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ESA / Davide Coero Borga
Today, a team Italian scientists announced that Mars has liquid water lakes under its polar ice caps. Although the evidence is somewhat indirect and requires a number of badumptions, the alternatives that the researchers considered to fit the data worsened. The announcement is likely to get scientists thinking about how to use existing instruments in orbit around Mars to examine this region more carefully.
Under the Poles
Most parts of Mars spend most of the time at too low temperatures on the surface. Adding salt – and we found a lot of traces of salt on Mars – can lower the freezing point of water by turning it into brine. But this is not enough to overcome the extremely low pressures of the Martian atmosphere, which would make any ice sublimate into steam instead of melting.
So the only realistic hope of significant amounts of liquid water is under the surface, at a sufficient level. depth to create enough pressure to allow salt to overcome low temperatures. Calculations have suggested that these conditions could be met at the Martian poles, where large ice caps composed of both water and frozen carbon dioxide exist. There are obvious parallels with Antarctica, where lakes have been discovered under glaciers in several parts of the otherwise frozen continent.
But up to here, no clear evidence of liquid water has been found, but new evidence suggests
Data comes from the Mars Express orbiter of the United States. ESA, which has a ground-penetrating radar instrument called MARSIS. Subsurface imaging is based on the fact that the boundaries between materials with different properties will bounce off part of the incoming radar signal. Thus, even if one layer of ice and one layer of rock can both be weakly reflective, the limit that occurs when the ice rests above the rocks creates a strong reflection that can be detected by a radar in orbit. The interface between ice and water acts in the same way, producing a luminous reflection.
MARSIS has now generated about 13 years of data, including multiple transitions on the polar regions. The badysis of these data had already identified a highly reflective region under the thickest part of the southern ice cap in an area called Planum Austral. But this characteristic had been attributed to a boundary between the ice containing dust that dominates the surface of Mars and a layer of ice of pure water that should exist below it. For this study, the Italian researchers re-examined this hypothesis and tried to determine if the ice of pure water was compatible with the radar data
Windows of opening
For their study, they would ideally use the radar signal to measure the properties the layer, which would determine how far the radar signals could cross it. The process requires a detailed understanding of the radar signals that were transmitted down to Mars by Mars Express. And unfortunately, we do not have this understanding – the instrument was just too big to open and calibrate on the ground before the launch of Mars Express.
The researchers therefore performed a rough calibration by recording the signal that bounced when the radar hit the atmosphere / ice dust limit on the surface of Mars, since the composition of these materials is fairly well understood. Combined with temperatures calculated at different depths, this calibration allowed researchers to calculate a value for reflections occurring at deeper levels. Although there are significant uncertainties badociated with these values, they were used to estimate a window in which the actual value probably resided.
Conveniently, the full range of these windows was superior to any previously observed material on Mars. In other words, even the lowest probable value was above the highest value we observed previously. And similar values on Earth are usually generated only by wet or liquid materials. In Antarctica, all similar values have been badociated with liquid water. And if you try the same technique on areas outside highly reflective areas, it produces values similar to those of dry volcanic rocks on Earth.
Lake gold pancake?
There are other possible explanations for this type of signal, but the authors consider them and reject them. To give an example, they consider a layer of carbon dioxide that has been liquefied by pressure. Although this layer may form under the conditions present at the base of the ice cap, its properties are such that you will not get as strong a reflection. Other conditions that they consider would produce similar reflections, but it is unlikely that they lie under an ice cover of a thickness of 1.5 km.
It is tempting to consider this feature as a lake of several kilometers in diameter. But radar reflections could be produced by a layer of water less than one meter thick, so it could be just a thin layer of liquid lining the base of the ice cap. And again, the liquid involved would be much saltier than the terrestrial oceans (and the salts are probably perchlorates, which are not as common on Earth).
Before we get excited about drilling 1.5km in the Mars poles to watch for ice, however, we will probably want some independent confirmation that this region of high reflectivity is consistently manifesting itself and behaving as if it were going on. it was liquid water. Ideally, there is another radar instrument called SHARAD orbiting the red planet on the Mars Reconnaissance Orbiter. SHARAD operates at different frequencies from MARSIS, but is also designed to detect underground features.
Obviously, if the results are good, they will have a big impact on how we look at life prospects on Mars. While a frozen, salty and high-pressure environment with no obvious energy intake is not a good place for life, it's possible that an already-formed ecosystem can scratch off using water. recovered chemical energy from the underlying rocks – we have examples of this process on Earth.
Science 2017. DOI: 10.1126 / science.aar7268 (About DOIs).
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