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The surface of the planet
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury (planet), Mercury. In the English language, Mars is named for the Mars (mythology), Roman god of war. Mars is a terr ...
appears reddish from a distance because of rusty
dust Dust is made of fine particles of solid matter. On Earth, it generally consists of particles in the atmosphere that come from various sources such as soil lifted by wind (an aeolian process), volcanic eruptions, and pollution. Dust in homes ...
suspended in the
atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
.NASA - ''Mars in a Minute: Is Mars Really Red?''Transcript
From close up, it looks more of a
butterscotch Butterscotch is a type of confectionery whose primary ingredients are brown sugar and butter, but other ingredients are part of some recipes, such as corn syrup, cream, vanilla, and salt. The earliest known recipes, in mid-19th century Yorkshir ...
, and other common surface colors include golden, brown, tan, and greenish, depending on minerals. The apparent colour of the
Martian Mars, the fourth planet from the Sun, has appeared as a setting in works of fiction since at least the mid-1600s. It became the most popular celestial object in fiction in the late 1800s as the Moon was evidently lifeless. At the time, the pred ...
surface enabled humans to distinguish it from other planets early in human history and motivated them to weave fables of war in association with Mars. One of its earliest recorded names, Har decher, literally meant "Red One" in
Egyptian Egyptian describes something of, from, or related to Egypt. Egyptian or Egyptians may refer to: Nations and ethnic groups * Egyptians, a national group in North Africa ** Egyptian culture, a complex and stable culture with thousands of years of ...
.Kieffer, Hugh H., Bruce M. Jakosky, and Conway W. Snyder (1992), "The planet Mars: From antiquity to the present," in Mars, University of Arizona Press, Tucson, AZ, p.

Its color may have also contributed to a malignant association in Jyotisha, Indian astrology, as it was given the names '' Angaraka'' and ''Lohitanga'', both reflecting the distinctively red color of Mars as seen by the naked eye.


Reason for red and its extensiveness

Modern observations indicate that Mars's redness is skin deep. The Martian surface looks reddish primarily because of a ubiquitous dust layer (particles are typically between 3 μm to 45 μm across) that is typically on the order of millimeters thick. Even where the thickest deposits of this reddish dust occur, such as the Tharsis area, the dust layer is probably not more than 2 m (7 feet) thick. Therefore, the reddish dust is essentially an extremely thin veneer on the Martian surface and does not represent the bulk of the Martian subsurface in any way. Martian dust is reddish mostly due to the spectral properties of nanophase ferric oxides (npOx) that tend to dominate in the visible spectrum. The specific npOx minerals have not been fully constrained, but nanocrystalline red
hematite Hematite (), also spelled as haematite, is a common iron oxide compound with the formula, Fe2O3 and is widely found in rocks and soils. Hematite crystals belong to the rhombohedral lattice system which is designated the alpha polymorph of . ...
(α-Fe2O3) may be the volumetrically dominant one, at least at the less than 100 μm sampling depth of infrared remote sensors such as the Mars Express OMEGA instrument. The rest of the
iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...
in the dust, perhaps as much as 50% of the mass, may be in
titanium Titanium is a chemical element with the symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resistant to corrosion in ...
enriched
magnetite Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With the ...
(Fe3O4). Magnetite is usually black in colour with a black streak, and does not contribute to the reddish hue of dust. The mass fraction of
chlorine Chlorine is a chemical element with the Symbol (chemistry), symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate betwee ...
and
sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...
in the dust is greater than that which has been found (by the
Mars Exploration Rovers NASA's Mars Exploration Rover (MER) mission was a robotic space mission involving two Mars rovers, ''Spirit'' and ''Opportunity'', exploring the planet Mars. It began in 2003 with the launch of the two rovers to explore the Martian surface and ...
''Spirit'' and ''Opportunity'') in the soil types at
Gusev crater Gusev is a crater on the planet Mars and is located at and is in the Aeolis quadrangle. The crater is about 166 kilometers in diameter and formed approximately three to four billion years ago. It was named after Russian astronomer Matvey Gusev ...
and
Meridiani Planum The Meridiani Planum (alternately Meridiani plain, Meridiani plains, Terra Meridiani, or Terra Meridiani plains) is either a large plain straddling the equator of Mars and covered with a vast number of spherules containing a lot of iron oxide or ...
. The sulfur in the dust also shows a positive correlation with npOx. This suggests that very limited chemical alteration by thin brine films (facilitated by the formation of frost from atmospheric H2O) may be producing some of the npOx. In addition, remote sensing observations of atmospheric dust (which shows slight compositional and grain size differences from surface dust), indicates that the bulk volume of dust grains consists of plagioclase
feldspar Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) feldsp ...
and
zeolite Zeolites are microporous, crystalline aluminosilicate materials commonly used as commercial adsorbents and catalysts. They mainly consist of silicon, aluminium, oxygen, and have the general formula ・y where is either a metal ion or H+. These pos ...
, along with minor
pyroxene The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents calcium (Ca), sodium (Na), iron (Fe II) ...
and
olivine The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickl ...
components. Such fine material can be generated easily via mechanical erosion from feldspar-rich
basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...
s, such as rocks in the southern highlands on Mars. Collectively, these observations indicate that any chemical alteration of dust by aqueous activity has been very minor.


