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| The surface topography of Mars' south pole is depicted in the panel on the left, with the black outline of the south polar cap. The region containing the inferred subglacial water is depicted by the green square and the light blue line, respectively. The area's ice is about 1500 meters thick. The surface undulation that was discovered by the Cambridge-led research team is depicted in the panel on the right. It is evident as the red area, which is elevated by 5-8 meters above the regional topography. Upstream, toward the top right of the image, is a smaller depression that is 2-4 meters below the regional topography. The orbiting radar's inference of the water's area is depicted by the black outline. Credit: University of Cambridge |
New evidence from a global team of researchers suggests that liquid water may have existed beneath Mars' south polar ice cap.
The University of Cambridge-led team used spacecraft laser-altimeter measurements of the shape of the ice cap's upper surface to identify subtle height patterns. They then demonstrated that these patterns correspond to predictions made by computer models regarding the effects that a body of water beneath the ice cap would have on the surface.
Their findings are consistent with earlier measurements made by ice-penetrating radar, which were initially interpreted to indicate a potential area of liquid water beneath the ice. There has been debate regarding how to interpret the radar data alone for liquid water, with some studies claiming that the radar signal is not caused by liquid water.
The findings, which were published in the scientific journal Nature Astronomy, are the first independent proof using data other than radar that Mars' south polar ice cap contains liquid water.
"Mars must still be geothermally active in order to keep the water beneath the ice cap liquid," said Professor Neil Arnold from Cambridge's Scott Polar Research Institute, who led the research. "The combination of the new topographic evidence, our computer model results, and the radar data make it much more likely that at least one area of subglacial liquid water exists on Mars today."
At both poles, Mars has thick water ice caps that are similar in volume to the Greenland Ice Sheet on Earth. The polar ice caps on Mars, on the other hand, were previously thought to be frozen solid all the way to their beds due to the cold Martian climate. This is in contrast to the ice sheets on Earth, which are supported by water-filled channels and even large subglacial lakes.
This was challenged in 2018 by evidence from the European Space Agency's Mars Express satellite. The ice-penetrating radar on the satellite, MARSIS, is able to see through the southern ice cap of Mars. It revealed a region that strongly reflected the radar signal at the base of the ice, which was interpreted as liquid water beneath the ice cap.
However, subsequent research suggested that similar reflectance patterns could be produced by other dry materials on Mars if they existed beneath the ice cap. Liquid water beneath the ice cap would require additional heat, such as geothermal heat from within the planet, at levels higher than those expected for present-day Mars, given the cold climate. As a result, additional independent evidence was needed to confirm this lake's existence.
Subglacial lakes alter the surface topography of the underlying ice sheet on Earth. The water in subglacial lakes affects the velocity of ice flow under gravity by reducing friction between the ice sheet and its bed. This, in turn, changes the shape of the ice sheet above the lake, often resulting in a depression and a raised area further downflow.
A variety of methods were used by the team, which also included researchers from the Open University, the University of Sheffield, the University of Nantes, University College, Dublin, and NASA's Mars Global Surveyor satellite, to examine data on the surface topography of the region of Mars' south polar ice cap where the radar signal was found.
Their research revealed a surface undulation that is 10-15 kilometers long and consists of a depression and a raised area that both deviate from the surrounding ice surface by several meters.The scale of this is comparable to that of undulations over subglacial lakes on Earth.
The team then investigated whether liquid water at the bed could account for the observed undulation on the ice's surface. They adapted computer model simulations of ice flow to Mars conditions. After that, they inserted a portion of the simulated ice sheet bed with reduced bed friction, which would allow the ice to slide and accelerate if water was present. Additionally, they varied the amount of internal geothermal heat. The team observed undulations on the simulated ice cap surface that were comparable in size and shape to those created by these experiments.
Both the model-created topographic undulation and the actual observations made by the spacecraft, in addition to earlier evidence from ice-penetrating radar, suggest that there is an accumulation of liquid water beneath the south polar ice cap of Mars. Additionally, magmatic activity has only recently occurred in the subsurface of Mars, allowing for the enhanced geothermal heating that is required to keep the water in a liquid state.
Arnold stated, "We can use it to answer really difficult questions about conditions on, and even under the planet's surface, using the same techniques we also use on Earth." "The quality of data coming back from Mars, from orbital satellites as well as from the landers, is such that we can use it to answer really difficult questions. "Using these methods to learn about planets other than our own is exciting."
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