Comparative planetary science or comparative planetology is a branch of space science and planetary science in which different natural processes and systems are studied by their effects and phenomena on and between multiple bodies. The planetary processes in question include geology, hydrology, atmospheric physics, and interactions such as impact cratering, space weathering, and magnetospheric physics in the solar wind, and possibly biology, via astrobiology.
Comparison of multiple bodies assists the researcher, if for no other reason than the Earth is far more accessible than any other body. Those distant bodies may then be evaluated in the context of processes already characterized on Earth. Conversely, other bodies (including extrasolar ones) may provide additional examples, edge cases, and counterexamples to earthbound processes; without a greater context, studying these phenomena in relation to Earth alone may result in low sample sizes and observational biases.
Background
The term "comparative planetology" was coined by George Gamow, who reasoned that to fully understand our own planet, we must study others. Poldervaart focused on the Moon, stating "An adequate picture of this original planet and its development to the present earth is of great significance, is in fact the ultimate goal of geology as the science leading to knowledge and understanding of earth's history."
Geology, geochemistry, and geophysics
All terrestrial planets (and some satellites, such as the Moon) are essentially composed of silicates wrapped around iron cores. The large outer Solar System moons and Pluto have more ice, and less rock and metal, but still undergo analogous processes.
Volcanism
Volcanism on Earth is largely lava-based. Other terrestrial planets display volcanic features assumed to be lava-based, evaluated in the context of analogues readily studied on Earth. For example, Jupiter's moon Io displays extant volcanism, including lava flows. These flows were initially inferred to be composed mostly of various forms of molten elemental sulfur, based on analysis of imaging done by the Voyager probes. However, Earth-based infrared studies done in the 1980s and 1990s caused the consensus to shift in favor of a primarily silicate-based model, with sulfur playing a secondary role.
Much of the surface of Mars is composed of various basalts considered analogous to Hawaiian basalts, by their spectra and in situ chemical analyses (including Martian meteorites). Mercury and Earth's Moon similarly feature large areas of basalts, formed by ancient volcanic processes. Surfaces in the polar regions show polygonal morphologies, also seen on Earth.
In addition to basalt flows, Venus is home to a large number of pancake dome volcanoes created by highly viscous silica-rich lava flows. These domes lack a known Earth analogue. They do bear some morphological resemblance to terrestrial rhyolite-dacite lava domes, although the pancake domes are much flatter and uniformly round in nature.
Certain regions further out in the Solar System exhibit cryovolcanism, a process not seen anywhere on earth. Cryovolcanism is studied through laboratory experiments, conceptual and numerical modeling, and by cross-comparison to other examples in the field. Examples of bodies with cryovolcanic features include comets, some asteroids and Centaurs, Mars, Europa, Enceladus, Triton, and possibly Titan, Ceres, Pluto, and Eris.
The trace dopants of Europa's ice are currently postulated to contain sulfur. This is being evaluated via a Canadian sulfate spring as an analogue, in preparation for future Europa probes. Small bodies such as comets, some asteroid types, and dust grains, on the other hand, serve as counterexamples. Assumed to have experienced little or no heating, these materials may contain (or be) samples representing the early Solar System, which have since been erased from Earth or any other large body.
Some extrasolar planets are covered entirely in lava oceans, and some are tidally locked planets, whose star-facing hemisphere is entirely lava.
Cratering
The craters observed on the Moon were once assumed to be volcanic. Earth, by comparison, did not show a similar crater count, nor a high frequency of large meteor events, which would be expected as two nearby bodies should experience similar impact rates. Eventually this volcanism model was overturned, as numerous Earth craters (demonstrated by e. g., shatter cones, shocked quartz and other impactites, and possibly spall) were found, after having been eroded over geologic time. Craters formed by larger and larger ordnance also served as models. The Moon, on the other hand, shows no atmosphere or hydrosphere, and could thus accumulate and preserve impact craters over billions of years despite a low impact rate at any one time. In addition, more searches by more groups with better equipment highlighted the great number of asteroids, presumed to have been even more numerous in earlier Solar System periods.
As on Earth, a low crater count on other bodies indicates young surfaces. This is particularly credible if nearby regions or bodies show heavier cratering. Young surfaces, in turn, indicate atmospheric, tectonic or volcanic, or hydrological processing on large bodies and comets, or dust redistribution or a relatively recent formation on asteroids (i. e., splitting from a parent body).
Examination of the cratering record on multiple bodies, at multiple areas in the Solar System, points to a Late Heavy Bombardment, which in turn gives evidence of the Solar System's early history. However, the Late Heavy Bombardment as currently proposed has some issues and is not completely accepted.
One model for Mercury's exceptionally high density compared to other terrestrial planets is the stripping off of a significant amount of crust and/or mantle from extremely heavy bombardment.
