The Starset SocietyBy Rhodilee Jean Dolor
Findings of a new international study on the iconic red color of Mars are bolstering the popular theory that the now dusty, cold, desert world used to be wet and possibly habitable.
The Red Planet
Mars is known for its red color, which has long fascinated astronomers. According to the National Aeronautics and Space Administration (NASA), the ancient Romans named the planet after their god of war because its reddish hue is reminiscent of blood. The Egyptians also called Mars the “Her Desher,” which means “the red one.”
Many scientists accept the idea that Mars owes its color to the presence of iron oxide. The red-tinted compound formed when iron in the planet’s soil known as regolith reacted with water or with water and oxygen in the air, a process similar to how rust forms on Earth.
Over billions of years, the iron oxide disintegrated into dust and dispersed by winds across Mars, literally turning the extraterrestrial world into a red planet.
“Even today, it is frequently called the ‘Red Planet’ because iron minerals in the Martian dirt oxidize, or rust, causing the surface to look red,” NASA said.
But questions on which of the different kinds of iron oxide led to Mars’ distinctive red color remain. Researchers previously considered hematite, an iron oxide that can form without water, to be the reason behind the planet’s distinct hue.
The idea stems from previous analyses of iron oxide on Mars based on space observations. Researchers did not find traces of water, which led to the belief that the compound formed when the planet’s ancient lakes and rivers had already dried up.
According to Briony Horgan, professor of planetary science at Purdue University in West Lafayette, Indiana, oxidized iron can form without water.
“There are ways to form oxidized iron without water, and some proposed dry processes include surface oxidation like the oxidation rinds that form in rocks in the Antarctic Dry Valleys and surface oxidation by abrasion as the surface is blasted with sand grains over long periods,” Horgan said.
Water-rich Ferrihydrite
Findings of a new study now point to a different kind of iron oxide. The research, which was published in the journal Nature Communications on February 25, suggests that among the different minerals found on the Martian dust, a water-rich iron oxide known as ferrihydrite likely caused Mars’ reddish dust.
For the study, the researchers analyzed data from multiple Mars missions, which include orbital observation from the instruments on NASA’s Mars Reconnaissance Orbiter, the European Space Agency’s (ESA) Mars Express and Trace Gas Orbiter along with ground-level measurements gathered by the NASA rovers Curiosity, Sojourner and Opportunity. They also conducted laboratory experiments that tested how light interacts with ferrihydrite particles and other minerals under simulated Martian conditions.
The analysis showed that Mars’ red color is better matched by ferrihydrite.
“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 study researcher Adam Valantinas, a postdoctoral fellow at Brown University in Providence, Rhode Island, who started the work as a Ph.D. student at the University of Bern in Switzerland.
Because the mineral commonly forms in the presence of cool water, the researchers believe that it was likely created when the Red Planet still had wet areas on its surface billions of years ago.
“From our analysis, we believe ferrihydrite is everywhere in the dust and also probably in the rock formations, as well,” Valantinas said.
Mars’ Potentially Habitable Past
Liquid water is a crucial ingredient for living organisms to form and thrive, but the Martian atmosphere is too thin and cold to support water for prolonged periods on the surface. This condition makes Mars unlikely to host life as we know it.
But despite turning into the desert world that it is today, the planet has features that resemble dried up rivers and lakes. Minerals that only form when water exists are also present on the planet suggesting that water used to be abundant on its surface.
“Mars appears to have had a watery past, with ancient river valley networks, deltas, and lakebeds, as well as rocks and minerals on the surface that could only have formed in liquid water. Some features suggest that Mars experienced huge floods about 3.5 billion years ago,” NASA said.
Scientists no longer expect to find living things on Mars, but they continue to look for signs of life that may have existed when the planet was warmer and covered with water.
The finding of the study that Mars’ color is primarily due to ferrihydrite indicates that the planet used to have a wetter and potentially more habitable past since this iron oxide forms in the presence of cool water. It also supports theories that Mars had an environment that could sustain liquid water before it became a dry planet.
“We demonstrate that ferrihydrite provides the best spectral match to the color of Mars,” Valantinas and colleagues wrote in their study. “Here, we show that poorly crystalline ferrihydrite (Fe5O8H · nH2O) is the dominant iron oxide-bearing phase in Martian dust, based on combined analyses of orbital, in-situ, and laboratory visible near-infrared spectra.”
Valantinas said that questions about whether Mars has hosted life call for understanding the conditions that were present at the time the ferrihydrite mineral appeared on the planet.
Since ferrihydrite could only have formed when water was still present on the surface of Mars, the result of the study suggests that the alien world rusted earlier than previously thought.
“What we know from this study is the evidence points to ferrihydrite forming and for that to happen there must have been conditions where oxygen from air or other sources and water can react with iron,” Valantinas said. “Those conditions were very different from today’s dry, cold environment.”
