My Research

I am broadly interested in understanding the compostions of icy moons throughout the solar system, and what this can tell us about the formation and histories of the gas and ice giant systems. I use ground- and space- based telescopes, alongside laboratory spectra to explore these icy worlds. See below for info on some of my recent work!

Callisto’s Ancient Surface

Coming soon! See our accepted Cycle 3 JWST Program.

Reconstructing the Histories of the Ice Giant Systems through Small Satellite Observations

Coming soon! See our accepted Cycle 3 JWST program.

Irradiation Chemistry on Europa

Europa’s trailing hemisphere surface composition reflects a complex interplay between endogenic material present within the recently geologically active chaos terrains and linea (possibly sourced from the subsurface salty ocean), exogenic material deposited by the Jovian magnetosphere, and irradiation-induced alteration of both the endogenically and exogenically sourced material. Using spectroscopic observations from telescopes and spacecraft, I am working to understand the irradiation processes which alter Europa’s surface composition.

Crater Chronicles: Using Europa’s Youngest Craters to Measure Irradiation Timescales

A classic example of irradiation-induced alteration of exogenic material on Europa’s trailing hemisphere is the so-called radiolytic sulfur cycle, where Iogenic sulfur ions captured in Jupiter’s magnetosphere are deposited on Europa’s trailing hemisphere and subsequent irradiation produces hydrated sulfuric acid, along with a variety of potential sulfur-bearing intermediary products. Because of the intense irradiation environment on Europa’s trailing hemisphere, these sulfur-bearing compounds are expected to be in radiation-induced equilibrium, with sulfuric acid continuously produced and destroyed in some steady state concentration. The timescale for reaching this equilibrium concentration on Europa’s trailing hemisphere is not well known, but various estimates based on laboratory experiments have suggested timescales on the order of a few thousand years. In my recent paper (Davis et al. 2024), I use Galileo/NIMS ovservations to show that both craters and their ejecta are depleted in sulfuric acid relative to nearby older terrains, suggesting that the radiolytic sulfur cycle has not yet had enough time to reach an equilibrium concentration of hydrated sulfuric acid. Based on estimates for the ages of these craters, we are able to place a lower limit of a few million years on the equilibrium timescale of Europa’s sulfur cycle, which is significantly longer than what estimates based on laboratory experiments have suggested!

On the other hand, the crater and dark proximal ejecta appear to be enriched in salts relative to the bright white ejecta blanket at both craters, suggesting that the dark material exhumed by the impacts is relatively salt-rich. Endogenic salty material within Europa’s other geologically young features like the chaos terrains, bands, and ridges become progressively darker and redder towards the trailing hemisphere apex, potentially due to irradiation induced reddening of salts. We expect that the salty material exhumed by the craters has also been darkened and reddened via irradiation based on their low-albedo and visually red coloration in the Galileo/SSI color images. We are therefore able to place an upper limit, of the age of Pwyll crater (~1 Myr), on the timescale over which this salty material is irradiation reddened on Europa’s trailing hemisphere. Apparantly irradiaition induced reddening of salts operates on a much faster timescale than the radiolytic sulfur cycle on Europa’s trailing hemisphere.

Europa’s Mysterious 2.07μm Absorption Feature

A small abosorption feature near 2.07μm on Europa's trailing hemisphere has been used as evidence for the presence of endogenic magnesium chloride (MgCl₂), possibly sourced from Europa's subsurface salty ocean. While the spectrum of MgCl₂ does not have this specific absorption feature, it could be altered via irradiation on Europa's trailing hemishere into either more complex magnesium chlorate/perchlorate salts (Ligier et al. 2016) or if combined with Io-genic sulfur, into a sulfur-bearing salt such as epsomite (MgSO₄⋅10H₂O) (Brown et al. 2013). Both of these possiblilties can explain the presence of an absorption feature at 2.07μm as arising from irradiation induced changes of endogenically sourced MgCl₂. However, if this feature does arise from an irradiation product of endogenic magnesium chloride, we would expect the absorption feature to be the strongest within Europa's young geologic features such as the recently disrupted chaos terrains, where the endogenic salty material is thought to be. Using archived VLT/SINFONI H+K band spectra, I showed that the presence and band area of the 2.07µm absorption feature is not spatially associated with the large-scale geology, but it is correlated with a bullseye-shaped irradiation pattern associated with Iogenic sulfur deposition (see Davis et al. 2023). This suggests that the absorption feature is unlikley to arise from the irradiation induced processing of endogenic MgCl₂, and therefore neither epsomite nor a combination of magnesium chlorate and perchlorate are likely to explain the spatial distribution of the absorption. Instead, we suggest that it may simply arise from some intermediary product of the radiolytic sulfur cycle, or perhaps form during the bombardment of water ice by electrons or the remaining (non-sulfur) ion species, independent of any endogenic material present within the recent geology. Unfortunately, we were not able to identify any candidate species within existing laboratory data and look forward to future laboratory experiments which may help identify the species responsible and provide insights into the nature of the sulfur radiolysis occurring on Europa.