets do not absorb all incident light; much gets reflected. The albedo is the fraction of incident light reflected, not absorbed. The albedo of the Earth is 0.37; that of Venus is 0.65; that of the Moon is about 0.12 (cloud is highly reflective, basaltic rock is not). You must multiply the solar irradiance by the albedo. This extends the inner edge of the habitable zone inwards

Another complication is that planets are not ideal black bodies. Carbon dioxide, water vapor, and other atmospheric gases are opaque in the near-IR (where the peak of the black body emission would be). A less-than-ideal radiator must be hotter than a black body to radiate the same amount of luminosity. This extends the outer edge of the habitable zone outwards. //taken from:http://abyss.uoregon.edu/~js/ast121/lectures/lec24.html//

From Wikipedia, the free encyclopedia
Jump to:[|navigation], [|search]This article is about the Sun and its planetary system. For other systems, see [|Planetary system] and [|Star system].[|Planets] and [|dwarf planets] of the Solar System. Sizes are to scale, but relative distances from the Sun are not. The **Solar System**[|[a]] consists of the [|Sun]  and those [|celestial objects] bound to it by [|gravity], all of which formed from the collapse of a giant [|molecular cloud] approximately 4.6 billion years ago. Of the retinue of objects that [|orbit] the Sun, most of the [|mass] is contained within eight relatively solitary [|planets] whose orbits are almost circular and lie within a nearly-flat disc called the [|ecliptic plane]. The four smaller inner planets, [|Mercury], [|Venus], [|Earth] and [|Mars], also called the [|terrestrial planets], are primarily composed of rock and metal. The four outer planets, [|Jupiter], [|Saturn], [|Uranus] and [|Neptune], also called the [|gas giants], are composed largely of hydrogen and helium and are far more massive than the terrestrials. The Solar System is also home to two regions populated by smaller objects. The [|asteroid belt], which lies between Mars and Jupiter, is similar to the terrestrial planets as it is composed mainly of rock and metal. Beyond Neptune's orbit lie [|trans-Neptunian objects] composed mostly of ices such as water, ammonia and methane. Within these two regions, five individual objects, [|Ceres], [|Pluto], [|Haumea], [|Makemake] and [|Eris], are recognized to be large enough to have been rounded by their own gravity, and are thus termed [|dwarf planets]. In addition to thousands of [|small bodies] in those two regions, various other small body populations, such as [|comets], [|centaurs] and [|interplanetary dust], freely travel between regions. The [|solar wind], a flow of [|plasma] from the Sun, creates a [|bubble] in the [|interstellar medium] known as the [|heliosphere], which extends out to the edge of the scattered disc. The hypothetical [|Oort cloud], which acts as the source for [|long-period comets], may also exist at a distance roughly a thousand times further than the heliosphere. Six of the [|planets] and three of the [|dwarf planets] are orbited by [|natural satellites],[|[b]] usually termed "moons" after Earth's [|Moon]. Each of the outer planets is encircled by [|planetary rings] of dust and other particles.

