China’s Mars rover finds evidence of catastrophic flooding

China’s Zhurong rover has peered deep beneath the surface of Mars and found evidence of two major floods that likely shaped the region the robot has been exploring since it landed in May 2021.

An analysis published in Nature today¹ is the first result from Zhurong’s radar imager, which can detect probes up to 100 meters below the surface. “It’s a very interesting paper, and I was particularly impressed by how deep they can see with this radar,” says Svein-Erik Hamran, a planetary scientist at the University of Oslo who analyzed the only previous ground-penetrating radar data used on the planet, collected by NASA’s Perseverance Rover.

The history of the Zhurong landing site — on Utopia Planitia, a vast plain in the northern hemisphere of Mars — has puzzled scientists. Some have theorized that water or ice was once a feature of the landscape. Observations from space have identified sedimentary deposits that suggest the region was once an ancient ocean or was inundated by massive floods, and geological features such as pitted cones resemble structures formed from water or ice. In May, researchers analyzed infrared images of the landing site taken by China’s Mars orbiter Tianwen-1 and found hydrated minerals that may have formed as groundwater rose through the rock or as ice melted.

But the region may also have been covered in lava, obscuring some of these subsurface hydrological processes. Eruptions of the Elysium Mons volcano to the east of the landing site or other volcanic activity may have covered the region with magma as observed in other parts of the Utopia Basin. By examining the radar data, the researchers hope to understand what happened and whether there may still be water or ice lurking beneath the rocks. “We want to know what’s going on beneath the surface,” says study co-author Liu Yang, a planetary scientist at the National Space Science Center in Beijing.

Under the surface

Zhurong is China’s first rover on the Red Planet, exploring the southern part of Utopia Planitia. The rover’s ground-penetrating radar transmits high-frequency radio waves, which can penetrate the surface to depths of 3 to 10 meters, and low-frequency waves, which can penetrate to 100 meters underground but offer poorer resolution. The study authors analyzed low-frequency data collected over more than 1,100 meters of terrain between May 25 and September 6 as Zhurong traveled south of his landing site. Radar signals reflect off subsurface materials, revealing the size of their grains and their ability to hold an electrical charge. Stronger signals typically indicate larger objects.

The radar found no evidence of liquid water to a depth of 80 meters, but it did detect two horizontal layers with interesting patterns. In a layer between 10 and 30 meters deep, the team said the reflection signals increased with depth. The researchers say this is likely due to larger boulders resting at the bottom of the layer and smaller rocks settling on top. An older, thicker layer between 30 and 80 meters deep showed a similar pattern.

The older layer is likely the result of rapid flooding that carried sediment to the region more than three billion years ago, when there was much water activity on Mars, says co-author Chen Ling, a seismologist at the Institute of Geology and Geophysics, Chinese Academy of Sciences Sciences, in Beijing.

The upper layer may have been created by another flood about 1.6 billion years ago, when there was a lot of glacial activity. According to Chen, the upper layer is unlikely to contain intact lava flows because it has less ability to hold an electrical charge than intact volcanic rock would be expected to have. In addition, the researchers did not see any sudden changes in stratification that would be expected when lava flows meet sedimentary material.

Volcanic or sedimentary?

But, says Chen, it’s possible that a thin layer of lava once covered the top layer and it was gradually broken down into smaller pieces. Radar data alone cannot definitively reveal whether the material is sediment or volcanoes, says Xu Yi, a planetary scientist at Macau University of Science and Technology.

Radar data is good at showing the stratification and geometry of underground material, but not so good at pinpointing its composition, including whether the material is ice or rock, Hamran says. Oftentimes, researchers rely on other clues, such as rocks sticking out of the surface, to form a picture of past events, he says. The authors say they cannot rule out the possibility that the region contains buried salt ice.

More radar results are expected from the mission, including data acquired during Zhurong’s continued traverse of Mars, results of the high-frequency radar measurements already taken and orbital radar observations of Tianwen-1 penetrating deep into the planet. They might help clarify details of the terrain. “It’s just the first step,” says Ling.