Participants: Per Nørnberg, Haraldur Pall Gunnlaugsson, Jonathan Merrison
"The red planet" has its name from the reddish colour by which Mars appears in the sky on clear evenings. The reason for this colour is a layer of surface dust on Mars consisting of iron oxides, and it was determined by Earth-based reflection spectroscopy and confirmed by optical studies on the Viking-missions and the Pathfinder mission in 1997 from which the photo below is taken (Pathfinder view).
A number of iron oxides are known from the Earth to be ochreous or reddish/yellowish coloured (Hematite, Maghemite, Ferrihydrite, Goethite and Lepidocrocite). Common to the reddish or yellowish iron oxides are that they contain iron in the oxidation state +III. The presence of iron oxides on the surface of Mars is no surprise since iron (Fe) is known to be the third most common element (after oxygen (O) and silicon (Si)) of the planet’s surface material.
However, the primary minerals at the surface of Mars are known to consists almost entirely of iron in the oxidation state +II. This is known partly from studies of Martian meteorites, which are assumed to be surface rocks ejected from Mars upon meteorite impacts at the planet, and partly from studies of reflection spectra taken of the Martian surface. Thus, during formation of the Martian surface dust, oxidation of the iron from +II to +III took place. The red colour is a much bigger challenge after the NASA Mars Exploration Rovers determined the chemistry of the dust captured from the atmosphere to be nearly the same as the basic rock.
Basically, two possibilities are discussed:
The dust particles at the surface of Mars, in their mineralogy, hold information on their formation history, and thus information on the earlier surface environmental conditions. However, today’s knowledge of the Martian dust operates with particles about 2 microns in size, with a complicated basaltic mineralogy. These are particles which by dissolution in an Earth environment would disappear within decades or a few hundred years. This is an important reason why missions to Mars study the dust in details.