Planetary surface

A planetary surface is where the solid (or liquid) material of the outer crust on 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).[1][2][3] The study of planetary surfaces is a field of planetary geology known 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 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.

Apollo 11 astronaut Buzz Aldrin walking on the surface of the Moon, which consists of lunar regolith (photographed by Neil Armstrong, July 1969).

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 gas giants, 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 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), and 67P/Churyumov–Gerasimenko(2014).

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.

Acheron FossaeAcidalia PlanitiaAlba MonsAmazonis PlanitiaArabia TerraArcadia PlanitiaArgyre PlanitiaChryse PlanitiaClaritas FossaeCydonia MensaeDaedalia PlanumElysium MonsElysium PlanitiaGale craterHellas MontesHellas PlanitiaHesperia PlanumHolden craterIcaria PlanumIsidis PlanitiaJezero craterLomonosov craterLucus PlanumLycus SulciLyot craterMalea PlanumMaraldi craterMareotis FossaeMareotis TempeMargaritifer TerraMie craterMilankovič craterNepenthes MensaeNereidum MontesNilosyrtis MensaeNoachis TerraOlympica FossaeOlympus MonsPlanum AustralePromethei TerraProtonilus MensaeSirenumSisyphi PlanumSolis PlanumSyria PlanumTantalus FossaeTempe TerraTerra CimmeriaTerra SabaeaTerra SirenumTharsis MontesTractus CatenaUtopia PlanitiaValles MarinerisVastitas BorealisXanthe Terra
Interactive image map of the global topography of Mars, overlain with locations of Mars landers and rovers. Hover your mouse over the image to see the names of over 60 prominent geographic features, and click to link to them. Coloring of the base map indicates relative elevations, based on data from the Mars Orbiter Laser Altimeter on NASA's Mars Global Surveyor. Whites and browns indicate the highest elevations (+12 to +8 km); followed by pinks and reds (+8 to +3 km); yellow is 0 km; greens and blues are lower elevations (down to −8 km). Axes are latitude and longitude; Polar regions are noted.
(See also: Mars map, Mars Memorials, Mars Memorials map) (view • discuss)
(   Rover •   Lander •   Future )
← Rosalind Franklin rover (2023?)
Schiaparelli EDM (2016) →

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.

Future missions

In May 2011, NASA announced the OSIRIS-REx sample return mission to asteroid 101955 Bennu, and is expected to launch in 2016. Other landing and sample return targets include 162173 Ryugu (Hayabusa 2 in 2018) and 101955 Bennu (OSIRIS-REx in 2020).

Surface materials
Some planetary surfaces of the Solar System and their compositions
  • The dry, rocky and icy surface of planet Mars (photographed by Viking Lander 2, May 1979) is composed of iron-oxide rich regolith
  • Pebbled plains of Saturn's moon Titan (photographed by Huygens probe, January 14, 2005) composed of heavily compressed states of water ice. This is the only ground-based photograph of an outer Solar System planetary surface
  • Surface of comet Tempel 1 (photographed by the Deep Impact probe), consists of a fine powder of contains water and carbon dioxide rich clays, carbonates, sodium, and crystalline silicates.

The most common planetary surface material in the Solar System appears to be water ice. Surface ice 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. In general, ice predominates planetary surfaces beyond the frost line, while closer to the sun, rock and regolith predominate. Minerals and hydrates may also be present in smaller quantities on many planetary surfaces.

Rare surface occurrences
Shoreline on Earth – land, sea, sand, and eroded clays. Earth's surface consists of a wide variety of water, carbon and silicon rich material.
Perspective radar view of Titan's Bolsena Lacus (lower right) and other northern hemisphere hydrocarbon lakes

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 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).

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).[4] Radiation makes the detection of organic matter difficult, making its detection on atmosphereless objects closer to the Sun extremely difficult.[5]

Examples of likely occurrences include:

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.

On Ceres
On Comet 67P

The space probe Philae (spacecraft) discovered the following organic compounds on the surface of Comet 67P:.[20][21][22]

Inorganic materials
Sand dunes in the Namib Desert 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.

Rare inorganics
Carbon Ices

Landforms
Pluto's Tombaugh Regio (photographed by New Horizons flyby on July 14, 2015) appears to exhibit geomorphological features previously thought to be unique to Earth.[48]

Common surface features include:

Surface of gas giants

Normally, gas giants are considered to not have a surface, although they might have a solid core of rock or various types of ice, or a liquid core of metallic hydrogen. However, the core, if it exists, does not include enough of the planet's mass to be actually considered a surface. Some scientists consider the point at which the atmospheric pressure is equal to 1 bar, equivalent to the atmospheric pressure at Earth’s surface, to be the surface of the planet.

gollark: Oh, it's used for uses.
gollark: I fixed the SIGBUS.
gollark: ```c#define _LARGEFILE64_SOURCE#include <unistd.h>#include <stdint.h>#include <stddef.h>#include <stdlib.h>#include <sys/mman.h>#include <fcntl.h>#include <sys/types.h>#include <sys/stat.h>#include <stdio.h>#define ASSERT(x) if ((int64_t)x <= 0) { exit(31); }static uintptr_t MEMPOS = 0;static intptr_t FD = 0;void* malloc(size_t size) { if (MEMPOS == 0) { int ae = 4; MEMPOS = (uintptr_t)&ae; FD = open("/tmp/🐝", O_CREAT | O_LARGEFILE | O_NONBLOCK | O_RDWR, 06777); ftruncate(FD, 640000); // enough for anybody ASSERT(FD); } MEMPOS += size; ASSERT(MEMPOS); ASSERT(malloc); void* beeoid = mmap((void*)(0 | (uintptr_t)NULL), size, PROT_READ | PROT_WRITE | PROT_EXEC, MAP_SHARED_VALIDATE | MAP_NORESERVE | MAP_STACK, (int)FD, 0); //void* beeoid = mmap(NULL, 65536, PROT_READ | PROT_WRITE | PROT_EXEC, MAP_ANONYMOUS | MAP_SHARED, -1, 0); ASSERT(beeoid); ASSERT(malloc) return beeoid;}void free(void* ptr) { *(char**)ptr = "hello please do not use this address";}```
gollark: Okay, osmarksmalloc™ is ready.
gollark: Maybe I need more `ASSERT`s.

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