2024-09-06 マックス・プランク研究所
＜関連情報＞
- https://www.mpg.de/23435699/dwarf-planet-ceres-origin-in-the-asteroid-belt?c=2249
- https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2023JE008150
ケレスのコンサスクレーター アンモニウムに富む塩水とフィロケイ酸塩の交換？ Consus Crater on Ceres: Ammonium-Enriched Brines in Exchange With Phyllosilicates?
A. Nathues, M. Hoffmann, R. Sarkar, P. Singh, J. Hernandez, J. H. Pasckert, N. Schmedemann, G. Thangjam, E. Cloutis, K. Mengel, M. Coutelier
Journal of Geophysical Research: Planets Published: 05 September 2024
DOI:https://doi.org/10.1029/2023JE008150
Abstract
Ceres is a partially differentiated dwarf planet located in the main asteroid belt. Consus crater (diameter ∼64 km) is one of the oldest impact features (∼450 Ma) on the Cerean surface that surprisingly still shows a large variety of color lithologies, including exposures of bright material, which are thought to be brine residues. Here, we present new results that help in understanding the structure and composition of the Cerean crust. These results were deduced by using newly processed Dawn Framing Camera (FC) color imagery and FC clear filter images combined with infrared spectral data of Dawn’s Visible and Infrared Spectrometer (VIR). Consus exhibits a variety of color lithologies, which we describe in detail. Interestingly, we found three spectrally different types of bright material exposed by a large old crater on Consus’ floor. One of these, the yellowish bright material (Nathues et al., 2023, https://www.hou.usra.edu/meetings/lpsc2023/pdf/1073.pdf) and its modification, shows spectral signatures consistent with ammonium-enriched smectites. We hypothesize that the ammonium in these smectites stems from contact with ascending brines, originating from a low-lying former brine ocean that has been enriched in ammonium during the differentiation and freezing process of the Cerean crust. This enrichment is mainly due to ammonium uptake by sheet silicates. If such an ammonium enrichment occurred over long-time scales on a global scale, this process may explain the vast presence of ammonium on the Cerean surface. Therefore, an outer solar system origin of Ceres is possibly not needed to explain the global presence of ammonium.
Key Points
- The old impact crater Consus still shows a large variety of color lithologies
- The yellow bright material lithology is ammonium-enriched
- We hypothesize that this ammonium stems from ascending originally low-lying brines
Plain Language Summary
Ceres is the largest object in the asteroid belt and surprisingly exhibits surface features formed by recent cryo-volcanism, although evolution models predicted an inactive, completely frozen body. Cryo-volcanism possibly stems from a brine layer at the crust-mantle transition. The global presence of ammonium on the Cerean surface led to the conclusion of a potential outer solar system origin of Ceres. Here, we investigate the structure of its crust, by studying one of the older, but still mineralogical diverse, impact craters that excavated originally low-lying materials. For our study, we are using data of the Dawn Framing Camera, spectral data of the Visible and Infrared Spectrometer as well as several geoscientific remote sensing methods. We describe “yellowish bright material” for the first time (Nathues et al., 2023, https://www.hou.usra.edu/meetings/lpsc2023/pdf/1073.pdf), that is spectrally unique in our color data and exhibits a strong absorption feature in the near-infrared caused by ammonium. We hypothesize that the ammonium enrichment of this surface material is mainly a consequence of an ammonium exchange between rising brines and near-surface phyllosilicates. If true, this would strengthen the case for the existence of a low-lying ammonium-rich brine layer, which in turn could weaken the hypothesis of an outer solar system origin for Ceres.
