Aqueous alteration of the Vastitas Borealis Formation at the Tianwen-1 landing site

Abstract

The Vastitas Borealis Formation is a Hesperian-aged sedimentary unit that mainly covers the Martian northern plains. Its environment of formation is uncertain but may be related to an ancient Martian ocean, outflow channel deposits, compaction and drainage activities, or subsurface ice-related processes. Here we investigate the geological evolution of the Vastitas Borealis Formation using geochemical and mineralogical data acquired by the Mars Surface Composition Detector on board the Zhurong rover at the Tianwen-1 landing site. We find that the fine-grained soil is very similar to the ubiquitous surface dust at other landing sites on Mars but mixed with calcium-rich and magnesium-poor local materials. These materials are mostly igneous minerals mixed with allophane and imogolite/opal and appear to have a low degree of chemical alteration. We suggest that these deposits likely formed by alteration of extensive volcaniclastic soils in limited or ephemeral water under cold conditions, which largely supports the hypotheses of a frozen ocean and sublimation.

In situ analysis of surface composition and meteorology at the Zhurong landing site on Mars

“…The average compositions of soils and TAR sands presented in this study agree within error with the LIBS results reported by Liu et al. (2022) [ 26 ].…”

“…The pulsed tetrorockets excavated the top layer of the regolith under the Tianwen-1 lander and showed the cemented and indurated layer, similar to those observed at the InSight lander site [ 3 ]. These cemented duricrusts potentially indicate brine activity and the presence of hydrated salts, which are consistent with the in situ observations of the Zhurong site [ 13 , 14 , 26 ] and those inferred for the Viking and InSight sites [ 17 , 18 , 20 ]. The intriguing lithified duricrusts have been inferred previously only at the Viking 1 site.…”

Ground ice-magma interaction resurfaced the landing region of the Zhurong rover during the Amazonian

“…The spectral absorptions have been interpreted to re ect the presence of sulfates/silica 14 or allophane/silica 15 . LIBS results indicate that the detected materials are rich in Ca 15 . Here we found that Ca is negatively correlated with Si (Fig.…”

“…The Chinese Zhurong rover landed in southern Utopia Planitia (25.066°N, 109.925°E), which has been resurfaced during the Amazonian with the formation of distinctive geomorphic features 11 , loose nearsurface material 12,13 , and hydrous minerals 14,15 . Most of the diverse distinctive landforms are thought to be related to a large reservoir of volatiles 16 .…”

“…However, the physical weathering rate during the Amazonian is too low 17 to form regolith with such thickness. In addition, hydrous minerals have been discovered at the surface of the loose Amazonian materials with laser-induced breakdown spectroscopy (LIBS) spectrometer and short-wave infrared spectrometer (SWIR) data collected by Zhurong 14,15 , but no liquid water was detected within the radar detection depth range 12 . Given that no groundwater was directly detected and that Amazonian Mars was generally considered icy and hyper-arid for billions of years, the source of the liquid water required for the formation of the hydrous minerals is a mystery.…”