When a radar signal bounces off underground layers, the strength of its reflection depends on what the subsurface is made of. Most materials let the signal slip through or absorb it, making the return faint. Liquid water is special in that it produces a very reflective surface, sending back a very strong signal (imagine pointing a flashlight at a mirror).
That’s the kind of signal that was spotted from this area in 2018 by a team working with the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument aboard the ESA (European Space Agency) Mars Express orbiter. To explain how such a body of water could remain liquid under all that ice, scientists have hypothesized it could be a briny lake, since high salt content can lower water’s freezing temperature.
“We’ve been observing this area with SHARAD for almost 20 years without seeing anything from those depths,” said Putzig. But once MRO achieved a very large roll over the precise area, the team was able to look much deeper. And rather than the bright signal MARSIS received, SHARAD detected a faint one. A different very-large-roll observation of an adjacent area didn’t detect a signal at all, suggesting something unique is causing a quirky radar signal at the exact spot MARSIS saw a signal.
“The lake hypothesis generated lots of creative work, which is exactly what exciting scientific discoveries are supposed to do,” said Morgan. “And while this new data won’t settle the debate, it makes it very hard to support the idea of a liquid water lake.”
Alternative explanations
Mars’ south pole has an ice cap sitting atop heavily cratered terrain, and most radar images of the area below the ice show lots of peaks and valleys. Morgan and Putzig said it’s possible that the bright signal MARSIS detected here may just be a rare smooth area — an ancient lava flow, for example.
Both scientists are excited to use the very large roll technique to reexamine other scientifically interesting regions of Mars. One such place is Medusae Fossae, a sprawling geologic formation on Mars’ equator that produces little radar return. While some scientists have suggested it’s composed of layers of volcanic ash, others have suggested the layers may include heaps of ice deep within.
“If it’s ice, that means there’s lots of water resources near the Martian equator, where you’d want to send humans,” said Putzig. “Because the equator is exposed to more sunlight, it’s warmer and ideal for astronauts to live and work.”
More about MRO
NASA’s Jet Propulsion Laboratory in Southern California manages MRO for the agency’s Science Mission Directorate in Washington as part of NASA’s Mars Exploration Program portfolio. Lockheed Martin Space in Denver built MRO and supports its operations. SHARAD was provided to the MRO mission by the Italian Space Agency (ASI).