A radar in Zhurong can penetrate the surface to a depth of 100 meters. Credit: Xinhua/Shutterstock
China’s Zhurong rover has looked deep beneath the surface of Mars, finding evidence of two major floods that likely shaped the region the rover has been exploring since landing in May 2021.
An analysis published in Nature today1 is the first result of Zhurong’s radar image, which can probe 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 Zhurong’s landing site, in Utopia Planitia, vast plains 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 submerged by great floods, and geological features, such as melt cones, resemble structures formed by 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 when groundwater rose through the rock or the ice melted.
But the region could also have been covered in lava, hiding some of these hydrological processes underground. Eruptions of the Elysium Mons volcano east of the landing site, or other volcanic activity, could have blanketed the region with magma, as has been observed in other parts of the Utopia Basin. By studying the radar data, researchers hope to understand what happened and whether water or ice might still be lurking beneath the rocks. “We want to know what’s going on below the surface,” says study co-author Liu Yang, a planetary scientist at the National Space Science Center in Beijing.
Below the surface
Zhurong is China’s first rover on the Red Planet and has been exploring the southern part of Utopia Planitia. The rover’s ground-penetrating radar transmits high-frequency radio waves that can penetrate the surface to a depth of 3 to 10 meters, and low-frequency waves that can reach up to 100 meters underground but provide a poorer resolution. The study authors analyzed low-frequency data taken between May 25 and September 6 over more than 1,100 meters of terrain as Zhurong traveled south of its landing site. Radar signals are reflected off materials below the surface, revealing their grain size and their ability to hold an electrical charge. Stronger signals usually indicate larger objects.
The radar found no evidence of liquid water up to 80 meters, but it did detect two horizontal layers with interesting patterns. In a layer between 10 and 30 meters deep, the team reports, the reflection signals strengthened with increasing depth. The researchers say this is likely due to larger rocks resting at the base of the layer and smaller rocks sitting on top. An older, thicker layer 30 to 80 meters below showed a similar pattern.
The oldest layer is likely the result of a rapid flood that brought sediment to the region more than three billion years ago, when there was a lot of water activity on Mars, says co-author Chen Ling, a seismologist at the Institute of Geology and Geophysics. , Chinese Academy of 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. Chen says the top layer is unlikely to contain intact lava flows, because it has a smaller capacity to hold an electrical charge than would be expected for intact volcanic rocks. In addition, the researchers did not see any sudden changes in the layers, which would be expected when lava flows meet sedimentary material.
Volcanic or sedimentary?
But, Chen says, a thin layer of lava may have covered the upper layer and gradually broken down into smaller pieces. Radar data alone cannot definitively reveal whether the material is sedimentary or volcanic, says Xu Yi, a planetary scientist at Macau University of Science and Technology.
The radar data is good at indicating the layers and geometry of subsurface material, but not so good at identifying its composition, including whether the material is ice or rock, Hamran says. Researchers often rely on other clues, such as rocks protruding from the surface, to build a picture of past events, he says. The authors say they cannot rule out the possibility that the region contains buried salt ice.
Further radar results from the mission are expected, including data taken during Zhurong’s continued flyby of Mars, results from high-frequency radar measurements already made, and Tianwen-1 orbital radar observations, which penetrate deep into the planet. They could help clarify details of the terrain. “This is just the first step,” says Ling.