Why Mars could be red

Mars has captivated scientists and the public alike for centuries. One of the main reasons for this is its reddish hue, which has earned it the nickname "Red Planet". But what exactly gives Mars its iconic color? Scientists have wondered this for as long as they’ve studied the planet. Today, they may finally have a concrete answer, one that ties into Mars’ watery past.  

The new study, which has just been published in the journal Nature Communications, calls into question the previous assumption that hematite, a dry, rust-like mineral, is responsible for the color of Mars. Instead, the team, led by first author Dr. Adomas Valantinas, identified the water-rich iron mineral ferrihydrite as the main cause of the reddish coloration. Valantinas was a PhD student in the Department of Space Research and Planetary Sciences (WP) at the Physics Institute of the University of Bern until December 2022 and worked there as a postdoc researcher until April 2023. He is currently a visiting researcher at Brown University (USA) thanks to the Postdoc.mobility grant from the Swiss National Science Foundation (SNSF) and conducts research in the laboratory of planetary scientist Prof. Dr. Jack Mustard, who is also co-author of the study. Valantinas started the work on the current study as a doctoral student at the University of Bern. Also involved in the study are Prof. Dr. Nicolas Thomas and PD Dr. Antoine Pommerol from the University of Bern, both from the Department of Space Research and Planetology (WP) at the Physics Institute.

Innovative methods and international synergies

First author Valantinas says: "The fundamental question of why Mars is red has been pondered for hundreds, if not thousands, of years." For the current study, the research team combined observational data from space probes and Mars rovers with novel laboratory methods. "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."

The team worked with data from several Mars space probes, including NASA's Mars Reconnaissance Orbiter and the European Space Agency's Mars Express and ExoMars. On board the ExoMars probe is the Color and Stereo Surface Imaging System (CaSSIS), a camera system developed and built by an international team led by Nicolas Thomas. "The initial finding was made with CaSSIS and pointed to ferrihydrite. Further research utilizing higher resolution data confirmed the initial CaSSIS finding," Valantinas explains. Nicolas Thomas adds: "CaSSIS has been observing Mars since April 2018 and provides high-resolution color images of the Martian surface. The fact that CaSSIS images are repeatedly used for studies speaks to the impressive scientific capabilities of the Bernese camera system."

The data from Mars' orbit was combined with measurements from NASA's Mars rovers such as Pathfinder, Curiosity and Opportunity and with laboratory analyses of synthetic, Mars-like materials. Antoine Pommerol explains: "Satellite data are invaluable in providing new insights into the surface and history of Mars, but ground truth is often needed for interpretation. This might come from in-situ rovers and landers, Martian meteorites or studies of Earth analogues, in the field or in the laboratory. Here, a combination of laboratory characterizations by different teams, including the unique reflectance measurement capabilities at the Ice Laboratory in Bern, strongly supports the theory that ferrihydrite is responsible for the reddish color of Mars."

Further experiments and measurements were carried out at the University of Grenoble, Brown University and the University of Winnipeg. Valantinas says: "Overall, our analyses show that ferrihydrite is widespread in Martian dust and probably also in the rock layers."

Waiting for samples from Mars

The discovery of ferrihydrite as the main component of Martian dust has far-reaching implications for understanding the history of Mars and whether there was ever life on Mars. 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," says Valantinas. He continues: "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."

"The study is really a door opening opportunity," said co-author Jack Mustard. "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."