What made Mars warm enough for water?
In 2013, NASA’s Mars Science Laboratory Curiosity rover discovered an ancient lake environment in Gale Crater, Mars. Sedimentological, stratigraphic and geochemical evidence confirmed the existence of a freshwater lake that persisted at Gale Crater for hundreds to thousands of years. Minerals, such as magnetite, and other compounds preserved in lacustrine mudstones, reflect redox gradients and neutral to alkaline pH conditions within sediment pore waters during and soon after deposition. The palaeoenvironment captured by these deposits shares a distinct set of physical and chemical characteristics with some lake systems on modern Earth, enabling detailed comparisons between the terrestrial and Martian sedimentary records of planetary habitability.
Nevertheless, the climate conditions that stabilized liquid water at Gale Crater remained unknown. Although CO2 is widely thought to have dominated the early Martian atmosphere, the Curiosity rover has only detected carbonate minerals at abundances significantly less than we would expect to find on Earth. At the same time, climate models demonstrate that without additional gases, CO2-rich atmospheres could not have maintained surface temperatures above freezing. Sulphur-bearing volcanic species were previously thought to trigger greenhouse warming, but recent models show that this would actually have the opposite effect, cooling the Martian surface rather than warming it.
To constrain atmospheric composition at the time mudstones were deposited at Gale Crater, researchers from the Department of Earth Sciences experimentally simulated chemical processes that formed new, or authigenic, minerals soon after deposition. Comparing experimental data with geological observations revealed that magnetite precipitation was likely triggered by the infiltration of groundwater into a lake equilibrated with a CO2-rich atmosphere. The experiments ealso showed that magnetite precipitation in Gale Crater mudstones would have been accompanied by the production of H2. Because H2 and its reaction products can raise annual mean surface temperatures above freezing in CO2-dominated atmospheres, magnetite authigenesis could have provided a short-term feedback mechanism for stabilizing liquid water once climate conditions allowed it to infiltrate surface and subsurface reservoirs.
Lead author, Professor Nick Tosca notes, “Our study shows that geological observations by the Curiosity rover are consistent with both estimated timescales and predicted climatic shifts associated with transient H2-induced warming.”
Tosca was assisted in this research by co-author and fellow Oxford researcher Dr Imad Ahmad, former postdoc Ben Tutolo, now at the University of Calgary, 4th year undergraduate alumna Alice Ashpitel, and Joel A. Hurowitz of Stony Brook University.
Paper: ‘Magnetite authigenesis and the warming of early Mars’ by Nicholas J. Tosca, Imad A. M. Ahmed, Benjamin M. Tutolo, Alice Ashpitel & Joel A. Hurowitz, was published in Nature Geoscience, doi.org/10.1038/s41561-018-0203-8
Image: Nick Tosca working in the ‘glove box’ for recreating Martian environments.