Rover on Mars discovers what lies beneath

Scientists have reconfigured a robot working millions of miles away to discover what lies beneath a Martian crater...

Mars is one of Earth’s closest neighbours and, after a 560 million kilometre journey, NASA landed a robotic rover in the centre of a huge crater, named Gale Crater, on the surface of the planet in 2012. On board the rover is the most advanced suite of scientific instruments ever to be carried to the Martian surface. The rocks and soil on Mars hold the secrets to the planet’s history, and unlocking those will help us understand what the environment was like in the past.

Since landing, the rover has been roaming the massive crater and is beginning the ascent of a 5 km high mountain in the middle called “Aeolis Mons”, or “Mount Sharp” for simplicity. But how did a huge mountain form in the middle of an even bigger crater?

To begin to find out, scientists recently reconfigured the navigation equipment on board the rover to measure changes in the gravitational field at the rock surface.  Planetary scientist Kevin Lewis, from Johns Hopkins University in Baltimore, Maryland, is part of the team that devised this clever idea that was recently published in the journal Science.

"We were able to repurpose some engineering sensors on the rover and use them to actually hack together a new science instrument, to do some science the rover wasn’t originally intended to do," explains Lewis. "The rover has a set of what are called accelerometers, to measure accelerations on the rover, we use these to figure out how far we’ve driven and to detect the orientation of the rover, much like the accelerometers that are built into modern smartphones."

So a piece of equipment originally meant to help navigate and drive the rover is what was unconventionally used to carry out experiments on the rocks below the surface. When the rover isn’t driving, they can use these accelerometers to detect the gravity field of Mars, and by looking for small changes in the gravity beneath the rover as it climbs Mount Sharp, it can determine the rock density, which is a measure of how porous the rock is.

"What we found was the rocks that make up Mount Sharp, which is a mountain made of sedimentary rocks, layered rocks, record the action of water and wind and other geological processes that occurred on Mars for millions of years, and we found that those rocks were surprisingly low density," says Lewis.

Scientists don’t know exactly how the mountain formed, and so far the rover has only driven a couple of hundred of metres up the base, but there still remains several kilometres of rocks above. One theory is that the crater was once filled to the height of the top of the mountain, but that would mean the rocks at the rover landing site would have been buried about 5 km deep. "We estimate that, if that were the case, you would have really compacted those rocks a lot more and crushed out some of that porosity, and so we infer from this study that the rocks were never buried to the full depth of Gale crater, which is a bit surprising,” explains Lewis.

The team of resourceful scientists highlight the value of measuring gravity on Martian surfaces and show how those measurements aid the understanding of subsurface geology. "It would be great to have future gravity instruments on surface missions, we do a lot of studies from orbiting spacecraft around Mars and other planets. But this is the first one to actually do gravity traverse on the surface of another planet."

So, by investigating the rocks below, scientists are getting answer to the rocks that lie above. "It’s a bit of a puzzle, but we’ve got more geology to understand as we climb the mountain and hopefully we can get some answers to how this strange mountain formed in the first place."