Theory 
Planetary Geology| CONTENT - A fine tutorial about how planetology works and how telluric or gazeous planets form and evolve |
Space age, with its probes and robots sent to other planets in the solar system, came as a complement to the study of planets from Earth-based observatories. Further advance in astronomy led to make science about the environment of stars and to understand that solar systems might be fairly common in the Universe. A lot of proto-planetary disks have been found about other stars. Planets are forming in such proto-planetary disks. Once formed, planets endure a series of processes which further shape them
| Planetary Tools Solar Systems Formation | Planets Evolution |
Planetary ToolsIt's mainly Hubble and other orbiting space telescopes which allowed to progress in the knowledge of how planets are forming about stars. Before that, it had already been hypothesized that our own solar system had formed from a proto-planetary nebula. Most recent advances are going further still than Hubble as a new field of astronomy is taking interest in what is called "exoplanets", i.e. planets found orbiting other stars
As far as our knowledge of planets is concerned, planetary geologists are using various methods. Data may be inferred from planets orbits or from other observations made at Earth-based observatories. Astronomers have additional tools too which are provided by probes or robotic missions. All these tools are applying both to planets of our solar system and to solar systems found at other stars. Here are various ways allowing to gather data about a planet's geology:
Planetary scientists stress that water plays a critical role in determining the tectonic behavior of early planetary surfaces, the melting point of planetary interiors and the location and eruptive style of planetary volcanoes, as volcanic glass samples ejected by explosive volcanism are of importance to return like samples from a planetary mission
Solar Systems FormationPlanets around a star form from a 'protoplanetary disk.' A protoplanetary disk is the remnant of the cloud from which a star formed. Under gravity, that contracts and begins to rotate until most of the dust and gas falls into a flattened disc swirling around a growing central protostar. Dust grains are born when individual atoms collide and stick to one another. If they pack together like snow into snowballs, they don't react much with light or heat. But if they instead link together into lacy, snowflake-like structures, they do much more. Pieces of a protoplanetary disk further eventually turns gravitationally unstable and collapse to start the planets' formation process. Pebbles got trapped into pockets at some point of the protoplanetary disk, and planets grew there. During the period of gravitational instability leading particles in a circumplanetary disk to clump, effects of the magnetic field formed by spirals arms -- themselves formed by gravity -- appearing in the disk, erode the angular momentum of forming planets and increase heat. The outer part of the disk might evolve more rapidly as the energy generated by the interaction of the forming magnetic field with gravity is acting outwards and driving a wind that throws matter out of the disk. 90 per cent of the mass might be lost in less than a million years. A wild variety of shapes, sizes and structures, including the likely effects of forming planets or the type of stars, is the mark of dusty disks surrounding young stars. A protoplanetary disk is composed mainly of cold molecular hydrogen gas, which is a highly transparent and essentially invisible as a small fraction of dust or other gases are mixed in the gas. Simple hydrogen molecules are the main gas component of planets, as hydrogen deuteride molecules containing deuterium, a heavier version of hydrogen, are the best light-emitting object to assess the disk's mass. Two models nowadays concern the formation of planets around a star. A one, the core-accretion model necessitates a metal-rich star, with dust and gas particles circling a young star clinging together and gradually become larger, forming rocks, boulders turning into terrestrial planets or the stony cores of gas giants (that theory of planetesimals aggregation was based upon a hypothesis formulated by Soviet astronomer Viktor Safronov in 1969). And the disk-instability model where gravitational attraction between gas molecules alone may lead to the formation of pure gas planets only. In that second theory, planets might have formed already as since as around earlier, metal-poor stars in the Universe, lacking elements heavier than hydrogen and helium. Heavy elements, or elements heavier than hydrogen and helium, such as iron, carbon and silicon generally are the materials out of which the first grains of the early planet formation coalesce. Findings by 2012 however are showing that some exo-solar systems lack of such heavy elements hinting to some alternate planet formation principles. The mass of material swirling in a protoplanetary disk is a key factor controlling planet formation. Around three million years into the solar system’s history, gas had disappeared from the primitive solar nebula, for example, only leaving solid material behind. The farther a forming planet from its parent star, the slower its orbital speed and the more the deficiency of material in the disk. Jupiter at 500 million miles from the Sun, formed in about 10 million years. A finding in 2015 that the formation of stars in the Universe yields a consistent distribution of the varied types of stars implies that the early Universe did not have as many heavy elements for making planets, because there would be fewer supernovae from massive stars to manufacture those
 | A protoplanetary disk is seen with material from the disk flowing along the star’s magnetic field lines and deposited onto the star’s surface. picture courtesy NASA/JPL-Caltech |
