Jezero crater, the destination of the Perseverance rover, is a promising place to look for evidence of extinct Martian life.
Sixteen years ago, Caleb Fassett, then a graduate student at Brown University, spotted an intriguing hole in the ground on Mars.
Mars today is cold and dry, but it was not always that way. Here was one of the places with clear signs that liquid water flowed when the planet was warmer and wetter.
The image, taken by Nasa's Odyssey orbiter, showed a sinuous dried-up river channel leading into one side of the crater. On the other side of the crater, part of the rim has collapsed, as if it had been swept away by flowing water.
In between these two features was a large circular depression.
"The only way that could form geometrically was for it to be a lake," said Fassett, now a planetary scientist at Nasa's Marshall Space Flight Center in Huntsville, Alabama.
This one-time lake named Jezero, a crater close to 48km wide, is the next stop on Nasa's search for possibilities of life elsewhere in the solar system. On July 30, the space agency's new Mars rover, Perseverance, is scheduled to launch on a 6-1/2-month trip to the red planet, arriving at Jezero in February.
Perseverance is a near clone of Curiosity, the Mars rover that landed eight years ago and almost immediately discovered unmistakable signs of a habitable lake. But Perseverance is outfitted with different instruments designed to answer a more difficult follow-up question: Could there have been Martians living on Mars long ago?
Jezero, Mars scientists decided, is the best place to look.
Happy homes for Martians
No one expects to find the Martian equivalent of dinosaur fossils, shark teeth or seashells. If life arose on Mars, it likely resembled what existed on early Earth — single-cell microbes in oceans, lakes and rivers.
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Even on Earth, these microscopic organisms did not leave behind recognisable fossils. The evidence of this ancient life is difficult to discern and sharply debated.
Still, Mars scientists think they might be able to detect patterns in rocks that could have been the work of microbes.
Liquid water is a requirement for life, and Jezero is but one of hundreds of former lakes on Mars. The feature that drew scientists to this particular crater was where the river flowed into the lake more than 3.5 billion years ago.
Even at the modest resolution of that Odyssey photograph, Fassett saw a fan of dirt and mud that had been disgorged by the river into the crater — similar to the slope of sediments where the Mississippi Delta slides into the Gulf of Mexico.
Kennda L. Lynch, a scientist at the Lunar and Planetary Institute in Houston, said this "beautiful deltaic deposit" in Jezero could preserve hints of life from three different environments: from streams and smaller lakes upstream; from the Jezero lake itself; or in groundwater pushed to the surface from below.
"We know on Earth that those kinds of deposits preserve organics," said Lynch, who has studied partially dried-up lakes in Utah that may resemble what Jezero used to look like.
Also seen at Jezero along what appears to have been the shoreline are deposits of minerals known as carbonates, almost like bathtub rings. The carbonates could be similar to limestone on Earth, which typically forms out of seafloor sediments and is often chock-full of fossils.
Although chemical reactions not involving biology can create carbonates, "This carbonate signature could indicate some kind of microbial life," Lynch said.
The fine lake sediments could have been a happy home for tiny Martians.
At least on Earth, layers of microbes can form at the bottom of a lake, often held together by slime secreted by the organisms. If anything like that lived within the lake at Jezero, the biological molecules of the microbes would likely have decayed away by now. But as one layer formed on top of another, they could have left wavy patterns in the rocks similar to what has been found in Earth rocks.
"Now, if you look at that rock, you wouldn't know for sure that it was a potential biosignature," said Kathryn Stack Morgan, one of the mission's deputy project scientists, during a news conference in June. "But when you couple the textures, as well as the chemical composition, the mineralogy and the distribution of organic carbon, you can start to build a case that that rock could only have formed under the influence of life."
A camera and a microscope on the rover will be able to see such patterns. Another instrument, shooting a beam of X-rays into the rock, could measure the elements within each layer and help determine if the layers consist of different minerals or just surface smudges.
Part of Perseverance's mission is to collect pieces of rocks that a follow-up spacecraft will bring back to Earth. Then scientists will be able to peer at the samples in exquisite detail for signs of past life.
Tournament of holes
Six years ago, Mars scientists began debates over where to send the rover, starting with more than 30 candidates. For each, scientists presented their arguments in favor like lawyers laying out a legal case.
Other candidates had names like Eberswalde (another dried-up lake bottom with a preserved delta), Holden (an old impact crater that was a lake the size of Lake Huron) and Mawrth Vallis (a mysterious valley that was likely wet but with no signs of where its water came from).
Timothy A. Goudge, who started as a planetary sciences graduate student a few years after Fassett, took the mantle as the champion for Jezero during the landing site workshops, which were like a Mars Madness tournament of holes.
"Definitely much more intense and high-energy than a typical scientific conference," he said.
Now a geosciences professor at the University of Texas, Austin, Goudge studied whether it would be possible to identify minerals in the delta sediments and figure out where they had originated upstream. He said this kind of "source-to-sink analysis is really common for understanding systems on Earth."
If that was possible, then he could investigate whether the minerals underwent chemical changes along the way. Or were they the pieces of rock that simply chipped off the outcrops and washed into crater pretty much unchanged?
Fassett had identified two deltas flowing into Jezero: a large prominent one to the west and a smaller, more eroded feature on the northern rim. Goudge found that the two deltas contained different minerals, reflecting different origins of the sediments.
The presence of clays and carbonates, minerals that typically form with the interaction of carbon dioxide and liquid water, strongly suggested that this was a freshwater lake that was not highly alkaline or highly acidic: a location that could have been friendly for life.
