NASA’s newest Mars rover, Perseverance, is going back in time to the bottom of a vanished lake. If all goes well, in February 2021 it will land in Jezero crater and pop the dust covers off its camera lenses. Towering in front of it, in all likelihood, will be a 60-meter cliff of mudstone: the edge of a fossilized river delta. These lithified martian sediments could hold answers to urgent questions about the earliest days of Earth’s chilly, parched neighbor: How did this pintsize planet, so distant from a faint young Sun, support liquid water on its surface? How much water was there, and how long did it persist? And did Mars ever spawn life?
The 45-kilometer-wide crater is an intriguing target. Billions of years ago, when life was just beginning on Earth, water broke through its western rim and spilled into its interior, carrying sediments that settled and piled up in thick, meandering braids that today can be seen from space, as plain as day. “It’s kind of like the Mississippi delta, but smaller,” says Raymond Arvidson, a planetary geologist at Washington University in St. Louis. The water filled the crater like a bathtub until, 250 meters deep, it breached the eastern rim. And then, just as mysteriously as it arrived, the water disappeared.
Scientists have traced the tracks of ancient water across Mars ever since the 1970s, when orbiters revealed branching valley networks that matched the dendritic shape of water-eroded valleys on Earth. In the 1990s, the Mars Global Surveyor zoomed in on deeply incised gullies that could only have been carved by powerful flows of water—and may even have glimpsed shorelines from an ancient ocean. Later orbiters found evidence of abundant clay-bearing minerals that need water to form. More recently, the Curiosity rover, Perseverance’s predecessor, has charted the existence of a long-lived lake at the bottom of its adopted home, Gale crater.
Some scientists believe the water shows ancient Mars was warm for millions of years, a favorable climate for life to emerge. Others say the climate was cold and dry, punctuated by sporadic bursts of water that only lasted for hundreds or thousands of years—a much more difficult environment for life to take root. Along with the question of past life, says Ken Farley, the mission’s project scientist and a geologist at the California Institute of Technology (Caltech), Mars’s ancient climate “is the biggest unanswered question.”
Perseverance will tackle both questions, although the search for life will take longer. The rover, developed by NASA’s Jet Propulsion Laboratory (JPL) and set for launch next month from Cape Canaveral Air Force Station in Florida, is also the start of an audacious campaign that will ferry to Earth about 30 samples of martian rock and grit. Perseverance will gather the samples, and NASA and the European Space Agency (ESA) are designing two follow-up missions to retrieve them, aiming for launches in 2026.
The complex mechanisms needed to drill and store these cores limited the room on board for tools to chemically analyze samples and look for organic molecules. Until the samples reach labs on Earth, the question of whether life once existed in Jezero will probably go unanswered. “We’ll have to be patient,” says Tanja Bosak, a geobiologist at the Massachusetts Institute of Technology and member of the rover’s science team.
The story of the martian climate, on the other hand, will be etched across Jezero’s surface, visible to an array of rover instruments. Scientists can only make a rough guess at the lake’s age, but they think it formed 3.8 billion years ago, about the same time as the valley networks, over hundreds or thousands of years. Unlike Curiosity’s target, Gale crater, which offers a snapshot of a moment some 3.5 billion years ago when Mars was likely drying out, Jezero and its surroundings will grant access to more than 500 million years of martian history, including some of the planet’s oldest terrain, says Bethany Ehlmann, a Caltech planetary scientist and member of the science team. “We have the potential for a really rich history of climate.”
By design, Perseverance borrows much from Curiosity: a six-wheeled chassis the size of a small SUV, an imaging turret, a radioisotope power source. “From the outside it looks the same,” says Allen Chen, one of the rover’s lead engineers at JPL. “But it’s got it where it counts.” That includes advanced new imaging instruments, landing capabilities, and a complex drilling system—innovations that led its budget, originally pitched as a bargain at $1.5 billion, to balloon and end up matching Curiosity’s $2.7 billion price tag, which includes operations.