The occurrence of nanophase ferric oxides (npOx) in dust

There are several processes that can yield npOx as an oxidation product without the involvement of free oxygen (O2). One or more of those processes may have dominated on Mars, since atmospheric modeling over geologic time scales indicates that free O2 (generated mostly via the photodissociation of water (H2O)) may have always been a trace component with a partial pressure not exceeding 0.1 micropascal (μPa). One oxygen-(O2)-independent process involves a direct chemical reaction of ferrous iron (Fe2+) (commonly present in typical igneous minerals) or metallic iron (Fe) with water (H2O) to produce ferric iron (Fe3+(aq)), which typically leads to hydroxides such as goethite (FeO•OH) under experimental conditions. While this reaction with water (H2O) is thermodynamically disfavored, it may be sustained nevertheless, by the rapid loss of the molecular hydrogen (H2) byproduct. The reaction can be further facilitated by dissolved carbon dioxide (CO2) and sulfur dioxide (SO2), which lower the pH of brine films increasing the concentration of the more oxidative hydrogen ions (H+). However, higher temperatures (c. 300 °C) are usually needed to decompose Fe3+ (oxy)hydroxides such as goethite into hematite. The formation of palagonitic tephra on the upper slopes of the Mauna Kea volcano may mirror such processes, as consistent with the intriguing spectral and magnetic similarities between palagonitic tephra and Martian dust. In spite of the need for such kinetic conditions, prolonged arid and low pH conditions on Mars (such as diurnal brine films) may lead to the eventual transformation of goethite into hematite given the thermodynamic stability of the latter. Fe and Fe2+ may also be oxidized by the activity of hydrogen peroxide (H2O2). Even though the H2O2 abundance in the Martian atmosphere is very low, it is temporally persistent and a much stronger oxidant than H2O. H2O2-driven oxidation to Fe3+ (usually as hydrated minerals), has been observed experimentally. In addition, the pervasiveness of the α-Fe2O3 spectral signature, but not of hydrated Fe3+ minerals reinforces the possibility that npOx may form even without the thermodynamically disfavored intermediaries such as goethite. There is also evidence that hematite might form from magnetite in the course of erosion processes. Experiments at th
Mars Simulation Laboratory
of
Aarhus University Aarhus University ( da, Aarhus Universitet, abbreviated AU) is a public research university with its main campus located in Aarhus, Denmark. It is the second largest and second oldest university in Denmark. The university is part of the Coimbra Gr ...
in
Denmark ) , song = ( en, "King Christian stood by the lofty mast") , song_type = National and royal anthem , image_map = EU-Denmark.svg , map_caption = , subdivision_type = Sovereign state , subdivision_name = Danish Realm, Kingdom of Denmark ...
show that when a mixture of magnetite powder,
quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...
sand, and quartz dust particles is tumbled in a flask, some of the magnetite converts to hematite, coloring the sample red. The proposed explanation for this effect is that when quartz is fractured by the grinding, certain chemical bonds get broken at the newly exposed surfaces; when these surfaces come in contact with magnetite, oxygen atoms may be transferred from quartz surface to magnetite, forming hematite.


Red skies on Mars

Approximately true-colour ''in situ'' images from the Mars Pathfinder and Mars Exploration Rover missions indicate that the Martian sky may also appear reddish to humans. Absorption of sunlight in the 0.4-0.6 μm range by dust particles may be the primary reason for the redness of the sky. An additional contribution may come from the dominance of photon scattering by dust particles at wavelengths in the order 3 μm, which is in the near-infrared range, over
Rayleigh scattering Rayleigh scattering ( ), named after the 19th-century British physicist Lord Rayleigh (John William Strutt), is the predominantly elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the ...
by gas molecules.


References


External links


NASA - ''Mars in a Minute: Is Mars Really Red?''


{{Portal bar, Solar System Surface features of Mars