Cloud Morphologies on Mars: A Closer Look through the HRSC Cloud Atlas

Kurzfassung

Thanks to a long-term atmospheric monitoring campaign, the High Resolution Stereo Camera (HRSC) onboard Mars Express provides an exceptionally detailed view of atmospheric phenomena on Mars. These observations have been compiled into a comprehensive database - the HRSC Cloud Atlas [1, 2] - which offers in-depth information on the types, distributions, characteristics, and morphologies of various clouds and storm systems across different Martian seasons. Covering data from 2017 to the present, this catalog provides fundamental knowledge about when and where specific cloud and storm types occur on Mars. This in turn allows conclusions on the underlying atmospheric conditions and surface-atmosphere interactions, as each of these phenomena forms under specific boundary conditions constrained by factors such as temperature, pressure, dust and aerosol content, circulation patterns, seasonal cycle, topography, and altitude [e.g., 3, 4, 5]. We will present a comprehensive overview of the cloud type classes identified in our study, the classification scheme developed for their categorization, and detailed close-up views to illustrate the remarkable variability and visual elegance of Martian clouds. Due to space and time constraints, this presentation focuses exclusively on cloud phenomena. Our analysis reveals that cloud types on Mars are significantly more diverse than previously reported in studies based on lower-resolution imaging instruments [e.g., 3, 6, 7], which nonetheless serve as the foundation of our classification scheme. The higher resolution of HRSC has allowed us to refine and extend existing classification systems, propose updated definitions, and introduce previously unidentified cloud types. One such discovery is a new class of Elongated Dust Clouds (EDCs) - enigmatic features that occur during a narrow seasonal window and within a specific southern latitude band. While they resemble trough clouds likely formed by katabatic jumps [8], EDCs appear farther from the South Pole and are not always associated with topographic troughs. Another notable finding is that the Aphelion Cloud Belt (ACB, [e.g., 2, 8]), does not only contain diffuse cirrus-type clouds, also forms as gravity wave fields as well as structured cloud street fields made up of small cumulus cloud cells arranged in linear or grid-like patterns - features previously observed only at high northern latitudes. Furthermore, it was possible to observe the daily cycle of the Arsia Mons Elongated Cloud (AMEC, [10]) in high resolution and at multiple local solar times, allowing us to visualize its development phases in great visual detail.