Protoplanetary disks are left remained from a star formation among a gas cloud. Such disks are typically a few hundred AU wide and made of gas and dust. Their mass is ranging from 1 to 10 percent of one solar mass. Midplane of a protoplanetary plane is chemically inert, which could be caused by early meridional flows. Water vapor, ice, and oxygen combine at different times during planet formation, and then with dust to form the mass that may one day become a planet. As a star grows in size, more material rains down toward it from the cloud, and the rotation flattens this material out into a turbulent disk. Magnetic storms in the gas orbiting young stars may explain why the stars give off more infrared light than expected. Such magnetic fields yield a kind of atmosphere' aroung the disk and make that gas and dust are suspended above the disks on gigantic magnetic loops and they add to infrared light. Such a stirring also contributes to planet-forming. Two types of developing planetary environments exist, generally. The first, known as a YSO disk, typically is less than 5 million years old, contains large quantities of gas, and often is found in or near young star clusters. The second planetary habitat, known as a debris disk, tends to be older than 5 million years, holds little or no gas, and possesses belts of rocky or icy debris that resemble the asteroid and Kuiper belts found in our own solar system. Vega and Fomalhaut, two of the brightest stars in the sky, host debris disks. When a star is young the material in the planet-forming disk falls into the star, which is a accretion process but when it hits the magnetic bubble around it, called the heliosphere, the material travels up and around the bubble, landing on the star from the top and bottom. This bubble could be halting, on a other hand, migrating planets on a other hand. A gap too forms between the star and the inner edge of its dusty disk. The measure of the availability of ingredients for making stars and planets in a molecular cloud is called 'metallicity.' Planets like Earth are composed almost entirely of elements such as iron, oxygen, silicon and magnesium, or heavy elements as giant planets are also linked to such a metallicity. Planets similar to Earth however may also appear in a mettalicity-poor environment. The "radial mixing" inside a protoplanetary disc is a transport mechanism which occurs as such a disc condense around a new star. The mixing is stimulated in varying amounts by winds and heat from the central star pushing materials away, along with temperature differences and turbulent motion created in the disc during planet formation. Protoplanetary disks evolve into planets within about 10 million years. As earlier theories had suggested that building a giant planet required a stately agglomeration of large chunks, each about a kilometre across, new studies by mid-2015 it could have taken much smaller objets otherwise the process would have taken more than a few million years. The reason is that embryonic giants whizzing around the Sun would have nudged one another as their gravitational fields interacted. This would have flung some of the protoplanets out of the plane of the dusty disk, effectively starving them of material; those remaining in the disk could gorge on pebbles to become true planets. Dust grains generally stick together building pebbles, pebbles collide making boulders (planetesimals), planetesimals form planets. Crustal rocks formed also on planetesimals within the first 2.5-million years of solar system formation -- long before the formation of terrestrial planets like Earth, Mars and Venus. A study by June 2013 solved the long-standing mystery about how dust particles in discs grow to larger sizes so that they can eventually form comets, planets and other rocky bodies. To get a planet, dust particles must have agglomerated into objects of the size of pebbles. Computer models had shown that dust grains, albeit sticking together kept colliding thence smashing to pieces, or because of friction with gas were moving inwards towards their parent-star. In both cases, they couldn't grow any further. A continuous growing needed what astronomers called 'dust traps," a region where bigger dust grains were trapped and can grow much larger by colliding and sticking together. Particles there are able to grow from millimeter to comet size. A dust trap forms as bigger dust particles move in the direction of regions of higher pressure, which can originate from the motions of the gas at the edge of a gas hole. In some cases, the cause of the dust trap is a vortex in the disc's gas', with a typical duration of life of hundreds of thousand of years. Even when the dust trap ceases to work, the dust accumulated in the trap takes millions of years to disperse providing ample time for the dust grains to grow larger. Harsh environment (high winds, high velocity -about 100 yards per second) should prevent such aggregation and growing. Electrostatics might be at work there too. Magnetic fields are thought to have played an important function in moving small, magnetised dust grains around in the infant Solar System, but they did not continue to play a significant role once the particles had agglomerated to form larger building blocks. The question of how small clumps of one centimeter, generally, grow beyond the 'aggregation barrier' once electrostatic forces and the Van der Waals forces -which are tiny forces due to the polarity of molecules- allowed dust particles to stick together is due to collisions endured as it might also that such small clumps are the fundamental building blocks with many asteroids made of small chondrules only. Chondrules are tiny, sphere-like particles found in meteorites and asteroids, as their formation is still a mystery. Scientists suggest that lightning agitate dust particles in protoplanetary disks, providing the energy necessary for chondrule formation. Once a certain size reached, these bodies retain enough gravity to hold every other particles or clumps passing by. They may still be torn back to dust by large impacts however. It is not until clumps reach Moon size that bodies tend to stabilize. As the cloud from which a star is forming has a temperature of about -400°F (-240°C), the proto-planeteray disk is usually hovering at about -175°F (-115°C). Planetesimals were long thought to form as irregularly shaped bodies and remained that way over their lifetimes. If such an object however formed early enough in the solar system's history, it could have harbored the kinds of radioactive material that would produce substantial heat over a short timescale, warming the interior. That has been observed at Phoebe, the main irregular moon of Saturn. That has been confirmed by 2012 with planetesimals, in their formation stage, possessing a subsurface magma ocean after they had undergone a almost complete melting. Planetesimals generally are thought to form quickly. Gas giants either form from a planetesimal which, since a certain size, is gathering gas only, or direct from a gazeous clump unto which other layers of gas come to add. Outbursts from a young star change the chemistry of the star's disk, from which planets may eventually form. Should the phenomenon be common, that would mean that planets may carry the chemical signatures of an ancient disk of gas and dust scarred by stellar outbursts. Late accretion generally is linked to the delivery of water and other volatile elements to Earth, for example
->A Alternate View to How the Solar System Could Have Formed
