New evidence of liquid water under the south polar cap of Mars

Sep. 29 () –

An international team of researchers has revealed new evidence of the possible existence of liquid water under the south polar cap of Mars.as announced in the magazine ‘Nature Astronomy’.

the researchers, led by the University of Cambridge (UK), 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 patterns match the predictions of computer models about how a mass of water located under the ice cap would affect the surface.

Their results, which provide the first independent line of evidence, using non-radar data, that there is liquid water under the south polar cap of Mars, are consistent with earlier ice-penetrating radar measurements, which were initially interpreted as as a possible zone of liquid water under the ice. Interpretation of liquid water from radar data has been debated, and some studies suggest that the radar signal is not due to liquid water.

“The combination of the new topographical evidence, the results of our computer model and the radar data make it much more likely that there is at least one area of ​​subglacial liquid water on Mars today, and that Mars must remain geothermally active to keep the water under the ice cap in a liquid state,” explains Professor Neil Arnold of the Scott Polar Research Institute in Cambridge, who led the research.

Like Earth, Mars has thick ice sheets at both poles, the combined volume of which is roughly equal to that of the Greenland ice sheet. However, unlike Earth’s ice sheets, which underlie water-filled channels and even large subglacial lakes, Until recently, the polar ice caps on Mars were thought to be frozen to their bedrock due to the cold Martian climate.

In 2018, tests by the European Space Agency’s Mars Express satellite challenged this assumption. The satellite has an ice-penetrating radar called MARSIS, which can see through the southern ice cap of Mars. It revealed an area at the base of the ice that strongly reflected the radar signal, what was interpreted as an area of ​​liquid water below the ice cap.

However, later studies suggested that other types of dry materials, which exist elsewhere on Mars, could produce similar reflectance patterns if they exist below the ice cap. Given the cold climatic conditions, liquid water under the ice cap 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 another independent line of evidence.

On Earth, subglacial lakes affect the shape of the underlying ice sheet, that is, its surface topography. Subglacial lake water reduces friction between the ice sheet and its bed, which affects the speed of ice flow under 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 an elevated area below.

The team, which also included researchers from the University of Sheffield and the Open University in the UK, the University of Nantes, France, and University College Dublin, Ireland, used a number of techniques to examine data from the NASA’s Mars Global Surveyor satellite over the surface topography of the south polar cap portion of Mars where the radar signal was identified.

Their analysis revealed a 10- to 15-kilometre-long surface undulation comprising a depression and a corresponding uplift, both offset from the surrounding ice surface by several metres. This is on a similar scale to the ripples of subglacial lakes on Earth.

Next, the team checked whether the observed rippling on the ice surface could be explained by liquid water in the bed. They performed computer simulations of ice flow, tailored to the specific conditions on Mars. They introduced a low-friction patch at 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 in the simulated ice surface that they were similar in size and shape to those the team observed on the surface of the actual ice sheet.

The similarity between the topographic undulation produced by the model and the actual spacecraft observations, along with previous ice-penetrating radar tests, suggest that there is an accumulation of liquid water under the south polar cap of Mars, and that the magmatic activity occurred relatively recently in the subsurface of Mars to allow for the increased geothermal heating necessary to keep the water in a liquid state.

“The quality of data coming from Mars, both from orbiting satellites and from landers, is such that we can use it to answer really hard questions about conditions on the planet’s surface, and even below it, using the same data. techniques that we use on Earth,” says Arnold. It’s exciting to use these techniques to find out about planets other than our own.”