=Origin of the Solar System=

It is impossible to "reverse" the equations of motion, chemistry and physical evolution of a complex system like the solar system to arrive back at its origins unambiguously. The current system is too chaotic, indeterminate and possibly lacking in some of its earliest 'ingredients'. Even the things we know very well have error bars on them or small builtin uncertainties which would build up such a large uncertainty when we tried to trace them back as tomake our conclusions worthless. So the only way to try to work out a viable theory of the origin of the solar system (and there is no guarantee this would be a unique solution) is to: > > > **Theories to date** Theories of the origin of the solar system, universe, earth etc are known generally as **cosmogonic theories**. The origin of the solar system is in many ways linked to the origin of the universe; certainly to that of the local star group and the galaxy, but we r group and the galaxy, but we shall restrict ourselves only to solar system origin, bringing only a few ideas from the "wider picture" where that will help us understand the solar system. The (serious) theories of the origins of the solar system can be divided into a number of different types, generally. There are //co-eval// and //non-co-eval// theories - that is those which postulate the planets' birth taking place at the same time as the central star of the system was forming, and those where the planets were "acquired" later. We shall briefly look at a non-co-eval theory later. The co-eval theories we can further subdivide into //nebula// and //non-nebula// theories - those where the planets formed out of the gas/dust nebula which was the birthplace of the star, or otherwise. Within the nebula theories there is a further sub-division into low and high mass theories. The non-nebula theories are currently out of favour, so we shall only look at the two types of nebula theories. These both start from the same basic mixture of dust and gas; they just differ in the details of the development of planetary formation. > **What evidence do we judge these theories against?** We have to decide first what sort of information we can judge the theories against. As we go through we shall see there is a plethora of facts about the physics and chemistry of the system against which we can measure these ideas. There are ese ideas. There are distributions of elements, the details of the dynamics, overall compositions etc which all need to be fitted in. But we can start by just looking at some of the basic outline information that we have. We know for instance that the planets can be divided into two main groups - the "terrestrial" planets, small rocky bodies with densities 3-6 times that of water in the inner part of the system, and gaseous, low density bodies in the outer part, divided from the terrestrial planets by the asteroid belt. We also know that the gaseous planets can be further subdivided: there is Jupiter and Saturn, the "Gas Giants" which are roughly the same size (though different in density), and Uranus and Neptune, the "Subgiants"again of similar size. Outside Neptune (and often inside) is the small, icy Pluto with its large companion Charon. This is an interesting exception which may be unimportant in deriving the overall structure of the solar system (as it may be a chance exceptional case) but which must be explained in any comprehensive theory. We do not have just the major planets to explain, however. Besides the asteroids we must explain the existence and occurrence of comets, and of the "Kuiper belt" bodies which form a large asteroid-like belt outside the Neptune/Pluto region. If the comets come from the "Oort cloud" (see later where we discuss comets) then we also have to explain how that cloud is formed. < is formed. There are elements of the morphology of the system which has to be explained: why are the planets all orbiting in or near the same plane - the ecliptic, why all in prograde orbits, and why most spinning in the same direction. (Conversely how important is it to our theory that a planet like Uranus with its spin axis virtually in the ecliptic plane differs from the general rule?) The satellites also generally tend to follow these same trends - orbiting in their primary's equatorial plane, in prograde orbits with prograde spin. It is important when we examine the theories that we know what are real trends and laws that the planets hold to, and which are spurious or more tenuous. Kepler's Laws are firmly established and "approved" by intimate connection with the basic laws of gravitation. The planetary motions can thus in principle be calculated and extrapolated virtually as far as we like by the accurate application of these principles. However, a good example of what is often called a "Law" but the accuracy of which is really in some doubt is the "Titus-Bode Law" of planetary distance from the sun. This states that the log of the disance from the sun is linear: > When this "law" was first posited we did not know of the existence of planets beyond Saturn. The fact that Uranus and Neptune, when discovered, fitted, when discovered, fitted so well to this curve seemed to give it some credence, though there are some minor discrepancies (in the positions of Mars and Venus) in even the inner part of the system. The discovery of the asteroids - near but generally inside their "Titus-Bode" position - and then of Pluto which is way off the line, cast the theory into doubt, though it could be argued these are explicable exceptions from an otherwise well-maintained rule. It has been pointed out that many satellites of the major planets also seem to follow a similar sort of rule. There have been attempts to explain why planets might tend to form in this sort of spacing, but it is still a matter of disagreement as to whether we should read any significance into this "rule" for the planets' spacing. > **The solar system - location and general information** > The solar system is located in the outer reaches of a minor arm of a spiral galaxy which is very similar to many others. "Our" galaxy is 3.2 109 AU across (50,000LY - LY = light-year). We are 34,000 LY from the centre. In the galaxy there are 1011 stars in an "interstellar medium" of gas and dust.
 * How do we arrive at a theory of Solar System formation?**
 * Postulate a theory of Formation
 * Develop that forward in time
 * Make predictions
 * Compare those predictions to the current system
 * Where there are seen to be differences or inconsistencies go back to the beginning with a new or modified theory.
 * Bode's Law:**