Meteorites which may found in the solar system or the Earth, those conservators of early system solar materials, and containing element magnesium 26, a radioactive decay of aluminum 26 might hint to how the solar system could have formed. A stray star, ejected from its original cluster through some gravitational effect, would have head towards the interstellar clouds which the solar system was to form from and ended there like a supernova. As that star had reached the Wolf-Rayet stage, where a star form heavy elements like aluminum, it spread those heavy, radioactive elements -like the berylium 12 too- into our original gas cloud. That event thus impacted the formation of our Sun and planets in two ways as it triggered the gravitational collapse of the interstellar gas cloud, and, on the other hand, it enriched the cloud with radioactive, heat-prone elements which likely helped to dry up in part the forming terrestrial planets, allowing those to eventually turn into celestial bodies with landmasses and oceans
Protoplanets may be orbiting on collision courses with each other, and as the gas in the protoplanetary disk subsided at that time, that makes moving around easier for those bodies, and not colliding with the disk like before. Largest protoplanets may kick smaller bodies into the star or out of the system. Planets' orbital planes in a planetary system are often misaligned, which may be ascribed to gravitational scattering by giant planets and/or compagnion stars, or flybys in a stellar cluster environment, as that may also be natal, the protoplanetary disk being originally warped (different rotational axis of the accretion gas, or misalignment of the rotation axis of the disk with the magnetic field direction might be a factor)
->Interstellar Objects, a Byproduct of Solar Systems Formation
1I/2017 U1 discovered by October 19th, 2017 was the first interstellar discovered in our solar system. The discovery of a interstellar object had been anticipated for
decades but the detected interstellar object was a asteroid, not a comet as expected. That celestial object was found then with a highly-elongated shape -- a length roughly ten times longer than the width and spinning every 7.3 hours -- as none of such bodies -- asteroids or comets -- in our own solar system have such a elongated shape. As far as our own solar system formation is concerned, a large percentage of the original planetesimals in the early system were ejected into interstellar space through encounters with giant planet Jupiter, or sent crashing unto Sun. The same likely occurred around other forming stars and the interstellar space was probably populated with billions and billions of planetesimals roaming around. Scientists will have to come up with new theories explaining how such an elongated object as 1I/2017 U1 could form, as that might hint to original conditions not possible in our own solar system. 1I/2017 U1's surface is somewhat reddish due to effects of irradiation from cosmic rays over millions of years and its composition similar to asteroids in the solar system or even more metallic. Scientists think that interstellar objects pass through our solar system all the time -- about several times a year -- but most are too small and too far away from Earth to be detected
Another major feature of planet formation inside a protoplanetary disk is the so-called 'snow line.' Protoplanetary disk is parted between a region, close to its star, where ice, water, and gases, are vaporized due to heat, and a region, far from star, where such elements exist under the form of ices, rock and metals. This feature explains why what are called "terrestrial planets" are smaller than outer, gas-giants planets. Ice, water and gases are providing much more material for planet formation at disk outer reaches as inner disk is providing dust and rocks only (rocky planets neither contain carbone). Gas giants like Jupiter are forming due to this abundance: they first form as an ice and rocky core, about the size of the Earth and then gather thick gas atmospheres. Water is thought to be present in the protoplanetary disks as soon as they form due to that ice is naturally present in the gas clouds from where the stars form. The ice reaching the disk is vaporized, as that water vapor freezes back and participate into the planets formation, as it's too freezing under the form of comets and asteroids. Moving radially from a hot inner protoplanetary disk with carbon and oxygen roughly stellar, to the cold outer disk, are to be encountered the snow lines of water, carbon dioxide and then carbon monoxide, according to the freeze-out temperatures of these molecules. At each stage the C/O ratio changes, generally increasing radially. The so-called snow line is not to be mistaken with the "habitable zone". The habitable zone, around a star, is the area where the temperature is just right to support life of the terrestrial sort. Also named like the 'Goldilocks zone,' it can be a wide band or a narrow one, and nearer the star or farther, depending on the star’s size and energy output. For small, red-dwarf stars, the habitable zone might lie close as, for gigantic, hot stars, the band must retreat to a safer distance. A other dividing line, called the 'frost line,' sits around Jupiter's present-day orbit. Even today, this is the approximate distance from the Sun at which the ice on most comets begins to melt and become active. A area called 'brown dwarfs desert' is a orbit interval at about 5 AU about a star in which brown dwarfs are theoretically rare or cannot exist. Such a zone exists due to the varied formation mechanisms of stars and their planets. A 'transition disk' is a protoplanetary disc with a giant hole in the center, and produced by the disk-forming planets interaction
Our solar system certainly built in a similar way to other exo-planetary systems which are seen building around other stars. Some questions remain about planet formation process: observations are showing that protoplanetary disks may be blown in about 100,000 years due to stellar winds of center or of nearby star(s). How is this consistent with an average 10 million years for terrestrial planet formation or several million years for gas giants? Likely because exo-solar systems may form along a given range of age. Most of exoplanets discovered up to now are gas giants, Jupiter-sized orbiting closer to their star than Mercury, e.g. How such massive planets may have form on this side of the "snow line" where conditions do not meet criteria for gas giants formation? Did gas giants formed in the appropriate snow zone and then moved gravitationally inwards? Astronomers were thinking until now that due to gravitational frictions and tugs most exoplanets were to have circular orbits. Most of them in fact have highly elliptical orbits. How did this occur? And why orbits in our solar system are circular? If a planet is more pliable, it can better dissipate its gravitational energy as heat. And the more heat that is dissipated, the faster the planet will