<P> The apparent colour of the Martian 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 . Its color may have also contributed to a malignant association in 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 . Modern robotic explorers have shown that not only the surfaces, but also the skies above may appear red under sunlit conditions on Mars . </P> <P> 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 . </P> <P> 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 (α - Fe O) 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 in the dust, perhaps as much as 50% of the mass, may be in titanium enriched magnetite (Fe O). Magnetite is usually black in colour with a black streak, and does not contribute to the reddish hue of dust . </P> <P> The mass fraction of chlorine and sulfur in the dust is greater than that which has been found (by the Mars Exploration Rovers Spirit and Opportunity) in the soil types at Gusev crater and Meridiani Planum . 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 H O) 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 and zeolite, along with minor pyroxene and olivine components . Such fine material can be generated easily via mechanical erosion from feldspar - rich basalts, 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 . </P>

Where did the iron on mars come from