Scientists Challenge Long-Held Theory on Why Mars Is Red

The fourth planet from the Sun is famously a rusty red—but scientists now believe we’ve been wrong about how it got its distinct hue.

The red coloration comes from iron minerals in Mars’ dust—no surprise there. But a team of ESA and NASA scientists now think that Mars rusted earlier in its ancient past than previously known, when liquid water was widespread on the planet’s surface.

Iron oxide—which is now dispersed across Mars—can form under different conditions. Scientists previously thought that the iron oxide on Mars’ surface was hematite, which likely formed through interactions with the planet’s atmosphere when Mars’ surface was dry.

But the new analysis offers a different origin—that Mars’ distinct redness seems to match more with the color of iron oxides that contain water, or ferrihydrite. Ferrihydrite keeps its water signature long after the water present during its formation disappears, and the research team detected its signatures in the reddish dust dispersed across the planet.

In the study, published today in Nature Communications, scientists attempted to replicate Martian dust in a laboratory setting, to match the dust seen in spacecraft observations of the planet.

“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,” said Adomas Valantinas, a researcher at Brown University and lead author of the study describing the findings, in an ESA release. “Mars is still the Red Planet. It’s just that our understanding of why Mars is red has been transformed.”

Though previous research suggested ferrihydrite might exist in Martian dust, the team’s laboratory experiments combined data from missions with hands-on testing to corroborate the idea. The researchers ground down the iron in the lab to be a similar size to Martian dust particles, and analyzed their lab dust with the same techniques used by spacecraft orbiting the Red Planet.

“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,” said Colin Wilson, ESA’s Trace Gas Orbiter (TGO) and Mars Express project scientist, in the same release.

“Some of the samples already collected by NASA’s Perseverance rover and awaiting return to Earth include dust; once we get these precious samples into the lab, we’ll be able to measure exactly how much ferrihydrite the dust contains, and what this means for our understanding of the history of water – and the possibility for life – on Mars.”

It may take some time before we know the exact makeup of Mars, but with scientists’ observations from above its surface and laboratories on Earth, we’re getting closer to the ground truth.