Geomorphic contexts and science focus of the Zhurong landing site on Mars

As part of the Tianwen-1 mission, the Zhurong rover successfully touched down in southern Utopia Planitia on 15 May 2021. On the basis of the new sub-metre-resolution images from the High Resolution Imaging Camera on board the Tianwen-1 orbiter, we determined that the Zhurong rover landed at 109.925° E, 25.066° N at an elevation of −4,099.4 m. The landing site is near the highland–lowland boundary1 and multiple suspected shorelines2–7. Under the guidance of the remote sensing survey, the Zhurong rover is travelling south for specific in situ investigation. Supported by the six payloads on board the rover8, its initial key targets are rocks, rocky fields, transverse aeolian ridges and subsurface structures along the path. Extended investigation will aim at troughs and cones in the distance. A better understanding of the formation mechanisms of these targets may shed light on the historical volcanism and water/ice activities within the landing area, as well as the activities of the wind. These results may reveal the characteristics and evolution of the ancient Martian environment and advance the exploration of the habitability of ancient Mars. The Zhurong rover has explored its landing site in Utopia Planitia, Mars, and is travelling south towards the highland–lowland boundary, focusing initially on the composition and physical properties of the rocks along the way.

Imaging Camera (HiRIC). The spatial resolution of the images taken from the altitude of 350 km is about 70 cm. The digital orthophoto map (DOM) and digital elevation model (DEM) of the entire area were obtained from the forward and backward stereo images from different orbits (Methods). On the basis of these data, the candidate landing area was further reduced to a safety ellipse of 56 × 22 km 2 with an inclination of 1.35° (Fig. 1a). At 7:18 (Coordinated Universal Time + 8) on 15 May 2021, the Zhurong rover successfully landed in this area and captured panorama images around the landing site using the onboard Navigation and Terrain Cameras (NaTeCams) (Fig. 1b). Using images acquired by HiRIC on 2 June, the landing site was eventually positioned at 109.925° E, 25.066° N at an elevation of −4,099.4 m (Fig. 1c, Methods).
To assist in the exploration of the Zhurong rover, the geomorphic and geological contexts within the 150 × 60 km 2 landing area were analysed from the HiRIC-derived DOM and DEM. At least five geomorphic features within the landing area are identified (Fig. 2, Fig.  3g and Supplementary Fig. 3a).
1. Rampart craters. Two distinct rampart craters appear at the north and south ends of the landing area (Fig. 1a). Figure 2a shows the northern rampart impact crater with a diameter of ~ 9 km and a depth of ~1.2 km. The overall ejecta flow morphologies of rampart craters have always been considered as evidence for high abundance of volatiles (ice/water) in the target materials 11,12 . 2. Cones. Cones are one of the most significant and controversial landforms in Utopia Planitia. Both isolated cones and clusters are found in the landing area ( Fig. 2b and Supplementary  Fig. 4). The cones in this specific region have been mostly interpreted as cinder cones 13 , mud volcanoes 14 or pingos 15 . However, the interpretation of the well defined cones in the landing area is inconclusive. 3. Ridges. The ridges distributed in the landing area range from approximately 0.59 to 12.6 km in length. Figure 2c shows one of the ridges ~42.7 km southeast of the landing site. In Utopia Planitia, the ridge-like features are probably attributable to tectonism 16 and volcanism 13 . 4. Troughs. Several troughs appear in the landing area, ranging from 1.2 to 10 km in length and from 1 to 20 m in depth. Two of Letters Nature astroNomy them are shown in Fig. 2d and Supplementary Fig. 3e. Troughs may be associated with polygonal terrains elsewhere in Utopia Planitia 17,18 , but this association is not clear for the cases in the landing area. Moreover, troughs are often modified by aeolian deposits 19 . 5. Transverse aeolian ridges (TARs). High-albedo TARs are common within the landing area, especially in some troughs and craters ( Fig. 3c and Supplementary Fig. 3a). Most TARs in the landing area run roughly in the east-west direction, indicating that the local wind was dominant in the north-south direction in the past. Their distribution, morphological characteristics, sediment composition and formation processes are indicators of the local ancient wind field and palaeoenvironment [20][21][22] .
These landforms identified by the HiRIC images reflect the abundance and complexity of the geological activities in the landing area, probably involving historical volcanism and tectonism, water/ ice activities, wind action and so on. Table 1 summarizes the possible geological factors and materials that could explain the formation of the landforms in the landing area. Such large-scale remote sensing surveys outline the fundamental geological settings of the landing area, strongly supporting the subsequent scientific exploration of the Zhurong rover. Since some landforms have multiple plausible origins and cannot be distinguished by remote sensing detections, in situ investigations using the Zhurong rover are indispensable to obtain additional diagnostic information.
To conduct detailed investigations of the landing area, the Zhurong rover is equipped with six payloads 8 , including the Mars Rover Penetrating Radar (RoPeR), Mars Rover Magnetometers (RoMAG), Mars Surface Composition Detector (MarSCoDe), Multispectral Camera (MSCam), NaTeCams and Mars Climate Station (MCS). The RoPeR consists of two channels. The low-frequency channel will provide a penetration depth of 10-100 m with a resolution of a few metres within the Martian soil. The higher-frequency channel is HH, HV, VH, VV polarization and will penetrate to a depth of 3-10 m with a resolution of a few centimetres within the Martian soil. The RoPeR will investigate the subsurface structures and even clues to underground water/ice distribution along the roving path 23 . The RoMAG will implement mobile magnetic field measurements on the surface of Mars 24 . Two identical triaxial fluxgate magnetometer sensors are respectively mounted at the top and bottom of the rover mast (67.5 cm apart). Each magnetometer measures the vector magnetic field with a resolution of 0.01 nT in the range of ±65,000 nT. Mobile magnetic field measurements on the Martian surface would obtain the fine-scale crust field and provide information about its remanent magnetization and any possible intrinsic  Fig. 2. b, The first panorama image around the landing site taken by the NaTeCams on board the Zhurong rover. c, View of the Zhurong rover, lander, heat shield and parachute on the HirIC image acquired on 2 June 2021. Credit: CNSA/GrAS.    atuRe aStRoNoMY | VOL 6 | JANUAry 2022 | 65-71 | www.nature.com/natureastronomy Letters Nature astroNomy magnetic field. The MarSCoDe uses laser-induced breakdown spectroscopy to vaporize rock specimens on the surface with a stand-off distance of 1.6-7 m, revealing their compositions 25,26 . Moreover, the near-infrared spectroscopy at 850-2,400 nm can conduct reflectance spectrum analysis of Martian soils and rocks, and a telescopic microimager can capture high-resolution images of research targets at different distances. The MSCam, with eight narrow bands and a band for solar imaging, will provide detailed

