Near-Surface Properties of Europa Constrained by the Galileo PPR Measurements

NASA's Europa Clipper mission will characterize the current and recent surface activity of the icy-moon Europa through a wide range of remote sensing observations. In particular, the Europa Thermal Emission Imaging System (E-THEMIS) will measure global, regional and local surface temperatures at three thermal infrared wavelengths, under various conditions of local time and emission angles [1]. These measurements will not only enable the detection and characterization of thermal anomalies on the surface, but also provide insight into the thermophysical properties of the regolith, such as particle size, block abundance, and subsurface layering.

Indeed, in the absence of endogenic heat-sources, Europa's surface temperatures are controlled by the albedo and thermal inertia of the surface. To date, these properties have been derived for Europa by comparing the surface temperature modeled by 1D thermal model with measurements from either ground-based observations [e.g., 2,3], Voyager 1 flyby [4] or by the PPR instrument onboard the Galileo spacecraft at the end of the 20th century [5,6].

The thermophysical properties of granular porous ice, as expected on Europa, differ fundamentally from silica-based trends usually considered by thermal models applied to rocky bodies [7]. Specifically, the bulk thermal conductivity of ice can vary by several orders of magnitude [7], depending on the microphysical properties of ice (crystallinity, ice crystal radius, porosity, contacts between ice crystals) and temperature (Fig. 4). Importantly, radiative conductivity within water ice fines is always neglected in models used to date [3,8-11], while it is a major contributor in the conductivity of particulate water ice, even at the low Europa temperatures [7]. Furthermore, [7] demonstrated that the contact conductivity for small-size ice crystals was controlled by the nature of contacts between ice grains. As a consequence, while the thermal conductivity for silica-based material decreases with decreasing grain size, [7] demonstrated that porous ice made of micrometer size ice crystals follows an opposite trend, and yields high thermal inertia (Fig. 4a), potentially inducing high nighttime temperatures compared to those expected with coarser material (e.g., +20 K in the case illustrated in Fig. 4b). Because the surface of Europa is thought to expose both crystalline and amorphous ice, with very small ice crystals [12,13], it is important to understand the differential thermal regimes that could results from various icy materials configurations, especially to avoid the erroneous identification of thermal anomalies as endogenic hotspots.

To account for this new knowledge, we are improving the KRC -K for k, symbol of thermal conductivity, R for rho (density), and C for Cp (specific heat) -  thermal model [8], which has been extensively used for planetary surfaces thermophysical studies, and which will be used for the interpretation of future E-THEMIS data. We have incorporated state-of-the art thermal inertia dependencies to ice properties described in [7], and we are currently implementing the emissivity dependency to ice temperature [e.g.,14], the penetration of solar radiation within the icy surface to simulate solid-state greenhouse effect [e.g., 15], and subsurface layering. At the conference, we will present model predictions for surface temperature signatures resulting from these physical processes, and compare them with PPR observations. This comparison enables us to place constraints on Europa's near-surface properties, including porosity, grain size, and potential layering. Ultimately, this work aims to refine the interpretation of upcoming E-THEMIS thermal observations and help distinguish between thermophysical and endogenic origins of thermal anomalies on Europa.

 

Figure 1: Effects of (a) the ice properties on thermal inertia [7] compared to silicate trends and (b) on Europa’s temperatures (computed with KRC). A surface composed of fine small crystalline grains can exhibit an unexpected high  thermal inertia, inducing a +20 K nighttime warming compared to the surrounding surfaces, and be thus  misinterpreted as a hot-spot.

References: [1] Christensen et al. (2024), The Europa Thermal Emission Imaging System (E-THEMIS) Investigation for the Europa Clipper Mission, Space Science Reviews; [2] Hansen (1973),  Ten-micron eclipse observations of Io, Europa, and Ganymede. Icarus; [3] Trumbo et al. (2018), ALMA thermal observations of Europa. Astronomical Journal; [4] Spencer (1987), The surfaces of Europa, Ganymede, and Callisto: an investigation using Voyager IRIS thermal infrared spectra (Jupiter). PhD thesis at the University of Arizona; [5] Spencer et al. (1999), Temperatures on Europa from Galileo photopolarimeter-radiometer: nighttime thermal anomalies, Science; [6] Rathbun et al. (2010), Galileo PPR observations of Europa: hotspot detection limits and surface thermal properties, Icarus; [7] Ferrari & Lucas (2016), Low thermal inertias of icy planetary surfaces, Astronomy and Astrophysics; [8] Kieffer (2013), Thermal model for analysis of Mars infrared mapping, JGR: Planets; [9] Spencer et al. (1989). Systematic biases in radiometric diameter determinations, Icarus; [10] Hayne et al. (2017), Global regolith thermophysical properties of the Moon from the Diviner Lunar Radiometer Experiment, JGR: Planets; [11] Thelen et al. (2024), Subsurface Thermophysical Properties of Europa’s Leading and Trailing Hemispheres as Revealed by ALMA, The Planetary Science Journal; [12] Berdis et al. (2020), Europa’s surface water ice crystallinity: Discrepancy between observations and thermophysical and particle flux modeling, Icarus; [13] Hansen, & McCord (2004), Amorphous and crystalline ice on the Galilean satellites: A balance between thermal and radiolytic processes, JGR: Planets; [14] Ferrari (2024), Infrared emissivity of icy surfaces. Sensitivity to regolith properties and water-ice contaminants, Astronomy and Astrophysics; [15] Brown & Matson (1987), Thermal effects of insolation propagation into the regoliths of airless bodies, Icarus.

Acknowledgement: This work was performed at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA (80NM0018D0004). LL’s research was supported by an appointment to the NASA Postdoctoral Program at the Jet Propulsion Laboratory, administered by Oak Ridge Associated Universities under contract with NASA.