Gravimetry — the measurement of tiny changes in gravitational fields — can be used to probe the internal structure of Earth and other planets. NASA’s Mars rover Curiosity carries a set of accelerometers normally used for navigation and attitude determination. A team of researchers led by Johns Hopkins University scientists recalibrated these instruments to allow them to be used for gravimetry. The team measured how the local gravitational field changed as Curiosity moved through Gale Crater and began to climb Mount Sharp, a 3.1-mile (5 km) high mountain within the crater and the main focus of Curiosity’s exploration. The resulting density of material under the crater shows that it is relativity porous, disproving a theory that the crater floor was once buried under several miles of rock.
A selfie taken by NASA’s Mars rover Curiosity on Sol 2291 (January 15, 2019) at the ‘Rock Hall’ drill site, located on Vera Rubin Ridge. Image credit: NASA / JPL-Caltech / MSSS.
Planetary researchers can learn a lot about a planet by measuring what lies beneath the surface at a particular location. By examining variations in gravity, they can calculate the density of the underlying rock, revealing all sorts of things about its history.
In addition to the overall pull of Mars’ gravity, higher density rocks in the subsurface exert a slightly greater downward gravitational force than lower density rocks.
Typically, to measure gravity on Mars and other planets, scientists rely on orbiting satellites like NASA’s Mars Reconnaissance Orbiter. But because the satellites are so far away from their targets, the spatial resolution is limited.
Johns Hopkins University’s Dr. Kevin Lewis and co-authors decided to try calibrating accelerometers on NASA’s Curiosity rover to measure surface gravity as the rover climbed Mount Sharp.
As it turned out, the rover had already collected hundreds of measurements over much of the mission that the team could use to measure subtle changes in gravitational acceleration.
“Curiosity, essentially, has a new science instrument six and a half years into its mission,” Dr. Lewis said.
“This allows us to get new information about the subsurface of Mars in ways the rover was never designed to do.”
First, the researchers took into account Mars’ rotation to accommodate expected changes in acceleration.
Next, they calibrated this raw information, accounting for the effects of temperature, the tilt of the rover, and other factors.
As Curiosity ascended Mount Sharp getting farther from the center of Mars, the gravitational pull of the planet became very slightly weaker. Measurements using the rover’s accelerometers showed, however, that the gravity was not falling off as fast as expected from its change in elevation.
The difference between the predicted and modeled measurements, the team determined, were the result of the additional downward pull of the mass of Mount Sharp itself, allowing the scientists to estimate the density of the rocks beneath the rover.
“What we were able to do is measure the bulk density of the material in Gale Crater,” said co-author Travis Gabriel, a graduate student at Arizona State University.
“Working from the rocks’ mineral abundances as determined by Curiosity’s Chemistry and Mineralogy instrument, we estimated a grain density of 2,810 kg/m3. However the bulk density that came out of our study is a lot less — 1,680 kg/m3.”
Scientists have long debated the origin of Mount Sharp. Mars craters the size of Gale have central peaks raised by the shock of the impact that made the crater. This would account for part of the mound’s height. But the upper layers of the mound appear to be made of wind-scoured sediments more easily eroded than rock.
Did these sediments once fill the entire bowl of Gale Crater? If so, they might have weighed heavily on the materials at the base, compacting them.
But the new findings suggest Mount Sharp’s lower layers have been compacted by only a half-mile to a mile (1-2 km) of material — much less than if the crater had been completely filled.
“There are still many questions about how Mount Sharp developed, but our study adds an important piece to the puzzle,” said Dr. Ashwin Vasavada, Curiosity project scientist at NASA’s Jet Propulsion Laboratory.
“I’m thrilled that creative scientists and engineers are still finding innovative ways to make new scientific discoveries with the rover,” Gabriel said.
“This is a testament to the utility of having a diverse set of techniques with the Curiosity rover, and we’re excited to see what the upper layers of Mount Sharp have in store.”
The results appear in the journal Science.
_____
Kevin W. Lewis et al. 2019. A surface gravity traverse on Mars indicates low bedrock density at Gale crater. Science 363 (6426): 535-537; doi: 10.1126/science.aat0738
