The surfaces of Solar System objects, other than the four
Outer Solar System giant planets, are mostly solid, with few having liquid surfaces. In general
terrestrial planets have either
surfaces of ice, or surface
crusts of
rock or
regolith, with distinct
terrains. Water ice predominates surfaces in the Solar System beyond the
frost line in the Outer Solar System, with a range of
icy celestial bodies. Rock and regolith is
common in the Inner Solar System until Mars. The only Solar System object having a mostly liquid surface is Earth, with its global
ocean surface comprising 70.8 % of
Earth's surface, filling its
oceanic basins and covering Earth's
oceanic crust, making Earth an
ocean world. The remaining part of its surface consists of rocky or organic carbon and silicon rich
compounds. (lower right) and other northern hemisphere hydrocarbon lakes Liquid water as surface, beside on Earth, has only been found, as
seasonal flows on warm Martian slopes, as well as past occurrences, and suspected at the
habitable zones of other
planetary systems. Surface liquid of any kind, has been found notably on
Titan, having large
methane lakes, some of which are the
largest known lakes in the Solar System.
Volcanism can cause flows such as
lava on the surface of geologically active bodies (the largest being the
Amirani (volcano) flow on Io). Many of Earth's
Igneous rocks are formed through processes rare elsewhere, such as the presence of volcanic magma and water. Surface mineral deposits such as
olivine and
hematite discovered on Mars by lunar rovers provide direct evidence of past stable
water on the surface of Mars. Apart from water, many other abundant surface materials are unique to Earth in the Solar System as they are not only
organic but have formed due to the presence of life – these include
carbonate hardgrounds,
limestone,
vegetation and
artificial structures although the latter is present due to probe exploration (see also
List of artificial objects on extra-terrestrial surfaces).
Extraterrestrial organic compounds Increasingly, organic compounds are being found on objects throughout the Solar System. While unlikely to indicate the presence of extraterrestrial life, all known life is based on these compounds. Complex carbon molecules may form through various complex chemical interactions or delivered through impacts with small solar system objects and can combine to form the "building blocks" of
carbon-based life. As organic compounds are often
volatile, their persistence as a solid or liquid on a planetary surface is of scientific interest as it would indicate an intrinsic source (such as from the object's interior) or residue from larger quantities of organic material preserved through special circumstances over geological timescales, or an extrinsic source (such as from past or recent collision with other objects). Radiation makes the detection of organic matter difficult, making its detection on atmosphereless objects closer to the Sun extremely difficult. Examples of likely occurrences include: •
Tholins – many Trans Neptunian Objects including Pluto-Charon, Titan, Triton,
Eris,
Sedna, 28978 Ixion, 90482 Orcus,
24 Themis •
Methane clathrate (CH4·5.75H2O) –
Oberon,
Titania,
Umbriel,
Pluto, 90482 Orcus,
Comet 67P On Mars Martian exploration including samples taken by on the ground rovers and spectroscopy from orbiting satellites have revealed the presence of a number of complex organic molecules, some of which could be biosignatures in the search for life. •
Thiophene () •
Polythiophene (polymer of ) •
Methanethiol () •
Dimethyl sulfide () •
Gilsonite On Enceladus •
Methylamine/Ethylamine (
CH3 NH2) •
Acetaldehyde •
Acetamide () •
Acetone (CH3)2CO •
Methyl isocyanate () •
Propionaldehyde ()
Inorganic materials on Earth (top), compared with dunes in Belet on Titan The following is a non-exhaustive list of surface materials that occur on more than one planetary surface along with their locations in order of distance from the Sun. Some have been detected by spectroscopy or direct imaging from orbit or flyby. •
Ice () – Mercury (polar); Earth-Moon system; Mars (polar);
Ceres and some asteroids such as
24 Themis; Jupiter moons –
Europa,
Ganymede and
Callisto;
Triton,; Venus, Earth-Moon system; Mars (and its moons
Phobos and
Deimos); asteroids (including
4 Vesta); Titan •
Nitrogen ice () –
Pluto–Charon,
Triton,
Kuiper belt objects,
Plutinos •
Sulphur () – Mercury; Earth; Mars; Jupiter moons – Io and Europa
Rare inorganics •
Salts – Earth, Mars, Ceres, Europa and Jupiter Trojans, Enceladus •
Clays – Earth; Mars; asteroids including Ceres and
Tempel 1; Europa •
Sand – Earth, Mars, Titan •
Calcium carbonate () – Earth, Mars •
Sodium carbonate () – Earth, Ceres
Carbon Ices •
Dry ice () – Mars (polar); Ariel; Umbriel; Callisto ==Landforms==