The gas and dust is thought to be the source of the stars. This gas and dust permeates the galaxy. The mass of this will have a gravitational attraction tending to make the cloud of material coake the cloud of material collapse in on itself, but usually internal forces like spin, electric and magnetic fields and thermal motions prevent this collapse from occurring. However, there is much chaotic behaviour in the complex gas/dust field and it is possible for regions to get isolated or change their characteristics. Two regions of slightly higher density might collide, for example, and the resultant shock might trigger collapse: > An alternative might be that two sub-critically-sized clouds merge and thence form a body which has too large a density to prevent collapse. Whatever starts the collapse, as it gathers momentum it will tend to be self-sustaining, as the gravitational force is concentrated by the material moving towards a central point: this collapse gathers pace until a distinct central body is formed - this is the origin of the central star of the system. (Hence planetary system cosmogony is usually linked to stellar system creation.) The largest densities and the lowest temperatures are the conditions most likely to produce planets in the clouds of molecular hydrogen which are though to be the birth places of stars and their planetary systems. The theories of formation usually concentrate on the //nebulous// regions, with the first problem being to explain what triblem being to explain what triggers the collapse, and the next major one being to explain how you segregate out the individual bodies. The nebulae are mainly composed of molecular Hydrogen, and are typically from 2 to 300 light years (LY) across, containing from 10's to 106 solar masses (MS) respectively of material. (1MS = 1.989 1030 kg). The temperature of the hydrogen cloud will be typically a few 10's of K, and the density 108 to 1012 H2 particles m-2. This latter may sound a lot but it represents just 0.003 kg in a 10km cube. This nebula will be typically cooler and denser than the rest of the interstellar medium so that it should be in equilibrium position-wise. There is also typically more dust - about 1% of the cloud. It generally also contains heavy elements from previous stellar explosions where complex nucleosynthesis took place. (You need a supernova to get heavier elements than Fe: also to give short-lived isotopes like Al26 and Co56 which we shall come across later.) The low temperature, high density is a good condition for leading to a contraction of the cloud. Holding it apart, however, will be thermal motions, and it will be "stiffened" against gravitational collapse also by **B** fields, spin and turbulence. Spin will be the biggest effect. Turbulence can act both ways - both helping to hold a h helping to hold a cloud apart with its chaotic motions, and also aiding compression through shock impact. Because of its size, the collapsing molecular cloud must fragment, or there would be one immense, very short-lived sun. (Sometimes this happens - large fragments of so-called "prestellar nebulae" with masses around 20MS, collapse to a star, which lasts about 1Ma and then goes nova. In comparison, note that a sol-type sun lasts 1010 years.) What causes the fragmentation? It could be turbulence, pressure waves or differential cooling. One can calculate that there is a minimum mass (at a given temperature and density) called the Jean's Mass, needed for an individual star to collapse. For a given medium, the larger the mass the more likely the collapse. Hence stars form in preference to planets, and we assume planetary formation follows formation of the sun. The fact that there is a tendency for fragmentation then stellar formation means that stars often form in "star clusters". There must be a dispersal mechanism though, at least in some cases, as 5-20% of stars are "singles" like the Sun. This is fortunate for planetary formation since it is very difficult to form a stable planetary system with star clusters. The collapsing cloud becomes denser and denser until it forms a "protostar", a compact infra-red emitting body surrounded by a "cloud". Sometimes the star is visibmes the star is visible at the centre of the cloud, like the T-Tauri stars. These stars are seen to be surrounded by tenuous gas and dust in violent motion (the //T-Tauri wind// - like a very strong solar wind). A lot of work is currently going in to understanding such systems, but they are still poorly understood. The collapse down into the protostar continues, with a subsequent rise in temperature, until eventually