image: View of Mount Sharp, Mars, with buttes showing main stratigraphy of the sulfate-bearing unit to be explored by the Curiosity rover, and expected ancient environments based on observed sedimentary structures.
Boulder, Colo., USA: Gale Crater's central sedimentary mound (Aeolis Mons or, informally, Mount Sharp) is a 5.5-km-tall remnant of the infilling and erosion of this ancient impact crater. Given its thickness and age, Mount Sharp preserves one of the best records of early Martian climatic, hydrological, and sedimentary history.
In this paper, published today in Geology, William Rapin and colleagues present the first description of key facies in the sulfate-bearing unit, recently observed in the distance by the rover, and propose a model for changes in depositional environments.
The basal part of this sedimentary sequence is ahead of the Curiosity rover traverse and was recently analyzed with unprecedented resolution by the rover cameras. The telescopic imager of the ChemCam instrument was used here in particular, and its images show sedimentary structures that reveal evolution of environments on Mars during the Hesperian age (3.7-2.9 billion years ago).
Analysis of the structures shows that on top of the ancient lake deposits currently explored by the rover (Murray formation), vast aeolian deposits were formed by a dune field during a prolonged dry climatic episode. Yet, higher up, the stratigraphy reveals the resumption of wetter climatic conditions.
The climate of Mars appears therefore to have fluctuated several times at high order between dry conditions and wet conditions in the Hesperian age, a period during which Mars' environment is thought to have changed globally due to the gradual loss of its atmosphere to space.