Evidence is hard to find on Earth, though. Much of it is long since folded back into the planet or washed away by other forces of erosion.
But orbital surveys of Mars reveal valleys and channels that go back millions and billions of years. Some researchers have long suspected superfloods were at work. Evidence has accumulated in recent years to bolster the idea, though no firm proof exists.
Kenneth Tanaka, an astrogeologist at U.S. Geological Survey in Flagstaff, Arizona, led a research team that spotted parts missing from the rim of a giant basin on Mars. The scientists think the material was stripped away when nearby volcanoes erupted, releasing surges of carbon dioxide -- the gas injected into soft drinks to create bubbles -- and perhaps water.
As lava welled up from below, it pressurized the carbon dioxide, creating a situation akin to a shaken soda, Tanaka told SPACE.com. When the pressure became too much for the overlying rock and dirt, the volcanoes blew their tops with extra fury, an explosion unlike anything on Earth in modern history.
The evidence is presented this month in Geophysical Research Letters, a journal of the American Geophysical Union.
Missing parts
Mars is thought by many researchers to have been largely stagnant for billions of years. Geologic activity -- earthquakes, volcanoes, shifting tectonic plates -- has been much more subdued than on Earth. Further, there are no oceans to cover ancient evidence, and scars of various origin are not overgrown with brush and trees.
One of the biggest scars is the Hellas Basin, 1,240 miles (2,000 kilometers) wide and 6 miles (9 kilometers) deep, created billions of years ago when an asteroid or comet smacked the Red Planet.
Around the basin's rim, Tanaka and his colleagues studied fields of lava that appeared to flow more recently -- probably well within the past billion years. Nearby, parts of the basin's rim are evidently missing. Specifically, regions of Malea and Hesperia Plana are several hundred yards (or meters) lower than adjacent parts of the rim. The lower areas also lack peaks that are seen nearby, outside the region of recent lava flow.
Perhaps, the scientists thought, volcanically powered floods carrying mud, boulders and other debris scoured away parts of the crater rim. Similar but less dramatic debris flows remade the landscape for miles around when Mount St. Helens erupted.
Recipe for disaster
The team analyzed what would happen if water ice, liquid water and carbon dioxide were trapped between surface rock and a deep underground well of hot molten rock, called magma, that was rising. Even water, pressurized and heated to steam, could fuel an explosion more massive than upwelling magma alone.
Carbon dioxide would be worse. And modern Mars is known to harbor significant amounts of carbon dioxide.
"Because of the enormity of the features on Mars and how much damage the floods have done, it makes sense that you might have had a fluid that is more erosive than water alone," Tanaka said. "Carbon dioxide is capable of doing more damage."
It is not certain, however, which liquid was at work. In fact, the whole hypothesis of the superfloods at Hellas Basin awaits confirmation. Tanaka said boulders and other debris might exist at the bottom of the crater, flood evidence he hopes to hunt for. Ultimately, surface rovers might be needed to analyze what kinds of minerals resulted from the interaction of fluid and rocks, he said.
USGS researchers Jeffrey Kargel, David MacKinnon, and Trent Hare also worked on the study, as did Nick Hoffman from the University of Melbourne in Australia. The researchers used topography data provided by the Mars Orbiter Laser Altimeter aboard NASA's Mars Global Surveyor.
These scientists, along with some other geologists, suspect other features on Mars may have been carved by
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