More than 20 years after NASA's Spirit rover began exploring Mars, scientists have uncovered a hidden clue in its archival data that adds to mounting evidence that the Red Planet was once rich in liquid water.

Researchers reanalyzed measurements collected by Spirit between 2004 and 2010, revealing widespread traces of crystalline hematite and altered magnetite in ordinary Martian soil — iron-bearing minerals that point to ancient chemical changes consistent with interactions between water and rock. The findings suggest liquid water may have been more widespread across Mars than scientists previously thought, according to a statement from Edith Cowan University (ECU) in Australia.

The team combined hundreds of individual measurements from 32 undisturbed soil sites inside Gusev Crater, where Spirit landed in January 2004, creating the most detailed iron-mineral profile yet assembled for typical Martian soil, according to the statement.

"One of the most important discoveries was finding crystalline hematite in ordinary Martian soil," Paulo de Souza, lead author of the study and an ECU professor, said in the statement. "This mineral's widespread presence suggests not only that water was present, but that significant areas of Mars may have once been covered by water."

Crystalline hematite has long been considered one of the strongest mineral indicators of past water on Mars. While it can also form through volcanic or hydrothermal activity, it commonly develops when iron-bearing rocks interact with liquid water.

The discovery is particularly significant because previous analyses concluded that crystalline hematite was largely absent from Spirit's landing site. According to the researchers, the mineral's signature was simply too faint to stand out in individual rover measurements. By combining thousands of spectra collected over the mission, however, the team was able to tease the signal out of the background noise.

The researchers also identified altered magnetite, suggesting the original volcanic minerals in the Martian soil were chemically transformed over time. Together, the altered magnetite and crystalline hematite provide new evidence that water played an important role in reshaping the planet's surface. And, because similar dust and soil blanket much of Mars, the findings could have implications well beyond Gusev Crater, suggesting water-related chemical processes may have been more widespread across the planet than previously believed.

The results add to a growing body of evidence that Mars was once far more habitable than it is today. During its mission, Spirit discovered water-altered rocks and minerals in the Columbia Hills, while NASA's Curiosity rover later found evidence that Gale Crater once hosted long-lived lakes. More recently, Perseverance has been exploring the remnants of an ancient river delta inside Jezero Crater, where scientists hope to determine whether the environment could once have supported microbial life.

Beyond revealing another chapter of Mars' watery past, the research also highlights the lasting scientific value of spacecraft data long after a mission has ended.

"This work shows that important discoveries are not always made by collecting new data," de Souza said in the statement. "Sometimes they come from looking at existing data with fresh eyes and better analytical techniques."

Their findings were published June 26 in the journal Interactions.