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information on the geological context, material composition and surface texture 8 . The NaTeCams are binocular stereo cameras with 270 mm stereo baseline, 13.1 mm focal length, 2,048 × 2,048 pixels and 46.5° × 46.5° field of view 27 . The MCS will measure the temperature, pressure, wind speed, wind direction and sound on the Martian surface 28 .

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After the successful landing in southern Utopia Planitia, the six payloads on board the Zhurong rover have been switched on and tested. Under the guidance of the remote sensing survey, the Zhurong rover is travelling south for specific in situ investigations, gradually approaching the direction of the HLB and the suspected shorelines. The Zhurong rover may find out if there are any traces of ancient oceans. For example, (1) the low dielectric constant compared with that of the typical volcanic materials may indicate low-density sedimentary deposits, massive deposits of ground ice or a combination of the two 29 ; (2) hydrated minerals, water-related sedimentary rocks and textures, and alteration minerals may reveal early water activities 5 ; (3) some of the anomalous chemistry (such as unusual abundances of S and Cl and their possible presence as sulfate minerals and chloride salts) may be related to the presence of former standing bodies of water 30 . Currently, the rover travels about 20-30 m per sol and had travelled about 700 m by the end of July 2021. Given the capacity and good condition of the rover, an accessible region was selected as the investigation region for the nominal and short-term extended mission phases, which is shown in Fig. 3c.
The major detection targets of the Zhurong rover are the rocks along the path (Fig. 3f,h). The onboard NaTeCams will preliminarily investigate the terrains around the rover and determine the locations and shapes of the target rocks. The rocks excavated by small impacts or covered in less dust will be the preferred targets. Then the powerful tools MSCam and MarSCoDe will be employed to identify the compositions and physical properties of the rocks. The fine-scale structures, particle sizes and alteration minerals will restrict the geological background of the landing area and may provide indirect evidence of the ancient ocean. Furthermore, the degree and characteristics of wind erosion on the rock surface are helpful to study the paleoenvironment around the landing area.
In the nominal mission phase, some intriguing features in the investigation region will also be targets, such as TARs and rocky fields (regions 1, 2 and 3 in Fig. 3c). The rover has encountered a barchan-like TAR ~200 m southwest of the landing site, with a height of ~0.6 m, ~50 m in length and ~8 m in width ( Fig. 3g and Supplementary Fig. 3a). The NaTeCams on board the rover will obtain its three-dimensional geomorphology, while MSCam and MarSCoDe will reveal the composition and particle size sorting of the sand. The MSC will further help monitor the current wind field nearby. This information is of great significance to establish the basic development model of Martian sand dunes. Two wide rocky fields showing rough textures with different albedo are present in the high-resolution images, located ~624 m and ~1,014 m from the landing site, respectively ( Fig. 3e and Supplementary Fig.  3b,c). Unlike nearby smooth terrains, there are many rock fragments exposed here. The difference in the subsurface structures and underground rock distribution between the rocky fields and smooth terrains will be revealed by the RoPeR.
Subsequently, the Zhurong rover will enter the extended mission phase. The short-term targets in this phase are another narrow rocky field and a trough with the maximum depth of ~10 m, which are located ~1.8 km and ~2.7 km south of the landing site, respectively (regions 4 and 5 in Fig. 3c and Supplementary Fig. 3d,e). In the longer term, we expect that the rover will travel further. From the DEM data, the elevation is constantly rising as the rover travels south. The height difference between the landing site and the highest point is ~60 m, and a scarp appears ~27 km away from the landing site. Many cone features are distributed near the scarp, especially clustered cones (Fig. 3a,b and Supplementary Fig. 4), indicating that this region may have been geologically active in the past. However, how the terrain formed remains unknown. Further research combining remote sensing data from the Tianwen-1 orbiter and in situ data from the Zhurong rover will address these questions. These results may reveal the characteristics and evolution of the ancient Martian environment and advance the exploration of the habitability of ancient Mars.

