The European Space Agency’s Mars Express orbiter captured this view of Mars’ south polar ice cap Feb. 25, 2015. Three years later, the spacecraft detected a signal from the area to the right of the ice cap that scientists interpreted as an underground lake. (Image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)
Planetary scientists have new clues about a long-standing mystery at the Martian south pole after a NASA spacecraft performed an unusual flip maneuver to investigate the possibility of an underground lake.
In 2018, the European Space Agency’s Mars Express orbiter detected unusually bright radar reflections beneath the Mars’ south polar cap. The discovery sparked excitement in the scientific community because the readings resembled what might be expected from a present-day subglacial lake.
However, proving the lake’s existence has been challenging, and the idea has remained a widely discussed, but unconfirmed, theory.
Scientists working with NASA’s Mars Reconnaissance Orbiter (MRO) published new findings on Nov. 17 in Geophysical Research Letters using MRO’s Shallow Radar instrument, known as SHARAD. Their results cast doubt on the subglacial lake theory.
Previous attempts by the SHARAD team to study this exact region had failed. According to NASA’s Jet Propulsion Laboratory (JPL), the radar antenna sits on the back of MRO, and the spacecraft’s body blocked its line of sight.
To overcome the problem, engineers at JPL and Lockheed Martin developed a set of commands to roll MRO by 120 degrees-essentially flipping the spacecraft -so that SHARAD could directly view the surface. The maneuver allowed the radar to probe an area about 12.5 miles in diameter buried beneath roughly 1 mile of ice.
If liquid water were truly present beneath this region, scientists would expect a strong return signal from the reflective surface. Instead, the pass SHARAD made in May revealed a very faint signal. The findings weaken the case for a subglacial lake and raise new questions about what, exactly, lies beneath Mars’ south polar ice.
This animation shows NASA’s Mars Reconnaissance Orbiter performing a “very large roll”: a 120-degree roll that can increase the capabilities of the spacecraft’s subsurface radar instrument, called Shallow Radar, or SHARAD. (Credit: NASA)
“A different very-large-roll observation of an adjacent didn’t detect a signal at all, suggesting something unique is causing a quirky radar signal,” said Than Putzig, SHARAD instrument scientist with the Planetary Science Institute.
Although the mystery isn’t solved, NASA says the maneuver could open the door to future radar studies of other regions where buried ice or unusual subsurface materials may be hiding.