On Thursday, NASA will attempt to land its latest rover, Perseverance, in an ancient lakebed on the surface of Mars. Perseverance, the biggest and fastest rover that has ever been sent to the red planet, is tasked with searching for signs of life at its landing site, Jezero Crater, which scientists think was filled with water more than three billion years ago.
Equipped with a small helicopter and a suite of powerful instruments, Perseverance is incredibly ambitious as a standalone mission. But the rover is just the first step of a broader endeavor that scientists have been working toward for decades: returning samples from Mars back to Earth.
Assuming Perseverance nails its landing, the rover will spend years collecting dozens of samples from Jezero Crater and hermetically sealing them into vials that will be cached in various locations across the crater. At some point within the next decade, the idea is for another rover to come collect them and load them into an ascent vehicle that will launch into Martian orbit and rendezvous with a capsule that will bring them back to Earth.
“Mars samples returned to Earth would represent a tremendous science resource because on Earth we can use the world’s most sophisticated instruments, including many instruments too large and complex to send to Mars, as well as techniques not even invented yet,” said Jeff Gramling, Mars Sample Return program director at NASA Headquarters, in an email. “Furthermore, Mars samples on Earth can be sorted and analyzed grain by grain and can be prepared in ways that are not possible on Mars.”
In addition to making Martian material available to the sophisticated instruments here on Earth, something that has never been done, a Mars Sample Return (MSR) mission would also provide a robotic dress rehearsal for any future round-trips to Mars involving humans.
“There are a bunch of elements of Mars Sample Return that are demonstrating key technologies required to safely send humans to Mars and bring them back,” said Alex Hayes, an associate professor of astronomy at Cornell University and co-investigator for Perseverance’s Mastcam-Z instrument, in a call.
While a MSR mission is not required to take place before a crewed mission to Mars, Hayes added that “it will probably make a lot of the folks doing the designs for Humans-to-Mars feel better once they’ve seen it demonstrated.”
Beyond its long-term implications for interplanetary human missions, MSR represents a key step toward understanding the Martian environment, both as it is today and how it was in the distant past, during an era when Mars was much more conducive to life.
In particular, Perseverance mission scientists are eager to explore an ancient delta that marks where a river once flowed into the lake within the crater basin. If life existed on the wetter, warmer version of Mars that existed some 3.6 billion years ago, fossilized evidence of it may well have been deposited in this delta feature.
“We have this planet that early in its history may have been more Earth-like, but not for very long—maybe the first half billion to two billion years,” said Marisa Palucis, an assistant professor and planetary scientist at Dartmouth College, in a call. “We're not going to find a dinosaur bone; it’s probably going to be the fossils of microbes, or small signs of life.”
“I think that's why it's so important that we can bring [samples] back,” she added, because it will be much easier to identify any lifeforms in labs on Earth, “just because we have more sensitive instruments.”
Perseverance is the first mission since NASA’s Viking landers, which touched down on Mars in 1976, to directly address the question of whether Mars has ever hosted life. But Hayes emphasized that it is, in itself, “not a life-detection mission.”
“In the case of Perseverance and Mars, we're looking for biosignatures, which are things that point towards the probability that life may have been there in the past, but don't definitively prove or disprove its existence,” he explained. “Instead, it tells us that this is the best thing to put in the vial and send back to Earth.”
In other words, collecting and caching tantalizing samples is step one. Step two is to send a second “fetch rover,” built by the European Space Agency (ESA), to Jezero Crater to pick up all these valuable vials and load them into a NASA ascent vehicle that will launch back into Martian orbit.
Step three is to send those samples back to Earth in a special capsule, also provided by ESA, which will be delivered to a desert area of Utah. The rough timeline is to complete all these steps in the next decade, meaning we may have juicy Martian samples on Earth sometime in 2031, though delays in space missions are always possible (if not probable).
Many spacecraft have already been sent into space to snag samples from extraterrestrial objects, including missions that have returned material from the Moon, asteroids, and a comet. But returning samples from Mars is much harder than any of these other celestial targets, due to its distance, its gravity, and concerns about planetary protection.