Landing area topography reconstruction and landing site positioning.
Tianwen-1 successfully entered the parking orbit of Mars on 24 February 2021. Before the Zhurong rover landed on the Martian surface, the HiRIC on the orbiter imaged the primary candidate landing area 21 times (including 11 forward-view images and 10 backward-view images to construct stereoscopic images). Each image has a width of about 11 km and a length of 170 km in the north-south direction. These images realized multiview stereo coverage of the 150 × 60 km 2 primary candidate landing area. On 2 June, to determine the accurate position of the Zhurong rover, HiRIC carried out a 10 s imaging with a width of about 11 km and a north-south length of about 43 km. The spatial resolution of the above images is about 0.6-1.2 m.
We produced a DOM with a spatial resolution of 0.7 m and a DEM with a spatial resolution of 3.5 m of the primary candidate landing area using these multiview stereo images by photogrammetric processing. The data processing flow is as follows.
1. The imaging geometric model of HiRIC (rational polynomial coefficients model) was constructed on the basis of its geometric calibration parameters and the orientation parameters of the Tianwen-1 orbiter, such as its position and attitude. Then the rigorous geometric relationship between the image points and their corresponding feature points on the Martian surface can be established. 2. Feature point matching of HiRIC images. Scale-invariant feature transform 31 feature matching is used here to extract connection points between the forward-view and backward-view images in the same imaging area, and in the overlapping areas between adjacent imaging areas. The mismatches were eliminated by a more precise matching process, such as least squares matching. 3. The Mars Reconnaissance Orbiter (MRO) CTX (Context Camera) image 32 with a spatial resolution of 6 m is used as the horizontal position control datum to extract dozens of control points between the forward-view and backward-view images. The elevation for these control points was adopted from the MRO Mars Orbiter Laser Altimeter (MOLA) terrain data 33 with a spatial resolution of 200 m, whose elevation accuracy is ±3 m. 4. HiRIC images were divided into blocks, and block adjustment was performed on the basis of the photogrammetric bundle adjustment technology 34,35 . The external orientation elements were optimized to realize the seamless mosaic and high-precision absolute positioning for the HiRIC images. Letters Nature astroNomy 5. Optimized exterior orientation elements were used to establish the epipolar geometric constraint between the forward-view and backward-view images so that corresponding image points could be matched pixel by pixel. The three-dimensional coordinates of each corresponding point were calculated by the forward ray intersection. Then a regular grid of DEM data was produced using inverse distance weighting, with a spatial resolution of 3.5 m. 6. According to the strict geometric imaging model of HiRIC, the forward-and backward-view images were orthorectified using DEM data to produce DOM data, with a spatial resolution of 0.7 m.
According to the photogrammetric adjustment results, the s.d. of the horizontal position error of the preselected landing area topographic data produced by the HiRIC images is about 0.4 m, and the s.d. of the elevation error is about 1.0 m. Compared with MRO CTX image data, the horizontal position deviation (root mean squared) is 15 m, while the elevation deviation (root mean squared) is about 10 m compared with MRO MOLA data.
According to the orthophoto image from the HiRIC obtained on 2 June, the accurate horizontal position of the landing site was determined, and its elevation was measured on the DEM mentioned above. The determined landing site of Tianwen-1 is 109.925° E, 25.066° N, with an elevation of −4,099.4 m. The coordinate system used here takes the Mars centre of mass as the origin; the reference surface of the elevation is the surface of a spheroid with a radius of 3,396.19 km and the reference plane is the equatorial plane of Mars. Its X axis, Y axis and Z axis are parallel to the corresponding three axes of the Martian fixed coordinate system, which is based on the International Astronomical Union orientation parameter model 36 .

Data availability
The Tianwen-1 data used in this work is processed and produced by the Ground Research and Application System (GRAS) of China's Lunar and Planetary Exploration Program, provided by China National Space Administration (http:// moon.bao.ac.cn). The data used in this manuscript are available at https://moon. bao.ac.cn/web/enmanager/zygj. Datasets generated or analysed during this study are available from the corresponding author upon reasonable request. Source data are provided with this paper. acknowledgements This work was supported by the China National Space Administration (CNSA). We thank the team members of the five systems of the Tianwen-1 mission: the probe system, the launch vehicle system, the launch site system, the Telemetry Track and Command System and the Ground Research and Application System. This research was also funded by the Key Research program of Chinese Academy of Sciences (grant no. ZDBS-SSW-TLC001).