“The lake hypothesis generated lots of creative work, which is exactly what exciting scientific discoveries are supposed to do,” said Gareth Morgan, SHARAD co-principal investigator. But after years of speculation, a bold engineering maneuver aboard NASA’s Mars Reconnaissance Orbiter (MRO) has cast serious doubt on the notion that a buried Martian lake lies beneath the planet’s south polar ice.
It started in 2018 when a radar controlled by the European Space Agency, called MARSIS, detected an extremely bright reflection from beneath the surface of Ultimi Scopuli, which is under almost 1,500 meters of layered ice and dust. On Earth, such a strong radar return generally means liquid water beneath glaciers. The finding sparked theories of a briny lake, kept liquid by high salt content lowering the freezing point. But holding liquid water stable in Mars’ frigid polar environment without geothermal heat seemed improbable, prompting alternative explanations ranging from exotic ice mixtures to clay-rich deposits.
For nearly 20 years, MRO’s Shallow Radar, provided by the Italian Space Agency, or SHARAD, failed to return data from the same region. Its higher-frequency signals, capable of finer vertical resolution than MARSIS, were dissipated before reaching the suspected lake’s depth. The limitation was due to the radar antenna’s placement on the spacecraft’s far side, partially blocked by the orbiter’s body. Standard operations allowed modest rolls of up to 30 degrees to improve signal strength but these were insufficient for deep penetration.
Engineers at NASA’s Jet Propulsion Laboratory and Lockheed Martin Space came up with a bold solution: a 120-degree “very large roll” that would align SHARAD’s antenna directly toward the surface and would provide more than 10 decibels increase in signal-to-noise ratio. The maneuver had to be carefully sequenced in the nightside eclipse of Mars to avoid misaligning solar arrays or jeopardizing sensitive instruments. On May 26, SHARAD performed the roll over the target zone and finally detected echoes from the base of the ice sheet.
The results were unequivocal. Instead of the bright, mirror-like signal MARSIS reported, SHARAD detected a faint return—about one-tenth of one percent of the surface reflection strength. A similar faint echo appeared in a nearby area with no history of anomalous signals, suggesting the MARSIS brightness may arise from local geology rather than liquid water. Morgan and colleague Nathaniel Putzig point to possibilities such as a rare smooth subsurface, perhaps an ancient lava flow, which would reflect radar more efficiently than rugged terrain without requiring water.
The implications go far beyond the debunking of one lake claim. This very large roll technique has opened SHARAD’s reach to hitherto inaccessible depths, enabling exploration of potential ice-rich deposits across Mars. Another prime target is the Medusae Fossae Formation-a vast equatorial deposit whose composition is still debated. Some studies, including recent radar soundings, suggest it may be hiding ice volumes rivaling Earth’s Red Sea beneath hundreds of meters of dust-resources that could prove priceless for future human missions in light of the equator’s milder conditions.
Equatorial ice detection aligns with mapping efforts, such as NASA’s Mars SWIM project, which has charted subsurface ice at various depths to guide the selection of landing sites. “If it’s ice, that means there’s lots of water resources near the Martian equator, where you’d want to send humans,” says Putzig. Such reserves could support drinking water, oxygen production, and fuel generation, reducing dependence on Earth resupply.
The contrast of SHARAD-MARSIS also underlines the complexity in radar data interpretation. While MARSIS can penetrate as much as 5 kilometers and detect deep interfaces with lower-frequency pulses, the resolution is coarser. At a higher frequency, SHARAD provides finer detail in the upper few hundred meters. However, with the help of a roll maneuver, it has been able to probe more than a kilometer in low-loss terrains. Such complementary capabilities are important in distinguishing between genuine water signatures and geological mimics.
Future plans call for the very large roll to be used on other scientifically rich regions, including midlatitude debris-covered glaciers, volcanic provinces such as Tharsis and Elysium Planitia, and polar zones plagued by “radar fog.” High on the list is a direct overpass of MARSIS’s brightest south polar feature to measure its power with SHARAD. Such a test could definitively confirm or refute the liquid water hypothesis. In altering one spacecraft’s orientation, NASA has gone beyond challenging a high-profile claim to expanding the toolkit available for peering into the hidden layers of Mars. It could offer sharper insights into the climate history, the distribution of volatiles, and the resource potential of the planet-all keys to both understanding Mars as a world and preparing it for human footsteps.