transition to a circular orbit, a process known as circularization. It has been found too that the stars featured with a protoplanetary disk are rotating slower, as their magnetic field gets tangled into the disk, with slows the star. Such slower-rotating stars are maybe those which are the most likely to have planets around them. As a comparison, our Sun is now rotating in about 28 days, as, when it was younger, it was rotating faster. As a star's spinning slows with the age, so too does starspot generation and the solar storms meaning less harmful radiation irradiating potentially nearby planets. When Jupiter formed just beyond the snow line, its powerful gravity prevented nearby material inside its orbit from coalescing and building planets. Instead, Jupiter's influence caused the material to collide and break apart. These fragmented rocks settled into an asteroid belt around the Sun. In fact, during the solar system's infancy, the asteroid belt probably had enough material to make another Earth, but Jupiter's presence and its small migration towards the Sun caused some of the material to scatter. Today, the asteroid belt contains less than one percent of its original mass. In a exo-system, generally, the presence of a Jupiter-like gas giant may, or may not trigger a asteroid belt. When at the exact location, it provides for a belt akin to the one of our solar system, when some further, the belt becomes too dense and when migrating too far inwards, it scatters the belt. A key stage in the birth of giant planets is that vast streams of gas are flowing across a gap in the disk of material around a young star created by giant planets as clear their orbit as they grow. The material which builds those is captured from inside the gap into the outer disk and forms bridges across the gap, between the outer and inner disk as the gas giant is lying just in the middle. A part of the planet's feed overshoots and poured into the inner disk. Diffuse gas is also found in the gap. Leftover material from the construction of a solar system can take the form of gas and dust, as well as small rocky and icy bodies. Debris disks can be broad, continuous disks or concentrated into belts of debris, similar to our solar system’s asteroid belt and the Kuiper Belt
Most planetary systems should yield planets mostly lying in former protoplanetary disk mid-plane and let after them a flurry of various leftovers, some having more irregular orbits and locations (comets, asteroids). When planets are young, they still glow with infrared light from their formation. On another hand, it seems sure than other leftovers of a planetary system formation are dust and boulders disks at a star system outer reaches. Kuiper Belt and Oort Cloud at Sun are best examples of such remainings of original protoplanetary disk. Types of planets planetesimals form has remained a inchoate diversity until now as the study of exoplanets, as it provides for a large number of planeteray outcomes might allow some patterns to emerge from that diversity. The outer part of a protoplanetary disk may feature a horseshoe shape from the gravitational effects of forming gas giants as the disk may be parted into a outer and a inner part by a gap in which gas giants are forming and streaming process occurring. Streams of gas flowing from the outer to the inner disk further, are likely replenishing the inner disk which otherwise would be swiftly devoured by the forming star. In our solar system, by about 4.5 billion years ago, collisions between large bodies between themselves kept on, yielding dust as Earth's moon or Pluto's moons system likely are byproducts of such a activity
Planets EvolutionTo reach to the usual spherical sphere form of planets, a object has to be 600 miles wide. Once formed, planets live their life, i.e. endure various processes which are determining their characteristics. Much of our planetary knowledge come from planets of our solar system, hence planet evolution is seen as parted into evolution at what is called terrestrial planets, and evolution at gas giants planets. Terrestrial planets are solar system small rocky, iron-rich inner planets as gas giants are large outer, mostly gas-composed planets. As far as exo-planets systems are concerned, conditions of exoplanets science has put a bias on discoveries. Until now discovered exoplanets have mostly been found gas giants. Terrestrial, rocky planets should be found in such systems however. A interesting feature of planetary -and moons- geology in the solar system is that both rocky, or icy bodies may display similarities in terms of geological features. Telluric planets, and satellites in the solar system, on a other hand, look like they all display a dichotomy between two faces, with one often more affected with volcanism, tectonics, or large impactors. It looks like terrestrial planets formed from the same materials as they then followed a different evolutionary path
| Terrestrial Planets | Gas Giants Planets |
Terrestrial PlanetsOnce a planet formed by accretion of planetesimals and once it reached Moon size, it endures heating and differentiation, where heat and gravity play a part. As planets start hot and almost completely melt, they then cool and crystallize various minerals. Rocky planets with no tectonics preserved rocks dating back to first few billion years of the Solar System. In some cases, minerals can separate to form different layers inside a planet. Earth’s Moon got such layers as Earth did not -- and is homogenous -- either because the minerals never separated, or because of tectonics which mixed everything up. Small terrestrial bodies in the solar system like minor planets in the Asteroid Belt quickly lose the heat from their formation. Chondritic meteorites have compositions similar to the Sun and are thought to be the building blocks of planets. It is unknown, on a other hand, whether asteroids have large cores. Astronomers, generally, believe that round bodies tend to have differentiated interiors. By learning about the layering of materials between those which rose and formed a crust and those which sank into the mantle and core, scientists can explain why some rocky planets turned into a 'Earth' rather than a 'Mars' or 'Venus.' Due to impacts, radioactivity or tidal gravitational effects, a part of primitive planet's material melts, or simply due to the kinetic energy produced by the accreting -and then sinking- materials. Melted material differentiates then into three layers due to density differences: heavier material sinks to planet's center and forms a planet core. Most cores are made of iron and nickel (in a planet liquid core, convection motions cause heat to be transferred to the mantle. Outer core is cooled in the process. It sinks, forming an inner solid core). Lighter material (mostly basalt and silicates) forms