For instance, sending a robot to the Moon and back can be done in weeks—as China’s recent Chang’e-5 mission demonstrated—whereas a round-trip to Mars requires over a year of spaceflight.
Sample return missions to asteroids and comets are by no means easy, but they don’t need to pack powerful ascent vehicles to contend with the atmospheres and gravitational pulls of their targets. Asteroids are so small that spacecraft, such as Japan’s two Hayabusa sample-return probes, don’t land on their targets like Mars rovers do; they just grab material in a split-second of contact and bounce back into space. In the case of NASA’s Stardust mission, which returned tiny dust grains from Comet Wild 2 in 2006, the spacecraft positioned itself in the comet’s atmospheric tail, rather than directly landing on it.
“It takes a lot of rocket fuel to launch a rocket off of our own planet and get all the way to Mars and we have to do that, basically, in reverse,” Palucis said.
Contamination of extraterrestrial samples is an issue no matter the source, but it is particularly problematic for Mars. Although Mars appears to be an irradiated and lifeless world today, there may be some pockets of extant life tucked away in subterranean reservoirs or other hidden habitats on the red planet.
For this reason, sample-return capsules to Mars need to be virtually indestructible to ensure that any speculative Martian life-forms do not escape into our world’s ecosystems—a threat known as backward contamination. But the more likely risk is forward contamination, which involves Earth life finding its way into Martian samples, a possibility that would make it difficult to differentiate between alien and terrestrial organisms.
“We do think that Mars had these environments that were potentially habitable in the past,” said Palucis. “There is caution around doing it right so we don't go and contaminate these samples.”
Gramling noted that much more “stringent planetary protection protocols” apply to Mars compared to most other solar system bodies.
“These regulatory constraints result in, for example, triple sealing the Mars samples for safe return to the Earth,” he said. “This adds mass and complexity to these systems unlike those utilized in sample return from the Moon, comets, or asteroids. The planned Mars Sample Return campaign is one of the most difficult robotic missions humanity has ever undertaken.”
One mind-boggling possibility is that biological cross-contamination between Mars and Earth has already occurred in the early days of the solar system. Life may have emerged on Mars first and then spread to Earth through interplanetary meteorites, a theory known as panspermia. Of course, this is completely speculative, but samples from Mars could shed light on this evocative question.
“There are a lot of big ‘ifs,’ but if we did find a preserved microbe, or signatures of microbes in a rock, and it's older than any evidence we have of microbes on Earth, that'd be really interesting,” Palucis said. “We have to decide: what does that mean? Maybe it is possible, then, that life originated on Mars and came to Earth.”
This essential question of how life emerged in our solar system—and the odds that it is widespread elsewhere in the universe—has always been a major incentive for an MSR mission. Now, the first step in this long-standing vision is, at last, slated to begin on Thursday, once Perseverance touches down on Mars (knock on wood).
But it would not be an isolated breakthrough, as sample-return missions to many other worlds are currently in the works, all of which could shed light on the story of our solar system and the creatures that have emerged inside of it, including, potentially, some that are not of this Earth. As sample-return missions become more common, scientists may discover that life, or at the very least the ingredients for life, may be widespread, which would have enormous implications not just for science, but for humanity’s conception of itself within the cosmos.
“We'd have to come to terms with the fact that maybe we're not so special, but also that we're maybe not alone,” Palucis said.
Given that new discoveries are still emerging from Moon rocks returned by the Apollo missions, thanks to constant advances in the techniques used to study them, any samples from Mars—or elsewhere—will remain a scientific goldmine for a long time to come.
“Mars Sample Return is a big deal,” said Hayes. “It's huge, but it's also the start of what we’re going to see being more sample return missions, in my opinion, over the next few decades.”
“Sample science really is the gift that keeps on giving,” he concluded.
Update: This article has been updated to include comments from Jeff Gramling, Mars Sample Return program director at NASA Headquarters