Differentiation
As a large body, Earth can efficiently retain its internal heat (from its initial formation plus decay of its radioisotopes) over the long timescale of the Solar System. It thus retains a molten core, and has differentiated- dense materials have sunk to the core, while light materials float to form a crust.
Other bodies, by comparison, may or may not have differentiated, based on their formation history, radioisotope content, further energy input via bombardment, distance from the Sun, size, etc. Studying bodies of various sizes and distances from the Sun provides examples and places constraints on the differentiation process. Differentiation itself is evaluated indirectly, by the mineralogy of a body's surface, versus its expected bulk density and mineralogy, or via shape effects due to slight variations in gravity. Differentiation may also be measured directly, by the higher-order terms of a body's gravity field as measured by a flyby or gravitational assist, and in some cases by librations.
Edge cases include Vesta and some of the larger moons, which show differentiation but are assumed to have since fully solidified. The question of whether Earth's Moon has solidified, or retains some molten layers, has not been definitively answered. Additionally, differentiation processes are expected to vary along a continuum. Bodies may be composed of lighter and heavier rocks and metals, a high water ice and volatiles content (with less mechanical strength) in cooler regions of the Solar System, or primarily ices with a low rock/metal content even farther from the Sun. This continuum is thought to record the varying chemistries of the early Solar System, with refractories surviving in warm regions, and volatiles driven outward by the young Sun.
The cores of planets are inaccessible, studied indirectly by seismometry, gravimetry, and in some cases magnetometry. However, iron and stony-iron meteorites are likely fragments from the cores of parent bodies which have partially or completely differentiated, then shattered. These meteorites are thus the only means of directly examining deep-interior materials and their processes.
Gas giant planets represent another form of differentiation, with multiple fluid layers by density. Some distinguish further between true gas giants, and ice giants further from the Sun.
Tectonics
In turn, a molten core may allow plate tectonics, of which Earth shows major features. Mars, as a smaller body than Earth, shows no current tectonic activity, nor mountain ridges from geologically recent activity. This is assumed to be due to an interior that has cooled faster than the Earth (see geomagnetism below). An edge case may be Venus, which does not appear to have extant tectonics. However, in its history, it likely has had tectonic activity but lost it. It is possible tectonic activity on Venus may still be sufficient to restart after a long era of accumulation.
Io, despite having high volcanism, does not show any tectonic activity, possibly due to sulfur-based magmas with higher temperatures, or simply higher volumetric fluxes. Meanwhile, Vesta's fossae may be considered a form of tectonics, despite that body's small size and cool temperatures.
Europa is a key demonstration of outer-planet tectonics. Its surface shows movement of ice blocks or rafts, strike-slip faults, and possibly diapirs. The question of extant tectonics is far less certain, possibly having been replaced by local cryomagmatism. Ganymede and Triton may contain tectonically or cryovolcanically resurfaced areas, and Miranda's irregular terrains may be tectonic.
Earthquakes are well-studied on Earth, as multiple seismometers or large arrays can be used to derive quake waveforms in multiple dimensions. The Moon is the only other body to successfully receive a seismometer array; "marsquakes" and the Mars interior are based on simple models and Earth-derived assumptions. Venus has received negligible seismometry.
Gas giants may in turn show different forms of heat transfer and mixing. Furthermore, gas giants show different heat effects by size and distance to the Sun. Uranus shows a net negative heat budget to space, but the others (including Neptune, farther out) are net positive.
Geomagnetism
Two terrestrial planets (Earth and Mercury) display magnetospheres, and thus have molten metal layers. Similarly, all four gas giants have magnetospheres, which indicate layers of conductive fluids. Ganymede also shows a weak magnetosphere, taken as evidence of a subsurface layer of salt water, while the volume around Rhea shows symmetrical effects which may be rings or a magnetic phenomenon. Of these, Earth's magnetosphere is by far the most accessible, including from the surface. It is therefore the most studied, and extraterrestrial magnetospheres are examined in light of prior Earth studies.
Still, differences exist between magnetospheres, pointing to areas needing further research. Jupiter's magnetosphere is stronger than the other gas giants, while Earth's is stronger than Mercury's. Mercury and Uranus have offset magnetospheres, which have no satisfactory explanation yet. Uranus' tipped axis causes its magnetotail to corkscrew behind the planet, with no known analogue. Future Uranian studies may show new magnetospheric phenomena.
Mars shows remnants of an earlier, planetary-scale magnetic field, with stripes as on Earth. This is taken as evidence that the planet had a molten metal core in its prior history, allowing both a magnetosphere and tectonic activity (as on Earth). Both of these have since dissipated. Earth's Moon shows localized magnetic fields, indicating some process other than a large, molten metal core. This may be the source of lunar swirls, not seen on Earth.
Geochemistry