Callisto Isn’t Boring After All: HST Spectroscopy Shows its Surface is a Complex Mix of Native and Delivered Material
Callisto’s ancient, heavily cratered surface might seem geologically dull at first glance. With no evidence for past volcanism, tectonics, or other forms of endogenic resurfacing, the moon is often considered the most geologically “inert” of Jupiter’s large satellites. But what it lacks in activity, it makes up for in preservation. Callisto’s surface has remained largely unchanged for billions of years, shaped entirely by impact craters and mass wasting (e.g landslides), recording a long and messy history of impacts, radiation exposure, and the accumulation of dark material from various sources. It may therefore preserve important clues about the history of the Jovian system and possibly the broader solar system as well.
Where does that dark material come from? Likely from a combination of processes. Some of it may be native to Callisto, leftover from ancient ice-rock mixtures excavated by large impacts and concentrated in low-lying areas as bright volatile ice sublimated away. Other components were likely delivered externally — dust from Jupiter’s irregular satellites spiraling inward, or material from the large comet-like impactors. Radiation from Jupiter’s powerful magnetosphere may have further altered all of these components, producing complex space weathering products like organic residues.
In our new paper, we used the Hubble Space Telescope (HST) Space Telescope Imaging Spectrometer (STIS) to take a closer look at Callisto’s surface — and it turns out, it’s far more dynamic (chemically, at least) than previously appreciated.
A Global Look in Ultraviolet and Visible Light
Using HST’s Space Telescope Imaging Spectrograph (STIS), we captured high-resolution, spatially-resolved spectra of nearly the entire surface of Callisto, covering wavelengths from 200 to 1000 nm (UV to near-infrared). This is the most detailed near-global UV-visible wavelength dataset ever obtained for Callisto, and it allowed us to explore how reflectance varies with geography and wavelength — a powerful window into the surface composition.
The Highlights:
Dark vs. Bright Regions: Bright areas, often enriched in water ice, tend to have relatively flat or even slightly blue spectral slopes and shallower UV absorption. In contrast, darker regions exhibit redder slopes and a much stronger UV downturn, likely due to non-ice species such as silicates, carbon-rich compounds, and complex organics.
Unique Chemistry in Impact Basins: The large Asgard and Valhalla impact basins stand out both visually and spectrally. We identified strong absorptions near 820 and 930 nm — likely linked to iron-bearing silicates or phyllosilicates — that are spatially confined to these basins, suggesting excavation of deep crustal material or formation via impact melt. A 320 nm UV absorption feature also appears unique to these basins and surrounding terrains and may be associated with a distinct class of organics not seen elsewhere on Callisto.
Mysterious UV Features: We detected two new UV absorption features near 230 and 450 nm. These might be from irradiated sodium chloride salt (NaCl) — a material that has been identified on Europa from these same two absorption bands and plausibly linked to its subsurface ocean, but are now tentatively popping up on Callisto. However, we couldn’t confirm whether these two bands are spatially correlated or if they originate from the same compound and other possible explanations for these features include species like organics or silicates.
Rethinking Sulfur: Sulfur dioxide (SO₂) has long been thought to be present on Callisto’s leading hemisphere based on a 280 nm feature seen in leading to trailing hemisphere ratio spectra. But our spectral maps show that this “feature” likely just results from dividing two unrelated absorption bands — one found predominantly on the leading hemisphere (320 nm) and the other on the trailing (275 nm). In other words, the 280 nm ratio “band” is probably just a spectral illusion and overall we find little evidence for any expected sulfur-related absorptions at these wavelengths.
Big Picture: A Chemically Patchy, Weathered World
What emerges from our mapping is a picture of a moon whose surface is a patchwork shaped by excavation, bombardment, and radiation. Some of Callisto’s non-ice material is delivered from elsewhere in the Jupiter system (irregular satellite dust or larger comet-like impactors), some is dug up from beneath the surface by impacts, and some is transformed by relentless exposure to charged particles from Jupiter’s magnetic field.
The composition of the dark material isn’t uniform, with a distribution that varies on local to global scales and different regions showing different combinations of sources of dark material and levels of space weathering. It contains spectral fingerprints that are consistent with silicates, carbonaceous compounds and organics, and perhaps even salts.
Surprisingly, despite years of speculation, we found very little spectral evidence for SO₂ or other sulfur-bearing compounds. Instead, the UV-visible spectrum seems to point more toward carbon-rich, silicate, and possibly salt-bearing materials. This conclusion agrees with other recent analyses of JWST observations of Callisto (Cartwright et al. 2024) and JUNO observations of neighboring Ganymede (Tosi et al. 2024).
Why It Matters
Callisto may be geologically quiet, but its surface holds vital clues about the Jovian system’s history. It also tells us something about what happens to planetary surfaces when they’re exposed to space weathering, radiation, and exogenic material for billions of years. The detection of distinct compositions in ancient impact basins suggests that we’re seeing materials from different depths or time periods — potentially even primordial crustal material preserved in the subsurface. The spatial variations in the UV and visible absorption features give us insight into how exogenic and endogenic materials have mixed over time as well as how radiation processes have modified Callisto’s surface. Understanding what’s on Callisto’s surface can help us piece together not just its own history, but the broader evolution of the Jovian system.
This study also sets the stage for upcoming spacecraft missions — such as NASA’s Europa Clipper or ESA’s JUICE mission, both of which will pass by Callisto on their journeys through the Jovian system. Our HST data can help guide their spectral observations and contextualize their findings. Stay tuned — we’re already working on the next chapter, using JWST/MIRI mid-infrared observations to hunt for silicates, complex organics, and other key components within Callisto’s enigmatic dark material.