If life ever existed on Mars, we may already have the answer at hand. In January NASA’s Perseverance rover deposited 10 tubes on the surface of Mars. Each contains a sample of Martian rock that was carefully selected for its potential to clarify chapters of the planet’s still-murky history. Those tubes “are capable of telling us whether Mars was habitable,” says Mitch Schulte, Perseverance’s program scientist at NASA Headquarters in Washington, DC “We see evidence of particular minerals that tell us there was water. Some of these minerals indicate there was organic material.”
But to know for sure, scientists need to bring these tubes back to Earth for closer study—an audacious endeavor known as Mars Sample Return (MSR), which is slated for the early 2030s via a follow-up robotic mission. These 10 tubes are only the opening course in a bigger awaiting feast, a backup cache in case Perseverance breaks down before it can fill and deliver the 33 additional tubes that it carries. These tubes will hold samples sourced from the area in and around Jezero Crater, the site of a four-billion-year-old river delta and the locale where the rover landed on February 18, 2021. Although many of the samples are yet to be gathered for a journey to Earth that remains years in the future, those already collected have whetted researchers’ appetite for their return home.
Scientists targeted Jezero for Perseverance because, on our planet, sprawling river systems like that found in the Martian crater build up enormous deposits of sediments. Washed in from a sizable swath of the surrounding landscape, these deposits contain various minerals that can be used to chart the Red Planet’s past geology. Also most anywhere water is found on Earth, life accompanies it. The same might hold true for Mars, meaning Jezero’s sediments could conceivably harbor biological remains. “We’re looking for signs of habitability—liquid water and the raw materials of life,” says Mark Sephton of Imperial College London, a member of the rover’s sampling team.
Perseverance collects most of its samples using a small drill, producing chalk-stick-sized specimens that each fit within cigarlike tubes measuring less than 15 centimeters long. Of the 43 sample tubes, 38 are slated for samples from the surface, with the remaining five being “witness tubes” to catch whiffs of Martian air and check for any contaminating gases that might vent from the rover. Collected in September 2021, the rover’s first sample is thought to be igneous rock from an ancient lava flow. Studying this material should allow scientists to date the crater more precisely. Since then, the rover has filled nearly half of its remaining tubes as it journeys several kilometers further up the ancient river’s channels toward Jezero Crater’s rim.
As a contingency, 10 of the samples are duplicates, each paired with another sample taken from the same location. These are the Perseverance tubes dropped on the surface as backup for potential future retrieval. In December 2022, in one of the last decisions he’d make at the space agency before reentering the private sector, NASA’s then science chief Thomas Zurbuchen made the call to drop that cache at a location called Three Forks. The surface drop-off was completed at the end of January, around the same time Perseverance officially began the “extended” phase of its mission—and after the science team agreed that those 10 samples alone could answer the question of past habitability if needed. MSR’s optimal plan calls for the rover to carry its remaining tubes to a yet-to-be-built lander slated to touch down in Jezero’s vicinity around 2030. Once the lander has secured those samples, it will launch them on a rocket back to Earth .
“We want the ones on the rover to come back,” Zurbuchen says. “But even the ones on the surface check all the boxes.” That includes igneous rock to date the crater and sedimentary rock and clays that may contain biosignatures, perhaps even fossilized evidence of microbial life. “They’re already worth the $10-billion investment,” Zurbuchen says, citing the MSR program’s estimated total cost. Some of the most promising samples are from a location called Wildcat Ridge, a meter-wide rock that contains evidence of sulfates. “Those are the ones we’re most excited about in terms of potential biosignatures,” says Kathleen Benison of West Virginia University, who is part of Perseverance’s sampling team. “Sulfate minerals can grow from groundwater. On Earth, those kinds of waters tend to have a lot of microbial life,” which can be entombed and preserved in sulfate minerals.
