Why Is Mars Red? – Sky & Telescope

Since the first Mars probes sent home images of the Red Planet, planetary scientists have thought its red color came from iron oxide — that is, rust. In 1965, Carl Sagan suggested the mineral was hematite, the most common terrestrial form of iron oxide. Now, laboratory tests suggest the red dust comes from a different form of iron oxide: ferrihydrite. This mineral includes water and doesn’t stick around for long on Earth, but survives on cold, dry Mars.

A key reason why Sagan and other early researchers suggested hematite was the absence of any signs of hydrogen or water in Martian spectra. A red durable compound containing two atoms of iron and three of oxygen, hematite contains no hydrogen.

But the origin of the hematite was still a bit of a mystery. Scientists think Mars’s early atmosphere contained little oxygen, explains Briony Horgan (Purdue), who wasn’t involved with the study. “It would have been hard to oxidize rocks at the surface,” she adds. So, while it was undisputed that the red color of Martian dust came from oxidized iron, iron oxide can take many forms, and which of them the oxidation produced on Mars has remained unclear for decades.

Solving the puzzle requires identifying which of the many types of rust colors Martian dust, says Adomas Valantinas (Brown University), who led a new study in Nature Communications.

“Nobody had ever looked at all iron oxide minerals to compare laboratory data with those from spacecraft,” Valantinas says. As a doctoral student at the University of Bern, he became curious about Martian dust and studied 10 different iron oxides in the laboratory. Then he decided to perform laboratory experiments on the ground to replicate the properties of Martian dust recorded by several spacecraft.   

In comparing the lab data to spectra from the European Space Agency’s ExoMars Trace Gas Orbiter, Valantinas thought that Martian dust most resembled ferrihydrite, a compound which contains water as well as iron and oxygen. On Earth, ferrihydrite forms in cool, moist environments but changes into other forms of iron oxide where it’s warmer or dryer.

In fact, Valantinas had personal experience with the orangish mineral, having collected samples from caves and lava tubes in the Azores. There, water percolating through iron-containing rocks at a triple junction of tectonic plates forms the mineral.

Valantinas spent 40 days testing how ferrihydrite would weather the Martian environment using a simulated Mars analog environment, operated by the University of Winnipeg. Exposed to intense ultraviolet light in a thin carbon-dioxide atmosphere where temperatures averaged -70°C (-94°F), ferrihydrite lost some water, but remained essentially frozen. Moreover, a kinetic model showed that at Mars-like temperatures, the compound would stick around for billions of years.

“All the evidence confirmed the initial hypothesis, which was surprising,” he says. To obtain the best match to the dust, he only needed to make one addition to the ferrihydrite: granulated basalt, a dark volcanic rock common on Mars and volcanic regions of Earth.  

Valantinas and colleagues conclude that ferrihydrite might have formed during an oxidative spell during the cold, wet period on early Mars, when liquid water and oxygen briefly became available for such reactions. The compound was then preserved even after the loss of liquid water, as the Martian climate cooled to the  current extremely dry deep freeze with thin air.

Horgan says their conclusion “that [Martian] dust contains ferrihydrite or something similar agrees with previous work.”

Next on the agenda, says Valantinas, is surveying Mars more thoroughly to find if the iron oxide in the dust might have originated in localized formations in specific areas, then scattered all over the planet by billions of years of windstorms.

“The best way to really solve this puzzle,” says Horgan, “would be to get a sample of Mars dust into our labs back on Earth. Indeed, some samples collected by The Perseverance rover are already sitting on Mars, waiting for the Mars Sample Return mission.