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Radiolysis on Icy Satellites

The icy Galilean satellites, Europa, Ganymede, and Callisto, orbit within Jupiter's massive magnetosphere, which results in the constant bombardment of their surfaces by energetic charged particles. Using ground- and space-based spectroscopy, I work to understand the resulting radiolytic chemistry, which is key to our understanding of the surface compositions of these moons and how they evolve over time. This understanding will, in turn, shed light on how these widespread processes operate throughout the solar system on icy bodies that are not so easily observed.

Production and Stability of H2O2

Hydrogen peroxide (H2O2) is part of an important radiolytic cycle on Europa, and potentially on other icy satellites. The bombardment of surface water ice by magnetospheric ions and electrons converts H2O to H2O2, losing H2 in the process and creating an oxidizing surface. Understanding this cycle is not only important to our knowledge of the chemical composition of Europa's surface and to the study of surface-magnetospheric interactions throughout the solar system, but it is also critical for our understanding of the potential chemical energy sources to Europa's ocean. Water-rock interactions at the seafloor can be a source of reductants, but the energy available for redox chemistry will likely depend on the supply of oxidants, such as H2O2, from surface. Using Keck Observatory and NASA's Near Infrared Telescope Facility (IRTF), I am working to better understand what controls its production and abundance on Europa, and by extension, its potential importance for bodies throughout the solar system.

The Keck results so far are surprising—contrary to the laboratory expectation that H2O2 should lie within Europa's coldest and iciest regions, they show nearly the exact opposite! Instead, I observe the most H2O2 at warm low latitudes within salty chaos terrain, and relative depletions toward the cold, ice-rich high latitudes. A possible explanation may be that chaos terrain contains abundant CO2, which has been shown in the lab to increase H2O2 concentrations. If this is true, then it could imply that Europa's CO2, or at least the carbon it contains, is endogenic and ultimately sourced from the interior ocean. But this hypothesis requires more work to evaluate. In the meantime, check out the paper here, and stay tuned for upcoming work combining IRTF and Keck data to address the mechanisms behind this species' formation and stability. 

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Figure from Trumbo et al. (2019) showing a single Keck NIRSPEC slit, which crosses both the most spectrally icy location on the surface and the salty, low-latitude chaos region Tara Regio. Contrary to the laboratory hypothesis that Europa’s H2O2 should follow the cold, icy terrain of the upper latitudes, the strongest absorptions fall nearly perfectly within the outlined bounds of Tara Regio, which is warm and ice-poor by comparison. An explanation may lie in the composition of chaos terrain, which may contain endogenic material that somehow increases the radiolytic yields and/or stability of H2O2 on the surface.

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Maps of Ganymede's 5773 Å O2 absorption from Hubble Space Telescope data taken from Trumbo et al. (2021). Somewhat counterintuitively, the largest absorptions appear at the comparatively warm low to mid latitudes of the trailing hemisphere. Destruction of O2 bubbles by sputtering ions deflected to the high latitudes by Ganymede's intrinsic magnetic field could explain this distribution. Discrepancies between overlapping regions from different observations suggest moderate temporal variability in the O2 that may reflect sensitivity to changes in the plasma environment. New observations are underway to constrain these effects!

Condensed O2 on Icy Moons

Molecular oxygen (O2) is a major product of the radiolysis of water ice on Europa, Ganymede, Callisto, and likely other icy bodies throughout the solar system. The bombardment of their surface ice by energetic particles within Jupiter’s magnetosphere dissociates the water molecules, resulting

in the subsequent formation of O2, which is a component of all three of their tenuous atmospheres. Strangely, though, condensed O2 has also been detected on the surfaces of all three satellites, despite the fact that their daytime surface temperatures are far too high and their surface pressures far too low for condensed O2 to be stable. Suggested explanations have ranged from “microatmospheres” of O2 bubbles trapped within ice defects, to O2-bearing mixed clathrates, and even to solid O2 within localized cold traps on the surface or in the near subsurface. The mechanisms controlling the production and stability of O2 on these bodies are similarly uncertain. Using spatially and temporally resolved observations of Ganymede from the Hubble Space Telescope and Gemini Observatory, respectively, I am working to understand the curious presence of this species on icy surfaces. Like hydrogen peroxide, radiolytically produced O2 is likely important for the potential habitability of these ice-covered ocean worlds.