When NASA’s Perseverance rover lands on Mars this week, it’ll begin one of the most ambitious scientific endeavors imaginable: Looking for evidence that life once evolved on an alien world. Scientists are pretty certain that there’s nothing living on Mars now, but they think there could have been at one point in the planet’s history – and the rover is visiting a site called the Jezero Crater to learn more.
You may have heard that Perseverance is searching for signs of ancient life, and you might even have heard that it’s heading to Jezero because that’s a prime target in that search.
But why are scientists so interested in going to this one particular location? How do you guess where life might have evolved millions or billions of years ago, on an alien planet? What makes Jezero so special?
We spoke to an expert in Mars geology, Katie Stack Morgan of NASA’s Jet Propulsion Laboratory, to find out.
The headline draw of the Jezero Crater is the nearby delta deposit. Millions of years ago, Mars had plentiful liquid water on its surface, and the landscape was dotted with rivers and valleys. That meant that craters like Jezero filled up with water, and when water flowed into the crater from a river, it formed a delta comparable to the Mississippi Delta on Earth.
Deltas are incredible targets for searching for signs of life, both because they provide a comfortable environment for life to emerge and because they concentrate organic matter in a way that makes it easier to detect.
However, like basically every aspect of Mars exploration, it’s not quite as simple as finding a structure that looks like a delta and hunting through that. That’s because it’s hard to tell the history of water on a planet that is now so dry.
Perseverance aims to land right in front of this delta to begin searching for signs of life.
When looking at indicators that there was once water, “the question that we have is: Was that water there for a long time?” Stack Morgan explained. In order to create conditions conducive to the emergence of life as we understand it, the best conditions would be warm, shallow water that stays for thousands of years or longer. A brief flood of water that quickly evaporates won’t cut it.
Stack Morgan compared this situation to a location in her state, Death Valley in California. It’s mostly dry there, but it does occasionally rain – and when it does, the water sits in pools for a few days and can form structures called alluvial fans before it evaporates.
After all the water has evaporated away, alluvial fan deposits look a lot like delta deposits. But they are formed by periods of water on the surface that are too brief to support the emergence of life. So that’s the big question: When we see these fan shapes on Mars, are they deltas or alluvial fans?
This is where Jezero’s secret weapon comes in. The crater has an outlet valley, a deep canyon carved by water that flowed out of the crater once it had filled up and overflowed. Because of the presence of this outlet valley, researchers can be confident that there wasn’t just a little bit of water in Jezero: There was enough for the crater to fill up and for the excess water to carve its way out over a long period of time.
“That’s what makes Jezero so exciting for us,” Stack Morgan said. “Because in addition to having what we think is a delta, we also have incontrovertible evidence that there was a lake there, because we have the outlet valley.”
That outlet valley is a special rarity. There are plenty of other craters on Mars with what seem to be deltas, like the Gale Crater where the Curiosity rover is exploring, but they don’t have outlets. As a result, researchers can never be entirely sure that what they’re seeing is really indication of water being present for a long time.
By contrast, in Jezero, researchers can be confident that the crater filled up with water and overflowed, and that it had water for what is referred to as a geologically significant period of time. When it came to the task of picking a landing spot for Perseverance, “that added certainty is what helped put Jezero over the finish line,” Stack Morgan said.
Perseverance aims to land right in front of this delta to begin searching for signs of life.
If Perseverance does find evidence there was once life on Mars, it is unlikely to look like a complete fossil of a complex organism like a trilobite. Life on the planet may never have progressed that far into its evolution. Instead, evidence of life would most likely take the form of a fossilized sheet of bacteria called a microbial mat.
“Microbes are capable of leaving behind signs that are larger than microscopic,” Stack Morgan explained. “That’s one of the great things about them.”
We’ve found comparable fossilized microbial mats on Earth, which form distinctive shapes in rocks in layers between sediment. There are other, nonorganic ways that these shapes can form, so it’s not easy to tell if a given shape was formed specifically by life. But by looking at clues like the thickness of various layers above and below the potential mat and whether these conform to what would be expected from the physical conditions, geologists can infer whether they were likely created by life forms.
“If we were to find with Perseverance a compelling candidate for a fossilized microbial mat, with organics alternating in different layers, with minerals like silica or iron, minerals that we know microbes like to use in their life processes and metabolism, and we saw that alternating in a way that was not otherwise expected, then I would be happy,” she said, before correcting herself. “Not just happy, that would be the understatement of the century! I would feel like we had found a sign of ancient life on Mars.”
The delta isn’t the only place that Perseverance will be hunting of life. Another feature nearby to Perseverance’s landing spot are deposits of carbonate minerals that have been identified from orbit. These salts form from reactions of carbon dioxide in the atmosphere and water on the surface.
“We have places on Earth where this happens, like the Bahamas,” Stack Morgan explained. “When you think about the Bahamas, it’s warm, shallow waters teeming with reef life. And while we don’t know that there were reefs on Mars, there’s a similar interest in carbonates as an astrobiology target because if carbonates form in water, that is conclusive to supporting life.” The presence of carbonates suggests the water that was in the Jezero Crater was not too acidic, and could have been a comfortable environment for life to flourish.
