Spectral properties of bright deposits in permanently shadowed craters on Ceres

TL;DR

This study provides direct spectral evidence supporting the presence of water ice in permanently shadowed craters on Ceres by analyzing narrowband images and spectral slopes, indicating high-purity ice deposits.

Contribution

It offers the first direct spectral characterization of water ice in Ceres' shadowed deposits, confirming their icy composition through detailed image analysis.

Findings

Bright deposits have blue spectral slopes consistent with water ice.

Spectral properties suggest ice is likely high purity with possible minor components.

No evidence of salts or cryovolcanic activity associated with the deposits.

Abstract

Bright deposits in permanently shadowed craters on Ceres are thought to harbor water ice. However, the evidence for water ice presented thus far is indirect. We aim to directly detect the spectral characteristics of water ice in bright deposits present in permanently shadowed regions (PSRs) in polar craters on Ceres. We analyzed narrowband images of four of the largest shadowed bright deposits acquired by the Dawn Framing Camera to reconstruct their reflectance spectra, carefully considering issues such as in-field stray light correction and image compression artifacts. The sunlit portion of a polar deposit known to harbor water ice has a negative (blue) spectral slope of $-58 \pm 12$ % $\mu$m$^{-1}$ relative to the background in the visible wavelength range. We find that the PSR bright deposits have similarly blue spectral slopes, consistent with a water ice composition. Based on the brightness and spectral properties, we argue that the ice is likely present as particles of high purity. Other components such as phyllosilicates may be mixed in with the ice. Salts are an unlikely brightening agent given their association with cryovolcanic processes, of which we find no trace. Our spectral analysis strengthens the case for the presence of water ice in permanently shadowed craters on Ceres.