A planetary surface is where the solid or liquid material of certain types of astronomical objects contacts the atmosphere or outer space. Planetary surfaces are found on solid objects of planetary mass, including terrestrial planets (including Earth), dwarf planets, natural satellites, planetesimals and many other small Solar System bodies (SSSBs). The study of planetary surfaces is a field of planetary geology known as surface geology, but also a focus on a number of fields including planetary cartography, topography, geomorphology, atmospheric sciences, and astronomy. Land (or ground) is the term given to non-liquid planetary surfaces. The term landing is used to describe the collision of an object with a planetary surface and is usually at a velocity in which the object can remain intact and remain attached. In differentiated bodies, the surface is where the crust meets the planetary boundary layer. Anything below this is regarded as being sub-surface or sub-marine. Most bodies more massive than super-Earths, including stars and giant planets, as well as smaller gas dwarfs, transition contiguously between phases, including gas, liquid, and solid. As such, they are generally regarded as lacking surfaces. Planetary surfaces and surface life are of particular interest to humans as it is the primary habitat of the species, which has evolved to move over land and breathe air. Human space exploration and space colonization therefore focuses heavily on them. Humans have only directly explored the surface of Earth and the Moon. The vast distances and complexities of space makes direct exploration of even near-Earth objects dangerous and expensive. As such, all other exploration has been indirect via space probes. Indirect observations by flyby or orbit currently provide insufficient information to confirm the composition and properties of planetary surfaces. Much of what is known is from the use of techniques such as astronomical spectroscopy and sample return. Lander spacecraft have explored the surfaces of planets Mars and Venus. Mars is the only other planet to have had its surface explored by a mobile surface probe (rover). Titan is the only non-planetary object of planetary mass to have been explored by lander. Landers have explored several smaller bodies including 433 Eros (2001), 25143 Itokawa (2005), Tempel 1 (2005), 67P/Churyumov–Gerasimenko (2014), 162173 Ryugu (2018) and 101955 Bennu (2020). Surface samples have been collected from the Moon (returned 1969), 25143 Itokawa (returned 2010), 162173 Ryugu and 101955 Bennu.
Distribution and conditions Planetary surfaces are found throughout the Solar System, from the inner terrestrial planets, to the asteroid belt, the natural satellites of the giant planets and beyond to the Trans-Neptunian objects. Surface conditions, temperatures and terrain vary significantly due to a number of factors including Albedo often generated by the surfaces itself. Measures of surface conditions include surface area, surface gravity, surface temperature and surface pressure. Surface stability may be affected by erosion through Aeolian processes, hydrology, subduction, volcanism, sediment or seismic activity. Some surfaces are dynamic while others remain unchanged for millions of years.
Exploration Distance, gravity, atmospheric conditions (extremely low or extremely high atmospheric pressure) and unknown factors make exploration both costly and risky. This necessitates the space probes for early exploration of planetary surfaces. Many probes are stationary have a limited study range and generally survive on extraterrestrial surfaces for a short period, however mobile probes (rovers) have surveyed larger surface areas. Sample return missions allow scientist to study extraterrestrial surface materials on Earth without having to send a crewed mission, however is generally only feasible for objects with low gravity and atmosphere.
Past missions The first extraterrestrial planetary surface to be explored was the lunar surface by Luna 2 in 1959. The first and only human exploration of an extraterrestrial surface was the Moon, the Apollo program included the first moonwalk on July 20, 1969, and successful return of extraterrestrial surface samples to Earth. Venera 7 was the first landing of a probe on another planet on December 15, 1970. Mars 3 "soft landed" and returned data from Mars on August 22, 1972, the first rover on Mars was Mars Pathfinder in 1997, the Mars Exploration Rover has been studying the surface of the red planet since 2004. NEAR Shoemaker was the first to soft land on an asteroid – 433 Eros in February 2001 while Hayabusa was the first to return samples from 25143 Itokawa on 13 June 2010. Huygens soft landed and returned data from Titan on January 14, 2005. There have been many failed attempts, more recently Fobos-Grunt, a sample return mission aimed at exploring the surface of Phobos.
Forms 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.
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