Detection of allophane by the Zhurong rover indicates water-limited alteration at Utopia Planitia, Mars

The Short-Wave Infrared Spectrometer (SWIR) in the Mars Surface Composition Detector (MarSCoDe) package onboard the Zhurong rover revealed the presence of hydrated phases. However, the exact types of phases are ambiguous and should be further constrained using laboratory spectroscopic studies. Therefore, we build a spectral dataset of binary mixtures by mixing pyroxene and potential hydrated phases (allophane and gypsum) in the laboratory to match the SWIR spectra and calculate the hydrated phase content. We find that the primary hydrated phase may be allophane according to the spectral similarity between the laboratory dataset and SWIR spectra. The relative abundance of pyroxene (∼82 wt%) and allophane (∼18 wt%) is determined using the models built by the integrated band depth (IBD) of binary mixture spectra. When the content of pyroxene is normalized to its content in Martian soil (∼30 wt%), the content of allophane is ∼7 wt%. The allophane may come from short-term fluid-rock interactions under cold climate involving ice and snow melt, and the content of allophane (∼7 wt%) represents a low-moderate degree of weathering at the Tianwen-1 landing site. We propose possible geological evolution scenarios of the Tianwen-1 landing site, i.e., the process of rapid aqueous alteration of volcanic materials under low-temperature conditions due to environmental changes, to explain the geomorphic features and widespread allophane observed by the Zhurong rover.