The subsurface of Mars has been mapped in detail using ground-penetrating radar from China’s Martian rover Zhurong (pictured) which has revealed shallow impact craters and other geologic structures in the top five metres of the planet’s surface.
Zhurong was sent to Mars as part of China’s Tianwen-1 mission and landed on the surface after nearly a year’s trip on 15 May 2021.
The rover was sent to a large plain in the northern hemisphere of Mars named Utopia Planitia, near the boundary between the lowlands where it landed and highlands to the south.
The region was chosen because it’s near suspected ancient shorelines and other interesting surface features, where the rover could look for evidence of water or ice.
A large body of underground ice was identified in a nearby part of Utopia Planitia in 2016 by radar from Nasa’s Mars Reconnaissance Orbiter.
After landing, Zhurong travelled about 1.9km south, taking pictures of rocks, sand dunes, and impact craters, and collecting data along the way.
Ground-penetrating radar detects features underground by sending electromagnetic pulses into the ground that are reflected back by any subsurface structures it passes over.
The rover uses two radar frequencies – a lower frequency that reaches up to 80 metres deep with less detail, and a higher frequency used for the latest study, which shows more detailed features but only reaches around 4.5 metres down.
Researchers hope that imaging the subsurface of Mars will help to shed light on the planet’s geologic history, previous climate conditions, and any water or ice the planet may host now or in the past.
They saw several curving and dipping underground structures in the Martian soil that they identify as buried impact craters, as well as other sloping features with less certain origins. They did not see any evidence of water or ice in the top five metres of soil.
Radar images of the deeper structures revealed layers of sediment left by episodes of flooding and deposition in the past, but also found no evidence of water in the present day.
This does not rule out the possibility of water deeper than the 80m imaged with the radar however.
The researchers contrast the data from Mars with ground-penetrating radar previously collected from the Moon, which shows a very different shallow subsurface structure.
While the Martian surface contains several distinct features that show up in the radar, the top 10 metres of the Moon has fine layers but no evidence of other structures like impact crater walls, despite also being subjected to meteorite bombardment.
The difference may be in the atmosphere, the researchers believe. While Mars’ atmosphere is just 1 per cent of the volume of Earth’s, the Moon has virtually no atmosphere.
With essentially no atmospheric protection, the Moon’s surface is bombarded by more of the smallest micrometeorites that rework the surface, eroding smaller-scale features and leaving behind fine layers of ejecta. By contrast, the surface of Mars is not being subjected to as many micrometeorite impacts because these smaller objects burn up in the atmosphere.
In the regions imaged by Zhurong, burial by wind-blown sediment may have also protected the impact craters from erosion. One of the craters imaged had its rim exposed at the surface, but the other crater was buried.
Yi Xu, the lead author on the study, said: “We found a lot of dunes on the surface at the landing site, so maybe this crater was quickly buried by the sand and then this cover reduced space weathering, so we can see the full shape of these craters walls.”
Images of the Martian subsurface are presented in a paper published in Geology.
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