FC colour images of dwarf planet Ceres reveal a complicated geological history
Introduction
The Dawn spacecraft (Russell and Raymond, 2012) carries two Framing Cameras (FCs) which obtained images in seven colours (centre wavelength at 0.43, 0.55, 0.65, 0.75, 0.83, 0.92 and 0.98µm) and one clear filter ranging between 0.45 and 0.92µm (Sierks et al., 2012), mapping the surface of Ceres at ~140m/pixel during the High Altitude Mapping Orbit (HAMO), i.e., from ~1400km above the surface (Russell et al., 2016). Image mosaics, derived via applying image processing tools (Nathues et al., 2015, Reddy, 2012), are used to determine the global surface colour characteristics of Ceres. All colour mosaics, forming a global image cube, have been photometrically corrected by applying Hapke's photometric model (Hapke, 1981, Hapke, 1999, Hapke, 2012 and references therein) using iteratively derived light scattering parameters from Approach, Survey orbit and HAMO orbit imagery. Potential endogenic resurfacing processes such as cryovolcanism (e.g., Castillo-Rogez et al., 2011; Ruesch et al., 2016) as well as impact-induced tectonics (Hiesinger et al., 2016) have mainly shaped the cerean surface, forming a few large basins and a multitude of craters, of which some have been modified by viscous relaxation as indicated by crater morphology (see Fig. 1A). Surface areas retaining original primordial composition several million years after accretion and partial differentiation (e.g., Mc Cord and Sotin, 2005; Castillo-Rogez and Mc Cord, 2010) may not exist anymore due to resurfacing, or they are at least challenging to identify. The most pristine materials can probably be found at locations of the most recent, unweathered surfaces: fresh crater ejecta and exposed crater interior materials. We are going to show apparent correlations of the local geologic history with colour properties. One area where craters >5km are absent, i.e., the surface is less modified by larger impacts but of older age, has been selected as our spectral standard site (Fig. S3D).
Section snippets
Surface colour units
The global clear filter mosaic (Fig. 1A) exhibits a large diversity of reflectances ranging between ~0.03 and 0.37; however most of the surface is on the lower end of this range. The darkest sites are associated with impact craters, for example, ejecta of Occator and Nawish, and floor material of Urvara, Yalode, and Ezinu. The brightest sites are located on the crater floors and rims of Occator, Oxo, Haulani, and Dantu.
Ceres' colours are more diverse than one could anticipate from ground-based
Prominent surface features
Occator (Fig. 3B) is the most prominent surface feature, exhibiting the largest reflectivity range and the brightest materials on Ceres (Nathues et al., 2015). It shows the highest spectral diversity and exhibits a dark ejecta field to its north-east. The central bright spot of Occator, a pit ~9km in diameter covered with bright material, shows signs of activity in the form of water ice sublimation (Küppers et al., 2014; Nathues et al., 2015), potential deposition of salts and clays (Nathues et
Surface ages
In order to infer a relative chronostratigraphic framework of the cerean crust, we measured crater size–frequency distributions (see supplementary information ‘methods’) on the floors of several craters, the ‘standard site’, and the ‘eroded crater’ (Fig. S6). Formation ages of floors are: Oxo, Haulani, both geologically recent, Occator ~6.9Ma, Dantu ~46–68Ma, Urvara ~78Ma, and Azacca ~84Ma. The ‘standard site’ (~570Ma) is much older than the measured crater floor materials, but considerably
Colour unit origin
In our current understanding the identified colour units are the products of internal (cryovolcanism, diapirism) and external (impact cratering, space weathering) evolution processes. Since Ceres is the largest carbonaceous chondritic-like body in the asteroid belt (Larson et al., 1979), it may be the most intact and the least stripped of primordial crustal material. Bright and bluish units are generally found at equatorial and intermediate latitudes, while they are virtually absent at high
Composition
We used absolute reflectance spectra to constrain the composition of the colour units, considering a wide range of geologically plausible materials related to carbonaceous chondrites (CCs). Colour spectra of the bright spots at Occator are consistent with mixtures of Mg-sulphates (kieserite), carbonates (magnesite) and phyllosilicates (phlogopite), whereby carbonates show the best global spectral matches (details in Fig. 4 and supplementary information). Due to the extended wavelength coverage
Stratigraphy
Based on the distribution of the identified colour units, their potential compositions, and the geomorphology of the related craters, we infer the cerean stratigraphy shown in the left-half of Fig. 5. The uppermost layer, which we have denoted as background material, is composed of carbonaceous chondritic-like material; more specifically this background material has been identified by (De Sanctis et al., 2015, 2016) to contain ammoniated phyllosilicates. Having been exposed by the impacts, we
Summary
The colours of the cerean surface are more diverse than anticipated by ground-based observations. Data obtained by the Framing Camera on-board the Dawn spacecraft led to the identification of five major colour units. The youngest of these units, the bright and bluish units, are exclusively found at equatorial and intermediate latitudes. Additionally, we identified correlations between the distribution of colour units, crater size, and formation age, and used these data to infer a cerean crustal
Acknowledgement
We thank the Dawn operations team for the development, cruise, orbital insertion, and operations of the Dawn spacecraft at Ceres. Also we would like to thank the Framing Camera operations team, especially P. G. Gutierrez-Marques, I. Hall and I. Büttner. The Framing Camera project is financially supported by the Max Planck Society and the German Space Agency (DLR).
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