To analyze samples in its onboard lab, Curiosity’s drill only needed to pulverize rock. Perseverance, in contrast, must drill intact cores, each about the size of a thick piece of school chalk, and store them within titanium tubes. The system also has to keep the cores safe and clean, to prevent Earthborne microbes and molecules from being mistaken for martian ones when the cores finally arrive back on Earth. In the end, engineers dreamed up a system involving two robotic arms, nine drill bits, 43 sample tubes, and a rotating carousel. “When you look at it, you won’t think of it being simple,” says Adam Steltzner, the rover’s chief engineer at JPL, “but it was the simplest we could imagine.”
Building and testing that system nearly delayed a mission straining to meet tight deadlines. In October 2019, engineers discovered the tubes seized up inside the drill bit when tested in martian conditions. “For me it was a moment of despair,” Farley says. “How were we ever going to fix this?”
The problem, it turned out, was that the rover was too clean. The tubes had been baked at 350°C for 1 hour, which not only sterilized them, but also vaporized a hydrocarbon film. The team hadn’t realized that the film, a patina that forms on nearly any metal exposed to Earth’s atmosphere, was needed as a grease. After several stressful months, they developed a cleaning routine that limited the baking to 150°C and included a series of chemical washes. That left a small amount of the film on the outside of the tubes but no trace inside, where it might contaminate samples. “We leave nothing behind, like a good hiker,” Steltzner says.
After the issue was resolved, another mote of organic material began to threaten the mission’s launch. By February, the coronavirus pandemic had postponed the launch of ESA’s Rosalind Franklin rover, which already had parachute problems, until 2022, the next Mars launch window. Determined to hit its window, NASA shuttled a skeleton crew to and from Florida for the rover’s final inspections, while most JPL engineers did what they could from their California homes. “It’s fascinating how much of it you can do from your living room,” says Jennifer Trosper, the rover’s deputy project manager for surface operations. “We’re used to remote operation. We just had to move it back a little earlier.”
In May, with the rover already stacked on the spacecraft that would ferry it to Mars, a C-130 transport plane delivered the cleaned sample tubes, quarantined in nitrogen-filled cases, to Cape Canaveral. Engineers loaded the tubes just before a heat shield sealed the rover within its landing capsule. A last-minute arrival of the tubes was always the plan to limit contamination risks. Also, Trosper adds, “We just finished them.”
On 22 July, a 3-week launch window opens up. Seven months after an Atlas V rocket puts it on a path to Mars, the rover will plunge through the barely there martian atmosphere. Just as for Curiosity, a “sky crane” hovering on retrorockets will unspool Perseverance on a tether and lower it to the ground. But there’s an important improvement: A camera on the rover’s belly will assess the landscape as it descends and compare it to a stored map of safe landing spots. The sky crane will fire its thrusters to divert to one of these zones, enabling the rover to land far closer to its target than Curiosity did in 2012, in a nearly circular, 8-kilometer-wide landing ellipse at the delta’s edge.
From that moment it will be a player in what Nature Geoscience dubbed a “war” over Mars’s ancient climate. What Curiosity saw at Gale crater convinced some geologists that ancient Mars remained warm for millions of years. Sediments probably built up more slowly on Mars than on Earth, so the thick sediments at Gale suggested “this lake almost certainly existed for tens of millions of years, maybe longer,” says John Grotzinger, the Caltech geologist who led Curiosity’s science for its first few years.
If so, the lake would have endured climate variations driven by chaotic wobbles in the planet’s tilt, which varies from 10° to 60°. Something must have kept the planet warm while the lake shifted between tropical and arctic latitudes. “Did we land in one weirdo place on Mars? Probably not,” Grotzinger says. But what warmed the climate is a mystery, he admits. “Something is missing, and we don’t know what that is yet.”
