The Red Dust on Mars Might Be a Different Mineral Than Scientists Thought, Shedding Light on the Planet’s Past

Humans have been training telescopes on Mars for hundreds of years and gazing up at it for thousands. Despite the fact that scientists have now sent more spacecraft to Mars than any other extraterrestrial planet, the world contains much more to learn—even regarding its most iconic feature.

Mars’ famous red color might come from a different mineral than previously thought, according to a study published Tuesday in the journal Nature Communications. Pinning down the chemical compound responsible for the red planet’s hue can shed light on what the Martian environment looked like billions of years ago—and in turn, it could answer questions about the existence of past life.

“What we want to understand is the ancient Martian climate, the chemical processes on Mars—not only ancient—but also present,” lead author Adomas Valantinas, an astrophysicist at the University of Bern in Switzerland, says in a statement from NASA. “Then there’s the habitability question: Was there ever life? To understand that, you need to understand the conditions that were present during the time of this mineral’s formation.”

Scientists widely accept that Mars is red because of the presence of iron oxide, which commonly forms when iron in the planet’s soil (known as regolith) reacts with either liquid water or water and atmospheric oxygen. Essentially, it’s rust. That red-tinted iron oxide disintegrated into dust and spread across the planet on wind currents for billions of years—as it continues doing today.

However, different environmental conditions form different kinds of iron oxides—so exactly which one has led to the distinctive, red Martian world has remained a hotly debated topic.

“This paper is trying to figure out which specific poorly crystalline iron oxide could be responsible for the red component of Mars dust, which would be helpful to work out as that could help us determine which process produced the dust and when that occurred,” Briony Horgan, a planetary scientist at Purdue University who was not involved in the study, tells CNN’s Ashley Strickland.

Previous studies of Martian dust suggested the specific mineral in question was hematite, an iron oxide that can form without water. That’s because spacecraft observations of the compounds had not identified traces of water, indicating the iron oxide must have appeared after all of Mars’ ancient lakes and rivers had evaporated.

The recent study, however, challenged this idea by conducting new analyses of various spacecraft observations as well as running lab experiments. The lab work involved replicating the size of Martian grains of dust (which are about 1/100th the width of a human hair) with an advanced grinder machine. They tested how light interacts with this simulated dust and compared it to spectral data collected on the planet. Ultimately, the team concluded that Martian iron oxide is actually ferrihydrite: an iron oxide compound that forms when iron reacts with oxygen and cool water.

“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 says in a European Space Agency (ESA) statement.

While the team is not the first to suggest that Martian regolith contains ferrihydrite instead of hematite, they say they are the first to have investigated the subject by using both observational data and lab experiments.

“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. Moreover, the ferrihydrite remains stable under present-day conditions on Mars,” adds Valantinas in the ESA statement.

Ferrihydrite’s association with water also carries implications for the search for Martian life.

“The presence of water does not signify that there is life, but water is thought to be an important ingredient for life,” Vashan Wright, a geophysicist at the Scripps Institution of Oceanography at the University of California, San Diego, said to the Guardian’s Nicola Davis last year.

Now, the researchers are eagerly awaiting the future arrival of Martian regolith samples collected by the Perseverance rover to continue their work.

Margherita Bassi | READ MORE

Margherita Bassi is a freelance journalist and trilingual storyteller. Her work has appeared in publications including BBC Travel, Discover magazine, Live Science, Atlas Obscura and Hidden Compass.