If you think Mars is an inhospitable place now, you're not wrong… but it seems the planet's current state is relatively mild in comparison to its distant past.
Roughly 4 billion years ago, a region of the red planet called Arabia Terra experienced thousands of powerful and explosive volcanic super-eruptions that filled the atmosphere with so much dust and toxic gases that each one would have altered the Martian climate for decades at a time.
"Each one of these eruptions would have had a significant climate impact - maybe the released gas made the atmosphere thicker or blocked the Sun and made the atmosphere colder," said geologist Patrick Whelley of NASA's Goddard Space Flight Center.
"Modelers of the Martian climate will have some work to do to try to understand the impact of the volcanoes."
Super-eruptions are the most powerful of all known volcanic eruptions, with a magnitude of eight - the highest scale on the Volcano Explosivity Index. A supervolcanic eruption spews more than 1,000 cubic kilometers of material (240 cubic miles) into the atmosphere and onto the surrounding terrain for up to thousands of kilometers.
Although it was a long time ago that Arabia Terra experienced this activity, Whelley and his team were able to uncover the evidence on the Martian surface from data recovered by the Mars Reconnaissance Orbiter's Compact Reconnaissance Imaging Spectrometer for Mars (CRISM).
The terrain is an interesting one, pocked with huge depressions that had been interpreted as impact craters. This is not unreasonable; Mars is covered with the things. But a 2013 paper suggested a different origin - not impact craters at all, but calderas. These are depressions left behind after a supervolcano has blown its stack; once the magma is evacuated, the rock above has no structural support, and collapses in a sort of sinkhole.
Whelley and his team were intrigued by this idea, but it can be hard to tell impact craters and calderas apart without looking at them more closely. So they looked for something else - the massive volume of volcanic ash that would have been deposited in these colossal eruptions.
That 2013 paper had also modeled how much material should have been ejected. Another paper suggested that the Arabia Terra terrain could contain ash deposits from ancient eruptions; and yet another worked out the effect the Martian atmosphere would have had on ash dispersal.
"We picked it up at that point and said, 'OK, well, these are minerals that are associated with altered volcanic ash, which has already been documented, so now we're going to look at how the minerals are distributed to see if they follow the pattern we would expect to see from super eruptions'," said volcanologist Alexandra Matiella Novak of the Johns Hopkins Applied Physics Laboratory.
Because Arabia Terra is eroded to show layers of rock, the team took their volcanic mineral profile and started analyzing. They found layered deposits indicative - and diagnostic - of altered volcanic ash throughout the region. These included aluminum-dominant minerals such as montmorillonite, imogolite, and allophane.
A three-dimensional topographic map of Arabia Terra then revealed how these minerals were layered. They were exactly where they were predicted to have fallen, 4 to 3.5 billion years ago. And, finally, the predicted volume of volcanic fallout allowed the team to figure out how many individual eruptions there had been - and it's huge, somewhere between 1,000 and 2,000 over half a billion years.
The researchers estimate just over half a dozen super volcanoes might manage this activity, if each erupted every couple million years.
This is really strange, the researchers said. Supervolcanoes here on Earth are not found in clusters like this; they appear in regions that include other types of volcanoes, too. And it's not like Mars only has one type of volcano, either. So why should Arabia Terra appear to exclusively host volcanic monsters? And why haven't we found any other supervolcanoes on Mars?
"People are going to read our paper and go, 'How? How could Mars do that? How can such a tiny planet melt enough rock to power thousands of super eruptions in one location?'" said geologist Jacob Richardson of NASA Goddard.
"I hope these questions bring about a lot of other research."
The research has been published in Geophysical Research Letters.