Study uncovers source of Mars' redness — a key indicator the planet (maybe) once had life

The first thing most people think of when they consider Mars is its deep red color. The Romans associated the planet with their god of war because it reminded them of blood while ​​the Egyptians called it "Her Desher," meaning "the red one." The planet didn’t get such a distinctive, rosy color by accident of course — but for the first time researchers have identified a single mineral they believe is responsible for Martian redness — and that mineral’s presence also indicates the potential presence of life.

A new study in the journal Nature Communications reveals that Mars is red for very much the same reason it may have once been home to life — namely, that it was a wet planet. This is in line with research from last year that detailed how Mars was covered in bodies of water in its distant past.

“Mars once had liquid water on its surface in rivers, lakes and possibly oceans,” Dr. Michael Manga, the chair of the University of California — Berkeley’s Department of Earth and Planetary Science, told Salon at the time. “We knew that the liquid water being buried deep in the subsurface was one possible solution to the question of where Mars' ancient liquid surface water went.”

Mars lost most of that liquid water, which is considered an essential ingredient for life anywhere in the universe. But importantly, this study notes that Mars' redness happened before the water left. So how did a wet planet become so red? 

It all comes down to ferrihydrite, a poorly crystalline mineral, or a substance where the atoms or molecules are not arranged in well-defined and repeating patterns. It contains iron oxide, a chemical compound humans traditionally associate with a different reddish-orange substance: rust. Indeed, this mineral exists on our planet, often in volcanic settings such as lava caves. Using state-of-the-art equipment, as well as firsthand analyses of Martian dust from the red planet’s surface, the researchers found that “ferrihydrite remains stable under present-day Martian conditions, preserving its poorly crystalline structure.”

This in turn suggests that the ferrihydrite “formed during a cold, wet period on early Mars under oxidative conditions, followed by a transition to the current hyper-arid environment.” In contrast to the conventional wisdom that Mars was continuously dry while its surface oxidized (or was exposed to oxygen), the new study suggests “ancient Mars experienced aqueous alteration before transitioning to its current desert state.”

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Dr. Adomas Valantinas, the paper’s lead author and a postdoctoral fellow at Brown University's Department of Earth, Environmental and Planetary Sciences, explained to Salon that the research team performed extensive spectral analyses on both orbital and rover data in their laboratory.

“We can now be quite confident that ferrihydrite is the dominant iron-bearing mineral causing Mars' distinctive ochre color,” Valentinas said. “Our research demonstrated that ferrihydrite provides significantly better fits than other iron oxides like hematite, goethite or akaganeite. We also employed Mars simulation experiments and theoretical calculations to show that ferrihydrite is thermodynamically stable on the Martian surface.”

Mars Dust Storm (Getty Images/MARK GARLICK/SCIENCE PHOTO LIBRARY)Similarly, Valentinas explained that the prevalence of this ferrihydrite proves that Mars was once covered in enough water that it was quite wet. That strengthens arguments suggesting Mars was once home to life.

“The finding is relevant to inferring the conditions of early Mars as the composition of minerals on the Mars surface tell us about the past climate,” Dr. Geronimo Villanueva, the associate director for Strategic Science of the Solar System Exploration Division at NASA’s Goddard Space Flight Center, and co-author of this study, told Salon. “Importantly, the new findings suggest a wetter and potentially more habitable past for Mars because ferrihydrite forms in the presence of cool water, and at lower temperatures than other previously considered minerals, like hematite.” 

Villanueva added that scientists already knew Martian dust contains a number of minerals, including iron oxides; this study narrows down the number of potential iron oxides that could cause the distinctive red color to just one, ferrihydrite.

“The presence of ferrihydrite tells us something specific about Mars' past environment,” Valentinas said, describing the cold and pH neutral waters that must have existed to oxidize the soil. “This suggests that rather than warm conditions, early Mars experienced a cold and wet environment.”  

To learn this, scientists at the University of Arkansas recreated the arid conditions which exist on Mars, in particular the average temperature of −70 °C and very low water vapor content. Over the course of 40-day laboratory experiments involving dehydration, the scientists learned that ferrihydrite loses some absorbed H2O while maintaining its poorly-crystalline structure. In addition to ferrihydrite the researchers used various quantities of iron oxide phases like magnetite, hematite, feroxyhyte and schwertmannite.

Dr. Avi Loeb, a Harvard University astronomer not associated with the study, emphasized the significance of discovering ferrihydrite.

“This material likely formed during water activity on early Mars,” Loeb said. “Subsequently, Mars became as dry as we see it today, preserving this mineral phase over its surface. The widespread presence of ferrihydrite in the Martian surface materials was not interpreted this way before, and was thought to be the result of dry oxidation late in Mars history.”

Loeb, who has long advocated that scientists seriously explore the possible existence of extraterrestrial life, added that he is impressed with the study’s potential implications on that question.

“On Earth, the majority of atmospheric oxygen is derived from biological activity, making the nature of surface oxidation important for understanding the potential for past life on Mars,” Loeb said.

Despite these impressive findings, Valentinas emphasized that more research needs to be done on this subject.

“Science always leaves room for testing and refinement, so we'll be able to test this hypothesis when the NASA-ESA Mars Sample Return mission brings actual dust samples back to Earth in the 2030s,” Valentinas said.

He added that scientists “still don't know the original source location of the ferrihydrite before it was distributed globally through dust storms, the exact chemical composition of Mars' atmosphere when the ferrihydrite formed, or the precise timing of Mars' oxidation.”

In addition to encouraging professional scholars to do research, Valentinas urged ordinary citizens to take a crack at looking at the Martian surface.

“The Mars rover and orbiter data is fully available to the public,” Valentinas said. “Anyone with access to the internet can download and view the images for themselves under NASA's Planetary Data System.”