Billions of years ago, Mars shifted from rivers and lakes to dry dunes; NASA says Curiosity’s hematite crystals may help date that climate shift in rock

Mars hasn’t always been the dusty, dry planet that we see in pictures today. NASA’s Curiosity rover has been climbing Mount Sharp in Gale Crater for more than a decade, passing through ancient lake beds, dried-up streams and fields of ancient sand dunes. For a long time, scientists have known that Mars transitioned from a warm, wet planet to a cold, dry one. What they didn’t have was a reliable way to date when this happened exactly. In a new study published in the journal Science, researchers Marek Szczerba and Tanya Peretyazhko may finally have found that timeline in a common rust-colored mineral called hematite.

A rusty clue buried in the rock

Hematite is a form of iron oxide, essentially rust, and iron reacting with water produces it: evidence that scientists have long used to confirm water once existed somewhere on Mars. What's new in this study is more specific: the tiny crystals that make up hematite grow to different sizes depending on how warm the water was and how long it lasted. That means hematite is more than just evidence of water flowing once on Mars. It may also provide a clue about how long that water hung around. The team studied 20 rock samples Curiosity drilled from different depths inside Gale Crater, NASA reports. Since the deeper layers of rock formed earlier in the history of Mars, scientists can travel up the crater wall to get a rock record that spans a long period of the planet's early climate.

How Curiosity reads the rocks like tree rings

The Curiosity rover carries an instrument called CheMin, short for Chemistry and Mineralogy. It fires X-rays at powdered rock samples and measures how the rays scatter, a technique called X-ray diffraction. This means scientists can measure the precise size of mineral crystals, something orbiting satellites cannot do from space. Thomas Bristow, the instrument's principal investigator at NASA's Ames Research Center, said the data reveals the size and shape of hematite crystals, as well as nearby minerals that only ground-level rover instruments can capture.

Deeper rocks stayed wet for millions of years longer

The study found that hematite crystals in higher, younger layers of rock remained below 10 nanometers. Crystals from the older, lower layers grew much larger, some reaching 65 nanometers. The team also noted that the mineral goethite, which is generally associated with hematite, was present in the upper layers but not in the lower layers. That growth didn't happen when the rock first formed; it happened afterward, as warm groundwater kept moving through those already-buried layers for millions of years, giving the crystals time to grow through a process called Ostwald ripening, where small crystals dissolve and feed the growth of larger ones. In this warmer, less acidic environment, goethite also slowly changes to hematite. Altogether, the findings suggest warm groundwater may have persisted for up to 4.7 million years in Gale Crater's deepest, oldest rock layers even as the surface of Mars was already cooling, according to the study.

"What we found was that warm and wet conditions were present for extended periods in buried rocks, despite Mars' climate becoming colder," said Peretyazhko, in comments to NASA Science.

Why the underground story matters more than the surface one

This aligns with what Curiosity has already seen on the surface. The rover’s climb up Mount Sharp has taken it from ancient lake-bed clays into rock layers formed by dry, windblown dunes, NASA’s Jet Propulsion Laboratory says. Ashwin Vasavada, who is the project scientist for the Curiosity mission, explains this phenomenon succinctly when he says that deposits from lakes eventually transitioned to those from dunes, sometimes with stream flow.

The hematite study adds a deeper layer of evidence: water may have kept moving underground for millions of years after the surface lakes had already dried up and vanished. This matters to anyone interested in whether Mars once supported life, because water deep underground, protected from cold and radiation, is often considered a more promising place to look for habitable conditions than the surface.

What this means for the hunt for life on Mars

None of this proves life existed on Mars. It gives scientists a new, physical way to trace how long water persisted in different layers of Mars' ancient rock, using samples the rover has already collected, rather than relying only on computer models. It also feeds the idea that Mars did not dry out overnight. The change was gradual, pockets of warmth and moisture lingering underground long after the surface had gone cold and bare.