Bethany L. Ehlmann, a professor at the California Institute of Technology, initially put forth Jezero as a possible landing site. But she then suggested that Goudge present the science at a workshop reviewing the candidate sites.
"She was like, 'I'm presenting a bunch of other stuff. So why don't you? You've been doing the most recent work on this,'" Goudge recalled. "And so I said yes."
In 2017, Perseverance mission scientists narrowed the possible landing sites to three. Jezero made the cut.
"I felt very proud," Goudge said. "Personal satisfaction of knowing that the science I'm doing is interesting and that people think the site is interesting."
The other two were the Columbia Hills region of Gusev Crater, previously explored by Nasa's Spirit rover, and Northeast Syrtis, an ancient volcanic site that may have possessed hot springs and melted snow, a potentially friendly environment for some microbes.
The science team later added a fourth finalist, which was given the unofficial name of Midway because it was roughly midway between Jezero and Northeast Syrtis. Midway possesses geology similar to that of Northeast Syrtis, but it is close enough to Jezero that Perseverance, if the rover lasts long enough, could explore both — a twofer science jackpot.
After a final workshop in October 2018, Nasa made its decision a month later.
Goudge, who did not receive advance notice of the winner, said he shut the door to his University of Texas office, sat down to listen to the announcement on his computer and "tried to not be too jittery as I was watching."
Thomas Zurbuchen, Nasa's associate administrator for science, did not drag out the suspense. "Hey everybody," he said, kicking off the news conference. "I selected Jezero crater as the landing site for Nasa's Mars 2020 rover mission."
Goudge said he did not yell or jump in joy
"I probably like slumped in my chair, mostly," Goudge said.
A deluge of congratulatory emails and text messages started flooding in.
Practice for perseverance
In February, as part of preparations for the mission, scientists practised how they would use Perseverance to search for signs of life on Mars. Of course, the rover and its instruments were not on Mars and not available; they were being packed up for their interplanetary journey.
Instead, Walker Lake, Nevada, which partially dried up tens of thousands ago, served as a stand-in for Jezero crater. Over two weeks, a team of seven people played the part of the rover to gather photographs and readings.
Each day, a team of 150 scientists — some at Nasa's Jet Propulsion Laboratory in California, which will operate Perseverance, others scattered around the world — reviewed the previous day's images and data and came up with a new set of science observations.
If the side of a cliff looked intriguing, the scientists might decide they wanted a closer look. Parts of Walker Lake are studded with stromatolites — bulbous structures that contain wavy patterns left behind by microbial mats.
The off-site scientists sent instructions to Nevada, where the team of seven set about to mimic the actions that the rover would undertake.
That included pressing a hand-held X-ray tool up to rock to generate data similar to an instrument on Perseverance, and rolling around a stroller-like contraption that held a ground-penetrating radar.
"Our job is to treat this like a movie set," said Raymond Francis, who led the team at Walker Lake. "And don't get anything in front of the camera that shouldn't be in front of the camera. One of our most important tools was a broom we bought at the dollar store that we used to erase footprints."
Francis said the remote teams of scientists did well identifying rocks that deserved a closer look, including a feature — little black specks in several layers of sediment — that he did not expect them to notice. The distant researchers asked for measurements of the composition. It was unusual — high in phosphorous.
The black specks were old fish bones.
"So yeah, they didn't miss much," Francis said. "If someone finds fish bones or seashells in Jezero crater, you know that's going to bring the mission to a screeching halt."
What's in a name?
One of the mysteries of Jezero has nothing to do with Mars. No one seems to remember who picked the name Jezero.
Ralph P. Harvey, a professor of geological sciences at Case Western Reserve University, was among those who first pushed for the assignment of an official name when he proposed the crater as a potential landing site for the earlier Curiosity rover.
"I got tired of calling it 'that crater in Nili Fossae,'" he said, referring to the wider fractured region surrounding the crater.
He turned to the International Astronomical Union, which has conventions for naming Martian craters. Those that are up to about 50 kilometres in diameter, are named after small towns with populations of 100,000 or less. That provides a large pool of potential names that are generally not controversial. And while a small town that shares its name with a crater may consider it an honor, the union does not intend the designation as a commemoration.
Harvey's suggestions were Kennan after a town in Wisconsin, and Novelty for one in Ohio.
Fassett suggested Tida, a town in Egypt, as a water-related pun.
Rita M. Schulz, who chairs the union's working group for planetary system nomenclature, said her records indicated that the original proposed name was Stolac, a small Bosnian town, but that "was not regarded as a safe choice," because of the destruction it suffered during the war that ravaged the country in the 1990s.
The records did not preserve who had offered the suggestion of Stolac. But Schulz said the replacement name of Jezero, another Bosnian town, must have come from Bradford A. Smith, a planetary scientist who was then chairman of a group that helped with assigning geographical names on Mars. Smith died in 2018.
Neither Fassett nor Goudge are among the 375 or so members of the Perseverance science team.
Fassett has largely moved on to other places in the solar system, the Earth's moon in particular, and Goudge is at a relatively early stage in his planetary science career. But both have submitted proposals to work with the Perseverance mission as what Nasa calls participating scientists. Neither has yet heard whether they have been accepted.
Although Fassett has been excited by all of Nasa's Mars missions, he closely follows Perseverance, and Jezero will be special.
"All of them are great," he said, "but my emotional connection to this particular site is pretty unusual, right?"
Written by: Kenneth Chang
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