The first scientific analysis of images taken by NASA's Perseverance rover
has now confirmed that Mars' Jezero crater—which today is a dry, wind-eroded
depression—was once a quiet lake, fed steadily by a small river some 3.7
billion years ago.
The images also reveal evidence that the crater endured flash floods. This
flooding was energetic enough to sweep up large boulders from tens of miles
upstream and deposit them into the lakebed, where the massive rocks lie
today.
The new analysis, published today in the journal Science, is based on images
of the outcropping rocks inside the crater on its western side. Satellites
had previously shown that this outcrop, seen from above, resembled river
deltas on Earth, where layers of sediment are deposited in the shape of a
fan as the river feeds into a lake.
Perseverance's new images, taken from inside the crater, confirm that this
outcrop was indeed a river delta. Based on the sedimentary layers in the
outcrop, it appears that the river delta fed into a lake that was calm for
much of its existence, until a dramatic shift in climate triggered episodic
flooding at or toward the end of the lake's history.
"If you look at these images, you're basically staring at this epic desert
landscape. It's the most forlorn place you could ever visit," says Benjamin
Weiss, professor of planetary sciences in MIT's Department of Earth,
Atmospheric and Planetary Sciences and a member of the analysis team.
"There's not a drop of water anywhere, and yet, here we have evidence of a
very different past. Something very profound happened in the planet's
history."
As the rover explores the crater, scientists hope to uncover more clues to
its climatic evolution. Now that they have confirmed the crater was once a
lake environment, they believe its sediments could hold traces of ancient
aqueous life. In its mission going forward, Perseverance will look for
locations to collect and preserve sediments. These samples will eventually
be returned to Earth, where scientists can probe them for Martian
biosignatures.
"We now have the opportunity to look for fossils," says team member Tanja
Bosak, associate professor of geobiology at MIT. "It will take some time to
get to the rocks that we really hope to sample for signs of life. So, it's a
marathon, with a lot of potential."
Tilted beds
On Feb. 18, 2021, the Perseverance rover landed on the floor of Jezero
crater, a little more than a mile away from its western fan-shaped outcrop.
In the first three months, the vehicle remained stationary as NASA engineers
performed remote checks of the rover's many instruments.
During this time, two of Perseverance's cameras, Mastcam-Z and the SuperCam
Remote Micro-Imager (RMI), captured images of their surroundings, including
long-distance photos of the outcrop's edge and a formation known as Kodiak
butte, a smaller outcop that planetary geologists surmise may have once been
connected to the main fan-shaped outcrop but has since partially eroded.
Once the rover downlinked images to Earth, NASA's Perseverance science team
processed and combined the images, and were able to observe distinct beds of
sediment along Kodiak butte in surprisingly high resolution. The researchers
measured each layer's thickness, slope, and lateral extent, finding that the
sediment must have been deposited by flowing water into a lake, rather than
by wind, sheet-like floods, or other geologic processes.
The rover also captured similar tilted sediment beds along the main outcrop.
These images, together with those of Kodiak, confirm that the fan-shaped
formation was indeed an ancient delta and that this delta fed into an
ancient Martian lake.
"Without driving anywhere, the rover was able to solve one of the big
unknowns, which was that this crater was once a lake," Weiss says. "Until we
actually landed there and confirmed it was a lake, it was always a
question."
Boulder flow
When the researchers took a closer look at images of the main outcrop, they
noticed large boulders and cobbles embedded in the youngest, topmost layers
of the delta. Some boulders measured as wide as 1 meter across, and were
estimated to weigh up to several tons. These massive rocks, the team
concluded, must have come from outside the crater, and was likely part of
bedrock located on the crater rim or else 40 or more miles upstream.
Judging from their current location and dimensions, the team says the
boulders were carried downstream and into the lakebed by a flash-flood that
flowed up to 9 meters per second and moved up to 3,000 cubic meters of water
per second.
"You need energetic flood conditions to carry rocks that big and heavy,"
Weiss says. "It's a special thing that may be indicative of a fundamental
change in the local hydrology or perhaps the regional climate on Mars."
Because the huge rocks lie in the upper layers of the delta, they represent
the most recently deposited material. The boulders sit atop layers of older,
much finer sediment. This stratification, the researchers say, indicates
that for much of its existence, the ancient lake was filled by a gently
flowing river. Fine sediments—and possibly organic material—drifted down the
river, and settled into a gradual, sloping delta.
However, the crater later experienced sudden flash floods that deposited
large boulders onto the delta. Once the lake dried up, and over billions of
years wind eroded the landscape, leaving the crater we see today.
The cause of this climate turnaround is unknown, although Weiss says the
delta's boulders may hold some answers.
"The most surprising thing that's come out of these images is the potential
opportunity to catch the time when this crater transitioned from an
Earth-like habitable environment, to this desolate landscape wasteland we
see now," he says. "These boulder beds may be records of this transition,
and we haven't seen this in other places on Mars."
Reference:
Nicolas Mangold, Perseverance rover reveals an ancient delta-lake system and
flood deposits at Jezero crater, Mars, Science (2021).
DOI: 10.1126/science.abl4051.
Tags:
Space & Astrophysics