Martian Surface
Encyclopedia
The study of surface characteristics (or surface properties and processes) is a broad category of Mars science that examines the nature of the materials making up the Martian surface. The study evolved from telescopic and remote-sensing techniques developed by astronomers to study planetary surfaces. However, it has increasingly become a subdiscipline of geology as automated spacecraft bring ever-improving resolution and instrument capabilities. By using characteristics such as color, albedo
, and thermal inertia and analytical tools such as reflectance spectroscopy
and radar
, scientists are able to study the chemistry and physical makeup (e.g., grain sizes, surface roughness, and rock abundances) of the Martian surface. The resulting data help scientists understand the planet’s mineral composition and the nature of geological processes operating on the surface. Mars’ surface layer represents a tiny fraction of the total volume of the planet, yet plays a significant role in the planet’s geologic history. Understanding physical surface properties is also very important in determining safe landing sites for spacecraft.
, which ranges from 0 for a body that reflects no sunlight to 1.0 for a body that reflects all sunlight. Different parts of a planet’s surface (and atmosphere) have different albedo values depending on the chemical and physical nature of the surface.
No topography is visible on Mars from Earth-based telescopes. The bright areas and dark markings on pre-spaceflight-era maps of Mars are all albedo features. (See Classical albedo features on Mars
.) They have little relation to topography. Dark markings are most distinct in a broad belt from 0° to 40° S latitude. However, the most prominent dark marking, Syrtis Major Planum, is in the northern hemisphere, outside this belt. The classical albedo feature Mare Acidalium (Acidalia Planitia
) is another prominent dark area that lies north of the main belt. Bright areas, excluding the polar caps and transient clouds, include Hellas
, Tharsis
, and Arabia Terra
. The bright areas are now known to be locations where fine dust covers the surface. The dark markings represent areas that the wind has swept clean of dust, leaving behind a lag of dark, rocky material. The dark color is consistent with the presence of mafic
rocks, such as basalt
.
The albedo of a surface usually varies with the wavelength
of light hitting it. Mars reflects little light at the blue end of the spectrum
but much at red and higher wavelengths. This is why Mars has the familiar reddish-orange color to the naked eye. But detailed observations reveal a subtle range of colors on Mars' surface. Color variations provide clues to the composition of surface materials. The bright areas are reddish-ochre
in color, and the dark areas appear dark gray. A third type of area, intermediate in color and albedo, is also present and thought to represent regions containing a mixture of the material from the bright and dark areas. The dark gray areas can be further subdivided into those that are more reddish and those less reddish in hue.
is a technique that measures the amount of sunlight absorbed or reflected by the Martian surface at specific wavelengths. The spectra represent mixtures of spectra from individual minerals on the surface along with contributions from absorption lines in the solar spectrum and the Martian atmosphere. By separating out (“deconvolving”) each of these contributions, scientists can compare the resulting spectra to laboratory spectra of known minerals to determine the probable identity and abundance of individual minerals on the surface.
Using this technique, scientists have long known that the bright ochre areas probably contain abundant ferric iron (Fe3+) oxides typical of weathered iron-bearing materials (e.g., rust
). Spectra of the dark areas are consistent with the presence of ferrous iron (Fe2+) in mafic minerals and show absorption bands suggestive of pyroxene
, a group of minerals that is very common in basalt. Spectra of the redder dark areas are consistent with mafic materials covered with thin alteration coatings.
Scientists can estimate the thermal inertia on the Martian surface by measuring variations in surface temperature with respect to time of day and fitting this data to numerical temperature models. The thermal inertia of a material is directly related to its thermal conductivity
, density, and specific heat capacity. Fortunately, rocky materials do not vary much in density and specific heat, so variations in thermal inertia are mainly due to variations in thermal conductivity. Solid rock surfaces, such as outcroppings, have high thermal conductivities and inertias. Dust and small granular material in the regolith have low thermal inertias because the void spaces between grains restrict thermal conductivity to the contact point between grains.
