Sandboxes are not just for kids. Giant ones — such as the Eurotank flume facility at Utrecht University in the Netherlands — can be used to study landscape and river evolution. And not just on Earth: Erin Kraal travelled to Utrecht to investigate landforms on Mars. But her big discovery came while she was taking time out from this work to show two high-school students how the tank can be used to study the formation of alluvial fans — the wedges of sediment left by rivers when they enter a basin. A simple demonstration in the 12 × 5-metre sandbox offered an unexpected explanation for how the red planet's 'stepped' deltas may have formed.

On completing her PhD, Kraal, now a research scientist at the Virginia Polytechnic Institute and State University in Blacksburg, was awarded an international fellowship from the National Science Foundation to spend her postdoc year at a foreign institution. She chose the home of the Eurotank. Her interest lay in the formation of alluvial fans, which provide a record of surface water flow. Understanding how these formations were produced on Mars is key to establishing not only where water flowed, but whether it was present for just decades or for millions of years.

Particularly intriguing were images of the martian surface that showed stepped, or terraced, fans. These are unlike any alluvial fan seen on Earth — here, such deposits have a single steep edge. Although Kraal wasn't focusing specifically on the origin of these stepped deltas, they were rarely far from her thoughts.

During her year at Utrecht, the editors of Copernicus, a European online science journal for young people (http://www.journal-for-young-scientists.net), approached Kraal to ask whether she might teach two high-school students about the Eurotank. She jumped at the chance. Clad in rubber boots, the trio carved extraterrestrial landscapes into the sand. They built a mock crater, fed a river to its rim and created an alluvial fan. The simple experiment was filmed, turned into a video and posted on Copernicus.

Afterwards, Kraal quickly drained the lake that had formed, because others were eager to use the facility. She was left with stepped deltas just like the ones in the images. “We were freaking out,” says Kraal. “It had been a seat-of-the-pants experiment and we hadn't made any measurements!” She immediately shifted her attention to developing control experiments, and recreating the crater and the unusual deposits.

On page 973, Kraal and her colleagues offer a model for the creation of martian stepped deltas. They propose that water was released suddenly from subsurface storage, carved out short canyons, cascaded over the rim of a crater, filled it, then rapidly drained away. These discharges, which would have been comparable in size to large rivers such as the Mississippi, would have to have occurred over a period of decades, not millions of years.

“We don't see stepped deltas on Earth,” says Kraal. “It is hard to imagine a situation that would have such a rapid release of water, but even if such a delta did form, it would not be preserved because there is so much rain here.” The group is now exploring what might have caused the water's rapid release on Mars. One possibility is a volcanic intrusion, which might have melted ice, mobilizing the water very quickly.