To the opposing camp, that’s grounds for skepticism about a warm early Mars. In 1991, James Kasting, a planetary scientist at Pennsylvania State University, University Park, reported that an atmosphere of carbon dioxide (CO2) and water vapor, both greenhouse gases, was not capable of keeping the ancient planet wet and warm for millions of years. The atmosphere would have been too thin, and the early Sun too weak. Mars “must have had a phenomenal greenhouse effect,” Arvidson says, double what exists now on Earth. To this day, even with more sophisticated models, “The climatologists haven’t figured out how to do it,” he adds.
That has led these scientists to argue that martian water flowed in bursts lasting just thousands of years—brief exclamations in an eternal deep freeze. That is a Mars that climate models can simulate, says Robin Wordsworth, a planetary scientist at Harvard University. Its ancient volcanoes could have belched a lot of hydrogen, a strong but short-lived greenhouse gas. Periodic bursts of water could have rusted iron-bearing minerals, releasing more hydrogen to the air. Or asteroid strikes, more common in that era, could have released hydrogen if they hit regions rich in ice or subsurface water. “For all of them you can make episodic warming work,” Wordsworth says. “But not warm and wet.”
The rocks in Gale crater can also support this view, Ehlmann says. They lack certain minerals that should be present if they were exposed to water for 1 million years or more. Jim Bell, a planetary scientist at Arizona State University, Tempe, has concluded that ancient Mars was probably like Antarctica, icy and dry, with spurts of melt. “More Earth-like does not mean like most of the Earth.”
Perseverance will need the head start provided by a precise landing to try to settle the issue. During its 2-year primary mission, it will take advantage of upgraded wheels and autonomous navigation capabilities to briskly traverse more than 15 kilometers—a distance Curiosity took more than 4 years to cover. The rover will collect its first 20 samples for an eventual return mission from the geologically diverse terrain it will cross.
The first samples are likely to be rocks thought to come from an eruption that covered parts of the crater after the lake dried up. Volcanic rocks contain trace radioactive elements that decay at a certain rate, a clock that lab scientists on Earth can use to date the eruption, putting a lower limit on the age of the lake. Mission scientists also hope to find outcrops of older volcanic rocks that sit below the delta mudstones, marking an eruption that occurred before the water arrived. Those would provide an upper age limit, making it possible to roughly bracket the lake’s existence. “When were these habitable environments in absolute time, and how quickly did they come and go?” Ehlmann asks.
As the rover rolls along the lake bottom, a ground-penetrating radar mounted on its belly will fire, recording echoes that reveal the textures of sediment up to 10 meters below the surface. “We’ll be creating a giant ribbon of data,” says David Paige, a planetary scientist at the University of California, Los Angeles, and the instrument’s deputy principal investigator. The reflections could help determine whether the lake was open water or covered in ice. Fine mud would suggest open water; anomalously large stones would suggest ice, which could have carried them to the middle of the lake before dropping them.
From there the rover will visit the fine-grained clay-bearing mudstones of the lower parts of the delta. Here the hunt for past life will take the lead. On Earth, such clays blanket living things and preserve them as fossils. In similar clays at Gale crater, Curiosity scientists detected traces of complex organic compounds that resembled kerogen, the feedstock of oil. But they could not determine whether the compounds, detected at levels of a few dozen parts per million, were produced by ancient life, or deposited on the martian surface by meteorites, which often contain complex organic molecules.
Two instruments mounted at the end of Perseverance’s main robotic arm may help tell the difference. One will fire an ultraviolet laser at the rocks; the other will bombard them with x-rays. The radiation re-emitted by atoms in the rocks could reveal organic chemistry. Mapping any organics in a rock could also say something about their origin. A uniform signal would favor meteoritic fallout, whereas a lumpier distribution, and the presence of minerals that hint at microbe-fueling reactions, could be a sign of life—and a green light to drill a sample.
As the rover forges a path up the delta, the fine mudstone will give way to rough sandstone. The team will keep an eye out for exposures of opal-like rocks that have recently been spotted from space. Opal forms from a solution of silica and water, and on Earth the deposits are classic fossil-hunting spots. That’s because the mineral creeps into organic layers and preserves fossil structures, Bosak says. “That’s where we find the most beautifully preserved microbial mats.” The rover’s cameras will search for such structures, but Bosak doubts they will be seen—even on Earth, they are not often apparent until polished in the lab.
