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 there are no astronauts on Mars yet, a group of clever researchers here on Earth realized there is a buggy there--and it has just the right tools for similar experiments.
In a research paper published January 31, 2019 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. Either way, it seems that Mars has had the capability to lay down significant amounts of low-density sedimentary rocks that record a complex environmental history."
Curiosity carries accelerometers and gyroscopes just like a smartphone. Moving a smartphone allows these sensors to determine the phone's location and orientation. Curiosity's sensors do the exact same thing 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."
The researchers used more than 700 measurements from Curiosity's accelerometers, taken between October 2012 and June 2017. These data were calibrated to filter out noise, such as the effects of temperature and the tilt of the rover during its climb. The team then compared its calculations to models of Mars' gravity fields to check their accuracy.
The team also compared their results with mineral density estimates from Curiosity's chemistry and mineralogy instrument, which characterizes the crystalline minerals in rock samples using an X-ray beam. That data helped to determine the porosity of the rocks.
There are many mountains nestled inside craters and canyons on Mars, but few approach the scale of Mount Sharp. Scientists still aren't sure how the mountain grew inside of Gale Crater. One idea suggests that the crater was once completely filled with sediment, and many millions of years of wind erosion eventually excavated all but the mountain at the center of the crater. If the crater had been filled to the brim, all that material would have compacted the many layers of fine-grained sediment beneath it.
But the new paper suggests that Mount Sharp's lower layers have been compacted by less than a mile of material--much less than the 3 miles that would have overlain the sediments if the crater was once completely filled.
"There are still many questions about how Mount Sharp developed, but this paper adds an important piece to the puzzle," said Ashwin Vasavada, Curiosity's project scientist at NASA's Jet Propulsion Laboratory in Pasadena, California. "I'm thrilled that creative scientists and engineers are still finding innovative ways to make new scientific discoveries with the rover."
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 press release was adapted from text provided by NASA's Jet Propulsion Laboratory.
The research paper, "A surface gravity traverse on Mars indicates low bedrock density at Gale crater," Kevin Lewis, Stephen Peters, Kurt Gonter, Shaunna Morrison, Nicholas Schmerr, Ashwin Vasavada and Travis Gabriel, was published in the journal Science on January 31, 2019.
This work was supported by NASA (Award Nos. 1548315, NNX14AQ92G and 18-PLANET18R-0036). The content of this article does not necessarily reflect the views of this organization.
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