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

Abstract

The Zhurong rover of the Tianwen-1 mission landed in southern Utopia Planitia, providing a unique window into the evolutionary history of the Martian lowlands. During its first 110 sols, Zhurong investigated and categorized surface targets into igneous rocks, lithified duricrusts, cemented duricrusts, soils and sands. The lithified duricrusts, analysed by using laser-induced breakdown spectroscopy onboard Zhurong, show elevated water contents and distinct compositions from those of igneous rocks. The cemented duricrusts are likely formed via water vapor-frost cycling at the atmosphere-soil interface, as supported by the local meteorological conditions. Soils and sands contain elevated magnesium and water, attributed to both hydrated magnesium salts and adsorbed water. The compositional and meteorological evidence indicates potential Amazonian brine activities and present-day water vapor cycling at the soil-atmosphere interface. Searching for further clues to water-related activities and determining the water source by Zhurong are critical to constrain the volatile evolution history at the landing site.

Keywords: LIBS; Tianwen-1 mission; Utopia Planitia; Zhurong rover; volatile.

Figure 1.

An overview of the landing site of the Zhurong rover, Tianwen-1 mission. (A) Regional context of the Zhurong landing site (marked by a star), with previous lander and rover missions within the latitude region (marked by circles). The base map uses the color hill-shade topographic data set from the Mars Orbital Laser Altimeter (MOLA). Dashed curves show the proposed and putative shorelines from the literature at the dichotomous boundary of Mars [40–42]. (B) The panorama of the touchdown site is composed of stitched images taken by the NaTeCams of the Zhurong rover. The Tianwen-1 lander and the Zhurong rover are shown to the right of the image. On the far horizon, a crater with a rocky rim, two TARs and the back shell of the Tianwen-1 lander can be identified. The descending plumes impinged the surface and removed dusts initially covering the surface, revealing a dark patch of gravels underneath the dust cover. The surface is mainly composed of gravels and clasts of rocks, soils and dust. The IDs of the NaTeCam and TMI images are tabulated in Supplementary Table S2. (C) Traverse path (white and black curves) during the first 110 sols (1130 m) since the landing on 15 May 2021. The base map is the HiRISE image (ESP_069 731_2055_RED). The red star marks the touchdown location and the white dashed lines outline the blast zone during landing. The Sol-100 marks the northern summer solstice (Ls = 90°), so the white curves from touchdown location to Sol 100 represent traversal during northern spring and the consequent traverse path in black curves represents northern summer. Blue to green dots show the elevation along the traverse extracted from the high-precision digital elevation model (DEM, DTEEC_069 665_2055_069 731_2055_A01) derived from the HiRISE image. Orange spots along the curves highlight the sols where MarSCoDe conducted the analysis.

Figure 2.

Representative rock targets observed and investigated during the 110-sol traverse. In subfigures (A–D), yellow rectangles or circles mark the targets selected in the NaTeCam images and consequently zoom in to the specific sampling spots by the LIBS marked by the red arrows and the micro-images. The sample names consist of the sol numbers and identification numbers listed in Supplementary Table S1. Note that although aiming to rock targets, most sampling spots are actually on dust/soil or cements coating the rock surface. (A) Two rock fragments analysed on Sol 43. Sample ‘Sol 43-06’ was a layered cement coating on the rock surface; sample ‘Sol 43-07’ was the dust/soils covering the rock surface. (B) Sample ‘Sol 45-06’ was the TAR sand at the foot of the eolian bedforms. Sample ‘Sol 45-07’ was measured on the rock materials indicated by an ablation pit after laser ablation. This is the only LIBS measurement of igneous rock during the first 110 sols and weathering rinds might be included in the collected spectra. (C) Two rock fragments measured on Sol 79. The LIBS laser hit the cemented coating on the rock surface and smashed the cements after ablation. (D) Two lithified duricrust samples analysed on Sol 100. Both samples show jagged etched with harder strength compared with the cements after laser ablation. The spectra of sample ‘Sol 100-06’ cannot be used for composition quantification. Micro-images were taken only after LIBS measurements before Sol 58 and, after that, micro-images both before and after LIBS measurements were available.

Figure 3.

Three Eolian bedforms (TAR) observed and investigated in sequence on Sols 47, 65 and 92. (A and B) TAR-47, (C and D) TAR-65, (E and F) TAR-92. TAR-47 and TAR-92 are simple crescent-shaped deposits. TAR-65 shows prominent secondary ridges (white dashed curves) on the primary ridges (dashed curve). Bright dust has accumulated on the troughs of the aeolian bedforms and fine secondary ridges have developed on the dust deposits, indicating that TAR-65 is stationary for a long time. The IDs of the NaTeCam images are tabulated in Supplementary Table S2.

