Perseverance’s new images, taken from inside the crater, confirm that this outcrop was indeed a mars lake or river delta. Based on the sedimentary layers in the outcrop, it appears that the river delta fed into a mars 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 mars lake’s history.
In particular, the delta itself provides many of the clues about past water in Jezero Crater. The rover took detailed images of the mars lake, which showed steep slopes called escarpments or scarps. The delta formed just like deltas on Earth, from accumulating sedimentary deposits. The images were taken by the rover’s left and right Mastcam-Z cameras as well as its Remote Micro-Imager (RMI), which is part of the SuperCam instrument.
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The new analysis, published on October 7, 2021, 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 mars lake.
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 mars lake was filled by a gently flowing river.
Such flows may have resulted from “glacial surges” or rainfall-induced flash floods like those that occur in some of Earth’s desert regions today, Mangold said. Regardless of the cause, the boulder-bearing deposits could point to a very different Jezero than the one that produced the earlier mars lake sediments.
“This helps us to constrain the mars lake level and will help us to build a scenario of the delta formation and mars lake activity along Perseverance’s traverse, and also identify the right layers to analyze and sample,” Mangold said.
The newly analyzed photos may provide an intriguing glimpse into this big shift. For example, Perseverance’s imagery also shows big boulders, some up to 5 feet (1.5 m) wide, in the upper (younger) layers of Jezero’s main delta outcrop. It took a powerful flow to transport such large rocks — likely a flood that moved up to 106,000 cubic feet (3,000 cubic m) of water per second, study team members said.
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In addition, even more signs of ancient water came from a rock outcrop called Kodiak. The outcrop is about a mile from the rover’s current position, the same distance as the scarps. By examining images of the outcrop, mission scientists determined the timing of different geological deposits. The outcrop is now “broken off” from the main delta but was once closer to the southern edge of the delta.
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 mars 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.
These results also have an impact on the strategy for the selection of rocks for sampling. The finest grained material at the bottom of the delta probably contains our best bet for finding evidence of organics and biosignatures. And the boulders at the top will enable us to sample old pieces of crustal rocks.
Perseverance has two main tasks during its $2.7 billion mission: hunt for signs of past Mars life and collect and cache dozens of samples for future return to Earth.
The Perseverance rover landed on the floor of Jezero crater on 18 February 2021. The landing site, informally named “Octavia E. Butler,” is 2.2 km from the SE-facing erosional scarp of the western fan deposits, a planned target for the mission.
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Jezero Crater was deemed a good place to do this work, based on data gathered by spacecraft such as NASA’s Mars Reconnaissance Orbiter. Orbital imagery showed a fan-shaped feature in Jezero that mission team members interpreted as a delta — a place where a river emptied into a mars lake about 3.7 billion years ago, depositing sediments that could harbor evidence of ancient Martian microbes, if any ever existed.
The 45-km diameter Jezero crater was selected as the landing site based on orbital images, which showed geomorphic expressions of two sedimentary fan structures (western and northern) at the edges of the crater.
Interestingly, Perseverance’s observations show that the ancient Jezero Mars Lake was about 330 feet (100 meters) lower than orbital data had suggested, “marking a phase of the delta well after the start of its formation,” Mangold said.
Perseverance will eventually get some up-close looks at the delta formation, if all goes according to plan. The team aims to drive the rover, which has traveled 1.62 miles (2.61 km) on Jezero’s floor to date, over to the delta outcrop and collect samples deposited during the calm-lake era.
Now, we know that not only was there a mars lake fed by a river, which created the delta, but there were also late-stage flash floods that carried large boulders into the delta.
Furthermore, the new evidence supports the previous thinking by scientists that the mars lake filled the crater up to the rim. However, it also indicates that the water levels fluctuated over time later in the history of the mars lake. By that time, the levels were about 330 feet (100 meters) below their highest levels. Ultimately, as with the rest of the planet, the mars lake dried out completely.
These results also have an impact on the strategy for the selection of rocks for sampling. The finest-grained material at the bottom of the delta probably contains our best bet for finding evidence of organics and biosignatures. And the boulders at the top will enable us to sample old pieces of crustal rocks.