Water flowed across the surface of Mars during its early history billions of years ago, as evidenced by the fluvial landforms that cover the Martian surface. During that time, between 4.5 and 4.1 billion years ago, the atmosphere of the Red Planet would have had to have been denser than it currently is and filled with greenhouse gases like carbon dioxide that supported a warm and wet climate.
In the eons that followed, however, much of the planet’s carbon dioxide escaped to space through physical processes like photochemistry, erosion from asteroid impacts, and surface weathering that sequestered the gas as carbonates in the soil. Yet water continued to flow, even after the carbon dioxide that warmed the atmosphere had escaped.
Scientists struggle to emulate the climate during that period of Mars’s history; without carbon dioxide in the atmosphere, climate models cannot warm the planet enough to support flowing water. This problem is a significant piece of the Martian climate puzzle.
In new research, Navarro-González et al. present what could be the missing piece to the climate puzzle: hydrogen. In the absence of carbon dioxide, hydrogen emitted from volcanoes could have kept the planet from freezing and could have allowed water to continue flowing freely, even after other greenhouse gases had disappeared. The authors keyed in on hydrogen after the Curiosity rover revealed nitrates and nitrites in sediments along the Gale crater. On Earth, plants fix these forms of nitrogen, but on Mars, the process was probably triggered by kinetic energy provided by volcanic lightning, ultraviolet light, or shock waves from bolide impacts and would have likely needed a hydrogen-rich atmosphere.
To investigate their hypothesis that hydrogen helped warm the ancient Martian atmosphere, the authors began by simulating Mars’s atmosphere in the lab, filling tanks with different gas compositions of carbon dioxide, nitrogen, and hydrogen. They then blasted laser beams into the gases to create shock waves and mimic energy inputs that would have jolted the Martian atmosphere.
The authors found that the amount of fixed nitrogen on Mars required a hydrogen-rich atmosphere, which supports the idea that hydrogen helped warm the planet billions of years ago. While investigating the sediments from the Gale crater, the authors also found evidence that the fixed nitrogen was initially deposited on the surface of the crater and then transported to the crater’s lake during wet times.
In younger sediments collected by Curiosity, the amount of nitrogen declined sharply. This finding suggests hydrogen eventually dissipated from the atmosphere. The loss of nitrogen would have created a crisis for soil microbes on Mars, similar to a nitrogen crisis that occurred during Earth’s early history.
The findings provide scientists with significant new clues about the Martian climate puzzle and also offer new insights into the nitrogen cycle on Mars. (Journal of Geophysical Research: Planets, https://doi.org/10.1029/2018JE005852, 2019)
—Aaron Sidder, Freelance Writer