Billions of years ago, Mars was a water world with rivers and lakes. Microbes might have swum in those waters, leaving their imprints on an ancient Martian lake bed called Jezero Crater.

NASA is sending a rover there to hunt for such fossils. The $2.4-billion, SUV-sized robot, called Perseverance, is set to land in Jezero Crater on Thursday.

More than 3.5 billion years ago, rivers spilled over the edge of the 28-mile-wide crater, keeping it filled with water. This alien pond was about the size of Lake Tahoe. The rivers probably carried clay minerals into Lake Jezero, and if microbes lived in the water, they could have gotten trapped. That would mean that today, there may be distinct fossil rocks called stromatolites at the bottom of the lake bed, along what used to be the shoreline, or in the dried-up river delta.

This makes Jezero Crater one of the best places in our solar system to search for evidence of alien life.

On Earth, the oldest signs of life are 3.5-billion-year-old stromatolites found in ancient, shallow lake beds — exactly what Perseverance will look for on Mars.

jezero crater mars lake water illustration
An illustration of Jezero Crater as it may have looked billions of years go on Mars, when it was a lake.
NASA/JPL-Caltech

"This is a tantalizing similarity," Ken Farley, the project scientist for Perseverance, said in a recent press briefing. "It would, of course, be a fabulous scientific discovery to find that life existed beyond Earth."

Perseverance is designed to scour Jezero Crater, collect about 40 samples that could contain signs of ancient microbes, and cache them in special tubes so that a future mission can bring them to Earth.

NASA had considered the crater as a destination for previous missions, but its steep cliffs, sand dunes, and boulder fields make it a dangerous place for robots to land. Now, new technologies equip Perseverance to take on the treacherous terrain.

A journey from the lake bottom to the crater rim

NASA has carved out a roughly 15-mile route for Perseverance on Mars that takes advantage of the diversified landscape of Jezero Crater.

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A possible route Perseverance could take across Jezero Crater.
NASA/JPL-Caltech

"This is a pathway that connects together all of the different habitable environments that we think existed within this lake and in its surroundings," Farley said. "This is a long traverse. It'll take many years for us to do this. The reason we do this, though, is that this will allow us to come up with the best possible set of samples to be brought back to Earth, to answer the major questions that we have about Mars and about life."

Perseverance aims to land near the cliffs of the fan-shaped river delta — a deposit of mud and clay that the river left as it flowed into the lake.

jezero crater river delta mars perseverance rover landing site
The remains of an ancient delta at the edge of Jezero Crater, captured by the ESA's Mars Express orbiter.
ESA/DLR/FU-Berlin

The rover might land atop the delta's cliffs, or it could first explore the muddy lake bottom for fossils and then climb to the delta. That depends on which spots its autonomous navigation system chooses for landing.

In the image below, the layer of green between the delta and the crater rim is where scientists think Lake Jezero's shoreline was. It appears to be rich with carbonates: minerals that are especially good at trapping microbes to form stromatolites.

nasa mars 2020 rover landing site jezero crater
NASA's Mars Reconnaissance Orbiter captured colorful spectral data showing clays and carbonates deposited across the Jezero Crater river delta. The green indicates carbonate minerals.
NASA/JPL-Caltech/MSSS/JHU-APL

If that's the case, Perseverance could simply amble along the carbonate beaches of Lake Jezero, scanning for stromatolites.

The rover's primary mission to look for signs of life lasts one Martian year (two Earth years). If the robot is still kicking when that's over, its extended mission would involve climbing the 1,600-foot-high rim of the crater.

Jezero Crater was most likely originally created when an object (probably a meteorite) collided with Mars. The crash exposed rock layers deep within the planet's crust. So at the crater rim, Perseverance aims to study these layers to learn more about Martian geology.

perseverance mars rover jezero crater
An illustration of Perseverance beneath the cliffs of Jezero Crater.
NASA/JPL-Caltech

The heat from that impact may have also given rise to hot springs, which would have deposited their own minerals that could also hold signs of aliens.

"That's why we're so excited about Jezero Crater, because it has so many different ways that it could preserve signs of life," Briony Horgan, a geologist on the Perseverance science team, said in the briefing.

Even if Perseverance finds no fossils whatsoever, that will be a major non-finding. To date, every habitable environment on Earth that scientists have examined has hosted life.

"If we do a deep exploration of Jezero Crater with the rover and its instruments... and we find no evidence of life, we will have shown that in at least one place, there is a habitable environment that is not inhabited," Farley said. "If that's what we find, it would tell us something important: that habitability alone is not sufficient, that something else has to be present — some, perhaps, magic spark — that causes life to occur."