Thermal inertia values for most of the Martian surface are inversely related to albedo. Thus, high albedo areas have low thermal inertias indicating surfaces that are covered with dust and other fine granular material. The dark gray, low albedo surfaces have high thermal inertias more typical of consolidated rock. However, thermal inertia values are not high enough to indicate widespread outcroppings are common on Mars. Even the rockier areas appear to be mixed with a significant amount of loose material. Data from the Infrared Thermal Mapping (IRTM) experiment on the Viking orbiters identified areas of high thermal inertia throughout the interior of Valles Marineris and the chaotic terrain, suggesting that these areas contain a relatively large number of blocks and boulders.
Dispersion of the returned radar echoes from Mars shows that a lot of variation exists in surface roughness and slope across the planet’s surface. Wide areas of the planet, particularly in Syria
and Sinai Plana, are relatively smooth and flat. Meridiani Planum, the landing site of the Mars Exploration Rover
Opportunity
, is one of the flattest and smoothest (at decimeter-scale) locations ever investigated by radar—a fact borne out by surface images at the landing site. Other areas show high levels of roughness in radar that are not discernable in images taken from orbit. The average surface abundance of centimeter- to meter-scale rocks is much greater on Mars than the other terrestrial planets. Tharsis and Elysium, in particular, show a high degree of small-scale surface roughness associated with volcanoes. This extremely rough terrain is suggestive of young, ʻaʻā
lava flows. A 200-km-long band of low to zero radar albedo ("stealth" region) cuts across the southwest Tharsis. The region corresponds to the location of the Medusa Fossae Formation, which consists of thick layers of unconsolidated materials, perhaps volcanic ash or loess
..
Ground-penetrating radar instruments on the Mars Express
orbiter (MARSIS
) and the Mars Reconnaissance Orbiter
(SHARAD
) are currently providing stunning echo-return data on subsurface materials and structures to depths of up to 5 km. Results have shown that the polar layered deposits are composed of almost pure ice, with no more than 10% dust by volume and that fretted valleys in Deuteronilus Mensae
contain thick glaciers covered by a mantle of rocky debris.
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...
, and thermal inertia and analytical tools such as reflectance spectroscopy
Spectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...
and radar
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
, scientists are able to study the chemistry and physical makeup (e.g., grain sizes, surface roughness, and rock abundances) of the Martian surface. The resulting data help scientists understand the planet’s mineral composition and the nature of geological processes operating on the surface. Mars’ surface layer represents a tiny fraction of the total volume of the planet, yet plays a significant role in the planet’s geologic history. Understanding physical surface properties is also very important in determining safe landing sites for spacecraft.
Albedo and Color
Like all planets, Mars reflects a portion of the light it receives from the sun. The fraction of sunlight reflected is a quantity called albedoAlbedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...
, which ranges from 0 for a body that reflects no sunlight to 1.0 for a body that reflects all sunlight. Different parts of a planet’s surface (and atmosphere) have different albedo values depending on the chemical and physical nature of the surface.
Classical albedo features on Mars
The classical albedo features of Mars are the light and dark features that can be seen on the planet Mars through an Earth-based telescope. Before the age of space probes, several astronomers created maps of Mars on which they gave names to the features they could see. The most popular system of...
.) They have little relation to topography. Dark markings are most distinct in a broad belt from 0° to 40° S latitude. However, the most prominent dark marking, Syrtis Major Planum, is in the northern hemisphere, outside this belt. The classical albedo feature Mare Acidalium (Acidalia Planitia
Acidalia Planitia
Acidalia Planitia is a plain on Mars. It is located between the Tharsis volcanic province and Arabia Terra to the north of Valles Marineris, centered at...
) is another prominent dark area that lies north of the main belt. Bright areas, excluding the polar caps and transient clouds, include Hellas
Hellas Planitia
Hellas Planitia, also known as the Hellas Impact Basin, is a huge, roughly circular impact basin located in the southern hemisphere of the planet Mars. It is the second or third largest impact crater and the largest visible impact crater known in the Solar System...
, Tharsis
Tharsis
The Tharsis region on Mars is a vast volcanic plateau centered near the equator in Mars’ western hemisphere. The region is home to the largest volcanoes in the Solar System, including the three enormous shield volcanoes Arsia Mons, Pavonis Mons, and Ascraeus Mons, which are collectively known as...
