For years, a razor-thin dip in Mars’s reflected sunlight stumped planetary scientists. It kept showing up in data from a NASA orbiter but nothing on Earth’s mineral lists quite fit.

Now, a team led by Janice Bishop, senior research scientist at the SETI Institute and NASA’s Ames Research Center, has a compelling match. They argue the signal comes from a new type of ferric hydroxysulfate mineral — a compound that forms when certain iron-rich salts are warmed and exposed to oxygen. The finding hints that parts of Mars stayed chemically lively and warm enough, perhaps long after the planet dried out.

Clues in the Chaos

“We investigated two sulfate-bearing sites near the vast Valles Marineris canyon system that included mysterious spectral bands seen from orbital data, as well as layered sulfates and intriguing geology,” said Bishop.

The clue appears in two places: Aram Chaos, a shredded landscape that likely saw ancient floods, and the plateau above Juventae Chasma, a high country of old channels and wind-cut rock. In both, the unusual signal lines up with thin, pale layers that contain sulfate salts. The team reprocessed the orbiter’s images to sharpen those layers and compared them with fresh laboratory measurements. The match — especially at Aram Chaos — is striking.

At Aram Chaos, the suspect layers sit low in the stack, tucked beneath other sulfates. That layout suggests warmth rose from below, like a buried hot spot that baked the rocks after the landscape collapsed about three billion years ago. Erosion later peeled back the surface, exposing the altered layers where the odd signal is strongest.

On the Juventae Plateau, the process seems to have occurred in reverse. There, the sulfate layers are sandwiched between two volcanic rock units. That arrangement points to heat from above — lava or ash that briefly capped the area — partly “cooking” the salts underneath before winds shaved the cap away.

“Investigation of the morphologies and stratigraphies of these four compositional units allowed us to determine the age and formation relationships among the different units,” said Catherine Weitz, a co-author on the study.

How the Team Cracked the Mystery

Back on Earth, the researchers warmed common Martian sulfates. Only when they added heat and oxygen did the powders transform into material that mirrors the Martian signal, according to Johannes Meusburger of NASA Ames. The reaction changes the mineral’s structure just enough to give it a distinct infrared “voice,” which is what the orbiter hears.

The lab work also set the temperature bar: the change kicks in above roughly the boiling point of water, far warmer than Mars’s usual surface chill. That makes brief volcanic episodes or longer, low-grade geothermal warmth the most likely heat sources. The team argues the alterations probably happened during the Amazonian period, within the last three billion years.

“The material formed in these lab experiments is likely a new mineral due to its unique crystal structure and thermal stability,” Bishop said. “However, scientists must also find it on Earth to officially recognize it as a new mineral.”

This discovery plugs into a broader Martian geological development. Since NASA’s Opportunity rover found the mineral jarosite in 2004, and Curiosity later traced a broad “sulfate unit” in Gale crater, sulfates have served as breadcrumbs of ancient water. The new compound, for instance, hints that while water left the salts behind, later heat from volcanism above or geothermal activity below altered those salts. That makes ferric hydroxysulfate a marker for places where Mars stayed geochemically active after it dried, and a smart target for future missions hunting past habitability.

If the team is right, there may be more of these warm-altered layers hiding in and around Valles Marineris, waiting for wind and time — and the next sharp-eyed spacecraft — to bring them to light.

The findings appeared in the journal Nature Communications.