The color red and the planet Mars go together like Earth and its oceans or Saturn and its rings. However, our understanding of how the Red Planet got its signature hue might be wrong. A new hypothesis focusing on ferrihydrite is detailed in a study published February 25 in the journal Nature Communications. The ferrihydrite might have formed back when there was still water on the planet–potentially when it was habitable.
Seeing red
In the decades that astronomers have been studying Mars, we’ve learned that its distinct color comes from rusted iron minerals in the dust that covers the planet. At some point in the Red Planet’s 4.5 to 4.6 billion year history, the iron bound up in its rocks reacted with water and oxygen in the air, similar to how rust forms here on Earth. The resulting iron oxide was then broken down into dust and was spread around the planet by winds.
However, iron oxides come in different varieties. Astronomers have debated the exact chemistry of Martian rust for years. Figuring out how this rust forms is a crucial window into Mars’ environmental conditions at the time of rusting. Closely linked to that environmental question is whether Mars has ever been habitable.
Earlier studies of the iron oxide component in Martian dust that were based on spacecraft observations alone did not find any evidence of water within it. This led researchers to conclude that this particular type of iron oxide must be a mineral called hematite.
[ Related: Europe’s first Mars rover will use nuclear-powered heaters. ]
Which iron oxide is it anyway?
This new analysis combining spacecraft observations with some newly developed laboratory techniques shows that Mars’ red color is better matched by iron oxides containing water–known as ferrihydrite–and not hematite.
Ferrihydrite typically forms quickly around cool water, which means it must have formed when Mars still had water on its surface. The ferrihydrite has kept this watery signature to the present day, despite being constantly ground down and spread around the planet for billions of years.
While trying to create a replica of martian dust in the lab with different types of iron oxide, the team from this study was in for a surprise.
“We found that ferrihydrite mixed with basalt, a volcanic rock, best fits the minerals seen by spacecraft at Mars,” Adomas Valantinas, a study co-author and postdoctoral researcher at Brown University, said in a statement.
While Mars is still undoubtedly the Red Planet, this new finding changes our understanding of why it is red.
“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,” said Valantinas. “Moreover, the ferrihydrite remains stable under present-day conditions on Mars.”
Dust in the (Martian) wind, but on Earth
Previous studies have suggested that ferrihydrite might be present in martian dust, but this research offers some of the first comprehensive proof with a unique combination of space mission data and lab-based experiments.
In the study, they used an advanced grinder machine to create a realistic replica of the martian dust. The grinder can create a dust grain equivalent to the size of 1/100th of a human hair. The team then analyzed the samples using the same techniques that orbiting spacecraft will use in order to make a direct comparison. This pinpointed ferrihydrite as the best match.
“This study is the result of the complementary datasets from the fleet of international missions exploring Mars from orbit and at ground level,” Colin Wilson, the European Space Agency’s (ESA) Trace Gas Orbiter (TGO) and Mars Express project scientist, said in a statement.
Mars Express’ analysis of the dust’s mineralogy helped show how even highly dusty regions of Mars have water-rich minerals. Additionally, TGO’s unique orbit allows it to view the same region under different lighting and angles. This allowed the team to disentangle particle size and composition, which proved essential for recreating the correct dust size in the lab.
Other data from NASA’s Mars Reconnaissance Orbiter and ground-based measurements taken by NASA Mars rovers Curiosity, Pathfinder, and Opportunity, also helped make the case for ferrihydrite.
“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 Wilson. “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.”
However, the samples from Perseverance are not expected to make it back to Earth until 2040, pushed back from the original target of 2031. So for now, we will still be debating just how Mars got so red.
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