Mars surface color

Did You Know?

• The Martian sky may appear reddish to humans, primarily due to absorption of sunlight in the 0.4-0.6 μm range by dust particles.
• Photon scattering by dust particles at wavelengths around 3 μm may also contribute to the redness of the sky.

Reason for red and its extensiveness

The redness of Mars is only skin deep, caused by a ubiquitous dust layer typically millimeters thick and, even in the thickest deposits such as the Tharsis area, probably not more than 2 meters. This reddish dust is a thin veneer and does not represent the bulk of the subsurface. The reddish color is primarily due to nanophase ferric oxides (npOx) that dominate the visible spectrum, with nanocrystalline red hematite (α-Fe2O3) possibly being the volumetrically dominant mineral at the sampling depth of infrared remote sensors.

Up to 50% of the iron in the dust may be in titanium-enriched magnetite (Fe3O4), which is black and does not contribute to the red hue. The dust contains higher mass fractions of chlorine and sulfur than soils at Gusev crater and Meridiani Planum, and sulfur correlates positively with npOx, suggesting limited chemical alteration by thin brine films may produce some npOx. Remote sensing of atmospheric dust indicates the bulk volume consists of plagioclase feldspar and zeolite, with minor pyroxene and olivine, likely generated by mechanical erosion of feldspar-rich basalts. Overall, chemical alteration by aqueous activity has been very minor.

The occurrence of nanophase ferric oxides (npOx) in dust

Several processes can produce npOx without free oxygen, which has likely always been a trace component on Mars with a partial pressure not exceeding 0.1 μPa. One oxygen-independent process involves ferrous iron or metallic iron reacting with water to produce ferric iron, typically leading to hydroxides such as goethite under experimental conditions. This reaction, though thermodynamically disfavored, may be sustained by rapid loss of molecular hydrogen and facilitated by dissolved carbon dioxide and sulfur dioxide that lower pH.

Higher temperatures around 300 °C are usually needed to decompose goethite into hematite, and the formation of palagonitic tephra on Mauna Kea volcano may mirror such processes. Prolonged arid and low pH conditions on Mars could eventually transform goethite into hematite. Iron may also be oxidized by hydrogen peroxide, a stronger oxidant than water, even though its atmospheric abundance is very low. The pervasiveness of the α-Fe2O3 spectral signature, without hydrated Fe3+ minerals, suggests npOx may form without intermediaries like goethite.