New data from an international research team suggests that there may be liquid water beneath Mars’ south polar ice cap.
To find subtle patterns in the height of the ice cap, scientists from the University of Cambridge and other institutions used laser-altimeter measurements of the shape of the upper surface of the ice cap. Then, they showed how these patterns matched what computers had predicted would happen to the surface if there was a body of water under the ice cap.
Their findings are consistent with earlier ice-penetrating radar measurements, which were initially interpreted to indicate the possibility of a liquid water area beneath the ice. Radar data alone can’t be used to figure out where liquid water is, and some studies say that the radar signal is not caused by liquid water.
The results, which were published in the journal Nature Astronomy, are the first evidence other than radar that there is liquid water under Mars’ south polar ice cap.
According to Professor Neil Arnold of Cambridge’s Scott Polar Research Institute, who led the study, “the combination of the new topographic evidence, our computer model results, and the radar data makes 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 beneath the ice cap liquid.”
At its poles, Mars has thick water ice caps similar to those on Earth that are roughly equal in volume to the Greenland Ice Sheet. The polar ice caps on Mars, on the other hand, have until recently been thought to be frozen solid all the way to their beds due to the cold Martian climate, whereas Earth’s ice sheets are underlain by water-filled channels and even large subglacial lakes.
The 2018 data from the Mars Express satellite of the European Space Agency called into question this presumption. The satellite has a MARSIS ice-penetrating radar that can see through the southern ice cap of Mars. It showed a region at the bottom of the ice that strongly reflected the radar signal, which was taken to mean that there was liquid water there beneath the ice cap.
However, later research claimed that other dry substances that are present elsewhere on Mars might also reflect light in a similar manner if they are present beneath the ice cap. Given the frigid climate, liquid water beneath the ice cap would need a different heat source, such as geothermal heat from within the planet, at temperatures higher than those anticipated for the Martian climate at the time. This left another independent source of evidence needed to confirm the existence of this lake.
On Earth, subglacial lakes have an impact on the surface topography of the ice sheet that lies above them. Ice flow under gravity is influenced by the water in subglacial lakes because it reduces friction between the ice sheet and its bed. This then changes the shape of the ice sheet’s surface above the lake, often making a depression followed by a raised area further down-flow.
Researchers from the Universities of Sheffield, Nantes, and University College Dublin were also on the team. They used different methods to look at data from NASA’s Mars Global Surveyor satellite about the surface topography of the area of Mars’ south polar ice cap where the radar signal was found.
According to their analysis, there is a surface undulation that is between 10 and 15 kilometers long and consists of a depression and a corresponding raised area that are both several meters away from the surrounding ice surface. The size of this is comparable to the undulations over subglacial lakes on Earth.
The team then investigated whether liquid water on the bed of the ice could account for the observed undulation on its surface. They performed ice flow computer simulations that were tailored to the unique Martian environment. Then, they added a patch of reduced bed friction where, in the case of real water, the ice would be able to slide and accelerate. The amount of geothermal heat emanating from within the planet was also altered. These experiments made ripples on the fake ice surface that were the same size and shape as the ones the team saw on the real ice cap surface.
According to the similarity between topographic undulation produced by models and actual spacecraft observations, there may be an accumulation of liquid water beneath Mars’ south polar ice cap, and magmatic activity in the planet’s subsurface only recently enabled the enhanced geothermal heating required to maintain the water in a liquid state.
Arnold said, “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 is variable, from orbital satellites as well as from the landers. Utilizing these methods to learn more about planets other than our own is exciting.
The European Research Council helped fund some of the research.
