Planetary surface

Apollo 11 astronaut Buzz Aldrin walking on lunar regolith (July 1969)

The cold dry and rocky surface of Mars (taken by Viking Lander 2 May 1979)

File:Venera 13 - venera13-left.jpg

The oppressive surface of Venus (taken by Venera 13 March 1, 1982)

Pebbled plains of Titan (taken by Huygens probe January 14, 2005) - the only image from the surface of a planetary body outside the inner Solar System

Shoreline on Earth - land, sea, sand, and eroded clays.

A Planetary surface is where the solid (or liquid) material of the outer crust on certain types of astronomical objects comes to contact with atmosphere or outer space - planetary surfaces can be found on terrestrial planets, dwarf planets, natural satellites and planetesimals. The study of planetary surfaces is referred to as surface geology but also a focus of a number of fields including planetary cartography, topography, geomorphology, atmospheric sciences and astronomy. Land (or ground) is the term given to a planetary surface that is not covered by liquid, 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. The inner atmospheres of some astronomical bodies such as stars and gas giants may have a contiguous liquid phase but are generally not regarded as a surface.

Planetary surfaces and surface life are of particular interest to humans as it is the species primary habitat having evolved breathing of air and terrestrial locomotion. As such, it is a major focus of space exploration and space colonization.

The Earth and Moon are the only such surfaces to have been directly explored by humans. Due to the difficulty involved in exploration, relatively limited "on the ground" exploration has been undertaken. As conclusions are difficult to make from observation at distance through flyby or orbit, the exact composition and properties of many planetary surfaces remain unconfirmed. To date the only extraterrestrial planetary surfaces where probes have been successfully landed are those of Mars, Venus, Titan, 433 Eros and 25143 Itokawa. Mars is currently the only other planet to have had its surface explored by a mobile surface probe (rover).

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 gas 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 is 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 manned 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 in 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.

Future missions

In May 2011, NASA announced the OSIRIS-REx sample return mission to asteroid 1999 RQ36, and is expected to launch in 2016.

Common Surface Materials

The most common planetary surface material in the Solar System appears to be ice, it is found as close to the Sun as Mercury but is more abundant beyond Mars. Other surfaces include solid matter in combinations of rock, regolith and frozen chemical elements and chemical compounds. Minerals and hydrates may also be present in smaller quantities. Surface liquid, while abundant on Earth (the largest body of surface liquid being the World Ocean) is rare elsewhere, a notable exception being Titan which has the largest known hydrocarbon lake system (Lakes of Titan) while surface water, abundant on Earth and essential to all known forms of life is thought only to exist as Seasonal flows on warm Martian slopes and in the habitable zones of other planetary systems. 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).

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 abundance excluding minerals. Some have been detected by specroscopy or direct imaging from orbit or flyby.

• Ice - Europa, Triton, Earth (polar), Mars (polar), Titan, Mercury (polar), Enceladus, Ceres, Ganymede, Callisto, Miranda, Charob, Haumea, 28978 Ixion, 90482 Orcus, 50000 Quaoar, Umbriel, Oberon
• Silicate rock - Venus, Earth, Mars, Mercury, Ganymede, Callisto, Moon, Triton, asteroids
• Methane clathrate - Oberon, Titania, Umbriel, Pluto, 90482 Orcus
• Regolith - Moon, Mars, asteroids, Titan, Earth
• Tholins - many Trans Neptunian Objects including Titan, Triton, Eris, Sedna, 28978 Ixion, 90482 Orcus
• Nitrogen ice - Triton, Charon, Kuiper belt objects, Plutinos
• Sulphur - Io, Mercury, Europa, Mars
• Dry ice - Mars (polar), Umbriel
• Sand - Earth, Mars, Titan
• Clays - Earth, Mars, Ceres

Common Landforms

Common surface features include impact craters (though rarer on bodies with thick atmospheres, the largest being Hellas Planitia on Mars), rilles, mountains (the highest being Rheasilvia on 4 Vesta), escarpments, canyons and valleys (the largest being Valles Marineris on Mars) while subsurface features that puncture the surface include caves and lava tubes.