While one Martian rover, Opportunity, has died, its younger cousin Curiosity has just solved a mystery. It measured tiny changes in the Red Planet’s gravity, allowing its mission team back home at NASA to understand how a huge, 3.4-mile high mountain the rover is exploring, had formed.
The answer to that may seem inconsequential: it was probably formed as windblown sand and sediment got deposited higher and higher. But what’s more intriguing is how the rover actually measured the tiny gravity changes weaving through the mountain to get that answer – when none of its scientific instruments were designed to measure anything of the sort.
Curiosity arrived on Mars on August 5, 2012, landing in the Gale Crater, and started digging – to see whether our closest planetary neighbour is, or could have ever been home to, microbial life.
“Curiosity was designed more as a surface geology mission rather than a subsurface geophysics mission,” says Kevin Lewis, an assistant professor of earth and planetary sciences at Johns Hopkins University and a Curiosity mission team member. He’s also the lead author of the gravity study, published in Science. The rover was never engineered to peer deep below the rusty rocks under its wheels, he adds.
As part of its mission, Curiosity has been climbing Aeolis Mons – unofficially known as Mount Sharp after a late, renowned Martian researcher – since 2014. Mars’ sedimentary rocks, including those in Aeolis Mons, contain detailed information about the planet’s climate, much like they do on Earth.
Unlike Mars, however, Earth has active plate tectonics. With its ability to tear plates apart and consume others in subduction zones, much of Earth’s ancient sedimentary record is annihilated. The same processes just don't happen on Mars. That makes Aeolis Mons an extremely old registry of the long-lost environment of Mars that’s unlike anything on our own world. It’s “an unparalleled history book,” Lewis says, just waiting to be explored.
Understanding how it formed has occupied the minds of numerous geoscientists and triggered plenty of debate. For some time, two major hypotheses have been at loggerheads. One idea was that it was a mountain that built itself up from the base of Gale Crater through wind-blown dust sticking in place; another was that it was once buried by sediment, which then eroded away, leaving the mountain to stand alone.
One way to nail this down would be to take a look at its internal structure, but it didn’t look like Curiosity would be capable of doing that. A gravimeter, an instrument that looks at small changes in the local gravity field to determine the distribution of mass there, would have been perfect. However, as Fred Calef, a geospatial information scientist for Curiosity who wasn’t involved in the study explains, such a tool “must not have been considered a prime instrument for the mission” back when these calls were being made.
Curiosity also happens to be equipped with accelerometers. Much like those found in smartphones, these measure changes in movement and let the rover know how fast it’s moving and which way it’s pointing.
Gravity causes mass to accelerate, and so accelerometers can measure gravitational changes too. “You can download a phone app that can measure gravity,” says Lewis. “Not well enough to do science for the most part,” but it’ll still give you a reading.
Lewis thought exactly about that one day, and had a lightbulb moment. Why not, he thought, apply the same logic to Curiosity’s accelerometers? Serendipitously, one of Lewis’ colleagues had already been tinkering with the software of these instruments on the faraway rover. For some time now, he had been using it to detect shifts in the little craft’s orientation when it wasn’t moving, the sort created by changes in the surrounding environment.
After further rounds of calibrating, Curiosity was primed to detect gravitational fluctuations when stationary. As it climbed ever higher, the gravitational signal of the mountain stood out ever more, and the intricacies within its structure became apparent.
Gravimeters on Earth are up to a thousand times more precise than the tweaked accelerometers on Curiosity, but they were still good enough to reveal that Aeolis Mons was full of tiny holes, just like a slice of swiss cheese, and was of a fairly low density. That indicates that it was never significantly buried by sediment, which would have squashed those holes out of existence.
The finding matches up with earlier work, says Lewis, when Curiosity’s drill easily broke through sediment on its slopes, hinting at a weak, perhaps porous structure. So far, with all data considered, it looks like the lower part of the mountain built up as layers settled in a wet lake environment within Gale Crater. Then, after a climatic transition, everything dried up, and the mountain continued to rise as wind-blown sediment flew in.
Despite lacking the proper tools, Calef says that this research really shows how resourceful the rover scientists are at “extracting new science from all possible sources”.
The rover is still climbing higher, collecting more data, and refining its gravitational fishing hooks, so in time the structure of the mountain will be more clearly nailed down.
Starting to solve the mystery of Aeolis Mons is exciting all by itself, but the fact that Curiosity is able to do so thanks to a hack from 34 million miles away does seem like a standout achievement. “This is one of the most fun studies I’ve ever taken part in as a scientist,” says Lewis.
This article was originally published by WIRED UK
