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Supervolcanoes erupted on ancient Mars

Accidental discovery Ancient Mars was home to giant volcanoes capable of eruptions a thousand times more powerful than the one that shook Mount St Helens in 1980, say scientists.

The discovery of irregularly shaped craters in the Arabia Terra, a region in the northern highlands of Mars, constitutes a previously unrecognised Martian volcanic province, they report in the journal Nature .

Their finding raises fresh questions about conditions on Mars in its early years, a time when scientists believe the planet was much more Earth-like with a thick atmosphere, warmer temperatures and water on its surface.

Major volcanic eruptions likely would have triggered climate shifts that toggled Martian temperatures between cold spells when ash blocked sunlight and heat waves when greenhouse gases filled the skies, the scientists say.

Supervolcanoes may have made it more difficult for life to evolve on the planet's surface, but underground steam vents and the release of water into the atmosphere also could have created niches for microbes to thrive, says geologist Joseph Michalski of the Planetary Science Institute in Tucson, Arizona.

Volcanic history

The discovery of supervolcanoes on Mars comes from analysis of images from a quartet of Mars orbiters over the past 15 years.

These types of volcanoes, also known as 'caldera' volcanoes, are ancient, collapsed structures rather than steep, cone-shaped or domed mountains like Olympus Mons on Mars, a so-called shield volcano that stands nearly three times taller than Mount Everest, the highest peak on Earth.

"We know a lot about the volcanic history of Mars over the last three to three-and-a-half billion years, but that still leaves about one billion years before that over which we don't really know anything about volcanism," says Michalski.

Some scientists theorised that the oldest Martian volcanoes had eroded away, but the new findings suggest a different kind of volcano existed long ago.

"If early Mars saw a lot more explosive volcanism, then the features that are left from that don't look like those shield volcanoes. That's maybe why we didn't see them," says Michalski.

Scientists say supervolcanoes erupt with about 1000 times the force of typical volcanoes like Mount St. Helens in the US state of Washington. The eruption of Mount St Helens in 1980 blasted the top off the mountain, killed 57 people and, according to the US Geological Survey, shot ash, steam, water and debris about 24,000 meters into the air.

Evidence of past supervolcanoes on Earth has been erased by plate tectonics and other geologic activities.

Accidental discovery

At the time of the discovery, Michalski was studying Martian impact craters, not looking for volcanoes.

"We made the discovery by accident," he says. "As I went through (the images) of this one region, I found a number of them that were simply not impact craters," he says.

"One was clearly a volcano. ... It is quite possible there are many more of these," Michalski adds.

Because the emission of gases from volcanoes helps create a planet's atmosphere, understanding the volcanic history of Mars is crucial to figuring out what the planet was like in its early years.

Additional evidence may come from NASA's Mars Curiosity rover, which is heading toward a five-kilometre-high mound of deposits called Mount Sharp.

The rover touched down inside a giant impact basin near the planet's equator in August 2012 to assess if Mars ever had the chemistry and environment to support and preserve microbial life.

"There are thousands of layers of rocks in Mount Sharp and they contain a long record of geologic history," says Michalski.

"There could be interlayered rocks that are ash beds, and we predict that and we hope that the rover can test it," he says.