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Today marks the Northern Hemisphere’s winter solstice. Millions of miles away from Earth, Mars’s northern hemisphere is already well into its winter.
On the eve of the Martian solstice this October, scientists from around the world began to gather in Glendale, California, for the fourth and final Mars 2020 Landing Site Workshop. Over the following three days, they discussed the merits and potential scientific opportunities of four candidate landing sites—Columbia Hills, Northeast Syrtis, Midway, and Jezero crater—for the next Mars rover, Mars 2020, which is slated to launch from Earth in July or August of 2020 and will land in February 2021. The latter three were candidates for the Mars Science Laboratory Curiosity Rover, which landed in landed in 2012 at a different site, Gale Crater.
At the workshop, scientists presented their analyses and opinions of certain sites and engaged in lively debates with workshop participants both in person and online. At the end, 158 scientists in attendance (full disclosure: I was one of them) voted on the four sites, ranking each in accordance to mission criteria. The rankings would help inform the final decision, to be announced before the year’s end.
Around lunchtime on November 19th, a month into the Martian winter, a NASA teleconference on the Mars 2020 rover’s landing site was set to begin. Tim Goudge, a planetary scientist and postdoctoral fellow at University of Texas at Austin, had spent the last four years becoming one of Jezero crater’s most well-known advocates in the Mars scientific community. When it came time to listen to the announcement, Goudge opted to lay low, rather than congregate with everyone else in his department who was listening.
“I decided I’d rather just sit in my office and listen to the news and try and soak it all in,” Goudge says. When he left the workshop in Glendale a month earlier, he wasn’t sure which way the final decision would go: “I was pretty much 50-50 on whether or not it was going to be Jezero.”
When the teleconference began, the decision was announced immediately: Jezero crater would be the landing site for the Mars 2020 rover.
The 28-mile-wide crater is located on the western margin of the Isidis Planitia region in the northern hemisphere of the Red Planet and once hosted a lake nearly 820 feet deep, fed by a nearby ancient river delta. The resulting Jezero catchment today may contain a rich archive of the Martian past, including the clay minerals and some of the oldest crustal materials on the surface of Mars.
“Those materials have been picked up and moved and collected all in one place at this deltaic deposit,” Goudge says, making Jezero a convenient location to study a diverse set of mineral deposits from around the site. Carbonate minerals at the site could also help track the evolution of the Martian atmosphere over the planet’s history
For the astrobiologically-inclined, Jezero crater is a tantalizing place to search for evidence of ancient life on Mars. “I think that has to be the hands-down most exciting thing that this site has to offer,” Goudge says. In fact, finding out whether Mars hosted life and whether it was ever habitable in the first place is one of the key goals of the new rover’s mission.
“Delta environments on Earth are very good at collecting and concentrating organic matter,” Goudge says. “Going to look at those parts of the outcrop to see if there’s any indication of biosignatures from potential ancient Martian biosphere is really, really exciting, and will be an incredible exploration feat.” Earlier this year, the Sample Analysis on Mars instrument, part of the Curiosity Rover, detected three-billion-year old organic molecules in Martian mudstones. But their origins—whether they’re derived from living things or not—can’t be determined using Curiosity’s instrumentation.
That’s where the Mars 2020 rover comes in. While some of the science analyzing the deposits at Jezero will be done on site, other analyses will occur on Earth. Tiny cores will be drilled from the Martian surface and cached for return to Earth through a later mission. Kennda Lynch, a postdoctoral researcher and astrobiologist at the Georgia Institute of Technology, studies the geobiology of saline-rich paleolake sediments on Earth as analogs for ancient lake systems on Mars.
"There’s no one smoking gun test that tells us that something’s biogenic,” or produced by living organisms, Lynch says. “That’s exactly the reason why we need to bring samples back, because we can’t do all the tests that we need to do [on Mars].”
Analyses of Martian samples in labs on Earth would not only be more detailed and complex, but could also help refine our estimates of the ages of geologic features on planetary surfaces. Currently, the ages of planetary surface features throughout our solar system are determined by a method called crater counting, which assumes that planets begin without impact craters and accumulate them over time. By estimating the rate at which the craters accumulated, scientists calculate ages of features, which are currently calibrated using ages of samples from Luna- and Apollo-era Moon missions. Bringing back samples from Mars could further that calibration.
“It will actually help calibrate our understanding of cratering rates for all the planetary bodies in our solar system,” Goudge said.
Deposits at Jezero also “preserve critical information that can help us reconstruct paleoclimate,” Lynch says—and they could give scientists a glimpse of how climate changed over time on Mars.
“I think we’ve chosen a really great site,” Lynch says. “I’m excited for all the work that we’re going to be able to do in the next few years to try to choose the best samples and best data to bring back to Earth to try to understand life on Mars.”