Mars May Have Been Wet When It Rusted, Upending Decades of Thinking

Scientists have discovered that Mars’ iconic rust-red appearance likely developed in a wetter environment than previously thought, challenging a longstanding theory about the planet’s geological history.

For decades, researchers believed the Red Planet got its distinctive hue from anhydrous hematite, an iron oxide that forms in dry conditions. But a new study published in Nature Communications suggests the dominant mineral in Martian dust is actually ferrihydrite, a hydrated iron oxide that typically forms when liquid water is present.

“Mars is still the Red Planet. It’s just that our understanding of why Mars is red has been transformed,” explains lead author Adomas Valantinas, a postdoc at Brown University who began this work at the University of Bern. “The major implication is that because ferrihydrite could only have formed when water was still present on the surface, Mars rusted earlier than we previously thought.”

The research team arrived at this conclusion by combining observations from European Space Agency (ESA) and NASA missions with novel laboratory techniques in which they created Mars dust replicas using different iron oxides.

“We were trying to create a replica martian dust in the laboratory using different types of iron oxide. We found that ferrihydrite mixed with basalt, a volcanic rock, best fits the minerals seen by spacecraft at Mars,” Valantinas notes.

Unlike previous studies that relied solely on spacecraft observations, the team used an advanced grinding machine to achieve dust particles approximately 1/100th the width of a human hair – matching the ultrafine texture of actual Martian dust. They then analyzed these samples using the same spectroscopic techniques employed by orbiting spacecraft, allowing for direct comparisons.

The finding contradicts the widely accepted model that proposed Mars’ red color developed through continuous oxidation in water-poor conditions during the Amazonian period, which spans from approximately 3 billion years ago to the present.

Instead, ferrihydrite likely formed during a cold, wet period on early Mars, possibly during the late Hesperian period around 3 billion years ago, when volcanic activity was intense and could have interacted with liquid water or ice.

What makes this discovery particularly significant is that ferrihydrite has remained stable on Mars’ surface despite the planet’s current hyper-arid conditions. According to the researchers, ferrihydrite can lose some adsorbed water while maintaining its poorly crystalline structure, as demonstrated by a 40-day laboratory dehydration experiment under simulated Martian conditions.

Colin Wilson, ESA’s Trace Gas Orbiter and Mars Express project scientist, emphasized the collaborative nature of the research: “This study is the result of the complementary datasets from the fleet of international missions exploring Mars from orbit and at ground level.”

The team’s analysis drew upon multiple sources, including the ESA’s Mars Express and Trace Gas Orbiter missions, NASA’s Mars Reconnaissance Orbiter, and ground-based measurements from NASA’s Curiosity, Pathfinder, and Opportunity rovers.

The research also revealed that the ferrihydrite in Martian dust is likely associated with sulfates, which aligns with a transition toward more acidic and arid conditions during the late Hesperian period.

This new understanding of Mars’ rusty appearance provides valuable insights into the planet’s climate history and potential habitability. The presence of ferrihydrite suggests Mars experienced aqueous alteration before transitioning to its current desert state.

“We eagerly await the results from upcoming missions like ESA’s Rosalind Franklin rover and the NASA-ESA Mars Sample Return, which will allow us to probe deeper into what makes Mars red,” Wilson adds.

The Mars Sample Return mission could be particularly revealing, as samples collected by NASA’s Perseverance rover include dust that, once returned to Earth, will allow scientists to measure exactly how much ferrihydrite is present.

For now, the Red Planet’s ochre hue continues to be admired from afar – but with a deeper understanding of its watery past.