New data has cast doubt on the presence of liquid water on Mars
Scientists have long known that ancient Mars had plenty of water, but in the current conditions - cold and dry - the existence of liquid water on the planet seems unlikely.
All the more interesting was the discovery of the MARSIS radar on board the Mars Express station: it detected a very bright radar signal under the southern polar cap of Mars in an area about 20 kilometres across. Such a reflective "spot" looked like a possible sign of an under-ice lake.
If there really was liquid water under the ice, it would greatly increase the chances of Mars being habitable - if only in the past or in the form of exotic microbial ecosystems. But for the water under the Martian ice to remain liquid, you need either extremely salty brines or localised sources of heat - such as recent volcanic activity. This seemed like too much of a stretch to many researchers, and they began looking for "dry" explanations for the bright signal:
a layered alternation of CO₂ ice and water ice,
a mixture of salt ice and clay,
layer structures that enhance reflectivity.
A new view from a different radar
The Mars Reconnaissance Orbiter carries a different instrument, the SHARAD radar, which operates at a higher frequency than MARSIS. This gives better "resolution" on the structure of the layers, but is worse at penetrating to greater depths. Previously, the SHARAD signal was simply not enough to "reach" to the base of the ice cap in the area where MARSIS saw a suspicious bright spot.
The team tested a new manoeuvre - very large roll (VLR): the station turns around the longitudinal axis at 120 ° (previously the maximum was about 28 °). This allows to amplify the reflected signal and "illuminate" the ice more deeply.
Gareth Morgan and colleagues, authors of a paper in Geophysical Research Letters, analysed 91 SHARAD observations crossing the high reflectivity zone from MARSIS data.
The result:
The basal (i.e., from the ice-ground boundary) SHARAD signal could only be detected using the VLR manoeuvre.
But in contrast to the bright MARSIS signal, the SHARAD reflection was very weak.
For a real subglacial body of water, one would expect, on the contrary, a strong reflection at high frequency - and there is none. This makes the hypothesis of liquid water at this point unlikely.
What is there, if not water?
The authors of the paper offer a more prosaic explanation: there is likely to be a particularly smooth ground surface under the ice at this location. The smooth bottom can act as an effective mirror for the MARSIS low-frequency radar, giving the impression of an "anomalously bright" signal, while for SHARAD the reflection remains weak.
Completely contradiction between the data of the two radars is not yet removed - scientists will have to further understand how exactly the composition of ice, layering and bottom topography affect the reflected signal at different frequencies. But at the moment the version with liquid water under the southern polar cap of Mars looks less convincing than a few years ago.