stellar "ignition" takes place and fusion begins. The rapid collapse also starts the system rapidly spinning. The "ignition" of nuclear fusion takes place about 10Ma after maximum luminosity, and by then by most cosmogonic theories, the star probably has a planetary system. One suggested mechanism for triggering the collapse of some interstellar nebulae, and one that seems may be appropriate for ours, is that a nearby supernove produces a shock wave which "seeds" the collapse. Credence is leant to this theory by the excess of Mg26 found in meteorites like the "Allende meterorite". By an "excess" we mean that there is more Mg26 than the estimated proportion from the universal abundances - and this is produced by the decay of Al26. This Al isotope is very short lived on a cosmic timescale and so the fact it was there when the meteorite was formed suggests a source of heavy nuclei - presumably a supernova - immediately preceeded the period leading up to condensation out of the ation out of the first material. Thus supernova and condensation seem linked, suggesting it could easily be that the former triggered the latter. Of course the supernova will also supply heavy elements to the collapsing nebula. > So far, to this point, there is little to distinguish high and low mass nebula theories. A lot of the difference between them, however, resides in the mechanism for explaining where the angular momentum goes, since all theories of collapse show a very fast spin rate must be built up as the star is formed. To get from that state to our current situation where the sun spins relatively slowly and most of the angular momentum of the solar system resides in the planets (particularly Jupiter) requires a lot of dissipation of the angular momentum to have occured, and it seems only in the early days of formation of the system could this have happened. It is one of the major difficulties of theories of solar system formation to explain where the angular momentum went. > **Low Mass Nebulae** Formation from a "low-mass" nebula means that the mass of the cloud which contracts to form the star and planets is around 1.1 times the mass of the star, so in the case of the solar system, 1.1 MS. The spin-up as the collapse continues leads to a trade-off between gravitationade-off between gravitational and spin forces which produces a sheet ofmaterial in the spin plane. > The material settling in to this plane eventually takes up Keplerian orbits, and infra-red cooling leads to further shrinkage. The cooling is partly offset by dust capture. You get some accretion - electrostatic, electromagnetic and collisional mechanisms lead to the formation of //planetisimals//. The temperature on the fringes of the collapsed disc is 50K but closer in to the centre of mass it can be up to 1000K. With the higher temperatures you get more mixing of materials. As the sun heats up it volatilises and "blows away" the volatile material near to it. Within the planetisimals there is a competition between accretion and the disruption due to destructive collisions, with accretion gradually winning and the gas and dust gradually acculumating into large bodies. H2 and He is lost from the inner system due to the heating from the sun. Jupiter and Saturn form rocky cores early and so start to capture the Hydrogen and Helium - at least enough to retain similar compositions to the early nebula. Either Jupiter and Saturn swept up so much material that Uranus and Neptune, forming later, had less to accumulate, or Uranus and Neptune formed with smaller cores which were unable to attract the lighter gases as efficiently before they were loently before they were lost to the system. The sun meanwhile slowed down due to solar wind drag which deposited the angular momentum in the material which was then lost from the system. Once the inner system had been cleared by the solar wind and radiation pressures from the primordial sun, all that was left in a surrounding cloud outside the main planets was icy conglomerates with compositions akin to the early nebula. > **High Mass Nebula** A "High-Mass" Nebula is, considered to be one where the mass is greater than about 2MS. Theories considering the solar system were formed in such a region again start with the cloud contracting to the spin plane. The large angular momentum in this massive disc leads to turbulence and increased interaction of the constituents. It is possible also that solar tidal forcing plays a part, but the end result is that the planetary bodies start to separate out:
 * Forming stars from molecular Hydrogen clouds**
 * Heavy elements enter the primordial nebula**

