Supervolcanoes ripped up early Mars
Mars may have had giant explosive volcanoes in its ancient past that spewed billions upon billions of tonnes of rock and ash into the sky.
Vast areas of collapsed ground in a region of the planet called Arabia Terra are their likely remains, believe Joe Michalski and Jacob Bleacher.
The two scientists report their findings in this week's Nature journal.
They say such supervolcanoes would have had a profound impact on the early evolution of Mars.
Their gases would have influenced the make-up of the atmosphere and perturbed the climate. And the ashfall would have covered the landscape across great swathes of the planet. It is quite likely some of the deposits the rovers are now encountering on Mars have their origin in colossal blasts.
"Scientists know the planet must have been more active in its deep past, in its first billion years. But we've always struggled to find evidence for these early volcanoes. The supervolcanoes we report in Nature may solve this puzzle," said Dr Michalski, who is affiliated to the Natural History Museum in London, and the Planetary Science Institute in Tucson, Arizona.
Supervolcano is an informal term to describe an immense eruptive event that expels in excess of a 1,000 cu km of rock and ash.
Earth has experienced this type of cataclysm numerous times, although not for many thousands of years. The US Yellowstone National Park, for example, sits on top of a past supervolcano.
But these behemoths are not so straightforward to identify.
Unlike, familiar volcanoes - such as Etna or St Helens on Earth, or indeed Olympus Mons on Mars - supervolcanoes do not build mountains out of layers of lava. Rather, their great scale means that, when they erupt, the whole landscape lets go with multiple vents and fissures. And after the eruption, the ground will fall back into the void left by all the ejected material, producing a large bowl or caldera.
Michalski and Bleacher, from Nasa's Goddard Space Flight Center, have identified a number of potential supervolcanoes in an area of Mars's northern hemisphere known as Arabia Terra. It is assumed to be one of the oldest terrains on the planet.
The researchers say their calderas are unlikely to be old meteorite impacts sites because they are irregular in shape, and lack the raised rim and central peak often found at impact craters. Neither do they have an apron of ejected material, another classic impact trait.
"Our best example is Eden Patera," Dr Michalski told the BBC's Science In Action programme.
"Thousands of cubic kilometres of material would have been erupted, and that's a minimum. In terms of size, it would be very similar to Yellowstone - about 70km across. So, imagine driving for an hour; it would take you most of that time to get across this hole in the ground."
The supervolcanoes' gases would certainly have influenced the composition of the early atmosphere and the evolution of the climate thereafter. But they may also have had an impact on Mars' potential for habitability, by pulling up substantial quantities of water and the essential elements needed for life from deep inside the planet.
These are ideas that can be tested by Nasa's Curiosity rover, which is heading for a large mountain in the middle of Gale Crater on Mars' equator.
No-one knows how this mountain formed, but some fraction of its layers may well be ashfall, possibly from the Arabia Terra supervolcanoes; and if not these particular volcanoes then perhaps other giants that have yet to be identified in satellite images.
"If future work shows that supervolcanoes were present more widely on ancient Mars, it would completely change estimates of how the atmosphere formed from volcanic gases, how sediments formed from volcanic ash and how habitable the surface might have been," emphasised Dr Michalski.
Commenting on the research, Mars expert Dr Peter Grindrod, from Birkbeck, University of London, said: "Understanding ancient terrains on Earth is extremely difficult because of subsequent processes hiding or removing the evidence - attempting to understand the earliest periods of Mars' history is even more challenging, as in addition we are limited to data returned from spacecraft.
"However, this study is a fascinating example of just what can be achieved with data from recent missions. This study is important because, if correct, it not only tells us something about Mars' early geological history, but can also help explain the source of later sedimentary deposits, many of which are important in studies of habitability on Mars."
And planetary geologist Dr Dave Rothery, from the Open University, proffered: "It is entirely plausible that the few, slightly anomalous, large craters that Michalski and Bleacher have highlighted are volcanic rather than impact craters. If so, then their size would suggest that they could well have erupted in the style of supervolcanoes on Earth.
"We know that such eruptions have had profound but short-lived effects on the Earth's climate.
"Were such an event to happen on Mars, it could either cool the globe (because of a global ash and aerosol cloud) or warm it (because of erupted greenhouse gases). The latter could lead to episodes of surface water flow, responsible for carving some of the medium-size valley systems on Mars."
Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos