Rock coatings, found abundantly in surface weathering environments, are crucial for studying how rocks, organisms, and the environment interact within Earth's surface system (Dorn, 1998; Dorn, 2009; Lu et al., 2019). Rock varnish, characterized by its deep brown to jet-black color, composition of iron and manganese oxides, and glossy appearance, represents a significant type of rock coatings. Additionally, NASA's Mars rovers have identified formations on Mars that resemble terrestrial rock varnish, prompting geologists to study Earth's rock varnish to better understand their Martian counterparts and their potential role in protecting microorganisms (Marnocha, 2017; VanBommel et al., 2022).
This study, as the foundational research in our series on rock varnish, aims to explore the structure and composition of rock varnish in the Beishan regions of Gansu and Inner Mongolia, China. Using scanning electron microscopy, energy-dispersive X-ray spectroscopy, and laser Raman spectroscopy, the focus is on observing and describing the basic structures, elemental compositions, and element distributions of rock varnish and "rock varnish analogs" found in this area. Combining the evidence from this study with other research, we systematically classifies four types of rock varnish and "rock varnish analogs", which termed rock varnish, calcium-rich varnish, calcium-iron-rich varnish and iron-rich varnish, proposing hypotheses about their origins and the factors contributing to their differences. The Beishan region, characterized by its semi-arid climate, predominant wind-blown sedimentation, and ample sunlight, serves as a natural laboratory for simulating Martian conditions. This research on rock coatings lays the groundwork for understanding those similar Martian rock varnish and prepares for potential future sample studies following Mars sample return missions in China.
 

Rock coatings,Rock varnish,Martian analogs