, and Arabia Terra
Arabia Terra
Arabia Terra is a large upland region in the north of Mars in that lies mostly in the Arabia quadrangle. It is densely cratered and heavily eroded. This battered topography indicates great age, and Arabia Terra is presumed to be one of the oldest terrains on the planet...
. The bright areas are now known to be locations where fine dust covers the surface. The dark markings represent areas that the wind has swept clean of dust, leaving behind a lag of dark, rocky material. The dark color is consistent with the presence of mafic
Mafic
Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...
rocks, such as basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
.
The albedo of a surface usually varies with the wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
of light hitting it. Mars reflects little light at the blue end of the spectrum
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
but much at red and higher wavelengths. This is why Mars has the familiar reddish-orange color to the naked eye. But detailed observations reveal a subtle range of colors on Mars' surface. Color variations provide clues to the composition of surface materials. The bright areas are reddish-ochre
Ochre
Ochre is the term for both a golden-yellow or light yellow brown color and for a form of earth pigment which produces the color. The pigment can also be used to create a reddish tint known as "red ochre". The more rarely used terms "purple ochre" and "brown ochre" also exist for variant hues...
in color, and the dark areas appear dark gray. A third type of area, intermediate in color and albedo, is also present and thought to represent regions containing a mixture of the material from the bright and dark areas. The dark gray areas can be further subdivided into those that are more reddish and those less reddish in hue.
Reflectance Spectroscopy
Reflectance spectroscopySpectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...
is a technique that measures the amount of sunlight absorbed or reflected by the Martian surface at specific wavelengths. The spectra represent mixtures of spectra from individual minerals on the surface along with contributions from absorption lines in the solar spectrum and the Martian atmosphere. By separating out (“deconvolving”) each of these contributions, scientists can compare the resulting spectra to laboratory spectra of known minerals to determine the probable identity and abundance of individual minerals on the surface.
Using this technique, scientists have long known that the bright ochre areas probably contain abundant ferric iron (Fe3+) oxides typical of weathered iron-bearing materials (e.g., rust
Rust
Rust is a general term for a series of iron oxides. In colloquial usage, the term is applied to red oxides, formed by the reaction of iron and oxygen in the presence of water or air moisture...
). Spectra of the dark areas are consistent with the presence of ferrous iron (Fe2+) in mafic minerals and show absorption bands suggestive of pyroxene
Pyroxene
The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...
, a group of minerals that is very common in basalt. Spectra of the redder dark areas are consistent with mafic materials covered with thin alteration coatings.
Thermal Inertia
Thermal inertia measurement is a remote-sensing technique that allows scientists to distinguish fine-grained from coarse-grained areas on the Martian surface. Thermal inertia is a measure of how fast or slow something heats up or cools off. For example, metals have very low thermal inertia. An aluminum cookie sheet taken out of an oven is cool to the touch in less than a minute; while a ceramic plate (high thermal inertia) taken from the same oven takes much longer to cool off.Scientists can estimate the thermal inertia on the Martian surface by measuring variations in surface temperature with respect to time of day and fitting this data to numerical temperature models. The thermal inertia of a material is directly related to its thermal conductivity
Thermal conductivity
In physics, thermal conductivity, k, is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction....
, density, and specific heat capacity. Fortunately, rocky materials do not vary much in density and specific heat, so variations in thermal inertia are mainly due to variations in thermal conductivity. Solid rock surfaces, such as outcroppings, have high thermal conductivities and inertias. Dust and small granular material in the regolith have low thermal inertias because the void spaces between grains restrict thermal conductivity to the contact point between grains.
Thermal inertia values for most of the Martian surface are inversely related to albedo. Thus, high albedo areas have low thermal inertias indicating surfaces that are covered with dust and other fine granular material. The dark gray, low albedo surfaces have high thermal inertias more typical of consolidated rock. However, thermal inertia values are not high enough to indicate widespread outcroppings are common on Mars. Even the rockier areas appear to be mixed with a significant amount of loose material. Data from the Infrared Thermal Mapping (IRTM) experiment on the Viking orbiters identified areas of high thermal inertia throughout the interior of Valles Marineris and the chaotic terrain, suggesting that these areas contain a relatively large number of blocks and boulders.
