In 2018, NASA decided that the landing site for its Mars 2020 Perseverance rover would be the Jezero Crater. At the time, NASA said the Jezero Crater was one of the “oldest and most scientifically interesting landscapes Mars has to offer.” That assessment hasn’t changed; in fact it’s gotten stronger.
A new research paper says that the Jezero Crater was formed over time periods long enough to promote both habitability, and the preservation of evidence.
The Jezero Crater is a dried up paleo-lakebed, with a preserved river delta and sediments. It contains at least five different rock types that can be sampled. The crater also holds geological features that are approximately 3.6 billion years old. It’s an excellent feature to study, and hopefully to collect samples from for eventual return to Earth. Scientists are hopeful that the Perseverance Rover may find fossilized evidence of early, single-celled life.
A new study based on the analysis of satellite imagery reinforces Jezero’s scientific desirability.
The study is titled “The Pace of Fluvial Meanders on Mars and Implications for the Western Delta Deposits of Jezero Crater.” It’s published in the journal AGU Advances. The lead author is Mathieu Lapôtre, an assistant professor of geological sciences at Stanford’s School of Earth, Energy & Environmental Sciences. The other author is Alessandro Ielpi from Laurentian University.
One of science’s main roadblocks to understanding Mars’ history is timing. With telescopes, orbiters, landers, and rovers, we’ve learned a lot about Mars. Over the past couple decades especially, scientists have uncovered compelling evidence showing that Mars was once warm, wet, and habitable. But questions of timing remain.
There’s ample evidence of ancient river-beds on Mars, and some of the timing questions revolve around those rivers. How long did rivers flow on Mars, and how often? How long ago? How long did it take form deltas like the one in Jezero Crater? Mars was likely habitable at the same time that life was evolving on Earth, and understanding the age of Mars’ ancient rivers, and how long they lasted, is one key to understanding habitability.
In their paper the authors write, “Here we develop a new model to calculate the pace of shifting Martian rivers, which, when applied to orbital observations of the Jezero delta, allows us to determine a minimum duration for delta formation.” Combined with other modelling and the work of other scientists, the pair of authors say that “…our results suggest that the delta took a few decades to form over a total timespan of, most likely, hundreds of thousands of years.”
During that hundreds of thousands of years, there were many dry, arid periods. They say that the river that flowed into the Jezero Crater likely flowed for only one day every 15 to 30 years; maybe a little more often. On Earth, sediments preserve organic molecules, and the same is likely true on Mars. So if the sediments at Jezero were buried quickly, there’s a strong possibility that organic molecules are preserved there, as well.
“There probably was water for a significant duration on Mars and that environment was most certainly habitable, even if it may have been arid,” said lead author Mathieu Lapôtre in a press release. “We showed that sediments were deposited rapidly and that if there were organics, they would have been buried rapidly, which means that they would likely have been preserved and protected.”
That paper showed that single-threaded sinuous rivers without plants stabilizing their banks drift sideways ten times faster than the same type of rivers with banks stabilized by plants. That sideways movement of river channels is called meander migration.
The tendency of rivers to meander migrate has been studied for a long time. The authors say in their 2019 paper that river meander is “among the most unequivocal indicators of hydrologically mature planets.”
Based on the likely fact that Martian rivers did not have plants to stabilize their banks, and accounting for the gravity on Mars, the pair of researchers say that the Jezero delta took at least 20 to 40 years to form, but that length of time was intermittent, and spread out over about 400,000 years.
And that brings us back to the time problem again.
“This is useful because one of the big unknowns on Mars is time,” Lapôtre said. “By finding a way to calculate rate for the process, we can start gaining that dimension of time.”
On Earth, single-threaded meandering rivers are most often found with vegetation on their banks. Only recently were these types of rivers detected without plants, and prior to that, scientists thought that before plants appeared on Earth, only braided, multi-threaded rivers existed. But now scientists have found many single-threaded rivers without vegetated banks.
“This specifically hadn’t been done before because single-threaded rivers without plants were not really on anyone’s radar,” Lapôtre said. “It also has cool implications for how rivers might have worked on Earth before there were plants.”
All rivers can go through drier spells, and it’s the wet spells that created sediment build up in deltas. The researchers think that on Mars, the dry spells were 20 times more frequent than on Earth today. “People have been thinking more and more about the fact that flows on Mars probably were not continuous and that there have been times when you had flows and other times when you had dry spells,” Lapôtre said. “This is a novel way of putting quantitative constraints on how frequently flows probably happened on Mars.”
If there was life at Jezero Crater, most scientists seem to think that it never evolved much, and was restricted to single-celled organisms. With this new understanding of how the sediment deposits in Jezero Crater were formed, and how it likely preserved evidence of life, it makes the Perseverance Rover mission even more exciting to look forward to.
Life on Earth began about 3.5 billion years ago, at about the same time that Jezero Crater was formed. Any life on Earth would have been single-celled when the crater was formed. If single-celled life was present at Jezero long before multi-cellular life evolved on Earth, then something stalled Martian life, depleting the atmosphere and sterilizing the planet.
Since Earth is such a geologically active planet compared to Mars, a lot of ancient evidence for life has been erased. But that never happened on Mars. In that sense the Jezero Crater may be a kind of time capsule, waiting to be opened by NASA’s Perseverance Rover.
It’s possible, that we might finally, unequivocally, have evidence for past life on Mars.
“Being able to use another planet as a lab experiment for how life could have started somewhere else or where there’s a better record of how life started in the first place – that could actually teach us a lot about what life is,” Lapôtre said. “These will be the first samples that we’ve seen as a rock on Mars and then brought back to Earth, so it’s pretty exciting.”
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