Why is Mars red?

…the fourth rock from the sun its best-known nickname — the “Red Planet.” But what exactly gives the planet its iconic color? Scientists have wondered this for as long as they’ve studied the planet. Today, they may finally have a concrete answer, and one that ties into Mars’ watery past.

Results from a study published in the journal Nature Communications and led by researchers from Brown University and the University of Bern suggest that the water-rich iron mineral ferrihydrite may be the main culprit behind Mars’ reddish dust. The team’s theory — which they reached by analyzing data from Martian orbiters, rovers and laboratory simulations — runs counter to the prevailing theory that a dry, rust-like mineral called hematite is the reason for the planet’s color.

Ferrihydrite is an iron oxide mineral that forms in water-rich environments. On Earth, it is commonly associated with processes like the weathering of volcanic rocks and ash.

via Brown University