Why is Mars red? New research reveals answer to ancient mystery
Mars has shone red in the night sky for as long as humans have gazed up at the cosmos, fascinating people from the ancient Romans to the present day.
"The fundamental question of why Mars is red has been pondered for hundreds, if not thousands, of years," said Brown University researcher Adomas Valantinas, lead author of a new study published Tuesday.
Now, he says, "our understanding of why Mars is red has been transformed." The new explanation for the reddish color – the rusty-colored dust known as ferrihydrite – also will help our understanding of the history of water and the possibility of life on Mars.
Rusty red dust
Scientists have long surmised that rusty iron oxide is what made Mars red: Thanks to the fleet of spacecraft that have studied Mars over the past few decades, scientists knew that the red color is because of iron minerals in the soil that have undergone the rusting process.
Over billions of years, this rusty material has been broken down into dust and spread all around Mars by the wind, a process that continues today.
But iron oxides come in many flavors, and the exact chemistry of Martian rust has been intensely debated, the study reports.
Hematite or ferrihydrite?
Because of the absence of liquid water on Mars’ surface today, those rusty red minerals were thought to arise from dry iron oxides present in the dust, such as hematite.
But new analysis of spacecraft observations along with laboratory techniques suggests Mars’ red color is better matched by iron oxides containing water, known as ferrihydrite. Ferrihydrite typically forms quickly in the presence of cool water and so must have formed early on ancient Mars when the planet was still wet.
"We are not the first to consider ferrihydrite as the reason for why Mars is red, but it has never been proven as we have now, using observational data and novel laboratory methods to produce Martian dust analog in the lab," Valantinas said.
Far-reaching implications for life on Mars
The discovery has far-reaching implications for understanding the history of Mars and whether there was ever life on Mars, the new study suggests. Unlike hematite, which forms in warm or dry conditions, ferrihydrite forms in the presence of cool water.
"This suggests that Mars once had an environment where liquid water was present, which is an essential prerequisite for life," Valantinas says. "Our study reveals that ferrihydrite formation on Mars required the presence of both oxygen – whether from the atmosphere or other sources – and water capable of reacting with iron. These conditions were very different from the dry, cold environment of Mars today."
'A door-opening opportunity'
"The study is really a door-opening opportunity," study co-author Jack Mustard said in a statement. "As exciting as the new findings are, we realize that our results can only be verified by samples from Mars, which are currently being collected by NASA's Perseverance rover. When we get those back, we will be able to check whether our theory about ferrihydrite is correct."
The study was published Tuesday in the peer-reviewed British journal Nature Communications.