Recently, researchers of the Space Science Climbing Plan Team at Shandong University found evidence of a low degree of alteration at the Tianwen-1 landing site by the in-situ detections of the Zhurong rover. They proposed that a short-lived frozen ocean (Figure 1) could have been present in the northern hemisphere of Mars. This extensive study is recently published online in Communications Earth & Environment, entitled “Aqueous alteration of the Vastitas Borealis Formation at the Tianwen-1 landing site”. Research assistant Liu Changqing is the first author of this paper. Professor Ling Zongcheng, the PI of the Planetary Science Group at Shandong University, is the corresponding author for this work.

Figure 1. Scheme of the frozen ocean on the northern hemisphere of Mars.

China’s first Mars exploration mission, Tianwen-1, landed and released the Zhurong rover in southern Utopia Planitia on the northern hemisphere of Mars on May 15, 2021. The landing site is a brand-new location within the Vastitas Borealis Formation unit (VBF) and Zhurong rover has travelled more than 300 sols on this geologic unit (Figure 2). VBF is one of the most widespread sedimentary units on the northern hemisphere of Mars which covers most of the northern plains. Previous work proposed that this unit may be deposited from an ancient ocean, but others argued that the ocean might be absent on Mars. Fortunately, the in-situ detections of the Zhurong rover can provide new clues to constrain the existence of the ancient ocean. The Zhurong rover carries a Mars Surface Composition Detector (MarSCoDe) instrument suite, consisting of a laser-induced breakdown spectrometer (LIBS), a short-wave infrared spectrometer (SWIR), and a micro-imaging camera (MI). This instrument can investigate the elemental compositions (Si, Al, Fe, Ca, Mg, K, Na, O, H, etc.), mineralogy, and morphology of rocks and soils.

Figure 2. Distribution map of the VBF unit, Tianwen-1 landing site, traverse map of Zhurong rover.

Chemical compositions and mineral assemblages of detected targets by the Zhurong rover can reflect the aqueous alteration at the landing site. For example, the mobile elements (e.g., Fe, Mg) can be taken away by water from the primary rock crystallized from magma, contributing to the high degree of alteration and producing secondary phases (e.g. allophane). If the ocean existed, materials encountered by the Zhurong rover would show a high degree of alteration. The chemical compositions and mineral information acquired by MarSCoDe can provide new constraints on the aqueous alteration degree at the Tianwen-1 landing site.

In this paper, researchers jointly interpreted the LIBS and SWIR spectra, deriving chemical compositions and mineral phases of rocks and soils. They found that the chemical compositions of fine-grained soils are similar to the ubiquitous surface dust at other landing sites (e.g.Gale crater) but mixed with calcium-rich and magnesium-poor local materials (Figure 3). More importantly, they found a low degree of alteration of detection targets according to their low Chemical Index of Alteration (CIA) values (<39%±9%) as well as their mineral assemblage, i.e. pyroxene mixed with amorphous aluminosilicates (allophane and imogolite/opal). These amorphous deposits are likely formed by the alteration of extensive volcaniclastic soils in limited or ephemeral water under cold conditions, which largely supports the hypotheses of a frozen ocean and sublimation. Overall, this work obtained the chemical composition and mineral assembles of the VBF targets by the joint interpretation of LIBS and SWIR spectral datasets and found in-situ direct evidence for the weak water-rock interaction at the Tianwen-1 landing site.

Figure 3. Geochemical results of detected VBF targets.

This work was supported by funding from the Pre-research project on Civil Aerospace Technologies, the National Natural Science Foundation, and the Natural Science Foundation of Shandong Province. In recent years, the Planetary Science Group at Shandong University has focused on the spectral studies that contributed to the Tianwen-1 mission. Recently, they built a Mars Environment Chamber and a LIBS system in the laboratory and a LIBS standard library containing more than 400 samples, and investigated the LIBS spectra acquired by Curiosity and Zhurong rovers. They have published 12 papers in academic journals like Communications Earth & Environment, Spectrochimica Acta Part B: Atomic Spectroscopy, and Remote Sensing, etc.

Link to this paper:https://www.nature.com/articles/s43247-022-00614-3