4,5,5,7,9,10,11,- 1NASA Postdoctoral Program Fellow at Jet Propulsion Laboratory (JPL), California Institute of Technology, Pasadena, CA, USA (lucas.lange@jpl.nasa.gov).
- 2Jet Propulsion Laboratory (JPL), California Institute of Technology, Pasadena, CA, USA.
- 3University of Colorado, Boulder, CO, USA
- 4European Space Agency (ESA), European Space Astronomy Center (ESAC), Villanueva de la Cañada, Spain
- 5University Paris-Saclay, GEOPS, UMR 8148, CNRS, Orsay, France.
- 6LATMOS/IPSL, Sorbonne Université, UVSQ, CNRS, Paris, France
- 7Institut Universitaire de France, IUF, France
- 8Cornell University, Ithaca, NY, USA.
- 9Southwest Research Institute, Boulder, CO, USA
- 10University of Oxford, Oxford, UK
- 11Northern Arizona University, Flagstaff, AZ, USA
- 12Arizona State University, Tempe, AZ, USA
Thermal measurements provide key constraints on the physical properties of icy satellite surfaces, including grain size, porosity, and regolith structure. On the icy moon of Jupiter Europa, previous analyses [e.g., 1,2] of the Galileo Photopolarimeter–Radiometer (PPR) dataset revealed heterogeneities in thermal inertia, but the limited spatial resolution and coverage prevented a detailed characterization of the thermophysical properties of the surface. Yet, the determination of these thermophysical properties is crucial to predict the surface temperatures of Europa that can be used to search for endogenic activity as hot spots, one of the main scientific targets of the upcoming NASA’s Europa Clipper mission [3].
We derived high-resolution maps of Europa’s surface albedo and thermal inertia, and inferred the microphysical properties of its icy regolith, through a reanalysis of the Galileo PPR dataset. We specifically investigated the spatial variability of these properties to discuss the processes controlling the thermophysical evolution of Europa’s surface. To do so, we used the KRC thermal model [4] to analyze the PPR brightness temperatures and retrieve the albedo and thermal inertia that best fit the observations. These values were then interpreted using theoretical conductivity models of porous ice [5] to constrain grain size and porosity and to investigate possible sintering processes affecting the surface.
We will present at the conference our main results: we derived a mean Bond albedo of 0.64 ± 0.06 (standard deviation of 1σ) and a mean thermal inertia of 56 ± 17 J m−2 K−1 s−1/2 (1σ). The thermal inertia shows significant spatial variations, including a band of low thermal inertia at the equator (39 ± 7 J m−2 K−1 s−1/2, 1σ) and higher values (56 ± 11 J m−2 K−1 s−1/2, 1σ) at mid-latitudes on the leading hemisphere (0°–180° W). The equatorial region of the trailing hemisphere (180° W–360° W) also exhibits higher thermal inertia (63 ± 17 J m−2 K−1 s−1/2, 1σ) than the leading hemisphere, likely related to compositional differences. Interpreting the thermal inertia with conductivity models indicates a porous icy regolith with grain sizes ranging from a few micrometers to a few centimeters and an average porosity of 0.61 ± 0.1 (1σ).
Interestingly, the thermal inertia distribution shows little correlation with geological units, the Pwyll ejecta being a notable exception, with markedly higher values than the surrounding terrain. In contrast, the good agreement between the thermal inertia distribution and modeled sputtering rates suggests that sputtering-driven sintering may play a fundamental role in controlling the thermophysical properties of Europa’s surface. The absence of a high thermal inertia equatorial band analogous to the PacMan anomaly observed on Saturn’s icy moons [e.g., 6] indicates that electron-driven sintering is inefficient on Europa, while temperature-gradient metamorphism may instead enhance grain growth at depth, potentially explaining the absence of large grains at the surface. In addition, modeled surface temperatures range from ~67 to 148 K at mid to low latitudes, with peak daytime temperatures counteracting radiolytic amorphization, while limiting the stability of volatile species. During the Europa Clipper mission (2031–2034), temperatures are expected to be slightly lower, ranging between 67.6–141.2 K. Our predictions provide a framework for interpreting future observations by the Europa Thermal Emission Imaging System (E-THEMIS) onboard Europa Clipper and the Submillimetre Wave Instrument (SWI) on JUICE. These future thermal measurements will provide key constraints to test these hypotheses and refine our understanding of the evolution of Europa’s icy regolith as well as search for active hot spots.
Acknowledgements
LL’s research was supported by an appointment to the NASA Postdoctoral Program administered by Oak Ridge Associated Universities at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004). Part of this work was performed at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004). Some of the computational analyses were run on Northern Arizona University’s Monsoon computing cluster, funded by Arizona’s Technology and Research Initiative Fund. © 2026. All rights reserved.
References
[1] Rathbun et al., 2010, Icarus, 210, 763–769
[2] Rathbun & Spencer, 2020, Icarus, 338, 11350
[3] Pappalardo, R. T., Buratti, B. J., Korth, H., et al. 2024, SSR, 220
[4] Kieffer, H. H. 2013, JGR: Planets, 118, 451–470
[5] Ferrari & Lucas, 2016, A&A, 588, A133
[6] Howett et al., 2011, Icarus, 216, 221–226
How to cite: Lange, L., Piqueux, S., O.Hayne, P., Mergny, C., Le Gall, A., Schmidt, F., Rathbun, J., Spencer, J., Sorli, K., Howes, S., Howett, C., Edwards, C., and Christensen, P.: Thermophysical Properties of Europa’s Surface Constrained by Galileo Photopolarimeter-Radiometer Temperature Measurements, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-122, https://doi.org/10.5194/epsc2026-122, 2026.