what is called a "mantle", which is a large, semi-melted, dense rocky layer formed by chemical segregation and thermal convection. Lightest material floats atop and cools, forming a thin crust, made of light rocks (mostly silicates). The planetary core eventually settles into a more or less spherical shape as crust eventually solidifies, giving a planet its definitive and well-known aspect: a liquid core, a mantle, a rigid crust. A strong, rock-dominated crust can remain unchanged over the 4.5-billion-year-old age of the solar system, while a weak crust rich in ices and salts would deform over that time. A round shape at a planet is usually the proof that its interior endured differentation even for a smaller body, like a minor planet, for example. This last stage is one of planet's cooling: heat is evacuated by convection (magma currents; hot material rises, cool sinks; convection may be acted by volcanism or plate tectonism) or conduction (transmission) up to surface and from there further evacuated into space (radiation). The larger the cooling, the thicker the crust. Smaller terrestrial planets heat and cool off more quickly, hence reaching their internal temperature peak earlier than larger ones. That also has a role in terms of water found on such bodies, with the minor planets, for example, possessing a low quantity of water only because they formed early, when radioactive material was more abundant and was a source of heat. Larger planets become hottest later, and cool then more slowly and later. According to this model, large terrestrial planets are Earth and Venus, small are Mercury and the Moon, as Mars is a middle-sized planet. On another hand, it may be known too that convection motions inside a planet's liquid core may yield a planet magnetic field. In turn magnetic field of a planet may be imprinted into planet's crust. At Earth this occurs mainly each side of oceans ridge when plate tectonism induced new rocks conserve a magnetic remanence of what the magnetic field polarity was when they formed. Original, natural radioactiviy then is weakening. On Earth, for example, radioactivity has been divided by 4 in 4.5 billion years. One knows too, generally, that our solar system endure a important re-organization of the orbits of its planets several hundreds million years after it formed
->A rotating, celestial body tends to stabilize its rotation axis. Should any mass discrepancy exists, the body will tend to readjust its axis, with an excess of mass adjusted near the equator, and a region of lesser mass adjusted near a pole. Should a blob of lesser density appear on a body, the axis of rotation will change so that the blob be positioned at the southern pole. Such blobs (of melted ice, or of rock) are likely triggered by tidal forces acting on the body from its parent-body. The re-arrangement of the equilibrium of the body lets the axis itself oriented like it was before the event: it's the body itself which shifts the concerned different mass along that axis
->A trick about this classical view of how planets form, is that astronomers now think that two supernovae exploding at the time of the solar system's formation might have peppered the latter with isotopes which triggered a differentation inside small planetoids (the trace of which may be seen in asteroids like Vesta which, by themselves, are too small to gravitationally trigger any such melting of a core and subsequently the emergence of a silicate crust). Hence larger planets, like Earth might have differentiated from those smaller, already differentiated bodies. Some asteroids, like Ceres, which contains much water ice, on the other hand, may hold clues to why some bodies in the solar system ended up, like Earth, with water, and some with none
Planetary geological second main stage consists of modification processes: impactors-created craters and volcanism, with lava flows are shaping first planet's features. Huge impacts have dominated the early history of Earth’s Moon and other solid worlds, like Earth, Mars, and the satellites of the outer solar system. They were extremely disruptive, world-altering events that caused substantial fracturing, melting and shaking of crusts, also blasting out material that fell back to coat older features. Comets or asteroids, which are leftovers of protoplanetary disk, are swept by newly born planets. The latter endure heavy bombardment periods. When the Solar System formed, for example, it was too hot in its central regions for water to have condensed at the locations of the terrestrial planets. Instead, it is thought that water was delivered there during that spate of asteroids and comets. Such a era at Earth is called the 'Heavy -or Late- Bombardment Period.' The influx of material by impactors depends upon a body's gravity and the angle of impact. The late heavy bombardment likely occurred because of some unusual gravitational dynamics in the early Solar System, like the migration of the solar system's outer planets jostling icy comets about and sending some of them flying inward. Gravitational interplay between Jupiter and Saturn, or a change in the orbital inclination of Jupiter is thought to have been responsible for disrupting a once highly populated Kuiper Belt, and triggering the Heavy Bombardment Period. During the Late Heavy Bombardment, the migration of Jupiter, Saturn, Uranus and Neptune generally, deflected dust and small bodies into the Kuiper and asteroid belts as a lot more dust in our solar system existed. The pulverization of the material, rather than mere surface scars was caused by the impactors as it extended deep within the crust. Materials brought by asteroids and meteorids, on the other hand might have mixed in the atmosphere with volcanic ash clouds, and spread easier on Earth. Some observations, on the other hand, might let think that the impacts at 3.9 billion years would only be the trail off from a earlier peak of impacts. Some are contending that planets in the solar system even gained the final portions of their mass from some large comet or asteroid impacts, of the size of Pluton or 1,500-2,000 miles (2,400-3,200 km) in diameter, more than 4.5 billion years ago, adding under 1 percent of the planet's mass less than one percent of the planets' mass and gold, platinum and palladium like metals. That process is called 'late accretion.' Those Moon-sized planetesimals added rock-forming minerals all the way down to planets' core, and ricocheted back into space. As far as the Earth is concerned such a accretion occurred even after Moon's formation. Platinum, iridium and gold tend to bond chemically together with metallic iron. Pummelled by at least four gigantic impactors, the early, Hadean Earth would have been resurfaced by molten rock from the mantle as a spike in asteroid impacts occurred around likely occurred 