# Spectroscopic Mapping of Callisto with HST/STIS and Implications for its Surface Composition

**Authors:** M. Ryleigh Davis, Samantha K. Trumbo, Michael E. Brown, Matthew Belyakov  
**arXiv:** [2506.00151](https://arxiv.org/abs/2506.00151)  
**Date:** 2025-05-30  
**Summary page:** https://www.summarizepaper.com/en/paper/2506.00151v1/

## Key points

- Comprehensive analysis of global, spatially resolved ultraviolet-visible spectra of Callisto obtained with HST/STIS
- Mapping the strength of a widespread downturn towards the near-UV and NIR spectral slope ranging from 700 to 1000 nm to determine possible compositions of Callisto's surface material
- Variations in spectral slope across Callisto's surface, with some regions showing slightly blue reflectance and others exhibiting red reflectance
- Bright water-ice-rich regions tend to have neutral or blue NIR slopes and a shallower near-UV downturn, while darker material is associated with red NIR slopes and stronger near-UV absorption
- Identification of broad absorptions near 820 and 930 nm correlated with Asgard and Valhalla impact basins, potentially linked to iron-bearing silicates
- Absorption features near 230 and 450 nm attributed to irradiated NaCl, limited evidence for sulfur-bearing species at UV-visible wavelengths
- Spatial variations in spectral characteristics indicating regional variations in Callisto's dark material composition reflecting endogenic and exogenic sources as well as radiolytic alteration

## Summary

In their study, M. Ryleigh Davis, Samantha K. Trumbo, Michael E. Brown, and Matthew Belyakov present a comprehensive analysis of global, spatially resolved ultraviolet-visible spectra of Callisto obtained with HST/STIS. The researchers delve into the possible compositions of Callisto's surface material by mapping the strength of a widespread downturn towards the near-UV and the NIR spectral slope ranging from 700 to 1000 nm. They observe variations in this slope across Callisto's surface, with some regions exhibiting slightly blue reflectance decreasing from 700 to 1000 nm, while others show red reflectance increasing in this range. The team notes that bright water-ice-rich regions on Callisto tend to have neutral or blue NIR slopes and a shallower near-UV downturn. In contrast, darker material is associated with red NIR slopes and stronger near-UV absorption. They identify broad absorptions near 820 and 930 nm that are spatially correlated with the Asgard and Valhalla impact basins, potentially linked to iron-bearing silicates. Additionally, an absorption edge near 275 nm primarily maps to Callisto's trailing hemisphere, while a 320 nm absorption prevalent within and surrounding Asgard and Valhalla may be related to organics. Furthermore, the researchers report two new absorption features near 230 and 450 nm that could be attributed to irradiated NaCl. They find limited evidence for sulfur-bearing species at UV-visible wavelengths and suggest reinterpreting a previously identified 280 nm band as a consequence of dividing unrelated spectral features. Spatial variations in spectral characteristics indicate regional variations in Callisto's dark material composition reflecting endogenic and exogenic sources as well as radiolytic alteration. This detailed analysis provides valuable insights into the surface composition of Callisto based on spectroscopic mapping data obtained with HST/STIS. The findings contribute to our understanding of the geological processes shaping this Jovian moon's surface features and offer implications for future studies exploring its complex composition further.