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 the Mars Simulation Laboratory of Aarhus University in Denmark show that when a mixture of magnetite powder, 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 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 by gas molecules. References External links NASA - Mars in a Minute: Is Mars Really Red? ContactInContext - Mars, The "Golden Brown" (not Red) Planet? Surface features of Mars