The images reveal that the watery life cycle of the now dried-out lake at Jezero Crater - the site of Perseverance's touchdown - is more complicated and intriguing than originally thought.
The pictures also provide insight into where the rover could best hunt for samples, including those that may contain signs of past life, known as biosignatures.
Billions of years ago, when Mars had an atmosphere thick enough to support water flowing across its surface, river flows carried sand and gravel from the surrounding highlands towards Jezero's fan-shaped river delta. The lake in the crater – Lake Jezero – could have been up to 40km wide and tens of metres deep.
The researchers involved are excited to see how the findings could help Perseverance achieve its scientific missions: finding signs of bygone life and collecting samples for future return to Earth. The paper on Perseverance's imaging of Jezero Crater's Delta Escarpment, the first since it landed, is published in Science.
Perseverance
Perseverance landed at Jezero Crater on 18 February 2021. About a month later, the rover's Mastcam-Z cameras and Remote Micro-Imager (RMI) zoomed in for a closer look at one of the crater's most massive geologic features, the 'Delta Scarp'. The scarp contains the remnants of a river delta that formed where a 120-mile-long ancient river and a 21-mile-wide lake join.
The Perseverance science team detected a prominent hill, which they called Kodiak, near the scarp and discovered it contained distinctive geological structures. Within its cliff face, they observed metres of sloping rock beds sandwiched between horizontal layers that indicate rocky deposits from the ancient delta.
The team says this confirms the presence of the delta that built into a lake in Jezero crater and suggests that there was steady water flow into the lake, which is consistent with a warm and humid Martian climate 3.7 billion years ago.
Co-lead author on the paper Professor Sanjeev Gupta, of Imperial College London's Department of Earth Science and Engineering, said: "These results 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. Both are main objectives for sampling and caching rocks before the Mars Sample Return – a future mission to bring these samples back to Earth."
Flash floods
On Earth, river deltas are created by slow and steady build-up of sand- and gravel-sized sedimentary particles deposited where a river enters a large lake or other body of water. However, the team discovered similar but more dramatic deposits in the uppermost layers of the Delta Scarp on Mars, with layers containing large boulders which can only have been transported by high energy floods. This meant a drastic change in the flow conditions occurred after the delta was created. The slow meandering waterway that fed the crater's delta fan was transformed into fast-moving flash floods.
Lead author Dr Nicolas Mangold from Laboratoire de Planétologie and Géodynamique (LPG) in Nantes, France said: "We saw distinct layers containing boulders up to five feet across that we knew had no business being there."
Although the Kodiak hills are currently not in the rover's travel plans, Jezero's Delta is the starting point of its efforts to collect rocks to bring home. The base of the delta layers may contain fine-grained clay and mudstones — types of rock that could potentially preserve traces of ancient life, if it ever existed on Mars.
A fluctuating Lake Jezero
Early in Lake Jezero's history, the lake is thought to be tens of metres deep, with water levels reaching high enough to break through the eastern crater rim, where images taken from orbit show an outflow event occurred. The new paper describes that the size of Jezero's lake fluctuated greatly over time, with its depth rising and falling by tens of meters before eventually disappearing altogether.
While it is unknown if these swings in the water level were a result of flooding or more gradual environmental changes, the team determined that they occurred during a time later in the history of the Jezero delta when lake levels were at least 100 meters lower.
Professor Gupta said: "A better understanding of Jezero's delta is a key to understanding the change in hydrology for the area, and it could potentially provide valuable insights into why the entire planet dried out."
Sue Horne, Head of Space Exploration at the UK Space Agency, said: "Since before space travel was a reality, Mars has captured the imagination of people who have looked to the skies. With recent technological advances, UK scientists and engineers have been at the helm of missions to provide us with a richer understanding of the red planet.
"This latest discovery confirms Jezero crater, Perseverance's landing site, once hosted a lake and river on its surface. These ancient environments provide the perfect place for the Perseverance rover to search for signs of past life on Mars."
