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Theory Observation
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Planetary Geology
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
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| Terrestrial Planets | Gas Giants Planets |
Terrestrial Planets
Once 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. 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. Lightest material floats atop and cools, forming a thin crust, made of light rocks (mostly silicates). Crust eventually solidifies giving a planet its definitive and well-known aspect: a liquid core, a mantle, a rigid crust. 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. These ones 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
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. Comets or asteroids, which are leftovers of protoplanetary disk, are swept by newly born planets. The latter endure heavy bombardment periods. 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! A further, lighter, bombardment period, 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. 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
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. Plate tectonics is caused by crust being broken into plates. These 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
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. Water is 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). 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, most of the lunar new craterlets being of the order of several meters
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?
see an inner solar system planets synoptic table, with cut-away views
Gas Giants Planets
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
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
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
see a table of cut-away views of the outer solar system planets, with moons