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.
[1][2] The large outer Solar System moons and Pluto have more ice, and less rock and metal, but still undergo analogous processes.
Other terrestrial planets display volcanic features assumed to be lava-based, evaluated in the context of analogues readily studied on Earth.
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.
[3] 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.
[5][6] Mercury and Earth's Moon similarly feature large areas of basalts, formed by ancient volcanic processes.
Examples of bodies with cryovolcanic features include comets, some asteroids and Centaurs, Mars, Europa, Enceladus, Triton, and possibly Titan, Ceres, Pluto, and Eris.
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.
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.
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.
[17][18][19] One model for Mercury's exceptionally high density compared to other terrestrial planets[20] is the stripping off of a significant amount of crust and/or mantle from extremely heavy bombardment.
[21][22] 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.
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.
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 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.
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).
Earth's Moon shows localized magnetic fields, indicating some process other than a large, molten metal core.
Meanwhile, gases at small planets such as Venus and Mars have isotopic differences indicating atmospheric escape processes.
Rocks and metals shielded by atmospheres (particularly thick ones), or other minerals, experience less weathering and fewer implantation chemistries and cosmic ray tracks.
As the long distances result in low spatial and spectral resolutions, KBO surface chemistries are currently evaluated via analogous moons and asteroids closer to Earth.
Comets vary between negligible atmospheres in the outer Solar System, and active comas millions of miles across at perihelion.
Impacts of solar wind particles create chemical reactions and ionic species, which may in turn affect magnetospheric phenomena.
[37] Venus has negligible tilt, eliminating seasons, and a slow, retrograde rotation, causing different diurnal effects than on Earth and Mars.
[38] Venus and Titan, and to a lesser extent Earth, are super-rotators: the atmosphere turns about the planet faster than the surface beneath.
This may be defined to include fluids other than water, such as light hydrocarbons on Titan, or possibly supercritical carbon dioxide on Mars, which do not persist in Earth conditions.
[52] Extant Mars hydrology includes brief, seasonal flows on slopes; however, most Martian water is frozen into its polar caps and subsurface, as indicated by ground penetrating radars and pedestal craters.
Analogues of Mars landforms on Earth include Siberian and Hawaiian valleys, Greenland slopes, the Columbian Plateau, and various playas.
Analogues for human expeditions (e.g. geology and hydrology fieldwork) include Devon Island, Canada, Antarctica, Utah, the Euro-Mars project, and Arkaroola, South Australia.
[62] The Galileo mission, while performing a gravity assist flyby of Earth, treated the planet as an extraterrestrial one, in a test of life detection techniques.
Astrobiologists consider alternative chemistries for life, and study on Earth extremophile organisms that expand the potential definitions of habitable worlds.