Curiosity uncovers Mars' explosive volcanic past
Drill samples from Gale Crater show the Red Planet's history wasn't all slow, blobby lava flows. Belinda Smith reports.
It boasts a volcano more than twice the height of Mount Everest, but Mars today is geologically quiet and its volcanic history still largely mysterious.
Now planetary scientists claim to have uncovered the first evidence of silicic volcanism on the Red Planet – a violent, explosive process that sends plumes of silicon-rich magma into the air – which flies in the face of previous thinking which favoured slow streams of basaltic lava.
The work, by NASA's Richard Morris and colleagues from the US, Denmark and the UK, was published in the Proceedings of the National Academy of Sciences.
Since NASA's Curiosity rover landed in Mars' Gale Crater in August 2012, it's examined rock and dust samples to reveal clues about the planet's climate and geology.
The very first sample of soil "ingested" by the rover and analysed by its Chemistry and Mineralogy instrument (CheMin) showed the mineral makeup similar to that of volcanic basaltic soils found in Hawaii.
This type of volcanism is a relatively calm process, where magma low in silicon oxide (called "mafic magma") oozes onto the surface to cool and harden as basalt.
But more recently, Curiosity drilled into mudstone in another area and analysed the drill samples – and found something different.
The same CheMin instrument found the dust to be around 74% tridymite – a mineral that crystallises from high-silicon dioxide material.
The only way the sample could be so high in tridymite, Morris and colleagues write, is if magma from silicic volcanism hundreds of millions or billions of years ago.
Silicic volcanism, where "felsic magma" erupts from underground, differs from basaltic volcanism.
Unlike mafic magma, felsic magma is full of dissolved gases and silicon dioxide. It's also much thicker than mafic magma.
And because dissolved gases find it much harder to escape the thick felsic magma, pressure within the hot rock grows and it explodes as it bursts through the planet's crust.
The lava cools to become "rhyolitic" rocks, high in tridymite.
On Mars, over time, water and wind eroded rhyolitic rock into dust which settled at the bottom of a lake to become mudstone.
A similar region to Gale Crater on Earth, the researchers write, is Lake Tecocomulco in Mexico – a catchment basin that includes basaltic and rhylitic rocks.