The cores form from the nebula with roughly primordial compositions, but then gas is lost from the disc due to turbulence and the evolving Sun's action. Angular momentum is transferred away from the Sun by the turbulence and viscosity of the medium. As the Sun warms up it clears the system of its un-bound volatiles, starting with the inner solar system first. How do these theories fit the current hese theories fit the current morphology of the system? We know that the density of the planetary bodies tends to be higher in the inner part of the system where we expect the volatiles to have been depleted. (The terrestrial planets have a higher proportion of silicates - this gives way to water ice at larger distances from the sun and then to methane and other more volatile ices.) The nebula theories, both high and low-mass, have support in the Allende meteorite discussed above, but this does not distinguish between them. We can look more generally at meteoritic composition - a topic we shall return to nearer the end of the course, and see if the range of compositions and morphologies favours one theory over another. We can also see what material from elsewhere in the solar system might tell us. We have material from the Moon. This seems to be differentiated, volatile poor, similar to chondritic meteorites in composition, but sampling was poor (the Apollo landers came down all in a band near the lunar equator and all in fairly similar smooth terrain, for safety reasons).There is anyway the question as to whether the Moon is typical or representative. In the cases of Venus and Mars very little is known of their compositions. We have some surface chemistry information from a limited number of sites on both bodies, but no internal material with the possible exception of the SNC meteorites which are believed to be Martian in origin; even with these though we have no way of knowing how representative the sampling is. Mercury we know little about except that it is dense, probably contains a high proportion of iron, and that FeO has been detected spectroscopically: we conjecture it might have a core. We give more information on this and the other terrestrial planets in a later section, but overall the information is complex and not always reliable or telling as far as one theory or the other is concerned. Similarly with the gas giants - little is know of their interiors and in many ways the theories of their structure are dependent on how we think the solar system formed, rather than vice versa. On the whole, though, the tendency currently is to favour the low-mass nebula theory and the accretion of many small planetisimals. Whatever theory we favour, they tend to leave Pluto/Charon as an anomaly along with others like the large relative size of the earth's moon in comparison to its primary, Triton's retrograde orbit about Neptune, Uranus' large inclination etc. These "uncomfortable" facts require additional detail to be added, and each of these has to be explained as exceptions to the general theories. This should not be unexpected: the theories of solar system formation are all complex, and will all have a lot of scope for chaotic variation. We get some evidence to support the nebula theories from looking a to support the nebula theories from looking at other star systems. We see stars like Beta Pictoris which have disc-like nebulae, probably made of rock rubble and asteroidal-type bodies. IRAS (the Infra-Red Astronomy Satellite) saw many bright infra-red sources which appeared to be flattened dust discs about young stars. We have found large Jupiter-sized (and larger) planets in nearby star systems (by the "wobble" they impart to the motion of the parent star) and what is believed to be a small terrestrial-sized planet associated with a neutron star, but so far our technology has not allowed us to see small planets in other star systems. However, the evidence is growing that planetary systems are not uncommon, and we believe that our system may not be atypical. When we start to examine the nebula creation theories in detail we can ask more complex questions. We know from dating meteorites and other evidence that the building blocks of the system were created 4.6 109 years ago, but the models of the creation tell us that the nebula only lasted 105 years. What are the consequences of this? It certainly seems likely that this short time scale means that many of the chemical processes that we postulate as taking place did not have time to come to an equilibrium state, and we have to take that into account when looking at abundances and distribution of elements. From meteorites we see evidence of volatiles captured in volatiles captured in large, low-temperature grains, but we also have some material which shows evidence of //differentiation//, that is processes, probably driven by heat, which have allowed materials to separate out by density or other gradient. Primitive "chondritic" meteorites have a matrix of materials containing hydroxyl silicates, sulphides, halides and reactive iron oxides. Other meteorites with larger crystals (which would have been heated - origin inside asteroids?) have much smaller concentrations of these. The **Gases** that we find around bodies as we go out from the sun also seems to show trends that would fit with our general picture. Models of the equilibrium concentrations expected in a radial temperature gradient, given the nebula's initial composition, suggest that near the sun one would get a primarily CO and N2 mixture as the most stable components. We would expect this to dominate in the early nebula inside the radius where the temperatures are above 680K. Below this (and hence further out) CH4 and nitrogen would be the most stable combination, and then out beyond the point where the temperature drops to 330K ammonia and methane would be expected to dominate. Unfortunately this picture is drawn from models which assume equilibrium conditions, and as we discussed above, the 105 years we have for the material to settle to the final locations from the nebula may not hanebula may not have been enough for this equilibrium to have been properly established. However, it does seem to fit in a crude way with the way volatiles and gases are distributed through the current system. > **Non-co-eval Theories** Just for completeness we should note that there are also the non-co-eval theories of planetary formation. An example of these is the theory due to Woolfson, wheretwo molecular clouds, with protostars already forming or formed, meet head on, leading to turbulence and shock waves in both systems. These in turn lead to compression and condensation out of planetary bodies from the nebula: This model implies that a lone protostar retains its original angular momentum and would have no planets. Any lone star with planets must have captured them from a passing protostar by tidal forces. This "Tidal Capture Theory" can obviously take a number of varying forms, and there are several theories along these lines, though the nebulaic theories currently seem to be holding sway. To some extent explaining the origins of the the atmospheres of the terrestrial planets represents a more complex problem than the distribution of solid bodies throughout the system. The main question is, where did the volatiles come from? Deciding between these is complicated by the fact the current atmospheres might be the result of a long process of chemical change from the original atmospheres that were created. It is not very east to try to trace back any possibe temporal variation to the "source atmosphere". The earth's atmosphere, for example, is largely the result of biological processes, rather than the primordial constituents. Mg, Si, Fe and O are the most abundant terrestrial elements. The most common elements in the primordial nebula (based on solar abundances) are H, He,C,O (H2O) and N. On Earth the C,N and H2O are 104 less abundant relative to Mg, Si and Fe than they were in the solar nebula. So the earth, during or since formation, lost a great deal of the volatile material from the primordial nebula. The question is, why did it not lose all in the same process? Is the atmosphere a result of the little that was left, or was this acquired later to a body that was by then volatile-free? There are a number of possible mechanisms by which we could imagine the earth (and other terrestrial planets, or satellites) acquired their atmospheres:
 * Fitting the evidence to the theory**
 * Atmospheric origin**
 * Primary capture from the primordial cloud?
 * Outgassing?
 * Cometary CLI>Outgassing?
 * Cometary Capture?