Figure 4.

Preliminary compositions of surface materials analysed by LIBS on board the Zhurong rover. The samples are categorized into igneous rock, lithified duricrust, cemented duricrust, soils and TAR sand. (A) Mg/Si versus Al/Si mass ratios and (B) Ca/Si and Fe/Si mass ratios for terrestrial ultramafic rocks, Mars SNC meteorites, Martian meteorites and in situ Mars measurements (source data listed in Supplementary Table S3). The igneous rock sample at Zhurong contains less Al and higher Fe than basaltic rocks at previous landing sites. The lithified duricrust deviates from the sedimentary rocks at previous sites and contains higher Al, Ca and Fe contents. The igneous rock and lithified duricrust of the Zhurong site belong to two distinct categories. The average compositions of duricrusts, soils and sands are consistent with average soils at previous sites but with elevated Ca. In (A and B), hollow black squares represent Martian meteorites with individual names noted. The hollow blue squares represent selected basaltic rocks analysed in situ by using an alpha particle X-ray spectrometer. Hollow red stars each represent averaged soil compositions at previous landing sites and the solid orange star is the averaged composition of Mars Global Soil. The solid squares represent igneous rock (black), lithified duricrust (blue) and averaged cemented duricrust (purple) at the Zhurong site. The solid circles represent averaged soil (orange) and TAR sand (yellow). ‘GC’, ‘MP’ and ‘PF’ abbreviate ‘Gusev Crater’, ‘Meridiani Planum’ and ‘Pathfinder’, respectively. ‘CH’ and ‘HP’ represent ‘Columbia Hills’ and ‘Home Plate’ at Gusev Crater. ‘Burns’ represent the ‘Burns Formation’ at Meridiani Planum. ‘PF-SFR’ represents the derived soil free rock composition based on Pathfinder analysis. (C) Normalization of the other four categories of Zhurong surface materials against the igneous rock composition (Supplementary Table S4). The horizontal black line represents the igneous rock. Ratios of the average composition of lithified duricrust (blue), cemented duricrusts (purple), TAR sands (red) and soils (orange) to igneous rocks are plotted for comparison. The H₂O contents are in the order of TAR sands > soils ≈ duricrusts > duricrust (lithified) > igneous rocks. The igneous rock has high Si, which might be due to the presence of weathered rinds on the rock surface. Soils and sands are enriched in Mg and have relatively low Na and K contents compared with local igneous rocks. The duricrusts (not lithified) generally follow the patterns of soil and sand concentrations. Errors associated with the elemental concentration data are presented in Supplementary Table S1 and are not shown here for clarity. (D) The potential correlation between the H₂O content and MgO for different targets. Dashed and dotted lines indicate the mixing lines between a presumed endmember (MgO 2 wt% and H₂O 1.1 wt%) and a Mg sulfate endmember with varying amounts of water indicated in parentheses. The uncertainties of the concentrations presented are estimated using the root mean square errors (RMSEs) of the calibration models (shown in Supplementary Table S1). The H₂O-to-MgO ratios range beyond the Mg sulfates, suggesting that adsorbed water in addition to hydrated Mg sulfates may be present in these categories.

Figure 5.

Local meteorological conditions recorded by the MCS on board Zhurong and comparison with the modeling results of the Mars Climate Database. In (A and C), the colored bars represent the measured data and solid black squares represent the simulation results at the same time and same location. The length of the colored bars represents the values of the parameters and the color indicates the measurement time at the local true solar time. (A) Near-surface temperature. The measured T is ∼10 K higher than the modeled results, which is well within the expected ±10 K error of the MCD simulation. (B) Simulation results of the diurnal trend of surface temperature (T_surf), surface pressure and water ice contents on the ground at Ls 50° (i.e. Sol 0), 70°, 90° and 110° (i.e. Sol 137). The maximum T_surf ranges from 270.7 to 276.6 K and the minimum T_surf from 187.8 to 194.4 K. Therefore, water frost can form on the ground even during the night of the spring season and sublimate soon after sunrise. Meteorological source data from the previous Mars landing missions are available in the Data Availability Statement. (C) The air pressure data near the ground recorded by the MCS along the traverse and compared to the diurnally averaged air pressure values of Viking-1 (VL) and InSight under the same Ls conditions. TW-1, VL1 and InSight demonstrate a consistent and substantial decrease in air pressure as the northern lowlands turn from spring to summer. Note that diurnal VL1 and InSight data are averaged from day and night, and the data of the TW-1 site are only of measurements with a short duration of daytime on each sol. The data from the Pathfinder mission were not included because the mission did not contain seasonal data during Ls 50°–100°, although it was located within the latitude range and close to site VL1.