An in‐situ record of major environmental transitions on early Mars at Northeast Syrtis Major

The Noachian‐Hesperian transition on Mars was a period marked by changes in volcanic processes and styles of aqueous alteration. Understanding the timing and nature of environmental change requires the exploration of units recording both sets of processes. Herein, we report the compositional stratigraphy of distinctive Noachian to Hesperian units along the northeastern margin of the Syrtis Major volcanic flows. A layered, polyhydrated sulfate‐bearing unit with jarosite ridges has been discovered beneath the Syrtis Major lava flows and above the regionally‐extensive stratigraphy of Noachian plains units reported previously. Sequential clay‐, carbonate‐, and sulfate‐bearing units formed in‐situ and record a transition from alkaline pH to acidic pH waters. The sequence is chronologically bookended by the Isidis impact and Syrtis Major flows, and is one of the most temporally‐constrained and well‐preserved stratigraphic sections from early Mars available for landed exploration.