Curiosity rover finds Mars may not be as dense as thought

Curiosity rover uses on-board accelerometers to measure Mars' density and gravity

Rock layers on the lower slopes of Mount Sharp on Mars are porous and much less dense than thought, according to a study by scientists from NASA's Jet Propulsion Laboratory (JPL), University of Maryland, John Hopkins University, Carnegie Institution, and Arizona State University.

The discovery was made using the data from several accelerometers aboard NASA's Mars Curiosity rover. Accelerometers and gyroscopes on Curiosity rover allow scientists to precisely determine the orientation and the direction of the rover as it moves across the Martian surface.

In the current study, scientists used more than 700 measurements taken by Curiosity's accelerometers between October 2012 and June 2017. The research team calibrated the data to filter out noise, such as the effects of the tilt of the rover as it climbed Mount Sharp, a five-kilometre-high peak in the centre of the 154-kilometre-wide Gale Crater.

Calibration of the data essentially turned the instruments into gravimeters, allowing scientists to measure changes in the gravity as the rover climbed the peak.

Gravimetry refers to the measurement of small changes in gravitational fields, and can be used to weigh mountains.

"Curiosity, essentially, has a new science instrument, six and a half years into its mission," said Kevin Lewis, an assistant professor at Johns Hopkins University and the lead author of the study paper.

"This allows us to get new information about the subsurface of Mars in ways the rover was never designed to do."

Scientists measured the difference in gravity experienced by the rover as it climbed the rock layers on the lower slopes of Mount Sharp.

The results suggested that rock layers on lower slopes of the peak are porous and much less denser than earlier predicted. These findings challenge an earlier theory that Gale Crater was once completely filled with sediment, and was later excavated by wind erosion over millions of years, which left only Mount Sharp behind.

These results were also compared with those from the mineral density estimates from chemistry and mineralogy instrument on board Curiosity rover. That data also helped scientists to determine the porosity of the rocks.

"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," said Lewis.

The findings of the study are published in the journal Science.