Widespread Diagenesis at Unconformities in Gale Crater as Inferred From the Curiosity Rover and From Orbit

Abstract

NASA's Curiosity rover has found widespread evidence of alteration in sedimentary rocks in Gale crater, Mars driven by interactions with fluids both before and after lithification (early and late diagenesis). Most notably, Curiosity observed distinctive color, chemical, and mineralogical changes interpreted as evidence of diagenesis at the unconformity between Mt. Sharp group fluvial/lacustrine mudstones and Siccar Point group (SPg) aeolian sandstones, a part of the larger Mound Skirting Unit (MSU) that mantles Mt. Sharp. However, the distribution of diagenesis across Mt. Sharp beyond Curiosity's traverse is poorly constrained. In this study, we use orbital color images and spectroscopy to characterize diagenesis-driven alteration at the MSU unconformity elsewhere in Gale. We find that color variations similar to those observed by Curiosity appear at the MSU unconformity across Mt. Sharp and exhibit spectral properties consistent with hydrated silica, suggesting that some of the alteration observed by Curiosity below the MSU unconformity was extensive across Mt. Sharp. We hypothesize that fluid flow was extensive throughout the MSU, but diagenesis was locally enhanced by permeability differences across the unconformity. In this model, more permeable SPg/MSU sandstones provided a conduit for subsurface fluids that stagnated within and altered the upper few meters of less permeable (e.g., clay-bearing) Mt. Sharp group strata below. The extensive diagenesis observed in Gale implies that subsurface fluids were long-lived and widespread. Gaining a better understanding of what rock properties control and influence diagenetic fluid flow will help us improve the search for ancient aqueous environments and possible biosignatures on Mars.