The red color of our neighboring planet is unmistakable – both in recordings of room probes and in the night sky. But what does his rust -red color give him? It has been clear for a long time that a form of iron oxide must be responsible for it, which remained controversial. Now a combination of spectral analyzes of Marsonden and rods with laboratory tests reveals surprising: Unlike expected, the Marsian rust does not consist of the almost water -free mineral hematite, but of the hydrated iron oxide ferrihydrite. This discovery suggests that the Mars dust did not gradually oxidize over billion years under largely dry conditions, but rather much earlier and under moist conditions than previously assumed.
The Mars is covered by fine, red dust and also swirls these dust particles in the atmosphere and color the sky reddish. Even from the earth, our neighboring planet appears slightly reddish, which brought him the nickname “Red Planet”. But what gives Mars its iconic color? Because the crust of Mars mainly consists of basalt rocks, its surface should actually appear rather dark gray, similar to the Mary of the moon. However, the common theory assumes that the weathering of the rock has caused the red color. The plenty of iron in it was oxidized – it rusted. However, there are different variants of iron oxide, which are formed under different conditions. Knowing which this oxide gives the Mars its color is therefore also important for the reconstruction of its past.
According to the very fine dust of Mars, common theory consists primarily of hematite or the magnetite -like mineral Maghämit. “A broad mineralogical model suggested that these non -hydrated iron oxides in Mars dust were formed by persistent oxidation and weathering under water surface conditions,” explain Adomas Valantinas from the University of Bern and his colleagues. According to this theory, the rust formed in the course of billions of years during the cold dry era of the red planet, which continues to this day. However, spectral data of several spatial probes and telescopes showed that the red Mars dust in its spectrum has a striking absorption tip at around three micrometers wavelength. “It has been shown that this spectral signature is due to tightly bound water or hydroxyl groups in the mineral structure of the dust,” writes the team. This raised the question of whether the iron oxide can really be the non -hydrated hematite. However, the extremely fine grain of the Mars dust made it difficult to collect more precise data. “In the area of nanocrystals, the clear characteristics of the various iron oxides with shrinking particle size and crystallinity disappear,” explain Valantinas and his colleagues. As a result, the spectral lines appear increasingly widened and diffuse.
Ferrihydrite instead of hematite
In order to create more clarity, Valantinas and his colleagues have now evaluated spectral data that come from various Mars orbiter probes such as the Mars Reconnaissance Orbiter of NASA and the European probes Mars Express and Trace Gas Orbiter, but also from Marsrovern. The team then used these spectra to test in laboratory experiments which iron oxide particles generate these spectral signatures under simulated Mars conditions. To do this, they mixed hematite and other “dry” oxides as well as the hydrated iron oxide mineral ferrihydrit with basalt rock. “We used a special grinding machine to bring our components except for submikrometer sizes,” reports Valantinas.
The comparison analyzes showed that hematite and other “dry” iron oxides do not match the spectral data even when mixing with basalt. Instead, a mixture of basalt and the iron oxide hydroxide ferrihydrite (Fe The fact that it is not hematite, but Ferrihydrite is responsible for the red color of Mars, has far -reaching effects. Because it throws a new light on the conditions of origin of this “Marsrosts”. While hematite can form under dry conditions over the course of a long period of time, Ferrihydrit is created relatively quickly and in the presence of cold water. “Our study shows that to form Ferrihydrit on Mars, both oxygen – whether from the atmosphere or other sources – were necessary. This indicates that Mars once had an environment in which liquid water was present, ”says Valantinas. “These conditions differ significantly from the dry, cold surroundings of today’s Mars.” In addition, Ferrihydrite often occurs on earth together with volcanic rocks. According to the researchers, these factors suggest that the “Marsrost” did not develop over the millions of millions, but within a relatively short period of time during the Hesperian age around three billion years ago. At that time, Mars’s more lifeline had already ended and liquid water only occurred sporadically. There was an intensive volcanic activity that could have favored the formation of the Ferrihydrit. “Mars is still the red planet. But our ideas about why the Mars is red have changed, ”says Valantinas. “The most important implication is that Ferrihydrit could only be formed when there was still water on the surface. So the Mars was rusted earlier than we thought so far. ” He and his colleagues are now looking forward to the rehearsals that the NASA Rover Perseverance has collected on Mars and which are to be brought to earth in a future return mission. “Some of the rehearsals already collected by Perseverance contain dust. As soon as we have these valuable rehearsals in the laboratory, we can measure exactly how much ferrihydrite the dust contains and what this means for our understanding of the history of water – and the possibility of life – on Mars, ”says he does not involve the study ESA planetary researcher Colin Wilson. Source: Adomas Valantinas (University of Bern) et al., Nature Communications, DOI: 10.1038/S41467-025-56970-ZCold water and fast rusting