An international team of researchers has revealed new evidence for the possible existence of liquid water beneath Mars’ southern polar cap.
The researchers, led by the University of Cambridge, used spacecraft laser altimeter measurements of the shape of the ice sheet’s upper surface to identify subtle patterns in its height. They then showed that these models matched computer model predictions of how a body of water below the ice sheet would affect the surface.
Their results agree with previous ice-penetrating radar measurements that were originally interpreted to show a potential area of liquid water beneath the ice. There has been some debate about interpreting liquid water from radar data alone, with some studies suggesting that the radar signal is not due to liquid water.
The findings, reported in the journal Nature Astronomy, provide the first independent source of evidence, using data other than radar, that there is liquid water beneath Mars’ south polar ice cap.
“The combination of new topographical evidence, results from our computer model, and radar data make it much more likely that at least one area of subglacial liquid water exists on Mars today, and that Mars must still be geothermally active. in order to keep the water under the ice sheet fluid,” said Professor Neil Arnold of the Scott Polar Research Institute in Cambridge, who led the research.
Like Earth, Mars has thick ice caps at both poles, roughly equivalent in combined volume to the Greenland ice cap. Unlike Earth’s ice caps, which rest on water-filled channels and even large subglacial lakes, the polar ice caps on Mars were until recently thought to be frozen to their beds due to the climate. cold martian.
In 2018, evidence from the European Space Agency’s Mars Express satellite challenged that assumption. The satellite has an ice-penetrating radar called MARSIS, which can see through Mars’ southern ice cap. It revealed an area at the base of the ice that strongly reflected the radar signal, which was interpreted as an area of liquid water under the ice sheet.
However, subsequent studies have suggested that other types of dry material, which exist elsewhere on Mars, could produce similar reflection patterns if they exist beneath the ice sheet. Given the cold climatic conditions, liquid water beneath the ice sheet would require an additional heat source, such as geothermal heat from the planet’s interior, at levels higher than expected for Mars today. This left confirmation of the existence of this lake pending from another independent source of evidence.
On Earth, subglacial lakes affect the shape of the overlying ice sheet – its surface topography. The water in subglacial lakes reduces the friction between the ice cap and its bed, affecting the rate at which ice flows under the effect of gravity. This in turn affects the shape of the ice sheet surface above the lake, often creating a depression in the ice surface followed by a raised area farther downstream.
The team – which also included researchers from the University of Sheffield, University of Nantes, University College Dublin and the Open University – used a range of techniques to examine data from the Mars satellite NASA Global Surveyor on the surface topography of the part of Mars’ south polar cap where the radar signal was identified.
Their analysis revealed a surface undulation 10 to 15 kilometers long comprising a depression and a corresponding uplifted area, both of which deviate from the surrounding ice surface by several meters. This is similar in scale to the ripples on subglacial lakes here on Earth.
The team then tested whether the ripple observed on the surface of the ice could be explained by liquid water at the bed. They ran computer model simulations of ice flow, tailored to specific conditions on Mars. They then inserted a reduced bed friction patch into the bed of the simulated ice sheet where water, if present, would allow the ice to slide and accelerate. They also varied the amount of geothermal heat coming from the planet’s interior. These experiments generated ripples on the simulated ice surface that were similar in size and shape to those the team observed on the actual ice sheet surface.
The similarity between the topographic undulation produced by the model and actual observations from the spacecraft, as well as previous radar evidence penetrating the ice, suggests that there is an accumulation of liquid water under the south polar ice sheet. of Mars, and that magmatic activity has occurred relatively recently in the subsurface of Mars to allow for the enhanced geothermal heating needed to keep water in a liquid state.
“The quality of the data from Mars, orbiting satellites as well as landers, is such that we can use it to answer really tough questions about conditions on, and even below, the planet’s surface, using the same techniques that we also use on Earth,” Arnold said. “It’s exciting to use these techniques to discover things on planets other than our own.”
The research was partly funded by the European Research Council.
Surface topographic impact of subglacial water beneath the south polar cap of Mars, Nature