In today’s plenary lecture titled ‘Mars: Geology and Geochemical Cycling on a Once Habitable Planet’, Professor Bethany Ehlmann captures our imaginations by taking us on a whirlwind journey back in time and far away to the beloved red planet, Mars.

Ehlmann’s journey starts on modern Mars, a harsh environment, characterized by cold, dry, oxidative weathering, teetering on the edge of habitability with only hints of modern liquid water. By turning back the clock by as little as a few hundred thousand years to a time when the first homosapiens were wondering around on Earth, Ehlmann shows us that Mars was quite recently a habitable planet, with much more of its surface being able to support liquid water. Highlights from the trip into the Martian past include visits to ancient lakes and the layered sediments they left behind – images that are surprisingly reminiscent of varves to my eye – as well as shallow, brine-rich playa lakes, and muddy ground that dries to form beautiful mudcracks. Her tour also plunges beneath the surface to reveal just how pervasive groundwater was on Mars. She shows us evidence of its widespread involvement in diagenesis and fumarolic activity, processes well known here on Earth. I was particularly entranced by the evidence Ehlmann presents for mineral carbonation found in the nearly 4-billion-year-old rocks of the Isidis Basin, a process that sequesters CO2 in ultramafic rocks on Earth today and is used by geo-engineers to help us fight our human-driven climate change problem.

From the mineralogical and geochemical records highlighted in Ehlmann’s tour, it is clear that liquid water has been a major player in shaping the Martian surface. She leaves us with some key outstanding questions, such as:

1. What was the nature of Martian geochemical cycles and how much ‘cycling’ do they actually involve in the absence of plate tectonics?
2. What can the Martian rock record tell us about the effect of key planetary-scale events (e.g., magnetic field decline, impacts, stellar evolution, and atmospheric loss) on conditions at the surface?

Mars is the ideal field site for exploring these questions thanks to the absence of plate tectonics and its corresponding metamorphism. 50% of the surface preserves the first 3 billion years of Martian history. In contrast, this timeframe corresponds to <2% of Earth’s surface and it is certainly metamorphosed.

Finally, she gives us a promise for more answers thanks to new information coming in from the flotilla of orbiters and the rovers’ mobile analytical laboratories, as she described them. It won’t be long until we can be inspired by new discoveries from the 2020 Mars Rover Mission sample and return.

Video: https://www.youtube.com/watch?v=e0yU1vlkEE0&feature=emb_imp_woyt&ab_channel=goldschmidtconf