We Can Only Bring 30 Samples of Mars Back to Earth. How Do We Decide?

The Mars Sample Return Mission is one of the most ambitious missions ever conceived. Though the samples won’t be returned to Earth until 2033 at the earliest, the Perseverance Rover is busy collecting them right now. Ideally, Perseverance could gather as many samples as we like and ship them all back to Earth. But of course, that’s not possible.

There are limitations, and this means that choosing which samples to return to Earth is an extremely critical task.

Scientists have made enormous progress piecing together Mars’ history. Thanks to orbiters like the Mars Reconnaissance Orbiter, and especially to rovers like Spirit, Opportunity, Curiosity, and Perseverance, scientists know that liquid water once flowed across the planet’s surface. But for how long it flowed and if the planet ever supported life thanks to all that water are two unanswered questions.

The samples that Perseverance is collecting could answer those questions. But for that to happen, the rover has to gather the right samples. How are those decisions made?

The best case scenario is to gather the most information-rich samples that can clearly address scientists’ most pertinent questions. But there are limitations on the practical side. Perseverance doesn’t have unlimited mobility and unlimited reach. Compromises must be made, and that’s where the hard decisions have to be made.

Scientists don’t have to wait for the Perseverance samples before they can study Martian rocks. When powerful enough impactors struck Mars, they sent ejecta high above the surface. Mars’ gravity is significantly lower than Earth’s, so some of the ejecta reached escape velocity and were flung out into space. Some of it made its way to Earth, survived re-entry, and has been collected. There are over 270 Martian meteorites, and they’ve told scientists a lot. But they have their limitations. They’re from a younger Mars and don’t tell scientists much in terms of a bigger picture of Martian history.

None of the meteorites are sedimentary rock. And when you’re trying to understand Mars’ ancient water, climate, and potential ancient habitability, sedimentary rocks should hold important clues. Scientists would love to get their hands on some.

Perseverance has been addressing that shortfall. By landing in Jezero Crater, an ancient paleolake, the rover has ample access to sedimentary rocks, including some from a river delta. The Perseverance mission was designed, at least partly, to get access to sedimentary rocks that could hold fossilized evidence of ancient life.

Chris Herd is a ‘sample return scientist’ at the University of Alberta. He is a professor in the Department of Earth and Atmospheric Sciences at the U of A and the curator of the University’s meteorite collection. Herd is also a member of a group of scientists charged with selecting which of Perseverance’s samples will be returned to Earth.

“Those are even more interesting from an ancient biology perspective,” Herd said about the sedimentary rock samples. “That’s the reason we went to this landing site because the rocks were laid down by liquid water some three and a half billion years ago and could preserve evidence of ancient life.”

The rover has also collected samples of igneous rock, which scientists can compare with Martian meteorites. Together, they can help build a more complete picture of the planet’s geological history.

Perseverance has gathered about half of its samples so far. As the rover ascends away from Jezero Crater, it’ll continue to gather more. So while the Jezero Crater is an important sampling site, the region above it is also important. Water flowing across Mars delivered material to these sites, so sampling them is taking advantage of how nature delivered a wider variety of material to the area.

“Each of those 15 or 16 samples could be unique and could represent a bigger range of ages and rock types than we’ve seen inside the crater,” Herd said.

In terms of sampling, the mission is divided into smaller campaigns, and each campaign targets three to five samples. But the rover does more than collect samples. It also gathers detailed data from each sampling site, including what the rocks are made of and the details of the surrounding site.

As Perseverance goes about its business, scientists have to decide which samples to keep for return to Earth. After each drilling operation, sample scientists file a detailed report. The reports contain “everything from the map view to the outcrop to the details of what we’ve learned about the rock as we sample it,” Herd explained. These reports help decide which samples get returned to Earth. Seating is limited, and only VIP samples will make the trip.

The samples aren’t large. Each one is only about 10 grams. But that should be enough to make scientific headway. Scientists’ ability to study samples and extract information is formidable, thanks to modern technology and innovative methods.

“There are ways we can analyze a sample that gives us incredible detail about when the rock formed, how it was modified, whether there’s any organic matter that could be evidence of life,” says Herd. “There’s a host of things we can tell from tiny amounts.” Japan’s Hayabusa 2 spacecraft, for example, collected only five grams of dust from the asteroid Ryugu. But the sample was enough to show scientists that the asteroid had a rich complement of organic molecules.

One of the sites Perseverance will visit as it leaves Jezero Behind is called the “Curvilinear Unit.” It’s an ancient Martian sandbar made of sediment deposited in a bend of a river that flowed into Jezero Crater. The location will give scientists a look at outcrops of mudstone and sandstone. Visiting this region also increases Perseverance’s reach since it contains evidence of geologic processes well outside Jezero Crater.

While for most of us, the collecting of samples and the work of the rover itself catches our attention, there’s a lot of work behind securing those samples and preserving their scientific value. Contamination is a no-go, and procedures are in place to establish vigilance.

“We need to get this right, because this is answering a huge question.”

Chris Herd, University of Alberta

“There’s a lot that we have to do to make sure we don’t contaminate the samples with signatures of life from Earth and misinterpret that signature as life on Mars,” Herd says. It may entail building an entirely new facility, Herd says, because no existing facility has been built with this mission in mind and with so much at stake. There’s also the small risk that something from Mars could contaminate Earth. That could muddy the scientific waters so badly that it turns it all into a big mess. And it would be a long time before we’d get another crack at Martian samples. “We need to get this right,” he says, “because this is answering a huge question.”

Could the samples contain not only evidence of ancient life on Mars but a possible sample of simple life that exists there today? Nobody would bet on that, but it’s hard to argue that it’s not at least a possibility. “There’s still a non-zero probability that there’s extant life that has somehow managed to survive on Mars,” Herd adds.

For the rest of us, the prospect of getting Martian samples in Earthly labs and studying them until they’ve revealed all their secrets is not only exciting but possibly mind-expanding, depending on the eventual results. As we’ve pondered ancient Mars’ habitability and followed along as evidence accumulated showing the planet was warm and wet, the prospect of life there has come into focus. Irrefutable evidence of life on Mars would change a lot for humanity, though many would likely sleep-walk through the discovery.

But for Herd, even being a part of this ground-breaking mission is career-defining. “It’s absolutely phenomenal for me to be involved in such a huge mission, where we get to explore and get information about the rocks and the geology while at the same time sampling and looking forward to bringing those samples back,” Herd says. “That’s what sets this mission apart.”