Martian floods filled Jezero Crater

Researchers chose Jezero Crater as the landing site for NASA’s latest Mars rover, Perseverance, because the site appeared to be an ancient lake that was fed by a river roughly

3.7 billion years ago. Now, mission scientists have confirmed this — and also found that the region experienced flash floods, carrying boulders into the lake from up to tens of miles away.

The research, published Oct. 7 in Science, is based on images the rover took in the first three months after landing in Jezero Feb. 18, 2021. Its cameras zoomed in on two regions roughly 1.3 miles (2.2 kilometers) distant with exposed rocky outcrops.

One of these features, named

Kodiak, is a butte west of the rover with exposed rock layers up to 80 feet (25 meters) high. Several places on the butte feature a distinct structure: a set of layers at a slanted angle, sandwiched between parallel rock layers above and below.

These slanted rock layers, known as foresets, are characteristic of former river deltas. As a river dumps mud and clay into a body of water, that sediment forms an underwater fan-shaped mound that drops off sharply away from shore. Those submerged slopes eventually become angled rock layers.

“This is the key observation that enables us to once and for all confirm the presence of a lake and river delta at Jezero,” said Nicolas Mangold, the paper’s lead author and a researcher at the Laboratories de Planétologie et Géodynamique in Nantes, France, in a statement.

Perseverance also imaged four outcrops on the wall of the river delta itself, which rises 200 feet (60 m) above the crater floor. Its lower levels are made of fine-grained, angled rock layers, similar to those at Kodiak. But on top of them was a surprise: a rock layer containing “boulders up to 5 feet [1.5 m] across that we knew had no business being there,” said Mangold.

Evidently, the river flowed steadily in its early years, building up the delta’s lower layers. But at some later time, it experienced episodes of flash flooding — perhaps triggered by intense rainfall or volcanoes suddenly melting snow — capable of carrying the boulders.

The findings will also help mission planners. To search for samples that may harbor signs of ancient life, the team will guide Perseverance to the fine-grained material in the lower rock layers, as the steady buildup of material may have preserved organic matter or other biosignatures.

NASA/JPL-CALTECH LOST LAKE. An illustration shows how Jezero Crater’s lake was fed by a river flowing in from its northwest (upper left), while overflow streamed out of a channel at the lake’s eastern edge (at right).