How life-friendly and abundant in water Mars once was is still a matter of dispute. Now recordings and measurement data from the NASA rover Perseverance confirm that at least in the Jezero crater, its landing area, there was a large lake until around 3.7 billion years ago. What is new, however, is the finding that this crater lake was fed by a steadily flowing river for a long time, but then there was a change. Large boulders and coarser sediment suggest that there were violent flash floods in the late stages of the lake. They could show the transition from the humid and mild early phase of the Martian climate to a more changeable, drier climate.
The notched traces of old river valleys and deltas as well as clay minerals, which must have formed in the presence of water, suggest that our neighboring planet Mars must once have been richer in water than it is today. How much water there was in its early days up to around 3.7 billion years ago, however, is a matter of dispute. While some scientists believe large Martian lakes and even oceans are possible, others are more skeptical. In their view, even in the early days of Mars, the atmosphere was too thin and its climate too cold to allow for larger areas of water over the long term. They suspect that the fluvial relics are more likely to be traced back to temporary heavy rain and flash floods, perhaps only to meltwater. If this were the case, it would significantly reduce the hope of finding traces of former microbial life on Mars.
In order to find out more about the potentially life-friendly phase of the red planet and to search specifically for fossils and other traces of life, the NASA rover Perseverance landed in a particularly promising area of Mars in February 2021 – the approximately 45-kilometer Jezero crater. It is located 18 degrees north of the Martian equator on the edge of the enormous Isidis Planitia impact depression and thus in an area in which some of the oldest rock layers on Mars come to light. The spectral evidence of carbonate rocks and clay minerals by orbital probes as well as traces of an ancient river delta suggest that the crater was filled with water in the early days of the planet – a lake more than 250 meters deep could have been in it. But whether this assumption is correct and how the delta-like landscape forms actually came about could not be clarified from the orbit alone.
Continuous deposit
Perseverance is now supplying the necessary data. Although he initially remained stationary during his first three months on Mars and only examined his surroundings using his mast camera and the SuperCam Remote Micro-Imager (RMI), these first data already provide valuable information about the geology and history of this area. Nicolas Nangold from the University of Nantes in France and his colleagues evaluated the images of two special formations in the vicinity of the rover. These include Kodiak Butte, an isolated, flat-topped mound that towers in the southern part of the delta, as well as several areas in the higher southwest part of the river delta. In these formations, different sediment layers are clearly visible, the structure and sequence of which allow conclusions to be drawn about the water levels in the body of water, but also about the flow velocities of the rivers.
The Perseverance images show that the lower sediment layers consist mainly of fine-grained material, as is typical for the alluvial sand of leisurely flowing waters. The river delta in which the Kodiak Butte is located therefore flowed into the deep crater lake, which at least at this time had no outflow. “Our results suggest that Lake Jezero was closed at the time the delta was formed,” write Nangold and his colleagues. The water level of this lake was sometimes around 100 meters lower than previously assumed based on the orbiter data. Nevertheless, the researchers see clear evidence in the new images from Perseverance that there was a deep lake in the Jezero crater until around 3.7 billion years ago. “The rover was able to answer one of the big questions – whether this crater was once a lake,” says co-author Benjamin Weiss from NASA’s Jet Propulsion Laboratory. “Before we landed there and could now confirm this, this was always in question.”
Boulders are evidence of flash floods
The rover data also reveal, however, that there has been a dramatic change in the history of Lake Jezero and its tributary: In the late days of the lake, this area must have experienced violent flash floods. Evidence for this is provided by numerous boulders up to one meter in size and several tons in weight, which were deposited in the upper, southwestern part of the delta. The multispectral images of the rover show that these chunks do not consist of phyllosilicates and olivine like most of the sediment in the delta, but are dominated by low-calcium pyroxene. “The origin of these fragments must therefore either be the rim of the Jezero crater or the exposed areas of pyroxene-rich crust a good 60 kilometers upstream from the Jezero crater,” the scientists report. Such a long transport of such heavy and large stones is only possible if the water flows with the appropriate force.
From the positions of the chunks and the structure of the sediment, Nangold and his colleagues conclude that there must have been violent flash floods in the late period of the lake. In them, the water could have rushed down the slope at up to nine meters per second. The flow rate was up to around 3000 cubic meters per second. Apparently, the researchers suspect, there was a change from a uniformly humid to a more changeable climate, in which rain fell only sporadically but heavily – similar to today in some desert regions of the world. “Our results indicate a temporal transition in the energy regime of the fluvial systems in the western delta – from persistent wet activity, which created the delta deposits that sloped towards the Jezero crater, to episodes with strong flash floods that could carry away meter-sized boulders for kilometers” , write Nangold and his colleagues.
The Jezero crater could thus have preserved valuable evidence of the change that turned Mars from a humid, life-friendly world to a cold, dry desert planet. “Something very fundamental must have happened at this time in planetary history,” says Weiss. However, what triggered this change is still unknown.
Source: Nicolas Nangold (Université Nantes) et al., Science, doi: 10.1126 / science.abl4051