For the first time, a Mars rover has measured the mass of the rocks beneath its wheels. By taking gravity measurements as it climbed a Martian mountain, Curiosity discovered something surprising: Mount Sharp appears to have been built in two phases — one soggy, one dry.

The rover found that the rocks it is driving over are less densely packed than scientists expected. That suggests the mountain was not formed just from compressed lake sediments, which would have crushed the rocks at its base more thoroughly.

“I had no idea what to expect,” says planetary scientist Kevin Lewis of Johns Hopkins University, who reports the results in the Feb. 1 Science. “This is the first measurement of its type on Mars.”

Since landing on Mars in 2012, Curiosity has been driving across Gale Crater, which scientists think used to be a lake (SN Online: 12/8/14). After 753 Martian days, the rover began climbing the 5-kilometer-high Mount Sharp, also called Aeolis Mons, in the crater’s center.

“Mount Sharp is a big puzzle,” Lewis says. One idea is that the mountain could have been laid down layer by layer in the ancient lake, until it filled the entire crater. The weight of all those layers of rock would densely compact the rocks at the base.

But some features of Mount Sharp suggest that’s not what happened — the mountaintop rises above part of the crater’s rim, for one thing. Some scientists suggested that instead, winds dropped sand and sediment in the crater’s center after the lake dried up to build the mountaintop. If that’s the case, the foothills at the edge of the mountain’s base would not have been buried as deeply, and so would be less dense.

Measuring the density of the rocks could help test which scenario is correct. Geologists use accelerometers, like those found in a smartphone, to measure small variations in Earth’s gravity that result from different rock concentrations underground. The more mass in the rocks below, the more gravity measured. The Apollo 17 mission made similar measurements on the moon, revealing an ancient lava flow.

Lewis realized that Curiosity’s accelerometer had been making gravity measurements since landing, even though it wasn’t designed for that purpose. Every time the rover stops driving, it takes a five-minute accelerometer measurement to figure out which direction it’s facing.

Lewis and colleagues combined more than 700 measurements taken between the mission’s 60th and 1,743rd Martian days to see how much the gravity changed as Curiosity climbed. The rover has climbed only about 350 meters, or about 7 percent of the way up Mount Sharp. But that was enough to let the team calculate the rock’s density.

The researchers found that the rock beneath Curiosity’s wheels is less dense than its mineral composition led them to expect. It’s “more like the density of soil than a fully cemented rock,” Lewis says. That means the crater must never have completely filled with rock — the upper layers would have crushed the lower ones — and supports the windblown sands theory for how Mount Sharp formed.

That in turn suggests there were two different periods of mountain-building in Gale Crater, one that laid down lake sediments and a drier one that built Mount Sharp’s peak. Curiosity might find the transition point as it keeps climbing, Lewis says.

“It’s very elegant to repurpose the accelerometer,” says planetary scientist Edwin Kite of the University of Chicago, who was not involved in the new work. “They’ve done a great job adding a new science instrument to the rover.”

One thing still puzzles Kite, though. The measurements suggest that the rocks beneath Curiosity are riddled with holes. “But the rover doesn’t see any holes,” Kite says. Either the pores are too small for Curiosity to see, less than 10 micrometers wide, “or there’s something unusual about the rocks right at the surface where Curiosity is driving.”