New Radar Data Dashes Hopes for Subsurface Liquid Water on Mars
A recent study by Gareth Morgan and his team at the Planetary Science Institute has cast doubt on the existence of a briny "lake" near the Martian south pole, which was first discovered in 2018. The discovery had sparked excitement among space enthusiasts and workers at Pepperidge Farm, but the new data from the Mars Reconnaissance Orbiter (MRO) suggests otherwise.
In 2018, the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) instrument on the Mars Express orbiter detected a highly reflective signal beneath the ice sheet at the south pole, suggesting the presence of liquid water. This finding led to intense speculation about the possibility of an underwater lake on Mars.
However, MRO's SHARAD instrument, which operates at a higher frequency (15-25 MHz) than MARSIS, couldn't provide a clear signal under the ice caps. This limitation is due to the higher frequency's superior resolution but its inability to penetrate deeper into the ground, especially when the ground is primarily composed of water ice.
To address this issue, MRO underwent a significant upgrade, enabling it to perform a "Very Large Roll" (VLR) maneuver, turning the spacecraft 120° on its axis to eliminate interference caused by its metallic body. This risky move proved successful, allowing SHARAD to capture a clearer subsurface image in the south polar region just a few months after the first VLR.
Despite the improved resolution, SHARAD failed to detect any bright reflections, even though it could scan 1.5km more of the ice than before. This lack of signal strongly suggests that the site does not contain liquid water.
One of the key findings from the 2018 MARSIS data was the significant difference in reflection between the surface and the subsurface, indicating the presence of a "brighter" material in radar frequencies, typically associated with liquid water. However, SHARAD's signal from the subsurface was only 0.1% as strong as that on the surface.
To explain this discrepancy, MRO engineers proposed two theories. The first theory suggests that the ice might still absorb high-frequency signals from SHARAD, preventing the detection of liquid water. However, no significant attenuation was found that could limit the ability to locate the liquid water.
The second, more plausible theory, posits that the "lake" is actually a patch of unusually smooth dry rock, such as a crater floor filled with sediment. MARSIS would have seen this as a mirror, reflecting most of the radar signal, while SHARAD would not, accounting for the lowered signal strength.
This finding is not the first to challenge the existence of the Martian "lake". Critics argue that the extreme geothermal heat or salt required for liquid water to exist at that location is unlikely. Another study highlights that frozen clay materials, which contain water but are not liquid, can produce the same signal strength as MARSIS.
In conclusion, the new SHARAD data, facilitated by the VLR maneuver, seems to bury the theory of an underground lake on Mars. However, MRO operators are eager to explore more areas of the planet with their enhanced radar capability, including searching for ice near the Martian equator, which could make the warmer region more appealing for preliminary human exploration.
