An image of a Martian terraced fan taken by a camera on board the Mars Odyssey spacecraft.
To figure out an odd landscape feature on Mars, play in a big sandbox.
Enlist some high school students, too.
That’s what some scientists at the Utrecht University in the Netherlands did, and they believe they now know how sediment deposits spilling out of the mouth of some water channels on Mars were shaped in a series of terraces that look like terraced rice paddies.
But no similar natural formations have been seen in river deltas on Earth. Usually river sediments spill out in a smooth, sloping fan like the Mississippi delta.
Planetary geologists have been speculating about the terraced fans since they were first spotted by NASA’s Mars Global Surveyor eight years ago. About 10 stepped fans have been identified, most at the base of a steep slope emptying into a basin like an impact crater. (Most of the 200 sediment fans seen on Mars do not have the stepped structure. Another mystery is why many of the river channels seem to have no sediment deposit at all.)
Some scientists suggested the terraced fans were the result of repeated shore erosion as a lake in the basin dried up. Others thought repeated landslides might have formed the steps.
The sandbox experiment, reported in Thursday’s issue of the journal Nature, supports a third notion. The terraces form by the interaction of the sediment flow with the water’s edge, which is rising as the basin fills.
“Where that’s happening, you’re getting a little lip,” said Erin R. Kraal, the lead author of the Nature paper. Pulses of flow and sediment produced multiple terraces. “They’re just stacking one atop the other,” she said.
While a postdoctoral researcher at Utrecht, Dr. Kraal became intrigued by the terraced fans and mentioned them to her colleagues there. Utrecht has a set-up known as Eurotank, essentially a 16- by 40-foot sandbox for studying sedimentary dynamics.
High school students visiting the laboratory as part of an educational project saw the Mars pictures on the laboratory walls and were interested in helping on an experiment, which eventually turned into a short educational movie about the Martian fans.
The students dug a crater in the sandbox and shaped a water channel. Then they sent water down the channel — and the result was a terraced fan, just as on Mars.
“We didn’t expect it to be so successful the first time,” said Dr. Kraal, now a research scientist at Virginia Polytechnic Institute and State University. “We were really surprised they formed so quickly and so easily.”
Dr. Kraal and her colleagues, Maurits van Dijk, George Postma and Maarten G. Kleinhans later repeated the experiments more rigorously so they could correlate their sandbox results with the Martian terrain.
They estimate that the water necessary to form one of the Martian fans, which measure up to a dozen miles wide, would equal 10 years of Mississippi River flow. The whole structure appears to have formed in one event lasting perhaps tens of years, they said.
“It does look like she’s experimentally shown here that this type of deposit can form in a single event type of discharge,” said Rossman P. Irwin III, a geologist at the Smithsonian Institute’s Center for Earth and Planetary Studies who has also studied the terraced fans. “It offers some good experimental support for a type of feature that is basically unique to Mars and really was not well understood.”
