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Mars, often referred to as the “Red Planet” due to its striking rusty color, may finally have its distinctive hue explained, as scientists challenge longstanding theories with new findings.

Being one of the most explored bodies in our solar system, Mars has garnered extensive study thanks to its relative closeness to Earth and the various missions it has hosted over the years. Data gathered by orbiters and landers indicate that the planet’s red appearance is primarily due to iron-rich minerals that have oxidized, resulting in a rust situation similar to that observed on Earth.

The transformation happens when iron within Martian rock interacts with water or moisture in the atmosphere, resulting in the formation of iron oxide, akin to the rusting process we observe on Earth. Over millions of years, this iron oxide disintegrates into dust particles that are transported by Mars’ wind dynamics, which generate both dust devils and massive storms.

Earlier examinations of iron oxide on Mars, based merely on spacecraft observations, found no signs of water. This led scientists to conclude that the iron oxide present was hematite, a dry mineral predominantly formed through reactions with the Martian atmosphere over eons. Consequently, it was assumed that hematite emerged later in the planet’s history, after it was believed to have harbored lakes and rivers.

Recent studies, integrating findings from multiple missions along with replicated Martian dust, have introduced a different mineral that potentially forms in the presence of cooler water as the origin of the planet’s reddish appearance. This finding could reshape scientists’ perceptions of Mars’ historical climate and its possible habitability. The results were reported by a research team Tuesday in the journal Nature Communications.

“While Mars retains its identity as the Red Planet, our comprehension of why it is red has undergone a significant update,” stated Adomas Valantinas, the lead researcher and a postdoctoral fellow at Brown University, in his comments.

Understanding the exact makeup of iron oxide found in Martian dust has been a focal point for researchers. Grasping its formation will give insights into Mars’ historical environmental and climate conditions.

Nonetheless, the extensive dust cover on Mars poses research challenges, as highlighted by Briony Horgan, who is involved with the Perseverance rover mission and serves as a planetary science professor at Purdue University, Indiana.

“The oxidized iron particles are so tiny, often measured in nanometers, that they lack a definite crystal structure, hence they cannot be classified as traditional minerals,” Horgan explained. “Various methods can oxidize iron without water; some studied dry processes include surface oxidation—akin to oxidation rinds forming in Antarctic Dry Valleys—and oxidation through abrasion caused by sand over extended periods. Yet, there are multiple water-based oxidation methods, too, occurring in environments like soils and lakes.”

The new analysis indicates that another variety of iron oxide which contains water, known as ferrihydrite, is likely responsible for the red coloration observed on Mars. This mineral rapidly develops in cooler water and likely formed under climatic conditions when liquid water was present on the Martian surface before the environment became colder and less hospitable. **Previous studies** hinted at ferrihydrite as a possible contributor to Mars’ red shade; however, this is the first study to marry laboratory techniques with observational data to substantiate that claim.

“This paper aims to identify which specific poorly crystalline iron oxide could be attributed to the red hue of Martian dust, which will be instrumental in determining how this dust was produced and its timeline,” Horgan noted.

Valantinas and his research team leveraged data from the European Space Agency’s Mars Express orbiter, the ExoMars Trace Gas Orbiter, alongside information from NASA’s Mars Reconnaissance Orbiter as well as the Curiosity, Pathfinder, and Opportunity rovers.

The CaSSIS color camera aboard the Trace Gas Orbiter provided crucial insights into the size and composition of Martian dust particles, enabling the researchers to recreate their own Martian dust samples in the lab.

In the laboratory, the scientists engineered Martian dust using various iron oxide types, grinding the replica to produce particles the size of those found on Mars, measured at one-hundredth of a human hair in thickness.

The team conducted analyses of the dust using X-ray techniques and reflectance spectrometry, mirroring the methodologies applied by orbiters while they orbit Mars. Subsequently, the lab-derived data was compared against the spacecraft findings.

Findings from Mars Express’s OMEGA reflectance spectrometer indicated that even the dustiest regions of Mars reveal traces of water-rich minerals. Data from the CaSSIS camera, when compared to lab samples, implicated ferrihydrite, enhancing our understanding of Martian dust’s actual composition over hematite, Valantinas noted.

This instrument has been observing Mars since April 2018, capturing high-resolution color images of the planet’s surface, according to Nicolas Thomas, a professor at the University of Bern’s Physics Institute, who led the development of the camera.

“Our findings reveal that ferrihydrite combined with basalt, a volcanic rock, best replicates the minerals identified by observing spacecraft at Mars,” Valantinas explained. “This implies that since ferrihydrite can only form under conditions where water was prevalent, Mars may have oxidized much earlier than previously assumed. Moreover, the stability of ferrihydrite under current Martian conditions is noteworthy.”

The enigma surrounding Mars’ reddish tint has intrigued scholars for centuries, as Valantinas pointed out.

The Roman civilization named Mars after their deity of war due to its blood-like color, while the ancient Egyptians referred to it as “Her Desher,” meaning “the red one,” as noted by the European Space Agency.

The revelation that Mars’ hue may stem from a water-bearing rusty mineral like ferrihydrite, rather than hematite’s waterless oxidation, surprised the research team, Valantinas remarked. This discovery offers valuable insights into the planet’s geological and climate history.

“The fact that this water-rich rust encompasses much of the Martian surface suggests that ancient liquid water on Mars may have been more prevalent than previously believed,” Valantinas stated. “This indicates that Mars once had a liquid water environment, a crucial element for potential life. Our study indicates that for ferrihydrite to form on Mars, the presence of oxygen—whether atmospheric or derived from other sources—along with water that could react with iron, was essential.”

Although the study did not focus on pinpointing the precise timing of the mineral’s formation, it is plausible that ferrihydrite emerged approximately 3 billion years ago, correlating with a period when the planet was significantly warmer and wetter.

“This era coincided with notable volcanic activity on Mars that likely triggered ice-melting occurrences and interactions between water and rock, creating favorable conditions for ferrihydrite development,” Valantinas asserted. “This aligns with a time when Mars was transitioning from its wetter past to its current arid state.”

Furthermore, it’s possible that ferrihydrite exists, not just in the dust but also within layers of Martian rock. The best way to confirm this would be to collect actual samples of martian rocks and dust, an endeavor that the Perseverance rover is currently undertaking. NASA and ESA plan to implement a complex series of missions aimed at bringing these samples back to Earth by the early 2030s.

“Once we retrieve these invaluable samples and analyze them in our laboratories, we will be able to determine the exact ferrihydrite content in the dust and what this reveals about Mars’ historical water conditions and its potential for supporting life,” commented Colin Wilson, the project scientist for ESA’s Trace Gas Orbiter and Mars Express.

Meanwhile, these findings open new avenues for Valantinas and his team, such as unraveling the original source location of the ferrihydrite before it spread across Mars through dust storms and determining the chemical makeup of the Martian atmosphere during its formation.

Understanding when and where this dust originated could offer scientists essential insights into how the atmospheres of early Earth-like planets evolved, as Horgan emphasized.

“Ferrihydrite is abundantly found in soils on Earth with significant water movement, resulting from phenomena like snowmelt or intense rainfall periods in warmer regions,” Horgan noted. “We’ve also detected ferrihydrite in lake sediments at Gale crater, which is currently under investigation by the Curiosity rover. To truly solve this mystery, obtaining a sample of Martian dust for analysis here on Earth would be invaluable.”