Olivine above the terrace
SuperCam reads the Margin unit above -2350 m as a slow-cooled, crystalline, olivine-rich rock without significant water exposure. The ancient lake shoreline suggested by orbital data does not describe that rock.[1]
Below the second terrace the picture changes. Exposures near Neretva Vallis and the Western fan carry water-rock interaction and physical reworking. The same unit splits, by elevation, into two water histories.[1]
One shoreline falls short
The paper counts 3 alteration episodes. Neutral-to-alkaline CO2-rich fluids left carbonate-rich ridges. Jezero paleolake waters or a change in groundwater then moved carbonate and precipitated silica in pores. Late hydrothermal fluids left fluorite-bearing Ca-sulfate veins in younger fractures.[1]
A single lake shoreline does not explain the dry olivine-rich rock above -2350 m and the late Ca-sulfate veins below as one contact. Astrobiological interest shifts to the lower rock, where the lake and groundwater worked in sequence. The upper rock still lacks significant water exposure.[1]