The apparent color of the Martian

Mars

Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

 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 names, Har decher, literally meant "Red One" in Egyptian

Egyptian language

Egyptian is the oldest known indigenous language of Egypt and a branch of the Afroasiatic language family. Written records of the Egyptian language have been dated from about 3400 BC, making it one of the oldest recorded languages known. Egyptian was spoken until the late 17th century AD in the...

. Its color may have also contributed to a malignant association in Indian astrology

Jyotisha

Hindu astrology , also Jyotish or Jyotisha, from Sanskrit , from "light, heavenly body") is the ancient Indian system of astronomy and astrology...

, as it was given the names Angaraka

Mangala

In Jyotish astrology, Mangala is the name for Mars, the red planet. Mars is also called Angaraka In Jyotish astrology, Mangala (Devanagari: मंगल) is the name for Mars, the red planet. Mars is also called Angaraka In Jyotish astrology, Mangala (Devanagari: मंगल) is the name for Mars, the red...

and Lohitanga, both reflecting the distinctively red color of Mars as seen by the naked eye. Modern robotic explorers have shown that not only the surfaces, but also the skies above may appear red under sunlit conditions on Mars.

Reason for the color and its extensiveness

Modern observations indicate that Mars's redness is skin deep. The Martian surface looks reddish primarily because of an 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.

The reddish dust

Martian dust is reddish mostly due the spectral properties of nanophase

Nanophase material

Nanophase materials are materials that have grain sizes under 100 nanometres. They have different mechanical and optical properties compared to the large grained materials of the same chemical composition....

 ferric oxides

Iron(III) oxide

Iron oxide or ferric oxide is the inorganic compound with the formula Fe2O3. It is one of the three main oxides of iron, the other two being iron oxide , which is rare, and iron oxide , which also occurs naturally as the mineral magnetite. As the mineral known as hematite, Fe2O3 is the main...

 (npOx) that tend to dominate in the visible spectrum. The specific npOx minerals have not been fully constrained, but nanocrystalline red hematite

Hematite

Hematite, also spelled as haematite, is the mineral form of iron oxide , one of several iron oxides. Hematite crystallizes in the rhombohedral system, and it has the same crystal structure as ilmenite and corundum...

 (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

Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

 in the dust, perhaps as much as 50% of the mass, may be in titanium

Titanium

Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....

 enriched magnetite

Magnetite

Magnetite is a ferrimagnetic mineral with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name is ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe2O3, which is one part...

 (Fe₃O₄). Magnetite is usually black in color with a black streak, and does not contribute to the reddish hue of dust.

The mass fraction of chlorine

Chlorine

Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...

 and sulfur

Sulfur

Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...

 in the dust is greater than that which has been found (by the Mars Exploration Rovers Spirit

Spirit rover

Spirit, MER-A , is a robotic rover on Mars, active from 2004 to 2010. It was one of two rovers of NASA's ongoing Mars Exploration Rover Mission. It landed successfully on Mars at 04:35 Ground UTC on January 4, 2004, three weeks before its twin, Opportunity , landed on the other side of the planet...

 and 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...

) in the soil types at Gusev crater

Gusev crater

Gusev is a crater on the planet Mars and is located at . The crater is about 166 kilometers in diameter and formed approximately three to four billion years ago. It was named after Russian astronomer Matvei Gusev in 1976....

 and Meridiani Planum

Meridiani Planum

Meridiani Planum is a plain located 2 degrees south of Mars' equator , in the westernmost portion of Terra Meridiani. It hosts a rare occurrence of gray crystalline hematite...

. 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

Feldspar

Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....

 and zeolite

Zeolite

Zeolites are microporous, aluminosilicate minerals commonly used as commercial adsorbents. The term zeolite was originally coined in 1756 by Swedish mineralogist Axel Fredrik Cronstedt, who observed that upon rapidly heating the material stilbite, it produced large amounts of steam from water that...

, along with minor 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...

 and olivine

Olivine

The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....

 components. Such fine material can be generated easily via mechanical erosion from feldspar-rich 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...

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 npOx in dust

There are several processes that can yield npOx as an oxidation product without the involvement of free 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 H₂O) may have always been a trace component with a partial pressure not exceeding 0.1 µPa..

One O₂-independent process involves a direct chemical reaction of Fe²⁺ (commonly present in typical igneous minerals) or metallic Fe with H₂O to produce Fe³⁺(aq), which typically leads to hydroxides such as goethite (FeO•OH) under experimental conditions. While this reaction with H₂O is thermodynamically disfavored, it may be sustained nevertheless, by the rapid loss of the H₂ byproduct. The reaction can be further facilitated by dissolved CO₂ and SO₂, which lower the pH of brine films increasing the concentration of the more oxidative 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 geothite.

There is also evidence that hematite might form from magnetite in the course of erosion processes. Experiments at the Mars Simulation Laboratory of Aarhus University in Denmark

Denmark

Denmark is a Scandinavian country in Northern Europe. The countries of Denmark and Greenland, as well as the Faroe Islands, constitute the Kingdom of Denmark . It is the southernmost of the Nordic countries, southwest of Sweden and south of Norway, and bordered to the south by Germany. Denmark...

 show that when a mixture of magnetide powder, quartz

Quartz

Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...

 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 fructured 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

Recent, approximately true-color 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

Rayleigh scattering, named after the British physicist Lord Rayleigh, is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. The particles may be individual atoms or molecules. It can occur when light travels through...

by gas molecules.