4.1 billion years ago. By 3.7 billion years ago, the mantle had not yet fully equilibrated with late accreted material. Because smaller than the Earth, Moon was only hit by impactors 150 - 200 miles wide (240-320 km). Such projectiles may have modified the orientation of Earth spin axis by 10 degrees, or brought water to Moon's mantle. A enormous collision, for example, occurred 3.26 billion years ago and involved an asteroid 23 to 36 miles across, with a crater created about 300 miles (500 km) wide. The disruption of Earth's crust possibly spurred a transition from a early tectonic regime to the more modern plate-tectonic system of nowadays. Such impacts further posed a severe challenge for life on Earth, likely vacating niches that the survivors evolved to fill. Such cataclysmic events were contemporary to the colossal impact which created the Moon from the Earth, bringing to that the event took place among commonplace large impacts of the time. Such impacts in the solar system obviously profoundly affected the young Earth, with material flowing out from the Moon and landing Earth, as the impacts too occurred at a time when life was extant at the Earth! The Heavy Bombardment Period brought comets and asteroids into the inner solar system and crashing unto the recently formed terrestrial planets there! The period ended about 3.8 billion years, about 700 million years after the solar system formed. Anectodically, gold existed outside Earth's formation and produced by stars as it was brought during that Heavy Bombardment Period. Diamonds, as far as they are concerned, were later the consequence of tectonics at our planet. The halt into the impacts likely was due too to that Jupiter and Saturn had their orbits getting distant from the Sun, and in turn pushing those of Uranus, Neptune and, eventually the Kuiper Belt. Some objects of the Kuiper Belt, in the process, were even ejected into the inner solar system and participated into the Heavy Bombardment Period. Jupiter and Saturn, generally, protected the inner solar system from comets -or even asteroids- through their gravitational influence albeit, in some sense, both planets can inject some into there too. Over the lifetime of the solar system, Jupiter has attracted and disrupted small celestial bodies millions of times over billions of years. Small bodies attracted by Jupiter served like shielding the inner planets from more heavy bombardment impacts as the gas giant either takes small bodies and pulls them out of the solar system or throws them inward. Newest theory by 2012 is holding that such asteroids might have come from the Asteroid Belt and not farther as comets might have come from the outskirts of our solar system. Carbonaceous chondrite meteorites were key sources of early Earth's volatile elements, such as hydrogen and nitrogen. Planetary exploration missions over the past 50 years have revealed the extent of cratering throughout the solar system as the impact flux during that first billion years or so of heavy bombardment was at least a 100 times higher than the present rate. Every day, currently, Earth is bombarded with more than 100 tons of dust and sand-sized particles from space as once a year, an automobile-sized asteroid hits Earth's atmosphere, creating a spectacular fireball (bolide) event. For the past 290 million years, large asteroids have been crashing into Earth more than twice as often as they did in the previous 700 million years. The reason for that is unknown. It might be related to large collisions taking place more than 300 million years ago in the main Asteroid Belt. Impacts as large as the Cretaceous-Tertiary event 60 million years ago, which wipped the dinosaurs, have occurred, and may continue to occur, every 50 million to 100 million years. All the telluric planets, like the Earth, ou Moon, Mars, Venus and Mercury all were hit by impactors which left craters hundreds or even thousands of miles across. Such craters were further modified -or erased- by subsequent events. Massive asteroids only hit Mars once every several million years. Volcanism occurs at a planet only if planet's rigid crust is thin, allowing mantle material to break through. At Moon, e.g. once differentiation completed, a flurry of impactors struck rigid crustal surface as lava flows flooded some other parts. The discovery, in June 2008, that a large part of the low, northern plains of Mars -the 'Borealis basin'- were created due to a 1,200-mile (1,900-km) wide impactor, 3.9 billion years ago, are throwing a new light about the gigantic impacts which were part into shaping the planets in the early solar system. 1,200 miles is larger than the size of Pluto! The Heavy Bombardment Period was then followed with 70 giant asteroids of the same size or larger than the dinosaur killer asteroid, at 6 miles (10 kilometers) in diameter, which impacted Earth 3.8 to 1.8 billion years ago and four similarly-sized objects hit the Moon too. That also supports the theory Jupiter, Saturn, Uranus and Neptune formed in different orbits nearly 4.5 billion years ago, migrating to their current orbits about 4 billion years ago from the interplay of gravitational forces in the young solar system. This event triggered that solar system-wide bombardment of comets and asteroids called the 'Late Heavy Bombardment.' The innermost portion of the ancien asteroid belt became destabilized and could have delivered numerous big impacts to Earth and the Moon over long time periods. With a class of meteorites known as howardite, eucrite and diogenite meteorites which were studied -and are connected with- the minor planet Vesta, astronomers now better understand a other bombardment event known as the 'Lunar Cataclysm,' when a repositioning of the gas giant planets destabilized a portion of the asteroid belt and triggered a solar-system-wide bombardment. They discovered that the projectiles which impacted our Moon four billion years ago also impacted Vesta and perhaps other large asteroids as that period not only affected the inner solar system but the Asteroid Belt too. It was determined that the population of projectiles that hit Vesta had orbits that also enabled some objects to strike the Moon at high speeds as the Asteroid Belt lost a lot of mass 4 billion years ago, which bited both upon the surviving belt asteroids and the Moon at high speeds. A further, lighter, bombardment period, at last is thought to have occurred in the solar system follwing about 300-500 million years ago as another flux of asteroids (of a size over the one kilometer -0.6 mile) is seen to appear since 100 million years, as this increase of the threats is due to a collision which occurred in the asteroids belt, between Mars and Jupiter, between two of the bodies there. Like the case for such collisions, astronomers are able to trace all the bodies derived from