New Data: No Salt Lake Under Martian Ice

A few years ago, Italian researchers rejoiced, claiming to have found liquid water in the ground in the south polar region of Mars. New data collected by the Mars Reconnaissance Orbiter (MRO) probe refutes this assumption, as announced by the US space agency National Aeronautics and Space Administration (NASA).

A team led by Roberto Orosei from the Italian National Institute for Astrophysics in Bologna reported in 2018 that it had found liquid water on Mars. The instrument Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) on the European Mars Express probe had detected a lake about 1.5 kilometers beneath the ice of the Martian South Pole.

A team led by US planetary scientists Gareth Morgan and Than Putzig re-examined the region at the end of May this year. The data was collected by the SHARAD instrument on the US probe Mars Reconnaissance Orbiter (MRO). SHARAD is a radar designed to investigate what lies beneath the Martian surface. SHARAD operates in a different frequency range than MARSIS and therefore has a higher resolution.

Rock and Dust

The MRO data leads Morgan and Putzig to an entirely different conclusion than the Italian researchers: According to them, the structure discovered by MARSIS is more likely a layer of rock and dust, the researchers write in the journal Geophysical Research Letters.

Morgan and Putzig believe the strong signal could originate from a smooth formation, such as a lava flow. The south pole of Mars is dotted with craters covered by a layer of ice. Most radar images show many peaks and valleys. MARSIS might have found a smooth area there, which is rare in this region.

The target area is approximately a water ice sheet about 20 kilometers in size and about 1.5 kilometers thick. Hidden beneath it is the structure that the Italian team mistook for a lake of liquid saltwater.

Strong Signal

Many materials allow radar signals to pass through or absorb them, thus producing only a very weak reflection. Water, on the other hand, produces a very strong, clear signal. This is especially true for liquid water. MARSIS received such a strong signal, which Orosei's team took as evidence of liquid water. The team explained that it could be liquid at the prevailing temperatures in the region because it was very salty water with a lower freezing point.

To investigate the target area on Mars, according to NASA, a special maneuver was necessary because the radar antenna is mounted on the back of MRO, so the probe's hull obstructs the instrument's view. By rotating MRO by 120 degrees, the performance of SHARAD was improved.

Morgan and Putzig plan to conduct more of these Very-Large-Roll maneuvers in the future to better peer beneath the surface in other areas of Mars. This includes the Medusae Fossae formation at the equator of Mars, where subsurface ice deposits are suspected.

(wpl)