Sensors were repurposed on the Curiosity rover and turned them into gravimeters, which measure changes in gravitational pull. That enabled them to measure the subtle tug from rock layers on lower Mount Sharp, which rises 3 miles (5 kilometers) from the base of Gale Crater and which Curiosity has been climbing since 2014. The results? It turns out the density of those rock layers is much lower than expected.
Curiosity carries accelerometers and gyroscopes. Moving your smartphone allows these sensors to determine its location and which way it’s facing. Curiosity’s sensors do the same thing but with far more precision, playing a crucial role in navigating the Martian surface on each drive. Knowing the rover’s orientation also lets engineers accurately point its instruments and multidirectional, high-gain antenna.
Teaching Curiosity to do gravimetry
Gravimetry—the measurement of tiny changes in gravitational fields—can be used to weigh mountains. Large-scale gravimetric mapping can be done from orbit, but examining small details requires a vehicle on the ground. The Curiosity rover on Mars carries several accelerometers used for routine navigation. Lewis et al. recalibrated these accelerometers to allow them to be used for gravimetry. They measured how the local gravitational field changed as the rover moved through Gale crater and began to climb Aeolis Mons (Mount Sharp). The resulting density of material under Gale crater shows that it is relativity porous, disproving a theory that the crater floor was once buried under several kilometers of rock.
Gravimetry, the precise measurement of gravitational fields, can be used to probe the internal structure of Earth and other planets. The Curiosity rover on Mars carries accelerometers normally used for navigation and attitude determination. We have recalibrated them to isolate the signature of the changing gravitational acceleration as the rover climbs through Gale crater. The subsurface rock density is inferred from the measured decrease in gravitational field strength with elevation. The density of the sedimentary rocks in Gale crater is 1680 ± 180 kilograms per cubic meter. This value is lower than expected, indicating a high porosity and constraining maximum burial depths of the rocks over their history.