Rover footage shows a long history of rivers flowing into the Martian crater

Normally, it takes a while for NASA’s Mars rovers to return scientific data. The instruments must be calibrated and put into service, and the rover must leave the landing site towards a piece of landscape of obvious scientific interest. In the case of the Perseverance rover, there was an additional delay as it tested the Ingenuity drone for several weeks.

But this week brought a pleasant surprise: Perseverance’s first research article was published in Science. It turns out that some of the rover’s earliest photographs revealed features in some nearly vertical rock walls some distance from the landing site – details that tell us about the history of water flow in the Jezero crater.

Mars meets Kodiak

Jezero Crater was chosen as the landing site for Perseverance because images taken from orbit indicate that it was once home to a lake. Photos revealed an outlet channel that allowed water to flow through a breach in the crater walls and a site where rivers flowed to feed the lake. There was also what appeared to be a river delta extending into the crater from these sites. The rover’s mission profile includes material samples from this delta, which might once have been an inviting environment to life and would have collected any living material existing upstream.

A key part of the mission is to confirm that the deposit was in fact produced by a river delta. That’s the subject of the new article, and it goes considerably further by revealing what type of delta formed the deposits near the rover’s landing site.

Key to the discovery is a feature that has taken on the informal nickname “Kodiak,” a hillock located almost to the west of Perseverance’s current location. Its height from the crater floor is about the same as that of the larger delta fan adjacent to the crater wall, indicating that they were once part of the same formation but that the material connecting them has since eroded.

Critically, on the larger delta range, the structural details of the old delta are covered with mounds of rubble, making it difficult to interpret. In contrast, on Kodiak, some of the delta deposits are cleared from the rubble, which means they can be observed. And since these rocks are almost vertical, it is impossible to observe them from orbit. But Perseverance was able to get some excellent images of it, and those images turned out to be very revealing.

Parallel lines

In areas of Kodiak that are not covered with rubble, it is possible to detect general trends in the layers of sediment deposits that now form the body of the mound. And these layers show a consistent pattern. At the base, the layers are almost flat or gently sloping. Above are areas where the layers slope significantly vertically. And this section is topped with a second set of roughly horizontal layers.

These kinds of deposits, which form as a result of river deltas, have already been described on Earth. The lowest layers form at the bottom of a lake as new sediment is washed away and is deposited roughly evenly. As the high delta begins to sink further into the lake, the deposits tilt vertically as the sediments slide down the slope formed at the edge of the delta. Once the water crosses the top of the delta, the sediment begins to form roughly horizontal layers again.

For this to happen, the water levels in the lake must be reasonably stable for a relatively long period of time, although the duration will depend on the amount of sediment carried by the incoming water. In the case of Jezero Crater, however, the left and right sides of Kodiak reveal similar deposits at two different elevations from the crater floor. Since Mars is not tectonically active enough to have split the formation, this suggests that the lake was stable at at least two different depths at different times in its history.

Researchers who analyzed these photos also note that none of these levels appear to match the height of the apparent outlet channel in the crater wall, suggesting that the waters have at times reached a third elevation.

This third rise, however, could have been achieved during dramatic flooding. At the top of the delta, there are deposits that include large rounded boulders, suggesting that they have been moved by the flood waters. So late in its history, after the deposits of the delta were in place, Jezero Crater was likely the site of severe flooding, possibly including events after the lake itself dried up.

There are obviously still a lot of details to fill in. But remember, all of this information comes from a handful of photos taken early in Perseverance’s stay in the crater. Over time, the rover should have the chance to take a close look at some of the deposits to give us a clearer picture of the history of Mars.

Science, 2021. DOI: 10.1126 / science.abl4051 (About DOIs).