Hacking NASA’s Curiosity rover let it measure Martian gravity, even though it has no scientific instruments designed to do so. The measurements revealed a surprise at Gale crater.
We can use gravity to probe the interior structure of a planet because the gravitational pull of a particular area depends on the type and density of the rocks there. On Earth, this is used both for science and to find valuable substances like oil underground. On Mars, it could help us understand the planet’s climate history, but the Curiosity rover has no gravimeter — the type of instrument that measures gravity.
So Curiosity’s handlers just hacked one. Instead of a gravimeter, they used the accelerometers designed to help navigate the rover across the rough Martian terrain. The researchers took more than 700 measurements of the planet’s gravitational pull on the rover as it drove within Gale crater.
The results were far less precise than those from actual gravimeters, but they are the first gravity measurements we have from the surface of Mars. “If Exxon sent me to do a gravity traverse and I sent them this they would not be impressed with the precision,” says team leader Kevin Lewis at Johns Hopkins University in Maryland. “But for Mars, because we have so little data, it’s useful.”
Gale crater is an ancient lake bed with a five-kilometre-tall mountain, Mount Sharp, at its centre. There are hypotheses that suggest the crater used to be completely full of sediment, which was carved out to create Mount Sharp.
These measurements contradict those ideas. The team found that there was less gravity pulling on Curiosity than we thought, meaning the rock in the area is less dense.
“This study suggests that those rocks never filled the crater to a depth of five kilometres or else the rocks that we’re driving over would have been much more compacted,” says Lewis. In fact, the sediment under Curiosity seems to be about 40 per cent air.
This is consistent with several other observations of the area, including that the sedimentary layers of Mount Sharp are not flat, as we’d expect if they were built as part of a plateau, but sloped downward from the centre. Lewis says it’s possible that instead of being carved out, the mountain was actually built up by sediment-shifting winds — and this may apply to other mountains on Mars as well.
Journal reference: Science, DOI: 10.1126/science.aat0738