If water were acquired by local accretion it would mainly be in hydrated form, as seen in meteorites. N,C, O and H would be acquired in complex carbohydrates, and smallex carbohydrates, and small amounts of C and N "dissolved" in the metallic Fe phase. Some water would be derived from reduced hydrogen from organic compounds. As the planetisimals were forming and accretion taking place, the early sun would be "burning off" the volatile material near the sun, but material deflected by Earth and Venus would tend to homogenize the inner solar system. If accretion of volatiles was from bodies further out than the terrestrial bodies, this would require deflection inwards due to resonances with Jupiter and Saturn and other interactions further out. If the material that arrived was then asteroidal in type, then the water would be in hydrated form (with maybe some water from ice if the material was from far enough out) and C and N from organic molecules. In all cases you would expect the terrestrial planets all to have similar fractional amounts of volatiles. We can check this out by looking at the evidence in terms of the C and N in the different planets. This is difficult though because of the differing distributions and histories. On earth the C is mainly trapped in carbonates, whereas on Venus much of it is still in the atmosphere. Most of the N2 on Venus is in the atmosphere, but on earth some has been buried in organic material. The evidence for earth is particularly difficult to interpret as we have to take into account "recycling" in the crust/mantle. It does seem, though, that earth ough, that earth and Venus have similar mounts of C and N, but Venus is deficient in H. In trying to assess the evidence we have to try to guess how an accretion scenario would go. Early on the accretion would be fairly 'gentle' and all the volatiles would be 'held' by the bodies to which they accreted. Later accretion "collisions" would have more power and so would contribute to loss of the volatile material - we would then have a process that both adds and takes away volatile material. Larger bodies would degas more - anything over 0.3RE would heat up enough to outgas the trapped material at its centre - but would have larger gravitational fields to hold onto the volatiles and gases. It is thought that the earth while accreting could have reached a temperature of 1500K from the pressure-generated heat of collisions and redistribution. In the presence of iron in the bodies, some reduction reactions control the gases formed. Thus Carbon would be seen mainly as CO, N as N2, Hydrogen in H or H2 form. In the absence of iron we would get chiefly H2O, CO2 and N2. In addition to these considerations of the original components and reactions we also have to consider subsequent possible evolution of atmospheres - water can be extracted from the gas phase as ice and "locked in" to the body. Ultra-violet radiation can dissociate water to H2 and O2, and the hydrogen flow outwards (hydrodynamic escape) can take other particles with it. (Thus, there is evidence of preferential loss on earth of lighter isotopes of Ne.) A relatively new theory from NASA suggests that regular increases in the amount of oxygen in the earth's atmosphere is caused by the loss of large amounts of organic matter in large-scale tectonic upheavals. An extra complication is the fact we must also take into account the sun's early history. It was, for example, less luminous early on, but may have been a stronger u.v. radiator. The early earth may have needed more greenhouse gases in its atmosphere in order to keep its oceans liquid (as they have been know to be for at least 3 109 years.). Could there have been more CH4 and NH3? These could then have been converted to CO2 and N2 by photodissociation and loss of H2 But early volcanic activity producing H2 and CO would soon have destroyed the methane and ammonia. Some clues as to the origins of the terrestrial atmospheres can be obtained by looking at the proportions of inert gases in these bodies. One piece of evidence suggesting that they are not of pre-solar nebula origin is that the noble gases are not of solar abundances. The proportion of inert gases is more like that found in chondritic meteorites than in thc meteorites than in the sun. Mars is more deficient in the noble gases than earth, which is in turn more deficient than Venus. It is easiest to understand this overall deficiency if the acquisition was by "veneering" rather than initial accretion at the time of planetary body formation. Mars is depleted in H and N compared to earth. It might have acquired similar proportions of material but lost more by outgassing, it might have acquired from a less-volatile rich region, or it may have been incapable of attracting volatile rich bodies late in its history because if its size. Venus has about the same amount of C and N as earth but less hydrogen. If Venus has lost its hydrogen why hasn't the earth? In the case of Mercury and the Moon, did they accrete material but lose it almost immediately, or were they incapable of attracting sufficient volatile rich bodies?

