Normally, it takes some time for NASA’s Mars rovers to return scientific data. The instruments need to be calibrated and commissioned, and the rover needs to move slightly away from the landing site towards a landscape that has clear scientific instruments. In the case of the Perseverance rover, there was an additional delay as it tested the Ingenuity drone over the course of several weeks.
But this week brought a pleasant surprise, as the first research paper on persistence was published in Science. It turns out that some of the rover’s earliest photos revealed features in some vertical rock walls at some distance from the landing site—details that tell us about the history of water flow at Jezero Crater.
Mars meets Kodiak
Jezero Crater was chosen as Perseverance’s landing site because images taken from orbit indicate that it once hosted a lake. The photographs revealed an exhaust channel that allowed water to flow through gaps in the crater’s walls, the place where rivers flowed to feed the lake, and what appeared to be a river delta extending from these sites into the crater. The rover’s mission profile includes sampling material from that delta, which may have once been an inviting environment for life, and collecting any living material that once existed upstream.
A significant part of that mission would also confirm that the deposit was indeed produced by a river delta. That’s what the new paper is about, and goes a long way by revealing what type of delta formed the deposit near the rover’s landing site.
Key to this is a feature that has raised the unofficial nickname Kodiak, a butt located approximately west of the fortification’s current position. Its height relative to the crater floor is roughly the same as that of the large delta fan adjacent to the crater wall, indicating that they were once part of a single formation, but that the material that once connected them has since eroded. .
Crucially, on the large delta fan, the details of the former delta’s structure are covered by mounds of debris, making it difficult to interpret. In contrast, on Kodiak, some delta deposits are clear of debris, meaning they can be seen. And, since those rocks are nearly vertical, it’s impossible to see them from orbit. But Perseverance was able to get some excellent images out of them. And those photos turned out to be very revealing.
In areas of the Kodiak that are not covered by debris, it is possible to detect general trends in the layers of sedimentary deposits that now make up the body of the butt. And these layers show a consistent pattern. At the base, the layers are almost flat or gently sloping. Above this are the regions where the layers are significantly inclined towards the vertical. And that section is topped by a second set of roughly horizontal layers.
Such deposits have already been described on Earth, which are formed as a result of river deltas. The lowest layers form at the bottom of the lake, as new sediment is carried in and settles almost evenly. As the elevated delta begins to move into the lake, the deposits tend to slope vertically as the sediment moves down the slope along the edge of the delta. Once the water is flowing over the top of the delta, the sediments again begin to form almost horizontal layers.
For this to happen, the water level in the lake must be reasonably stable for a relatively long period of time, although much will depend on how much sediment is in the incoming water. In the case of Jezero Crater, however, the left and right sides of Kodiak reveal similar deposits at two different heights from the crater floor. Since Mars is not tectonically active enough to split the formation, this suggests that the lake was stable at at least two different depths at different points in its history.
The researchers who analyzed these photos also noted that none of these levels matched the height of the apparent outlet channel in the crater wall, which suggests that water sometimes reaches a third height.
However, that third height could have been reached during a dramatic flood. At the top of the delta, there are deposits that include large boulders that are rounded, suggesting that they were eroded by floodwaters over a long period of time. Therefore, late in its history, after the delta was deposited, it appears that Jezero Crater was the site of some violent flooding. It is possible that this includes events after the lake dried up.
Clearly there is still a lot to fill. But remember, all this detail came from a handful of photos taken in the early days of Perseverance’s stay in the crater. In time, the rover should have a chance to examine some of the deposits to give a clearer picture of Mars’ history.
Science, 2021. DOI: 10.1126/science.abl4051 (About DOI).