The sand grains, washed in by the long-lost river, could also say something about what caused Mars’s early warmth—whether steady or intermittent—to dissipate after the delta formed. Some of the sand grains, eroded from volcanic rocks, will contain radioactive isotopes that make it possible to date them. Scientists on Earth will also examine certain minerals to look for the frozen imprint of a magnetic field. Mars is believed to have had a magnetic field early in its history, generated by a molten dynamo in the planet’s interior. The field would have failed as the dynamo cooled and shut down, and some believe that explains Mars’s radical change in climate. A weakening field could have allowed charged particles from the Sun to erode the planet’s once-thick atmosphere. Water would have escaped to space, making the planet colder and drier. Magnetic signatures teased out of the sand grains could show whether the decline of the field preceded—and perhaps caused—the climate change.
After nearly 2 years of frantic drilling, the rover will climb one of the delta’s fingers to reach the shores of Jezero’s paleolake, fast against the crater’s edge. Orbiters have spotted a bathtub ring of carbonate rocks running around the crater rim in a narrow band, likely where the lake’s warm shallows were. On Earth, such deposits are known to preserve fossilized stromatolites, bumpy cauliflowerlike mounds formed by the growth of bacteria. “They are an ideal place to look for past life,” says Briony Horgan, a planetary scientist at Purdue University. That is, she says, if the deposits were formed by the lake, and not by hot water created by the crater-forming impacts.
If the lake was responsible for the carbonate deposits, they will offer a window on the ancient martian atmosphere, which supplied the CO2 that formed them. By comparing carbon isotopes from carbonates in the bathtub ring and in older rocks outside the crater, scientists could learn how levels of atmospheric CO2—and the greenhouse effect it drives—changed over this time.
The outcrops around these shallows, near the entry point of the river, could also betray something about the climate at the time the delta was laid down, says Timothy Goudge, a planetary scientist at the University of Texas, Austin. Layering in the outcrops will reveal how much water was needed to form the delta, how long it flowed, and whether it came in the brief floods or steady flows. Cracks in river bottom rocks could be wedges opened up by continuous freeze-thaw cycles—a sign of persistent frigid conditions.
The shallows will likely mark the end of the primary mission. But the rover ought to have many more years left on its odometer. Engineers want the rover, while still healthy, to drop some samples on flat, accessible terrain where a later mission can retrieve them. But it may drill some sites twice, and it will continue to collect—in case the rover itself is healthy enough to deliver samples to the Earth return mission.
After the primary mission, many on the team will be eager to escape the delta for the ancient, mysterious terrain to the west. Deposits of clay and carbonate seen there also needed water to form. If bands of water-weathered rock capped by water-deposited sediments are visible on its mesas, a temperate climate may have prevailed. Alternatively, as Ehlmann and others believe, this landscape could be what’s left of an icy subsurface that was heated by nearby giant impacts 4 billion years ago and turned into an underground hydrothermal system capable of fostering life. “That would point to an ancient Mars that was habitable, but not so warm,” she says.
Whatever answers the rover finds, it will mark the end of an era on Mars. For decades, NASA has dominated exploration of the planet’s surface, culminating in the increasingly ambitious rovers of the past 2 decades. “We’ve had the privilege and responsibility to do a systematic investigation of a planet,” says Jim Watzin, director of NASA’s Mars Exploration Program. Other nations will soon add their rovers, starting with China this year.
But Perseverance also kicks off a new era. The sample return effort it anchors “will be the first round trip to another planet by humanity,” Watzin says. Bringing Mars to Earth will enable scientists to probe the secrets of the Red Planet more deeply. If humans follow in the rover’s tracks in the coming decades, as the United States and China have vowed, the terrain they encounter may seem strange. But it will be familiar ground.