=Appendix 4: = =The Origin of the Solar System =


 * 1) === A cloud of interstellar gas and/or dust (the "solar nebula") is disturbed and collapses under its own gravity. The disturbance could be, for example, the shock wave from a nearby supernova. ===


 * 1) === As the cloud collapses, it heats up and compresses in the center. It heats enough for the dust to vaporize. The initial collapse is supposed to take less than 100,000 years. ===


 * 1) === The center compresses enough to become a protostar and the rest of the gas orbits/flows around it. Most of that gas flows inward and adds to the mass of the forming star, but the gas is rotating. The centrifugal force from that prevents some of the gas from reaching the forming star. Instead, it forms an "accretion disk" around the star. The disk radiates away its energy and cools off. ===


 * 1) === First brake point. Depending on the details, the gas orbiting star/protostar may be unstable and start to compress under its own gravity. That produces a double star. If it doesn't ... ===


 * 1) === The gas cools off enough for the metal, rock and (far enough from the forming star) ice to condense out into tiny particles. (i.e. some of the gas turns back into dust). The metals condense almost as soon as the accretion disk forms (4.55-4.56 billion years ago according to isotope measurements of certain meteors); the rock condenses a bit later (between 4.4 and 4.55 billion years ago). ===


 * 1) === The dust particles collide with each other and form into larger particles. This goes on until the particles get to the size of boulders or small [|asteroids]. ===


 * 1) === Run away growth. Once the larger of these particles get big enough to have a nontrivial gravity, their growth accelerates. Their gravity (even if it's very small) gives them an edge over smaller particles; it pulls in more, smaller particles, and very quickly, the large objects have accumulated all of the solid matter close to their own orbit. How big they get depends on their distance from the star and the density and composition of the protoplanetary nebula. In the solar system, the theories say that this is large asteroid to [|lunar] size in the inner solar system, and one to fifteen times the [|Earth]'s size in the outer solar system. There would have been a big jump in size somewhere between the current orbits of [|Mars] and [|Jupiter]: the energy from the [|Sun] would have kept ice a vapor at closer distances, so the solid, accretable matter would become much more common beyond a critical distance from the Sun. The accretion of these "planetesimals" is believed to take a few hundred thousand to about twenty million years, with the outermost taking the longest to form. ===


 * 1) ===<span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;"> Two things and the second brake point. How big were those protoplanets and how quickly did they form? At about this time, about 1 million years after the nebula cooled, the star would generate a very strong solar wind, which would sweep away all of the gas left in the protoplanetary nebula. If a protoplanet was large enough, soon enough, its gravity would pull in the nebular gas, and it would become a gas giant. If not, it would remain a rocky or icy body. ===


 * 1) ===<span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;"> At this point, the solar system is composed only of solid, protoplanetary bodies and gas giants. The "planetesimals" would slowly collide with each other and become more massive. ===

===<span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">**Note:** this was the theory of planetary formation as it stood before the discovery of [|extrasolar planets]. The discoveries don't match what the theory predicted. That could be an observational bias (odd solar systems may be easier to detect from Earth) or problems with the theory (probably with subtle points, not the basic outline.) === <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">
 * 1) ===<span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;"> Eventually, after ten to a hundred million years, you end up with ten or so planets, in stable orbits, and that's a solar system. These planets and their surfaces may be heavily modified by the last, big collision they experience (e.g. the largely metal composition of [|Mercury] or the [|Moon]). ===

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