Scientists assumed every cave on Mars formed from ancient lava, but a team from Shenzhen University has now found eight that water carved through soluble rock — a process never before confirmed on the Red Planet

For years, the simplest explanation for Martian caves was lava. Mars has enormous volcanic provinces, long lava flows and many pit-like openings that look like skylights into lava tubes. If a cave existed on Mars, the default assumption was often that molten rock had once made the empty space.

A new paper makes that picture less complete. In The Astrophysical Journal Letters, Ravi Sharma, Chunyu Ding and colleagues report eight accessible potential karstic caves in Hebrus Valles, a region of Mars with a complicated volcanic and fluvial history. Karstic caves are not carved by lava. They form when water dissolves soluble rock, opening voids and passages over time.

That distinction is the point. The paper does not show astronauts walking through Martian caverns. It does not turn every dark pit on Mars into a water-formed cave. Its title uses the careful phrase “potential karstic caves.” But if the interpretation holds, it would mark a different class of Martian cave: one shaped by water moving through rock that could be dissolved.

Why caves on Mars usually meant lava

On Earth, caves form in several ways. Lava tubes form when the surface of a lava flow cools and hardens while molten material continues to drain underneath. Karst caves form when water chemically dissolves soluble rocks, often limestone, gypsum or other materials that can be eaten away by fluid over long periods.

Mars has always made the lava explanation attractive. The planet hosts giant shield volcanoes, broad volcanic plains and collapsed pits that look like skylights into subsurface voids. In a 2019 chapter titled “Lunar and Mars Caves, Lava Tubes and Pits”, Manfred von Ehrenfried summarised the long-standing interest in lava tubes and pits as possible shelters, science targets and future exploration sites.

That background matters because the new claim is not simply “Mars has caves.” Possible caves and skylights have been discussed for decades. The new claim is about formation. If eight features in Hebrus Valles are best explained as karstic, then Mars may preserve water-carved subsurface spaces as well as volcanic ones.

Hebrus Valles as the test case

Hebrus Valles sits in a part of Mars where channels, fractures and volcanic terrain meet. It has already attracted attention as a possible exploration zone. Sharma and colleagues previously examined the region’s resource potential and exploration planning in a 2019 paper in Research in Astronomy and Astrophysics.

The new paper focuses on features interpreted as accessible potential karstic caves. The term “accessible” is important for future exploration because it suggests openings that could, in principle, be reached or observed more directly by a mission. The term “potential” is equally important because the current evidence is still orbital. Mars has not yet had a rover descend into one of these candidates, inspect the walls and confirm the full subsurface geometry.

From orbit, researchers have to work with surface shape, setting, context and remote-sensing clues. A karst interpretation asks whether the features fit a water-dissolution process better than a purely volcanic collapse or lava-tube origin. That is a more demanding claim than simply spotting a hole in the ground.

What water-carved would mean

If these features are karstic, they imply that liquid water once interacted with soluble materials below the Martian surface strongly enough to create cavities. On Earth, karst landscapes are records of chemistry as well as erosion. Water does not merely push sediment aside; it changes the rock itself.

For Mars, that would add another layer to the planet’s water history. Channels and valley networks already show that water once moved across parts of the surface. Hydrated minerals show that water altered rocks. But caves formed by dissolution would preserve a more specific subsurface story: water moving through soluble material and enlarging underground spaces.

That is why the habitability angle appears in the paper’s title. Subsurface spaces are interesting because they can be shielded from radiation, insulated from surface temperature swings and capable of preserving geological records. None of that proves life existed there. It simply explains why caves, especially water-linked caves, draw attention in astrobiology and exploration planning.

The claim needs restraint

The tempting version of the story is too clean: scientists thought all Martian caves were lava tubes, and now water caves have been found. The real version is more careful. Many Martian cave candidates have been interpreted through a volcanic lens because the planet’s geology makes that plausible. The new work argues that eight candidates in Hebrus Valles may instead be karstic, which would be the first confirmed or strongly argued water-dissolution cave class on Mars.

That difference matters. A paper based on orbital analysis can shift the map of possibilities, but confirmation of caves on Mars remains difficult. Subsurface voids are hard to measure from above. Pit openings can form through several processes. Similar-looking features may have different origins.

The strongest reading is not that the cave question is settled. It is that the lava-only assumption is no longer broad enough. Mars may have more than one way to make a cave.

A different kind of shelter

Future missions to Martian caves are often imagined in practical terms: shelter from radiation, stable temperatures, possible storage of ice, or protected access for robots and eventually humans. Lava tubes fit that conversation because they can create large, continuous voids.

Karstic caves would add a different scientific value. They would point back to water chemistry. Their walls and sediments could preserve information about past fluids, mineral reactions and environmental conditions below the surface. If accessible, they would not simply be shelters. They would be archives.

That is why eight possible caves in one Martian valley matter beyond their count. They suggest that some of Mars’ hidden spaces may have been made not by fire, but by water working through rock. For a planet whose history is often reconstructed from dry channels and frozen remnants, that is a meaningful change in the underground story.