Molecular Monday: Jarosite on Mars

Today’s post is part of a bi-weekly series here on Planet Pailly called Molecular Mondays, where we take a closer look at the atoms and molecules that make up our physical universe.

Over the last few months, I’ve been trying to get to know the planet Mars better. There’s some pretty clear evidence that Mars used to be a different kind of planet, a wetter and more watery sort of world… but perhaps not in the way you might have expected.

Many of Mars’s rocks seem to tell a similar story about that wet, watery past. That story usually begins something like this:

We’ve already looked at Mars’s sedimentary basaltic rocks. Such a thing would be almost a contradiction in terms on Earth. Sedimentary rocks are made by flowing water; basalt is destroyed by water. So how could a rock be both basaltic and sedimentary?

Now let’s take a look at another rock that’s almost self-contradictory: jarosite. Jarosite is a potassium/sulfur/iron-containing mineral with the chemical formula KFe3(OH)6(SO4)2. It typically forms in wet, acidic environments by the oxidation of iron sulfides. If you don’t understand what that means, that’s okay; I don’t fully understand it either. The important thing is that jarosite forms in water; but jarosite is also destroyed by water. It breaks down into simpler compounds if exposed to water for long periods of time. That makes jarosite pretty hard to find on Earth, but of course it’s surprisingly common on Mars!

In 2004, the Opportunity rover discovered jarosite—lots of jarosite—in the Meridiani Planum region (incidentally, the same region where Airy-0 is located). According to a NASA press release at the time: “Jarosite may point to the rocks wet history having been an acidic lake or an acidic hot springs environment.”

But whatever was happening in Meridiani Planum, the region must have dried up pretty quickly. According to the abstract of this paper, also from 2004, “[…] the presence of jarosite combined with residual basalt at Meridiani Planum indicates that the alteration process did not proceed to completion, and that following jarosite formation, arid conditions must have prevailed.”

Before I started all this Mars research, I imagined ancient Mars as a very Earth-like place, with standing oceans and lakes and gently flowing rivers. I guess there still could have been a time like that in the very beginning, but most of Mars’s history with water seems to have been far more sporadic and violent than that, with lots of flash flooding (perhaps caused by melting glaciers) and very brief wet spells followed by long periods of dryness.

At least that’s my impression at this point in my research.

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