Besides sulfates, life-seeking scientists are particularly eager to grab samples from mudstones—fine-grained sedimentary rocks that the Curiosity rover has seen in Gale Crater but that Perseverance has not yet spotted. “Microbe cells are tiny,” says Tanja Bosak of the Massachusetts Institute of Technology, who is also part of the sampling team. “The mineral grain size should be even finer to preserve the [fossil] shape instead of destroying it. If you roll a boulder over a person, you will smush that person into something unrecognizable. For a microbe, everything is a boulder—unless you’re talking about mudstones.” The team members are also keen to sample carbonates, similar to things like chalk and limestone on Earth, which could preserve biosignatures as well. “If there had been microbial life in the lake, [the carbonates] could have trapped microbial matter in it,” says Sanjeev Gupta of Imperial College London, who is one of the “long-term planners” who plot out the rover’s path. On March 30, Perseverance collected its first carbonate sample, from a rock named “Berea” thought to have formed from material washed into Jezero by the ancient river.
While Perseverance has been hard at work collecting samples on Mars, the return phase of the mission remains in flux. Originally, NASA had planned for a European-built “fetch” rover to land on Mars around 2030, collect the samples from Perseverance and return them to a capsule on the lander for launch. Once in orbit, the sample capsule would rendezvous with a European orbiter, which would ferry the samples back to Earth for a landing in 2033. These plans were complicated, however, by Russia’s invasion of Ukraine in 2022. In response to Russia’s aggression, European Space Agency (ESA) officials chose to step back from a partnership with the nation on another long-simmering Mars mission, the Rosalind Franklin ExoMars rover. Russia had been due to provide the rover’s nuclear power source, as well as the launch vehicle and landing platform. NASA has now agreed to supply such missing pieces and has sought funding to do so in its budgetary request to Congress last month. But this unanticipated assistance comes at the cost of the fetch rover. “We couldn’t do both,” Zurbuchen says. “We could not individually land the fetch rover and do ExoMars.”
The ExoMars mission, most everyone agrees, is eminently worth saving. The Rosalind Franklin rover will carry a drill that can augur two meters beneath the Martian surface, accessing a subterranean habitat for past and present life that is considerably less hostile than the surface. “Nobody has ever done that on Mars,” Zurbuchen says. “Our science community thinks it’s really important.”
Jorge Vago, ESA’s ExoMars project scientist in the Netherlands, was glad that NASA stepped in. To hit a target launch date of 2028, set forth by European member states in order to save the mission, “we need the American contributions,” Vago says. “It’s an amazing mission. If we find super interesting stuff that’s suggestive of a possible biological origin, I would expect we may want to have another sample return mission and bring back samples from the subsurface.”
NASA’s current MSR plan faces its own challenges. In a mid-March town hall hosted by NASA’s Science Mission Directorate, Jeff Gramling, MSR program director at NASA’s Goddard Space Flight Center, said that some aspects of the mission may need to be “descoped.” This would be a preventive measure to keep budgets under control. NASA’s annual request of nearly $1 billion for MSR is expected to grow in the next few years, raising fears that unchecked increases could force the space agency to siphon funds from unrelated missions. Descoping options include removing one of two “Marscopters” planned for MSR, which had been included to build on the wildly successful Ingenuity rotorcraft that is now approaching 50 flights on Mars. Among other tasks, MSR’s helicopters were added as a backup option for collecting the 10-tube sample cache at Three Forks. “The mission remains complex,” Gramling said during the town hall. “We’re working on our earliest possible launch date.”
Despite the overwhelmingly intricate logistics of seeking life on Mars, the scientific riches on offer have lost none of their luster. Perseverance’s returned samples will cumulatively be only about half a kilogram, but the weight of their implications is immeasurable. Will they reveal that a second genesis of life in the universe has unfolded on the surface of Mars? For that matter, will Rosalind Franklin, once it arrives, validate the long-held suspicion that Mars’s subsurface was—or still is—habitable, too? In our winding quest to determine if we are alone in the universe, the answer may be practically within our grasp, merely waiting for us to reach out to claim it. “We won’t know until we get the samples back,” Bosak says.