For years, the prospect of a vast liquid water lake buried beneath the south polar ice cap of Mars captivated the scientific world. Such a body of water would be the perfect candidate for hosting microbial life, making it a prime target for exploration. However, new, high-resolution radar observations from NASA’s Mars Reconnaissance Orbiter (MRO) have largely dissolved the liquid water hypothesis, suggesting the mysterious feature is more likely a layer of rock and dust.
Revisiting the ‘Liquid Water’ Mystery on Mars.
The long-running mystery began in 2018 when the European Space Agency’s Mars Express orbiter, equipped with the MARSIS (Mars Advanced Radar for Subsurface and Ionospheric Sounding) instrument, detected a powerful radar reflection thousands of feet below the ice. On Earth, liquid water, especially highly briny water, produces a very bright radar signal, leading scientists to conclude that a subsurface lake existed, kept liquid by dissolved salts and the pressure of the ice above.
This discovery spurred a flurry of research, as finding abundant liquid water would drastically impact how we view the Red Planet’s potential for past or present habitability.
MRO’s Breakthrough: The ‘Very Large Roll’ Technique.
To get a clearer picture, scientists turned to MRO’s more powerful SHARAD (Shallow Radar) instrument. However, initial attempts failed to penetrate deep enough due to the orbiter’s configuration.
In an innovative move, the MRO’s operations team developed a special maneuver called a “very large roll.” This complex technique involved rolling the spacecraft 120 degrees, which enhanced SHARAD’s sensitivity and allowed its signal to be directed deeper underground.
“We’ve been observing this area with SHARAD for almost 20 years without seeing anything from those depths,” said Than Putzig, a SHARAD instrument scientist. “But once MRO achieved a very large roll over the precise area, the team was able to look much deeper.”
The Faint Signal: A Different Conclusion for the South Pole.
The SHARAD observations, detailed in the journal Geophysical Research Letters, delivered a decisive finding: instead of the bright, mirror-like reflection expected from liquid water, MRO detected only a faint signal.
This faint reflection, or in some adjacent areas, no signal at all, is consistent with a non-liquid material. Researchers now conclude the feature is most likely a layer of rock and dust or perhaps a geological feature like a rare, smooth surface beneath the ice, such as an ancient lava flow.
While the faint signal won’t completely end the debate, it makes it “very hard to support the idea of a liquid water lake,” according to Gareth Morgan, another SHARAD instrument scientist.
New Technique, New Opportunities for Future Mars Missions.
Despite deflating the sub-surface lake hypothesis, the scientific achievement marks a significant step forward for Martian exploration. The success of the “very large roll” technique has provided scientists with a newly powerful tool for mapping the planet’s hidden resources.
Researchers are now eager to use this enhanced radar technique to re-examine other scientifically interesting regions of Mars, particularly those closer to the equator, such as the Medusae Fossae formation.
If the radar can locate significant deposits of buried ice in warmer, more accessible equatorial regions, it would be a critical discovery. These locations are considered ideal for human habitation and work due to increased sunlight and proximity to potential water resources, offering a tangible resource for future human explorers. The MRO’s innovative maneuvers are helping to pave the way for the next generation of astronauts to land on the Red Planet.