Ceres Landing Site Planning - Requirements for DTMs and Current Status

IntroductionThe conceptual development of a Ceres Sample Return mission has intensified interest in the detailed topographic characterization of Occator Crater, a 92 km-wide impact crater on dwarf planet Ceres. This geologically young [1,2] crater hosts carbon-rich bright deposits [3,4,5] - Cerealia Facula and Vinalia Faculae - that are surface expressions of cryovolcanic processes linked to a ~40–50 km-deep brine reservoir [6,7]. These deposits contain a unique combination of sodium carbonates, ammonium chloride, and other hydrated salts [3,4,5] that were only recently exposed to the surface [2,3,8] and may preserve a record of past subsurface aqueous activity. Several Ceres landing site assessments [9] and mission concepts [10] have already considered Occator, with particular attention on these bright faculae as scientifically compelling targets. The success of such missions will depend on accurate, high-resolution Digital Terrain Models (DTMs) to enable safe landing, mobility, and efficient surface operations.Figure 1. Pan-sharpened RGB orthomisaic of Cerealia Facula [2].BackgroundTo support ongoing and future mission development efforts, we present a new set of high-resolution DTMs of Occator Crater and its interior faculae [11]. These were generated using stereophotogrammetric (SPG) and multi-view shape-from-shading (SfS) techniques applied to Dawn Framing Camera [12] (FC)  data from multiple mission phases. The datasets include global coverage from the High and Low Altitude Mapping Orbit (HAMO, LAMO), as well as extremely high-resolution data from the highly elliptical XMO7 orbit acquired during Dawn’s second extended mission (XM2). This multitemporal, multi-geometry coverage forms the basis for precise terrain modeling at multiple spatial scales.MethodologyOur initial DTM products were derived using the USGS ISIS and NASA's ASP [13,14], generating terrain models at Ground Sample Distances (GSDs) of up to 17 m. The workflow incorporated radiometric correction, photometric normalization using Hapke parameters tailored to Ceres’ surface [15], manual tie-point generation for improved co-registration, bundle adjustment [16], and a careful exclusion of over-exposed images covering the faculae. ResultsAn important component of our study involved evaluating published DTMs of Occator Crater, including those produced by DLR [17] and JPL [18]. Our comparison demonstrated significant differences in effective resolution, vertical offsets, and absolute elevation, especially in the complex terrains of Cerealia and Vinalia Faculae. These findings underline the need for further investigation into the sources of these discrepancies, particularly with regard to co-registration accuracy, bundle adjustment stability, and the impact of surface albedo variations on photoclinometry. Our improved terrain models, are suitable for detailed analyses of surface slopes and terrain roughness - key parameters in landing site certification and mobility planning. As illustrated in Figures 2 and 3, Cerealia Facula exhibits highly variable slopes, with regions on and near Cerealia Tholus exceeding 30°, as well as rugged, fractured terrains that pose potential hazards. Only few regions of the surface exhibits slopes