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 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.
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.
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 outcrop that planetary
geologists guess 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.
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.
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 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 the researcher says the delta’s
boulders may hold some answers.