When water was put into a short, adjacent channel, the liquid pulled sediment out of the channel and transferred it into the basin.
But unlike the uniform deposits seen where rivers dump into lakes on Earth, the high volume of water sent the sediment out into sequentially wider layers as it advanced, forming the steps.
For that scenario to work, the water would have to come in a sudden rush—and it could happen only once per basin.
"We suggest that the water was released internally, such as a hydrothermal water source suddenly pushed to the surface," Kraal said.
(Related: "Mars's Water Could Be Below Surface, Experts Say" [January 25, 2007].)
Not So Fast
Alan Howard, a University of Virginia researcher, noted that fan-shaped features occur in other settings—including when Earth's rivers empty into lakes or the ocean.
"There is little other direct evidence of former lakes on Mars, although many Mars researchers, including myself, feel that lakes were numerous during early Martian history," he said.
But Howard isn't convinced that the floods happened quickly, only once, or from single sources of water. He believes water could have seeped into the basins from a variety of sources or could have come from precipitation.
At least one other researcher is happy to see the new findings, though.
"This paper could go a long way toward convincing the community that water-flow events on early Mars may have been short-lived and catastrophic," said Douglas Jerolmack, a researcher at the University of Pennsylvania.
He understands that skeptics will want to wait until the findings are backed up with detailed sediment transport models, which can only come from closer looks at the surface of Mars, he said.
Satellites could do the trick, he added—but he's hoping for another rover.
Study leader Kraal said further support for the new findings could also come from elsewhere in the solar system.
On Earth, she pointed out, depositional features are often shrouded by vegetation and therefore tough to study.
But Saturn's moon Titan, for instance, is a little more like Mars in that its surface is exposed—and it also boasts fan-shaped craters.
There, she noted, "the fundamental variables—gravity, the type of rock, the atmosphere—are so different.
"It is interesting to change the fundamental variables and look at such processes," she said.
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