In July 2020, China's Tianwen-1 mission entered orbit around Mars. It consists of six robotic elements: an orbiter, a lander, two deployable cameras, a remote camera, and the Zhurong rover. The first in a series of interplanetary missions by the China National Aeronautics and Space Administration (CNSA), the mission's purpose is to study the geology and internal structure of Mars, characterize its atmosphere, and search for evidence of water on Mars. Like the many orbiters, landers, and rovers currently exploring Mars, Tianwen-1 is looking for possible evidence of life on Mars (past and present).
During the nearly 1,298 days that the Tianwen-1 mission has been exploring Mars, its orbiter has captured countless remote sensing images of the Martian surface. Thanks to a team of researchers from the Chinese Academy of Sciences (CAS), these images have been combined to create the first high-resolution global color image map of Mars, with a spatial resolution of more than 1 km. This is currently the highest resolution map of Mars and could serve as a global base map that will one day support manned missions.
The team was led by Professor Li Chunlai of the National Astronomical Observatories of China (NOAC) and Professor Zhang Rongqiao of the Lunar Exploration and Space Engineering Center. They were joined by several colleagues from the Key Laboratory of Lunar and Deep Space Exploration, the Institute of Optics and Electronics, the University of Chinese Academy of Sciences and the Shanghai Institute of Technical Physics. The paper detailing their research, “A 76-m per pixel global color image dataset and map of Mars by Tianwen-1,” recently appeared in the journal Science Bulletin.
The optical camera (MoRIC) and imaging spectrometer (MMS) on board the Tianwen-1 orbiter were used to obtain remote sensing images of the entire Martian surface. Image credit and ©: Science China Press
Several global maps of Mars have been created using remote sensing images acquired by instruments aboard six previous missions, including the visual imaging systems on the Mariner 9 spacecraft, the Viking 1 and 2 orbiters, the Mars Orbiter Camera-Wide Angle (MOC-WA) on board the Mars Global Surveyor (MGS), the Context Camera (CTX) on board the Mars Reconnaissance Orbiter (MRO), the High-Resolution Stereo Camera (HRSC) on Mars Express (MEX), and the Thermal Emission Imaging System (THEMIS) on the Mars Odyssey orbiter.
However, these maps all had significantly lower spatial resolution than the one the CAS team created from images taken by the Tianwen-1 orbiter. For example, the MGS MOC-WA Atlas Mosaic has a spatial resolution of 232 meters per pixel (280 yards per pixel) in the visible band, and the Mars Odyssey mission's THEMIS Global Mosaic provides a spatial resolution of approximately 100 m/pixel (~110 feet/pixel) in the infrared band. While Mars' MRO Global CTX Mosaic covers 99.5% of the Martian surface (88° North to 88° South) in the visible band, it has a spatial resolution of approximately 5 m/pixel (5.5 yards/pixel).
Global color images of Mars with a spatial resolution of one hundred meters (110 yards) or more were also lacking. As for global color images, the Mars Viking Colorized Global Mosaic v1 and v2 have a spatial resolution of approximately 925 m/pixel and 232 m/pixel (~1010 and 255 yards/pixel), respectively. Meanwhile, during the more than 284 orbits of the Tianwen-1 orbiter, the MoRIC instrument acquired 14,757 images with spatial resolutions between 57 and 197 m (62 and 215 yards).
At the same time, Tianwen-1's Mars Mineralogical Spectrometer collected a total of 325 data strips in the visible and near-infrared ranges with spatial resolutions ranging from 265 to 800 m (290 to 875 yards). The collected images also covered the entire Martian surface. Using these data, Professor Li Chunlai, Professor Zhang Rongqiao and their colleagues processed the image data, resulting in this latest global map of Mars. The team also optimized the original orbit measurement data using bundle adjustment technology.
(a) Level 2C data product as input, (b) image corrected by atmospheric correction, (c) image corrected by photometric correction, and (d) image corrected after color correction. Image credit and ©: Science China Press
By treating Mars as a unified matching network, the team was able to reduce the positional deviation between individual images to less than 1 pixel and create a “seamless” global mosaic. The true colors of the Martian surface were achieved thanks to the data acquired by the MMS, while color correction enabled global color uniformity. This all culminated in the release of the Tianwen-1 Mars Global Color Orthomosaic 76 m v1, which has a spatial resolution of 76 m (83 yards) and a horizontal accuracy of 68 m (74 yards).
This map is currently the highest resolution true color global map of Mars and greatly improves the resolution and color authenticity of previous Mars maps. This map could serve as a geographic reference for other space agencies and partner organizations to map the Martian surface with even greater resolution and detail. It could also be used by space agencies to select sites for future robotic explorers that will continue to search for clues to Mars' past. It could also be useful when NASA and China send manned missions to Mars, which are scheduled to begin in the early 2030s or 2040s.
Further reading: Eureka Alert!, Science Bulletin
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