Perseverance will explore history of ancient lake


Last week, NASA's $2.7 billion Perseverance rover made a picture-perfect landing on the floor of Mars's Jezero crater, which scientists believe was filled to the brim with water 3.8 billion years ago. Two kilometers away looms the rover's primary target: a fossilized river delta, created as muddy water spilled into the crater—ideal for preserving signs of life. But before Perseverance starts the long climb up into the delta, to drill samples that will eventually be returned to Earth, it will examine the rocks beneath its six aluminum wheels. The rover landed near outcrops of rock layers that may have originally been laid down before and after the lake and the delta. The NASA team will probe them for clues to the nature and timing of the brief period when water flowed—and life might have flourished. Even the first images returned to Earth, grainy and taken from the underneath the rover, left the team elated, says Katie Stack Morgan, the mission's deputy project scientist at NASA's Jet Propulsion Laboratory (JPL). “We have enough for the scientists to really sink their teeth into.” The rover's arrival at Mars was filled with nail-biting drama, even as the precise, autonomous descent unfolded like clockwork. After the spacecraft plunged by parachute through the thin air, a rocket-propelled hovercraft took over, seeking a boulder-free spot before lowering the rover from nylon cords. The final moments, captured in breathtaking detail by cameras below the hovercraft, show the rover landing in a cloud of dust. “We did have a pretty clean run,” says Allen Chen, head of the rover's landing team at JPL, in a dry understatement. “It did what it had to do.” The touchdown marks NASA's ninth successful landing on the martian surface out of 10 tries. ![Figure][1] GRAPHIC: C. BICKEL/ SCIENCE After 3 days, the rover had executed 5000 commands and scientific instruments were certifying their health, says Jessica Samuels, an engineer and mission manager at JPL. “Everything is coming back exactly how we want it to.” The rover raised its camera mast 2 meters above the surface to capture a panorama of its surroundings. After several days updating software, the team plans to wiggle the rover's wheels and conduct a short test drive. The rover will also extend its five-jointed, 2-meter-long robotic arm, which carries the rover's coring drill and several more cameras, and put it through some calisthenics. A second robotic arm, designed to manipulate a cache of dust and rock samples inside the rover, will be run through its paces. Stored in 43 ultraclean tubes, those samples represent the start of a multibillion-dollar, multinational effort to collect martian rocks and return them for analysis on Earth; two follow-up missions to retrieve the samples are planned for later this decade ( Science , 22 November 2019, p. [932][2]). Within its first 2 years, the rover is expected to fill nearly half the tubes on its trek of more than 10 kilometers to the crater's rim. The rest will be filled in an extended mission, as the rover trundles beyond the crater to ancient highlands thought to have once held geothermal springs. Perseverance's primary mission is to search for evidence of past life, captured in the delta mudstones and other rocks likely to preserve organic molecules—or even fossilized life. But interpreting this evidence will also require a better understanding of Mars's climatic past, from clues that can be collected right away by the rover. The first opportunity to drill a sample could come within a few months, on the flat, pebble-strewn terrain where Perseverance landed. Some scientists believe these rocks are from an ancient lava flow that erupted long after the lake disappeared, arguing that they look the way Hawaiian flows might if bombarded by meteorites and whipped by winds for several billion years. But when Perseverance's predecessor, the Curiosity rover, explored similar rocks in Gale crater and its ancient lake, most of what scientists had thought were lava fields turned out to be sedimentary rocks: ground up volcanic bits ferried by water and deposited in layers, presumably in the vanished lake. The early pictures from Perseverance are difficult to interpret: Rocks riddled with holes could be pumice, porous from gas escaping from cooling lava, or they could be sedimentary rocks, perforated over time by water. Bigger boulders in the distance look like ancient volcanic rocks: dark and coated by a light-colored dust. Fortunately, Perseverance's scientific instruments are designed to pin down the rocks' origin. Cameras on the mast could spy distinctive angular striped layers, called cross-bedding, that only form when deposited as sediments. A camera mounted on the end of the rover's robotic arm for microscopic views could capture the grain of minerals: Sedimentary rocks, for example, are typically rounded by their watery travels. Two other instruments on the arm will fire x-rays and ultraviolet laser light at rock samples, provoking reactions that could reveal chemical fingerprints of volcanic or sedimentary rocks. It's a crucial distinction. If the rocks are volcanic—either lava deposits or, more likely, ash from a distant eruption—they'll contain trace radioactive elements that decay at a certain rate, so when samples are returned to Earth, lab scientists could date the eruption and put a bound on the age of the lake. Any date will also help pin down the highly uncertain overall martian timeline, currently dated by counting the number of craters on a given terrain. (Older surfaces are pocked with more craters.) Sampling such a volcanic rock would “provide a critical anchor to the timing of events we are looking at,” says Ken Farley, the mission's project scientist and a geologist at the California Institute of Technology. The rover's initial path is likely to cross another intriguing target just 250 meters away on the crater floor: outcrops that, from orbit, appear rich in both olivine, a volcanic mineral, and carbonates, which can form when olivine is exposed to water and carbon dioxide. If this layer is volcanic ash from an eruption that preceded the Jezero lake, radioactive dates from it and the potential volcanic layer deposited on the lakebed should bracket the lake's existence in time. Moreover, isotopes of oxygen in the carbonates could reveal the temperature of the water that formed the mineral; balmy water would suggest Mars was once warm and wet for millions of years at a time, whereas water near freezing would argue for sporadic bursts of warmth. The carbonate might even contain gas bubbles—samples of the ancient martian atmosphere, which could allow scientists to see whether it held methane or other greenhouse gases that would have warmed early Mars. “That obviously would be game changing,” says Timothy Goudge, a planetary scientist at the University of Texas, Austin, who led the team that made the case for Jezero as a landing site. There will be no drilling at the landing site itself. But there will be flying. After the monthlong commissioning phase is over, the team will find a nearby, flat spot to loose the 1.8-kilogram Ingenuity helicopter, which survived the landing attached to the rover's belly. With a fuselage the size of a tissue box, Ingenuity is a technology demonstration, a bid to fly a rotor-powered vehicle on another planet for the first time. After being dropped to the surface, the helicopter will furiously spin its rotors to ascend 3 meters in the air for 20 seconds. Four additional, higher flights could follow, over a total of 30 days, says MiMi Aung, Ingenuity's project manager at JPL. On later flights the helicopter could collect reconnaissance images for terrain off the rover's main path. “It will be truly a Wright brothers moment,” Aung says, “but on another planet.” [1]: pending:yes [2]:

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