What started as a simple inexplicable number (2,236 µm) has become a time capsule revealing how water, heat, and oxygen transformed Martian rocks billions of years ago. A group of NASA scientists led by J.L. Bishop has solved a mystery that had been puzzling them for over a decade without answer. The study has been published in Nature Communications: the light peak at 2,236 µm on Mars now has an origin. Thanks to CRISM mapping work from orbit around Mars, that wavelength peak was detected, which could not be explained, as it did not match any known mineral on Mars. This anomaly was persistent, especially in two regions: Aram Chaos and the Juventae Plateau in the Valles Marineris system.
The team recreated the conditions of Primitive Art in the laboratory, and it was in this way that they witnessed the appearance of a solid that had exactly the same conditions as the one that until now had no explanation. This fact demonstrated that the mineral responsible was ferric hydroxy sulfate (Fe³⁺SO₄OH), which does not form on its own, but requires other compounds such as hydrated ferrous sulfates, as well as high temperatures. This discovery not only allows for the identification and understanding of Martian volcanism, but also enables the creation of a timeline that allows for a better and more precise understanding of Mars’ climatic history.
Solving Martian riddles
What had been a mystery for more than a decade has finally been solved by a team of NASA researchers, led by J.L. Bishop. This study, which has been published in Nature Communications, has managed to identify a peak of light coming from Mars, which until now had no explanation. The data collected by the CRISM instrument was not new; it has been mapping minerals in its orbit around Mars, always based on how minerals reflect light at different wavelengths.
There was one piece of data that did not correspond to any mineral, neither common clays, nor gypsum or jarosite, nor other Martian salts and silicates. It was a wavelength of 2.236 microns (µm), which appeared recurrently in two specific areas: Aram Chaos and the Juventae Plateau, in the gigantic Valles Marineris system. One of the theories considered was that it came from a mineral that does not naturally occur without heat and oxygen: crystalline ferric hydroxy sulfate.
Laboratory test
To confirm or disprove this theory, it was necessary to recreate the exact Martian conditions. To do this, the team heated hydrated ferrous sulfates such as rozenite (FeSO₄·4H₂O) and szomolnokite (FeSO₄·H₂O) between 100 and 300 °C. As the temperature rose, the chemistry changed. An orange solid appeared: Fe³⁺SO₄OH, with the same collection of narrow bands that CRISM observes, including the one at 2.236 µm. This result matches the proposed crystal structure: when Fe²⁺ is converted to Fe³⁺, a proton is lost in bridging groups, and the OH that characterizes the new solid appears.
Stratigraphy also confirms it
The presence of sulfates manifests differently in the Juventae Plateau and in Aram Chaos. In Juventae, the mineral appears over the older sulfates, interspersed with basalts. This suggests that the heat came from above, possibly from a lava flow or falling hot ash that baked the underlying layers. In contrast, in Aram Chaos (associated with catastrophic water outflow), the mineral appears in the lowest stratum, near the bedrock. This indicates geothermal heating (from below).
Conclusions
The conclusions of this study establish that ferric hydroxy sulfate (Fe³⁺SO₄OH) has the ability to act as a thermometer and oxygen marker on Mars. Its presence on the Red Planet indicates that Mars experienced pulses of localized heat of volcanic or geothermal origin during the later periods of its geological history. The fact that this mineral only forms in the presence of water, salt, heat, and oxidation helps to be more precise in determining which geochemical conditions were crucial for understanding Mars’ past climate.