Not only that, but carbonates are also excellent at preserving signs of life. So hunting through these deposits is a great place to look for ancient life, but there’s another geological question on the line as well. The martian atmosphere is composed primarily of carbon dioxide and used to be thicker than it is today, and we know at one time there was plentiful liquid water on the surface. But deposits of carbonate on the surface are rare. “So there’s been this question of where are all the carbonates?” Stack Morgan said. “If we once had this thicker, CO2-rich atmosphere, there’s this missing carbonate question.”
Finding answers to that question can help us understand the history of the martian climate. “We study carbonates here on Earth to find out things like: Was it warm or cold in the Proterozoic, 3 billion years ago? Carbonates are really good at preserving climate signals,” she said. “So carbonates are really exciting to us, both from an astrobiology perspective and their connection to life, but also as recorders of the evolution of ancient climate on Mars.”
Finding evidence of ancient life on another planet would be an extraordinary scientific achievement, but there’s more Jezero can tell researchers. One enduring mystery about Mars is exactly how old its rock formations are, and exactly when various events in its geological history – like the period in which there was water on its surface – actually happened.
To try to understand the geological history of Mars, geologists look at craters like Jezero, which are formed by impact events, and try to model how old the impacts are likely to have been, based on impact craters we’ve observed on other places like the moon.
“We are able to date them in a relative sense using crater chronology from the moon and samples we brought back from Apollo,” Stack Morgan said, “but that’s an extrapolated thing that we’ve applied to Mars. There are a lot of questions about when did things actually happen on Mars”
To answer these questions, geologists are desperate to get their hands on a sample of volcanic rock. This is formed when molten lava hardens into a solid rock, and it’s invaluable for dating because they can read when this transition from lava to rock happened. That can give an accurate date for events like the two impacts that created the crater.
Jezero has these volcanic rocks right near to the river delta. So Perseverance will scoop up a sample and seal it in a tube for eventual return to Earth under the Mars Sample Return program, and geologists will be able to finally pin down a timeline of Mars history.
It’s not just the history of Mars that we can learn about, though. We might even learn about the history of the whole solar system.
Mars was very active in its early history, and it has some extremely ancient rocks still visible on its surface. We can see some of these around the rim of the Jezero Crater in enormous, house-sized deposits called megabreccia, which were launched into the air by the impact that created the crater. These rocks are thought to be in the neighborhood of four billion years old, making them not only some of the oldest rocks on Mars but potentially even older than the oldest rocks on Earth.
That’s because Earth has an active interior, with plate tectonics that recycle rocks and destroy much of the rock record. The interior of Mars, however, is tectonically inactive, so rocks there last much for much longer periods of time.
“On Mars, 50 percent of the planet is three-and-a-half billion years old or older. So there’s this extensive record of early solar system time preserved on Mars that just isn’t here on Earth,” Stack Morgan said. “Mars is a great place to go to learn about the early solar system.”
Each of the different environments have something to offer researchers: The delta for searching for ancient life, the carbonate deposits for learning about the martian climate, volcanic rocks for dating periods in Mars history, and the most ancient rocks to learn about the early solar system.
Deltas have another useful feature as well, as they are full of rocks from other locations that were carried by the river. “Deltas serve this really great purpose of bringing together rock samples from far distances, way outside the crater. In some ways, the river and the delta has done our rock collecting for us,” Stack Morgan said.
Although these rocks don’t have the context that an in-situ sample would, they allow the researchers to get a glimpse of the diversity of ancient rocks that existed in a much larger area than a rover could possibly explore.
And that’s the magic of Jezero – it has all of these targets, each of which would be invaluable on its own, all close enough to be visited by one rover.
“You combine the carbonates and the potential they have, the delta deposit and the lake deposits being a great place to look for signs of ancient life, and then you have the volcanic rocks. And this is all within traverses of the Perseverance rover,” Stack Morgan said. “You have all of these things within the reach of a single Mars mission.”
As such a special location, you might wonder why NASA hasn’t sent a rover to Jezero before – like the Curiosity rover that’s currently exploring the Gale Crater. That’s because Jezero was previously inaccessible due to unsafe landing conditions. Jezero has features like sand dunes, steep slopes, and lots of scattered rocks, which would have created a landing hazard for previous rovers.
But Perseverance is armed with a new landing system, called Terrain Relative Navigation, which uses a camera and onboard maps to identify a safe place to land even among these hazards. The landing technology has now become so sophisticated that scientists can pick the most interesting site for exploration, and engineers can say that they’re confident they can land the rover there.
Even so, a rover landing is still an intricate, massively complex operation for which everyone is keeping their fingers crossed. Stack Morgan said she was a “ball of nerves” about the landing, but is deeply excited for the rover to start its mission.
With so much potential discovery resting on the rover’s robotic shoulders, we’ll also be keeping our fingers crossed for a safe landing and a successful mission.
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