The importance of lake breach floods for valley incision on early Mars

The surface environment of early Mars had an active hydrologic cycle, including flowing liquid water that carved river valleys^1-3 and filled lake basins^4-6. Over 200 of these lake basins filled with sufficient water to breach the confining topography^4,6, causing catastrophic flooding and incision of outlet canyons^7-10. Much past work has recognized the local importance of lake breach floods on Mars for rapidly incising large valleys^7-12; however, on a global scale, valley systems have often been interpreted as recording more persistent fluvial erosion linked to a distributed Martian hydrologic cycle^1-3,13-16. Here, we demonstrate the global importance of lake breach flooding, and find that it was responsible for eroding at least 24% of the volume of incised valleys on early Mars, despite representing only approximately 3% of total valley length. We conclude that lake breach floods were a major geomorphic process responsible for valley incision on early Mars, which in turn influenced the topographic form of many Martian valley systems and the broader landscape evolution of the cratered highlands. Our results indicate that the importance of lake breach floods should be considered when reconstructing the formative conditions for Martian valley systems.

Sequestration of Martian CO2 by mineral carbonation

Carbonation is the water-mediated replacement of silicate minerals, such as olivine, by carbonate, and is commonplace in the Earth’s crust. This reaction can remove significant quantities of CO₂ from the atmosphere and store it over geological timescales. Here we present the first direct evidence for CO₂ sequestration and storage on Mars by mineral carbonation. Electron beam imaging and analysis show that olivine and a plagioclase feldspar-rich mesostasis in the Lafayette meteorite have been replaced by carbonate. The susceptibility of olivine to replacement was enhanced by the presence of smectite veins along which CO₂-rich fluids gained access to grain interiors. Lafayette was partially carbonated during the Amazonian, when liquid water was available intermittently and atmospheric CO₂ concentrations were close to their present-day values. Earlier in Mars’ history, when the planet had a much thicker atmosphere and an active hydrosphere, carbonation is likely to have been an effective mechanism for sequestration of CO₂.

The mechanism by which Mars lost its early dense and carbon dioxide-rich atmosphere remains relatively unknown. Here, via mineralogical analysis of the Lafayette Martian meteorite, Tomkinson et al. infer that carbonation was an effective carbon dioxide sequestration mechanism on an early, water-rich Mars.

Subsurface water and clay mineral formation during the early history of Mars

Clay minerals, recently discovered to be widespread in Mars's Noachian terrains, indicate long-duration interaction between water and rock over 3.7 billion years ago. Analysis of how they formed should indicate what environmental conditions prevailed on early Mars. If clays formed near the surface by weathering, as is common on Earth, their presence would indicate past surface conditions warmer and wetter than at present. However, available data instead indicate substantial Martian clay formation by hydrothermal groundwater circulation and a Noachian rock record dominated by evidence of subsurface waters. Cold, arid conditions with only transient surface water may have characterized Mars's surface for over 4 billion years, since the early-Noachian period, and the longest-duration aqueous, potentially habitable environments may have been in the subsurface.