The Zhurong rover, part of China’s Tianwen-1 Mars exploration mission, has discovered evidence of liquid water on dune surfaces at low Martian latitudes. The discovery was made by researchers from the Institute of Geology and Geophysics (IGG) of the Chinese Academy of Sciences (CAS), the National Astronomical Observatories of CAS, and the Institute of Atmospheric Physics of CAS. Previous studies had shown proof of a large amount of liquid water on early Mars, but with the escape of the early Martian atmosphere during the later period, the climate changed dramatically. Very low pressure and water vapor content make it difficult for liquid water to sustainably exist on Mars today. Nonetheless, droplets observed on the Phoenix’s robotic arm prove that salty liquid water can appear in the summer at current high latitudes on Mars. Numerical simulations have also shown that climatic conditions suitable for liquid water can briefly occur in certain areas of Mars today. Until now, however, no evidence has shown the presence of liquid water at low latitudes on Mars. The Zhurong rover, which landed on Mars on May 15, 2021, successfully filled this gap. The researchers used data obtained by the Navigation and Terrain Camera (NaTeCam), the Multispectral Camera (MSCam), and the Mars Surface Composition Detector (MarSCoDe) aboard the Zhurong rover to study the different-scale surface features and material compositions of dunes in the landing area. They found some important morphological features on the dune surfaces, such as crusts, cracks, granulation, polygonal ridges, and a strip-like trace. The analysis of spectral data revealed that the dune surficial layer is rich in hydrated sulfates, hydrated silica (especially opal-CT), trivalent iron oxide minerals (especially ferrihydrite), and possibly chlorides. The estimated age of the dunes (about 0.4-1.4 million years) and the relationship among the three phases of water suggest that the transfer of water vapor from the polar ice sheet toward the equator during the large obliquity stages of Mars’s late Amazonian period led to repeated humid environments at low latitudes. Therefore, a scenario of water activity has been proposed, i.e., cooling at low latitudes during Mars’s large obliquity stages prompts frost/snow to fall and subsequently results in the formation of crusts and aggregates on the salty dune surface, thus solidifying dunes and leaving traces of liquid saline water activity. The discovery provides key observational evidence of liquid water at Martian low latitudes, where surface temperatures are relatively warmer and more suitable for life than at high latitudes.
