NASA orbiter data challenges the idea that liquid water exists on Mars

Searching for water on Mars often feels like chasing a flicker of hope in a cold and quiet world. You read about dried riverbeds, ancient lakes, and canyons carved by water that vanished long ago. So when scientists reported in 2018 that a bright radar signal beneath the planet’s south polar ice cap might be a hidden lake, the news carried real weight. It stirred excitement because water links directly to the possibility of life.

But a new study now challenges that early claim with sharper radar data and a fresh explanation that feels more grounded in the planet’s harsh reality.

The excitement started when the MARSIS radar on the European Mars Express orbiter detected a patch of subsurface terrain about 20 kilometers wide that reflected signals with unusual strength. The reflections came from about 1.5 kilometers beneath the layered ice and dust of Mars’ southern cap. On Earth, reflections that strong often hint at liquid water sitting beneath glaciers. Researchers called the site the High Reflectivity Zone, or HRZ, and suggested it might be a salt-rich lake kept liquid against the odds.

But there was a problem. Holding liquid water stable in such frigid conditions requires either extreme salt levels or heat from nearby volcanic activity. Neither scenario matched what scientists know about the south pole today. Other teams began searching for dry explanations that might mimic the radar signal.

Some proposed stacked layers of carbon dioxide and water ice. Others pointed to salty ice or clay that could scatter radar waves in unusual ways. All these ideas shared one prediction. The radar signature should change when viewed at different frequencies.

That prediction made NASA’s SHARAD radar, flying on the Mars Reconnaissance Orbiter, a valuable tool. SHARAD uses higher frequencies than MARSIS. In theory, it could help confirm whether the HRZ held liquid water. In practice, its signals faded before reaching the base of most south polar ice, leaving scientists unable to compare the two instruments.

The barrier came from the way the spacecraft carried SHARAD’s antenna. It sits along the vehicle’s far side, limiting how much radar power reaches the surface. Engineers could tilt the spacecraft slightly to improve things, but shallow rolls still left deep layers hard to see.

That changed when mission engineers developed a bold maneuver called a very large roll, or VLR. Instead of a small tilt, the orbiter rotates 120 degrees during data collection. The shift boosts radar power by more than ten decibels, which is enough to expose faint echoes from deep below the ice. Four of these large rolls have been flown so far, including two that sweep directly across the mysterious HRZ.

The VLR data finally brought the base of the south polar layers into clear view. On two matching tracks, the difference was striking. Normal passes showed no deep signal. The 120-degree rolls revealed a definite basal reflection.

On the VLR4 track, which crosses the heart of the HRZ, SHARAD detected the long-hidden boundary beneath 1,500 meters of ice. This was the first time the instrument had ever seen the base of the layered deposits in this region.

But the strength of the echo told a powerful story. The signal returning from the HRZ measured only about one-tenth of one percent of the reflection from the surface. That value was nearly identical to a second basal reflection about 80 kilometers away, in a region with no history of bright radar signals or suspected lakes.

Earlier MARSIS data had shown the HRZ reflecting signals more than three times stronger than the surface. The gap between the two instruments’ results now stands at roughly 30 decibels, a difference of about 1,000 in power. That strong frequency contrast is not what liquid water would produce.

The research team modeled many combinations of ice temperature, dust content, and potential subsurface materials. They found that the faint SHARAD echo fits only if the ground beneath the HRZ has a modest permittivity, similar to dusty rock mixed with ice. Liquid water, even if very salty, would need a permittivity five to ten times larger. That high value would create a bold SHARAD signal, not a faint one.

Reconciling the old lake interpretation would require a special column of unusually lossy ice sitting only above the supposed lake. That would mean a sudden spike in dust content within layers that otherwise stay smooth across hundreds of kilometers. The data show no sign of such abrupt changes.

Instead, the study points to a cleaner explanation. The surface beneath the HRZ may simply be smoother than the rugged terrain around it. Rough ground scatters radar signals in many directions, weakening the return. A smoother surface, like an old lava plain or sediment-filled crater, would send more energy back toward the spacecraft. The reflection would stay faint but detectable, and no lake would be needed.

That idea also agrees with a puzzling trend. MARSIS has detected similarly bright reflections across wide areas of Mars, including regions far from the poles where liquid water cannot survive. Models show that layered ice can create these bright signals over different kinds of dry ground.

For now, the HRZ story is shifting from hidden lake to hidden landscape. The new SHARAD results make the liquid water explanation hard to support, yet they deepen the mystery of why MARSIS sees such bright reflections. The truth may come from future radar passes that combine both instruments’ strengths.

These findings reshape how scientists search for water on Mars. They show that bright radar reflections do not always point to liquid water, which means researchers must be more cautious when interpreting subsurface signals. The new very large roll technique also strengthens SHARAD’s role in exploring buried ice across the planet.

It could help identify resources that future astronauts might use, especially near the equator where warmer conditions make human activity easier.

Understanding Mars’ layered ice also brings you closer to tracing the planet’s climate history and learning how water shaped its past.

Research findings are available online in the journal Geophysical Research Letters.

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