Mars is easily identifiable in the night sky by its prominent red hue.
Thanks to the fleet of spacecraft that have studied the planet over the last decades, we know that this red color is due to rusted iron minerals in the dust.
That is, iron bound up in Martian rocks has at some point reacted with liquid water, or water and oxygen in the air, similar to how rust forms on Earth.
Over billions of years this rusty material — iron oxide — has been broken down into dust and spread all around the planet by winds, a process that continues today.
But iron oxides come in many flavors, and the exact chemistry of Martian rust has been intensely debated because how it formed is a window into the planet’s environmental conditions at the time.
And closely linked to that is the question of whether Mars has ever been habitable.
Previous studies of the iron oxide component of the Martian dust based on spacecraft observations alone did not find evidence of water contained within it.
Planetary researchers had therefore concluded that this particular type of iron oxide must be hematite, formed under dry surface conditions through reactions with the Martian atmosphere over billions of years — after Mars’s early wet period.
However, a new analysis of spacecraft observations in combination with novel laboratory techniques shows that Mars’ red color is better matched by iron oxides containing water, known as ferrihydrite.
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