Using carefully selected terrestrial rocks, engineers try to figure out how to work with crumbly rocks like the one the rover encountered on its first sampling attempt.
When NASA’s Perseverance Mars rover tried to collect its first rock core sample last August, the outcome presented a puzzle for the mission team: The rover’s sample tube came up empty. But why?
Not long after, Perseverance successfully gathered a sample the size of a piece of chalk from a different rock. The team concluded that the first rock they had chosen was so crumbly that the rover’s percussive drill likely pulverized it.
But engineers at NASA’s Jet Propulsion Laboratory in Southern California, which manages the mission, want to understand why that first sample, nicknamed “Roubion,” turned to dust. The mission’s scientists and engineers had run extensive test campaigns on dozens of rock types prior to launch, but they hadn’t seen any react exactly like Roubion. So a new test campaign was started – one that would include a field trip, a duplicate of Perseverance’s drill, and JPL’s unique Extraterrestrial Materials Simulation Lab. Answers remain elusive, but here’s a closer look at the process.
Re-creating the unique physical properties of Roubion would be key to the test campaign.
“Of the rocks we’ve seen, Roubion had the most evidence of interaction with water,” said Ken Farley of Caltech, Perseverance’s project scientist. “That’s why it fell apart.”
Rocks altered by water can be more susceptible to falling apart; they’re also highly valuable to Perseverance’s scientists. Water is one of the keys to life – at least on Earth – which is why Perseverance is exploring Jezero Crater. Billions of years ago, Jezero contained a river-fed lake, making it an ideal spot to look for signs of ancient microscopic life now. Perseverance is collecting samples that future missions could bring back to Earth to be studied in labs with powerful equipment too large to be sent to Mars.
To find Roubion stand-ins, a handful of rover team members were granted permission to hunt rocks in the Santa Margarita Ecological Reserve, a two-hour drive from JPL. The team was searching for rocks that filled a geological sweet spot: weathered enough to be Roubion-like, but not so fragile that they would fall apart at the slightest touch. They eventually selected a half-dozen rocks.
“It was very physical work,” said JPL’s Louise Jandura, chief engineer for sampling and caching, who has been leading the test campaign. “We were chipping away with rock hammers and crowbars. A couple rocks were big enough that it took all five of us holding on to a stretched-out canvas to get it into the bed of our truck.”
Next step: testing at JPL. One of the places where that happens is the Extraterrestrial Materials Simulation Lab, a kind of service center that prepares materials for testing elsewhere at JPL.
A Rock Superstore
The low-slung building sits on a hillside above the Mars Yard. Barrels out front contain reddish dust called Mojave Mars Simulant, a special recipe for re-creating the messy conditions rovers travel in. Piles of rocks – some peppered with drill holes – are strewn about a forbidding industrial saw near the entrance. In back stands a concrete bunker with rock bins labeled with names that sound like Mad Libs for geologists: Old Dutch Pumice, China Ranch Gypsum, Bishop Tuff.
“I like to say we do artisanal selection and preparation of materials,” said Sarah Yearicks, a mechanical engineer who leads the lab. “Testing them is part manufacturing and part mad science.”
Yearicks is one of the people who picked out the rocks at the Santa Margarita Ecological Reserve excursion. For the testing on Roubion-like rocks, Yearicks’ team worked with a construction-grade drill – not a coring drill – along with other tools, while Jandura’s team used a “flight-like” duplicate of Perseverance’s drill.The teams passed the rock samples back and forth, testing them in different ways.
Put to the Test
Jandura’s team ran their flight-like drill a few millimeters at a time, stopping to check that a core was still forming; if it had crumbled, they’d look at variables that might be the cause. For instance, the engineers tweaked the drill’s rate of percussion and the weight placed on its bit. They also tried drilling into the rock horizontally instead of vertically, in case the build-up of debris was a factor.
For every adjustment they made, it seemed, a new wrinkle would emerge. One was that fragile samples can still resist the percussive drill. When Jandura’s team reduced the force of percussion to avoid powderizing the sample, the drill bit couldn’t penetrate the surface. But choosing a spot that holds up to stronger percussion means choosing one that likely interacted less with water.
Perseverance has so far captured six samples from highly weathered, water-altered rocks, and the team knows it’s capable of many more. But their experience with Roubion has prepared them for some of the extremes Mars will throw at Perseverance in the future. If they find more rocks like Roubion, the Extraterrestrial Materials Simulation Lab will be ready with its menagerie of Mars-worthy materials.
More About the Mission
A key objective for Perseverance’s mission on Mars is astrobiology, including the search for signs of ancient microbial life. The rover will characterize the planet’s geology and past climate, pave the way for human exploration of the Red Planet, and be the first mission to collect and cache Martian rock and regolith (broken rock and dust).
Subsequent NASA missions, in cooperation with ESA (European Space Agency), would send spacecraft to Mars to collect these sealed samples from the surface and return them to Earth for in-depth analysis.
The Mars 2020 Perseverance mission is part of NASA’s Moon to Mars exploration approach, which includes Artemis missions to the Moon that will help prepare for human exploration of the Red Planet.
JPL, which is managed for NASA by Caltech in Pasadena, California, built and manages operations of the Perseverance rover.