We now turn to volcanos on Mars. And specifically, we’re going to talk about a point in time when some of those volcanoes may have chosen violence.

First, some background information. Almost twenty years ago, there were two little rovers that landed on Mars – Spirit and Opportunity. They were both planned out as ninety-day missions, but as I’m sure a lot of you recall, they lasted far longer than that. And about sixteen years ago, Spirit explored Gusev crater and found bedrock that was rich in olivine. Cool, olivine. If you’re like me, you probably are asking, “What’s olivine?” I’ve actually asked that question.

Olivine is a green mineral that comes from magma in the mantle, both on Mars and here on Earth. It’s not really unexpected, but it is definitely a clear signal of some kind of volcanic activity occurring that brought those rocks to the surface.

Fast forward to the past year on Mars and a newer, much bigger rover named Perseverance. You know, the rover the helicopter is running around with. Anyway, there are also olivine-rich rocks over in the Nili Fossae region where Jezero crater is being explored by Percy. And both of these locations, despite not being close to one another, have the highest abundance of this mineral so far found on Mars. Which, of course, intrigued scientists as to how craters ended up with similar rocks.

Enter a team of scientists trying to link the two locations by their volcanic history. Their work, led by Steve Ruff and published in the journal Icarus, involved testing the leading hypothesis that the olivine was deposited as part of gently falling ash. They analyzed images from Spirit’s Microscopic Imager and discovered rocks with an interesting texture. By comparing those images to a catalog of Earth rocks, they found similarities, but those similarities mean the eruptions were far more violent than expected.

What Ruff and the team noticed were flame-like structures that held flattened pumice fragments in their cracks. And that matches up with a type of igneous slash sedimentary rock found here on Earth called ignimbrite. In fact, Ruff notes: That was a eureka moment. I was seeing the same kind of textures in the rocks of Gusev crater as those in a very specific kind of volcanic rock found here on Earth.

And that ‘specific kind of volcanic rock’ has a very specific formation history. It forms as the “result of flows of pyroclastic ash, pumice, and blocks from the largest volcanic explosions known on Earth.” Or, as Ruff goes on to explain: Imagine a ground-hugging cloud of hot gases and nearly molten ash and pumice flowing through the landscape for dozens of miles and piling up in layers up to hundreds of feet thick in just a few days.

One place on Earth where we can find ignimbrites? Yellowstone. That’s a pretty scary and explosive supervolcano, and apparently, it had cousins on Mars. We’re definitely not talking about ‘gently falling ash’ anymore.

And that olivine-rich composition is particularly telling. As Ruff says: The olivine-rich composition is unusual for most ignimbrites on Earth, but there is evidence for this composition in the oldest ones. Now with the strong evidence for ancient olivine-rich ignimbrites on Mars, maybe this points to a style of volcanism, cataclysmic explosive eruptions of olivine-rich magma, that happens in the early geologic evolution of a planet.

So billions of years ago, Mars woke up and chose violence. And now we just need to get those rock samples back from Perseverance and find out just what they are made of and how they formed.

More Information

ASU press release

“Olivine and carbonate-rich bedrock in Gusev crater and the Nili Fossae region of Mars may be altered ignimbrite deposits,” Steven W. Ruff et al., 2022 March 18, Icarus