Why is Mars red? Science finally has answers

The Red Planet’s distinctive color may hint at a much wetter – and more habitable – past.

A water-rich iron mineral ferrihydrite may be the main culprit behind Mars’ reddish dust, according to a new study led by researchers from Brown University and the University of Bern.

The study's results challenge the prevailing theory that a dry, rust-like mineral called hematite is the reason behind the planet’s color.

“We’re not the first to consider ferrihydrite as the reason for why Mars is red, but it has never been proven the way we proved it,” said Adomas Valantinas, a postdoctoral fellow at Brown who started this work as a Ph.D. student at the University of Bern.

The researchers based their analysis on observational data from Martian orbiters and rovers, as well as simulations that “essentially make Martian dust in the lab,” according to Valantinas.

“From our analysis, we believe ferrihydrite is everywhere in the dust and also probably in the rock formations, as well,” he said.

Ferrihydrite is an iron oxide mineral that forms in water-rich environments. On Earth, it is typically associated with processes like the weathering of volcanic rocks and ash, which plays a crucial role in soil formation, releasing nutrients and minerals that support plant growth.

Hints of life on Mars

Unlike hematite, which forms under warm and dry conditions, ferrihydrite forms in the presence of cool water. According to researchers, this may hint at Mars’ wetter and potentially more habitable past.

The findings, published in Nature Communications, suggest that the Red Planet may have had an environment capable of sustaining liquid water, an essential ingredient for life, and transitioned from a wet to a dry environment billions of years ago.

“What we want to understand is the ancient Martian climate, the chemical processes on Mars – not only ancient – but also present,” said Valantinas, who is working in the lab of Brown planetary scientist Jack Mustard, a senior author on the study.

“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 formation,” he said.

“As Martian winds spread this dust everywhere, it created the planet's iconic red appearance.”

The researchers used data from multiple Mars missions, including NASA’s Mars Reconnaissance Orbiter and the European Space Agency’s Mars Express and Trace Gas Orbiter.

However, as certain as researchers are about their findings, only samples from Mars itself can confirm them – a task that may not be possible until at least 2035, the earliest expected return date of NASA’s sample-collecting Perseverance rover.

“When we get those back, we can actually check and see if this is right,” senior author Mustard said.