the collision to it, through their orbit. That family of bodies is called the 'Baptistina family'. The dinosaur-killer asteroid, 70 million years ago, is part of that family as is the asteroid which created the Tycho Crater on the Moon, as Mars and Venus were also visited and hit by such large NEOs. Another period of intense bombardment is seen too at 35 million years ago, which brought a sharp fall in global temperatures as far as the Earth is concerned, bringing the formation of the Antarctic ice sheet. The famed Siberian and Chesapeake asteroids hit at that same time. Some theories state that the solar system might be prone to periodical bombardment by comets or large asteroids each 26 million years due that by a dark, distant companion of the Sun periodically perturbs comets in the Oort Cloud, sending some into the inner solar system, or to that the solar system as a whole is moving in and out, at interval, of the Milky Way Galaxy plane. As far as the origin of water and oceans at terrestrial planets, especially the Earth, is concerned, it might that water be extant since the origins at Earth, like a chemical among others, as any water extant on Earth since the origins being blasted off by the possible impact which created the Moon 4.5 billion years ago. The water still extant today could have been brought back Earth through comets and asteroids only. Recent studies however, in April 2010, are showing that water held on comets is a different isotope than most of the water on Earth but that numerous asteroids would have been coated with up to 20-30 percent water and organic molecules as until now that theory was thought impossible due to that asteroids are closer to the Sun than comets, thus leading to a evaporation of that water. Asteroids today in the Asteroid Belt, are still coated with ice, those studies found, or the largest asteroid in the solar system, Ceres, might harbor a vast amount of frozen water buried beneath a rocky, dusty surface. Researchers agree that water must have been delivered to Earth by small bodies at a later stage of the planet’s evolution. It is, however, not clear which family of small bodies is responsible. There are three possibilities: asteroid-like small bodies from the region of Jupiter; Oort cloud comets formed inside of Neptune's orbit; and Kuiper Belt comets formed outside of Neptune's orbit. The ESA Rosetta mission by late 2014 found that a comet's water vapor is significantly different from that found on Earth which makes it more likely that Earth mostly got its water from asteroid-like bodies closer to our orbit and/or that Earth could actually preserve at least some of its original water in minerals and at the poles. Water on a planet is recycled as water from the oceans is pulled into the mantle from the crust due to geological activity, and water released from the mantle through volcanic activity. A lesser erosion from the farthest of the solar system also occurs at atmosphere-less bodies. Accumulation of tiny metallic particles containing iron dulls the fluffy outer layer, for example as that perpetual contamination with material is a common process that changes many solar system objects. Ice tables made of water, or other elements, ice are extant at Mars, Neptune’s moon Triton and on Pluto, the principle of which is similar to water table at our Earth
A third and last stage is resulting of slower processes: erosion by wind or possible water, volcanism or plate tectonics are slowly shaping and modifying a planet's surface. Like on Earth (2.4 billion years ago), Venus, Mars (possibly 3.5 billion years ago), Mercury and our Moon (as early as 3.8 billion years ago) all endured enormous volcanic eruption events, or 'large igneous provinces' (LIPs). Without plate tectonics to keep the surface active however, those eruptions eventually ceased. Plate tectonics is caused by crust being broken into plates. Mascons, or density anomalies linked to large impacts, at the ancient Earth, might perhaps have triggered plate tectonics or create first ore deposits. Mascons also have been identified in association with impact basins on Mars and Mercury, a modification of early planetary crusts. Such tectonic plates are moving relative to each other, floating on mantle convection currents. Some plates get further from others and new material is created there; or some plates collide, creating mountain ranges. For example, Venus has no oceans, and no evidence of plate tectonics, either. This might be a clue that water is needed for plate tectonics to work. One theory proposes that without water, the asthenosphere of Venus will be more rigid and unable to sustain plates, suggesting internal heat is released in some other way, maybe through periodic eruptions of global volcanism. The space environment is harsh which can cause material exposed to space to change chemically and darken over time, including impacts from microscopic meteorites and the effects of the solar wind stream of electrically conducting gas blown from the surface of the Sun
An atmosphere and water process is found too at terrestrial planets, interacting with previous stages. At each terrestrial planet, a primitive atmosphere is created during the cooling phase: volatiles escaping from planet's interior, volcanic outgasing, are forming oceans and an atmosphere. Heat and gases are evacuated from planet's interior. Such a primitive atmosphere is always, at any planet, mostly composed of carbon dioxide and of a small part of nitrogen as such primitive atmospheres interacted with planets' surface materials. As Earth was mostly a dry rock after it coalesced 4.5 billion years, for example, water at terrestrial planets was due to volcanic process or may be brought in large quantities by infalling comets during heavy bombardment periods. Any primitive atmosphere yields a natural greenhouse effect: incoming star light makes its way to surface, heats planet's surface, as surface infrared back radiation is trapped by atmosphere. Planets benefits of warmth. Depending on how this primitive atmosphere is evolving has effects about planet's evolution. At Earth, e.g. more heat brought more water evaporation, hence more natural greenhouse effect, as carbon dioxide removal: water evaporation yielded rains which brought carbon dioxide down to surface. Further, photosynthesis created oxygen. At some other planets an increased clouds cover increased greenhouse effect, insulating planet further. Better example is at Venus which endured a runaway greenhouse effect leading to excessive surface temperatures. Mars is another example where a weaker gravity let atmosphere escape. Planet became colder and a weaker atmospheric pressure let water vaporize. Once an atmosphere stabilized, climate processes appear: atmosphere circulation, clouds, rain-cycles, possible oceans, winds, etc