Radar Investigations
Radar studies provide a wealth of data on elevations, slopes, textures, and material properties of the Martian surface. Mars is an inviting target for Earth-based radar investigations because of its relative proximity to Earth and its favorable orbital and rotational characteristics that allow good coverage over wide areas of the planet’s surface. Radar echoes from Mars were first obtained in the early 1960s, and the technique has been vital in finding safe terrain for Mars landers.Dispersion of the returned radar echoes from Mars shows that a lot of variation exists in surface roughness and slope across the planet’s surface. Wide areas of the planet, particularly in Syria
Syria Planum
Syria Planum is a broad plateau on surface of Mars, forming part of Tharsis region. It is located at the summit of the Tharsis bulge, and was the center of volcanic and tectonic activity in Martian history from the Noachian to the late Hesperian. It has been confirmed that low shield volcanoes...
and Sinai Plana, are relatively smooth and flat. Meridiani Planum, the landing site of the Mars Exploration Rover
Mars Exploration Rover
NASA's Mars Exploration Rover Mission is an ongoing robotic space mission involving two rovers, Spirit and Opportunity, exploring the planet Mars...
Opportunity
Opportunity rover
Opportunity, MER-B , is a robotic rover on the planet Mars, active since 2004. It is the remaining rover in NASA's ongoing Mars Exploration Rover Mission...
, is one of the flattest and smoothest (at decimeter-scale) locations ever investigated by radar—a fact borne out by surface images at the landing site. Other areas show high levels of roughness in radar that are not discernable in images taken from orbit. The average surface abundance of centimeter- to meter-scale rocks is much greater on Mars than the other terrestrial planets. Tharsis and Elysium, in particular, show a high degree of small-scale surface roughness associated with volcanoes. This extremely rough terrain is suggestive of young, ʻaʻā
Lava
Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...
lava flows. A 200-km-long band of low to zero radar albedo ("stealth" region) cuts across the southwest Tharsis. The region corresponds to the location of the Medusa Fossae Formation, which consists of thick layers of unconsolidated materials, perhaps volcanic ash or loess
Loess
Loess is an aeolian sediment formed by the accumulation of wind-blown silt, typically in the 20–50 micrometre size range, twenty percent or less clay and the balance equal parts sand and silt that are loosely cemented by calcium carbonate...
..
Ground-penetrating radar instruments on the Mars Express
Mars Express
Mars Express is a space exploration mission being conducted by the European Space Agency . The Mars Express mission is exploring the planet Mars, and is the first planetary mission attempted by the agency. "Express" originally referred to the speed and efficiency with which the spacecraft was...
orbiter (MARSIS
MARSIS
MARSIS is a low frequency, pulse-limited radar sounder and altimeter used on the ESA Mars Express mission...
) and the Mars Reconnaissance Orbiter
Mars Reconnaissance Orbiter
Mars Reconnaissance Orbiter is a NASA multipurpose spacecraft designed to conduct reconnaissance and Exploration of Mars from orbit...
(SHARAD
SHARAD
SHARAD is a subsurface sounding radar embarked on the Mars Reconnaissance Orbiter probe. It complements the MARSIS instrument on Mars Express, providing lower penetration capabilities but much finer resolution .SHARAD is developed under the responsibility of the Italian Space Agency SHARAD (Mars...
) are currently providing stunning echo-return data on subsurface materials and structures to depths of up to 5 km. Results have shown that the polar layered deposits are composed of almost pure ice, with no more than 10% dust by volume and that fretted valleys in Deuteronilus Mensae
Deuteronilus Mensae
Deuteronilus Mensae is a region on Mars 937 km across and centered at . It covers 344° -325° West and 40°-48° North. Deuteronilus region lies just to the north of Arabia Terra and is included in the Ismenius Lacus quadrangle. It is along the dichotomy boundary, that is between the old,...
contain thick glaciers covered by a mantle of rocky debris.