Fe-depleted, weathered rocks in the ancient Martian crust point to an early, reduced atmosphere
Climate models used to explain warming on ancient Mars rely on reduced greenhouse gasses such as methane and hydrogen, but until now physical evidence that such a reduced atmosphere actually existed has been lacking. Our results document Fe-mobility in a reduced surface environment on ancient Mars.
Iron-rich materials turn to rust, or Fe-oxide very quickly on this planet due to our oxidizing atmosphere. But this wasn’t always the case on Earth. In the first ~2 billion years after the planet formed, Earth’s atmosphere was a noxious mixture of reduced volcanic gasses such as methane, carbon monoxide, ammonia, and hydrogen, in addition to nitrogen and carbon dioxide. Though the the abundances of these various gasses on the early Earth are actively debated, it is well recognised that together they created a reduced greenhouse that warmed the our planet's atmosphere, despite the Faint Young Sun’s lesser radiance.
Earth scientists know well that our planet underwent the most substantial change in its history around 2.45 billion years ago when it shifted from a reduced planet to an oxidizing one in what is known as the Great Oxygenation Event or GOE. Whether the planet “skidded,” “hopped” or gradually transitioned in its global oxygenation is open to debate, but we mostly agree on the rough timing and main aspects of the major transition.
Like Earth, modern Mars is an oxidized world, full of rust. Scientists have understood for some time that the red-ochre color of Mars is attributed to fine-grained Fe-oxides that envelope the surface in a layer of dust and hematite-armored sand. It is a desolate, frozen, hyperarid hellscape today. But about 3-4 billion years ago, the planet was at least occasionally warm enough to allow water to flow freely, carving river channels that persist today. The questions of how and when these climate excursions occurred has driven a large fraction of my career as a planetary geologist.
My scientific journey has led from Arizona to Pasadena, USA, on to Paris, France, London, UK and eventually to Hong Kong, and along the way has always been focused on the exploration of Mars’ surface mineralogy using infrared remote sensing with the goal of understanding ancient climate, aqueous alteration processes and habitability. My path started as a graduate student under Professors Phil Christensen and Tom Sharp at Arizona State University 20 years ago. At that time, we struggled to evaluate thermal infrared emission data, which seemed to suggest the surface was widely weathered. But some in the community were convinced that Mars was never warm enough to allow alteration minerals to form. The major breakthrough came when the European Space Agency launched the Mars Express spacecraft in 2003, carrying with it the Observatoire pour la Mineralogie, l’Eau, les Glaces et l’Activité (OMEGA). Discoveries made by the OMEGA team showed definitively that clay minerals were widespread in the ancient crust of Mars, implying that Mars was warm enough to allow sustained interaction between primary rocks in the crust and surface or subsurface water. Since then (c.a. 2006), the Mars science community has spent significant time, energy and resources to determine exactly how the puzzle pieces of surface mineralogy fit together to form a picture of Martian climate.
For years, scientists have argued that in order to allow for liquid water on the surface of Mars, an ancient greenhouse must have been present. But, the only models that could produce a climate warm enough to result in the geology we see required that the ancient Martian atmosphere was a reduced greenhouse with appreciable amounts of methane and hydrogen, among other gases. Until now, we could not prove that the reduced ancient environment of Mars actually existed.
In a recent article published in Nature Astronomy, we have investigated chemical weathering profiles of ancient bedrock exposures on Mars using hyperspectral infrared imaging data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) and high spatial resolution false color images from the High Resolution Science Experiment (HiRISE) camera. On Earth, we know that soil profiles and chemical weathering profiles imposed on bedrock exposed to the air and water result in distinct distributions of various cations, anions and the minerals they reside within. On Mars, we have investigated this same concept to explore how the mineralogy and geochemistry have been reorganized and redistributed due to the effects of chemical weathering in the Noachian (>3.5 billion years ago).
My PhD student Jiacheng LIU carried out many detailed analyses of spectral data from Mars. Importantly, he thoroughly studied rocks on Earth that are analogous to those on Mars. He applied reflectance spectroscopy and other geochemical techniques to systematically analyse a long drill core extracted from a thick sequence of chemically weathered basalt form Hainan Island, China. Though the tropical climate of Hainan is extremely warm and wet, the trends observed in the weathered basalt, which is analogous to volcanic rocks that cover much of Mars, provide invaluable insights to how the spectral trends are linked to mineralogical and chemical trends resulting from weathering. Jiacheng was able to show some valuable spectral indices that can quantify chemical weathering trends, which we published in Applied Clay Science late in 2020. The comparison of these datasets and application to Mars remote sensing data revealed a striking discovery.
CRISM spectral data and HiRISE color data show striking patters in the uppermost crust of Mars. The spectral data show that the weathered horizons representing the ancient surface of Mars are rich in aluminium, but this part is not surprising. Previous work by John Carter and colleagues at the Institute d’Astrophysique Spatiale, in Orsay, France where I had previously worked had already shown the Al-enrichment. The key missing puzzle piece that we revealed is that this material is quite depleted in iron.
Element mobility means a lot to geologists investigating weathered rocks. In the case of weathered Martian rocks, the abundance of aluminium is not too surprising. Though Al is not abundant in Martian basalts when they crystallize from lava, the Al-content increases as other elements are removed from the system by dissolution and leaching during weathering. Aluminium is immobile under most reasonable conditions, and so the removal of other more mobile elements results in an accumulation of residual Al in the weathered rock. But, the Martian weathered rocks are also missing Fe, which under modern conditions on the Earth or Mars, should also be immobile just like Al.
Under oxidizing conditions, Fe occurs in its ferric form (Fe3+), which is insoluble. But in reducing conditions, Fe occurs in its ferrous form (Fe2+), which is soluble. Jiacheng’s work shows that Fe is depleted in the weathered Martian rocks, which means it had to be removed in its ferrous form. Both Fe and Al are immobile in oxidizing conditions, but Fe is mobile and would be separated from Al in reducing conditions. This is the basis of Jiacheng’s discovery.
This hypothesis that Mars once had a reduced climate but underwent a period of oxidation is testable not only with further remote sensing and laboratory studies, but also with rovers on the surface – and the timing couldn’t be better. As NASA’s Curiosity Rover continues to operate in Gale Crater, the NASA Perseverance Rover is in its final approach to Mars where it will land on February 18th 2021 and China's Tianwen-1 spacecraft will land on the surface later this spring. Though it will take some time to fully test our hypothesis, we suggest that data collected by the rover can be used to search for Fe-mobility in ancient, weathered rocks, which will provide critical addition clues to the ancient climate of Mars.
Liu, J., HP He, J. Michalski, J. Cuadros, Y. Yao, W. Tan; X. Qin, S. Li; G. Wei (2020), Reflectance Spectral Study of Clay Mineralogical Evolution and Weathering Intensity of A Thick Basaltic Weathering Sequence in Hainan Island, South China, Applied Clay Science.