Dawn XMO7 Clear Filter, RGB and Pan-Sharpened Orthomosaics

Introduction

In this document, which forms part of the Supplementary Online Materials accompanying the Icarus paper with the title given above, we present additional cartographic material that should be understood as contextual mosaics. 

These mosaics are intended to provide interested researchers with a broader spatial overview of the two cryovolcanic structures within Occaror crater (Cerealia Facula and Vinalia Faculae) that are discussed and analyzed in the main paper referenced below.

Within the framework of the Dawn mission (Russell & Raymond, 2011), these features were imaged multiple times by the Dawn Framing Camera (Sierks et al., 2011) during progressively lower orbital phases, at successively higher spatial resolutions, primarily using the clear filter but also, in part, through the narrow-band filters. Owing to the high-resolution digital terrain models (DTMs) of Occator Crater that we generated in our previous publication (Neesemann et al., 2025), and the iterative co-registration approach described therein, we were now able to register all image data from the various mission phases with high precision and subsequently orthorectify them. This process enabled the creation of the derived data products shown in Figures 1-3, that are also provided as GIS-ready GeoTIFFs.

Where earlier publications exhibited chromatic mismatches in their color mosaics, caused by imperfect registration of the use of
lower-resolution DTMs, we were able to minimize such inconsistencies to below the level of visual detectability. The final result is presented in Figure 3 as a IHS pan-sharpened RGB orthomosaic compiled from CSL/CXL (LAMO / ~34 m GSD) RGB and XM2 (XMO7 / ~8.5 m GSD) clear filter data.

Data

Occator crater was imaged during the Dawn mission across three polar orbits. During the Ceres Science Survey (CSS) orbit (altitude: ~4402 km, GSD: ~410.6 m) and the Ceres Science HAMO (CSH) (altitude: ~1480 km, GSD: ~138 m) and Ceres Extended Juling (CXJ) orbit (altitude: ~1489 km, GSD: ~138.9 m), the entire Occator crater was observed using both the clear filter (F1) of the FC2 camera and its seven narrow-band filters (F2 - F8) (Sierks et al. 2011). The crater was also completely imaged through FC2 clear filter data during the lowest-altitude polar science orbits, namely the Ceres Science LAMO (CSL) (altitude: ~386.9 km, GSD: ~36.1 m) and the Ceres Extended LAMO (CXL) (altitude: ~392 km, GSD: ~36.5 m), though narrow-band coverage was incomplete. Both Cerealia Facula and Vinalia Faculae, however, are fully covered by narrow-band data. A small LAMO RGB data gap located northwest of Cerealia Facula was filled using HAMO RGB data. As the northwest of Cerealia Facula exhibits little spectral variability, this substitution has no significant impact on the dataset‘s consistency.

During the second and final mission extension (XM2), Occator Crater was re-imaged within the highly elliptical XMO7 orbit at low altitude and thus higher spatial resolution, primarily using the clear filter of the FC2. Due to the limited predictability of these late-mission orbital geometries, image resolution among individual XMO7 orbits is less homogeneous compared to the earlier polar phases.

To maintain an approximately constant resolution ratio between the lower-resolution LAMO narrow-band data and the higher-resolution XMO7 clear filter data for the final data product (the pan-sharpened RGB orthomosaic), we primarily selected XMO7 images with a GSD of approximately 8.5 m, corresponding to a ratio of roughly 1:4. This choice also helped to preserve consistent illumination conditions across the orthomosaic, as corresponding images were acquired under similar illumination conditions. A small data gap in the western part of Vinalia Faculae, where no XMO7 images at this resolution were available, was filled using higher-resolution XMO7 data (~3.5--5 m GSD), which were downsampled using a 3 x 3 pixel box filter to visually match the sorrounding 8.5 m data. Another minor gap west of Cerealia Facula, where no ≤8.5 m XMO7 data existed, had to be filled using XMO7 imagery with a GSD of ~14 m. Nevertheless, both Cerealia Facula and Vinalia Faculae are covered by XMO7 data with GSDs better than 8.5 m. The individual FC2 images used to generate both the clear filter and RGB orthomosaics, together with the selected metadata, are listed in Tables 1 and 2.

Methods

The cartographic products presented here are based on the high-resolution DTM published in Neesemann et a. 2025, as well as the corresponding LAMO- und XMO7-based clear filter orthomosaics. These datasets are available through Zenodo research data repository under doi: 10.5281/zenodo.14531596.

Onto this base orthomosaic, we co-registered the 39 CSL/CXL (LAMO) and 6 CSH (HAMO) narrow-band images acquired with filters F2 (green, 550 +/-2 nm), F5 (IR, 980 +/-2 nm), and F8 (blue, 430 +/-2 nm). Due to the large differences in GSD between individual XMO7 orbits, their co-registration was performed iteratively. Forst, we co-registered all available XMO7 nadir-pointing data with GSDs >17 m and generated an initial orthomosaic. This intermediate product then served as the basis for co-registering the XMO7 nadir-pointing images with GSDs of ~8.5 m, which in turn provided the reference for the highest-resolution XMO7 data with GSDs down to 3.5 m.

For data processing, we used the freely available open-source software ISISa (Integrated Software for Imagers and Spectrometers) developed by the USGS. Using a custom routine and reconstructed SPICE kernels, we automatically generated equidistant GCP tie-point networks consisting of 289 GCPs per image, corresponding to approximately one GCP every 58 pixels. These were manually co-registered to the orthomosaics step-by-step using the ISIS tool qtie. Subsequently, the spacecraft position (SPK) and camera pointing (CK) kernels were bundle-adjusted via the ISIS tool Jigsaw (Edmundson et al. 2012) using our GCP networks. The bundle-adjustment quality metrics (σ0 values; see Edmundson et al. (2012) for details) are reported in Tables 1 and 2. For photometric correction of the individual filter images, we applied the reflectance model established for Ceres by Schröder et al. 2017, using the corresponding phase and disk function and their published parameter values.

Despite the circular nature of the science orbits, the acquired image data exhibits a certain latitude-dependent resolution range due to Ceres‘ slightly ellipsoidal shape. HAMO images acquired at the equator have a GSD of approximately 136 m. A global longitude-spanning mosaic at this resolution and assuming a sperical reference body with radius 470 km would theoretically correspond to 21,713.95 pixels in the x-direction. To match this as closely as possible, we set the GSD of our HAMO data products in Neesemann et al. (2025) to 2×π×470000 m)/21714 px ≈135.999 m. LAMO data in the Occator region have GSDs of 34 - 35 m, approximately a factor of four higher spatial resolution than the HAMO data. Accordingly, we set the GSD of the LAMO products to 2×π×470000 m)/(4×21714) px ≈33.999 m and that of the XMO7 data to 8.499 m.

Image Previews

Figure 1. RGB Orthomosaic of Cerealia Facula and Vinalia Faculae composed of LAMO (~34 m GSD) narrow band filters F5 (IR),  F2 (GREEN), and F8 (BLUE).

Figure 2. XMO7 clear filter orthomosaic processed to ~8.5 m GSD.

Figure 3. Pan-sharpened RGB Orthomosaic of Cerealia Facula and Vinalia Faculae composed of LAMO (~34 m GSD) narrow band filters F5 (IR), F2 (GREEN), and F8 (BLUE) and XMO7 clear filter images at ~8.5 m GSD.

Data Tables

Table 1. Dawn FC2 XMO7 data used to compile the F1 clear filter orthomosaic shown in Figure 2.

Table 2. CSL/CXL and CSH narrow-band filter data used to compile the (I)RGB orthomosaic shown in Figure 1.