At last, various astronomical forcings may be at work: axis inclination and orbit length may determine a seasons cycle, or longer cyclical variations (orbit eccentricity, apsides variations) may yield climate modifications like ice ages. Meteors, on the other hand, when the planet's atmosphere is not strong enough -like at Earth- to shield most of them, are getting to hit the planets' surface all along the life of it. It has been found, for example, that about 860 meteors a century likely are hitting the surface of Mars, leaving traces between 7 and 486 ft (2-148 meters). Mars nowadays is bombarded by more than 200 small asteroids or bits of comets per year forming craters at least 12.8 feet (3.9 meters) across as such asteroids or comet fragments typically are no more than 3 to 6 feet (1 to 2 meters) in diameter. New craters are spotted for cause of a darker color or blast zones. During a period of 10 years, on the other hand, Martian dust covered some marks of a mission landing site. A meteoroid may broken up upon atmospheric entry and fragment into two larger masses along with several smaller fragments, spawning at least twenty or so smaller impact craters. A good estimate of rate at which new craters appear serve as scientists' best yardstick for estimating the ages of exposed landscape surfaces on a planet. Or that about 1,100 meteors are hitting the surface of the Moon during the same duration, as even pebbles can blast craters several feet (above 1 meter) wide and 1.3-ft (0,43 m) deep, most of the lunar new craterlets being of the order of several meters. Such meteorites, in the absence of an atmosphere, are hitting a planet at a speed of 78,000 mph (126,000 km/h). Moon experiences a heavier bombardment by small meteoroids than models had predicted like discovered by 2016. Before the NASA's LRO mission, it was thought that churning of the lunar regolith (soil) from meteoroid impacts typically took millions of years to overturn the surface down to about 0.8 inches (2 centimeters). With the new estimated rate, Apollo astronaut tracks, for example, will be gone in tens of thousands of years rather than millions. 99 percent of the Moon's surface generally, would be overturned by small impacts formation after about 81,000 years, or over 100 times faster than previous models. During that NASA's LRO mission unfolded, or 7years, the team identified over 200 impact craters ranging in size from about 10 to 140 feet (approximately 3 to 43 meters) in diameter as over 47,000 small surface changes called splotches were likely caused by small impacts, some of them secondary
One goal of scientists is to have a comprehensive view of the evolution of the four inner solar system planets (Mercury, Venus, Earth, and Mars) to refine the understanding of what processes are at work during planets formation and evolution. The inner planets are now seen to have had similar formation processes but to have eventually turned out different. Scientists are aiming to understand what elements most impacted such an evolution. The Earth, Venus, and Mars are now well-known as NASA mission MESSENGER, which is to reach Mercury about 2008-2011, will complement the view we have of the inner planets. Such a mission will be able e.g. to answer this question: why Mercury turned out mostly iron with few rocky crust as it formed in the same medium than Earth, Venus or Mars? Is it due to more iron elements found in the protoplanetary disk near the Sun, or to extreme heat, or to a giant impact?
check a synoptic table of the inner solar system planets, with cut-away views
Gas Giants PlanetsCollisions likely were common in the early Solar system with planetary embryoes, for exemple, colliding with forming gas giants like Jupiter or Saturn and having consequence to the structural shaping of those. Due to their formation and composition -a solid core accreting gas- gas giants planets are mostly gas bodies. Having no solid surface, their geological history is reduced to a minimum and it might be possible that such planets had no real evolution once their core formed, and differentiated, and accreted gas organized in layers. Recent studies to explain why a exoplanet the size of Jupiter was much more heavy has had a theory to re-surface that gas giants core, like Jupiter's might be indeed not a rocky but a liquefied one, and mixing with the rest of the planet. Interior of such gas giants thus would be a turbulent mixture of elements without strictly defined borders. That would also have the heavier elements to wander in the planet, and up to the upper layers. Liquefied parts of a gas giant's core may have trouble reaching the outer envelope due to double diffusive convection as the heavy elements in Jupiter's core may have trouble gaining enough energy to move upward. Two gas giants colliding together also might explain for that theory
Gas giants have a rigid core about which various layers of gas (helium and hydrogen) have accreted. Main difference is between Jupiter and Saturn on one hand as they have an icy, rocky, and iron core, with accreted gas parted into an inner (liquid metallic hydrogen) and an outer (liquid molecular hydrogen) envelope, and Uranus and Neptune on another hand, which have rocks only core, and accreted gas parted into an icy mantle and a liquid molecular hydrogen envelope. Both groups have an additional external gaseous atmosphere layer, and both have had their cores differentiated. Interestingly all these planets are radiating more energy than what they receive from Sun. Some traces elements are found other than hydrogen and helium, like methane, ammonia or water but most of them are added with hydrogen yielding molecules. While the source of water in the lower layers of gas giants can be explained as internal, the presence of this molecule in the upper atmospheric layers is puzzling due to the scarcity of oxygen there. At Jupiter that was found due to what was brought during the collision with the fragments of comet Shoemaker-Levy in 1994. Astronomers have investigated several possible other candidates that may have delivered water to these planets, from icy rings and satellites to interplanetary dust particles
Main features of gas giants might be related to the fact that they are surrounding by large satellites systems, members of which range from planet-sized bodies to mere rocks, and display a range of features and characteristics. All of gas giants have thin rings located in their equatorial planes. Gas giants are cold worlds (about -300° F --147° C) and their atmosphere is seen enduring large weather systems with east-west winds or large, long-lasting storms. Gas giants present a differential rotation (planet rotates quicker at equator, slower at higher latitudes) leading to a pole flattening
check a table of cut-away views of the outer solar system planets, with moons
