K+/Na+-smectite indicates a lake 3500 million years ago in Gale crater on Mars

Abstract

In Gale crater, the Curiosity rover has documented a sequence of ∼3500 million year old sedimentary rocks containing smectite, indicating the presence of liquid water. However, the paleoenvironment in Gale crater, particularly the question of whether a paleolake ever existed, remains hotly debated. Here, we compare the interlayer cations of smectite from terrestrial basalt weathering sequences and marine environments. Marine smectites are considered key analogs for smectites detected in martian paleolakes due to perceived similarities in formation processes and geochemical contexts between terrestrial marine settings and ancient lacustrine environments on Mars. Numerous observations show that K+/Na+-rich smectite is associated with marine environments, whereas Ca2+-rich smectite is typical of terrestrial basalt weathering sequences. We find that the basal spacings of K+- and Na+-smectite are ∼10.0 Å, whereas the basal spacings of Ca2+- and Mg2+-smectite range from 13.1 to 13.8 Å at a relative humidity (<1%) equivalent to that of the Curiosity rover on Mars. The XRD profiles measured by Curiosity in Gale crater show a basal spacing of ∼10.0 Å, which is attributed to K+/Na+-rich smectites. Our results support the idea that the presence of K+/Na+-rich smectites from Yellowknife Bay to Glen Torridon indicates that a lake existed in the 3500 million-year-old Gale crater, confirming that the habitable state lasted longer than previously thought.