Apollo 17 astronauts drove a moon buggy across the lunar surface in 1972, measuring subtle changes in gravitational pull with an instrument called a gravimeter. Although no astronauts have landed on Mars yet, a group of clever researchers here on Earth realized that a more modern buggy there now has just the right tools for similar experiments.space buggies

In a paper published yesterday in the journal Science, the researchers detail how they repurposed data from navigational sensors aboard NASA’s Curiosity rover, essentially turning the sensors into gravimeters. This enabled the research team—which includes University of Maryland geology Assistant Professor Nicholas Schmerr—to measure the subtle tug from rock layers on the lower slopes of Mount Sharp, a peak that rises more than 3 miles from the center of Gale Crater.

The results suggest that these rock layers are much less dense than scientists had predicted. The findings call into question a competing theory that Gale Crater was once completely filled with sediment, then later excavated by erosion, leaving only Mount Sharp behind.

“This study represents the first gravity traverse and measurement of rock density on Mars. The low density of rocks in Gale Crater suggests that they did not undergo deep burial,” Schmerr said. “This could mean that Mount Sharp was not excavated by erosion, but rather was constructed by wind deposition and other processes.”

Curiosity carries accelerometers and gyroscopes just like a smartphone. Its sensors do the same job of determining location and orientation, but with far more precision, enabling the rover to navigate the Martian surface. Knowing the rover’s orientation also allows engineers to point its instruments and multidirectional, high-gain antenna.

By happy coincidence, the rover's accelerometers can be used just like Apollo 17's gravimeter, because the accelerometers detect Mars’ gravity whenever the rover stands still. Using engineering data from the first five years of the Curiosity mission, the researchers measured the gravitational tug of Mars. As Curiosity ascends Mount Sharp, which it has been doing since 2014, the mountain tugs on the rover’s sensors—but not as much as scientists expected.

"The lower levels of Mount Sharp are surprisingly porous," said the study’s lead author, Kevin Lewis of Johns Hopkins University. "We know the bottom layers of the mountain were buried over time. That compacts them, making them denser. But this finding suggests they weren't buried by as much material as we thought."

Lewis said that Mars holds plenty of mystery beyond Mount Sharp. Its landscape is like Earth’s, but sculpted more by wind and blowing sand than by water. They’re planetary siblings, at once familiar and starkly different.

“To me, Mars is the uncanny valley of Earth,” Lewis said. “It’s similar but was shaped by different processes. It feels so unnatural to our terrestrial experience.”

This news release was adapted from text provided by NASA’s Jet Propulsion Laboratory.