Dr Carly Howett

Abstract:

Saturn’s ocean moon Enceladus is among the most compelling targets for future astrobiology-focused missions. Material erupted from the moon’s south polar “Tiger Stripe” fractures is thought to originate from a global subsurface ocean, providing a natural pathway for ocean-derived compounds to be sampled without penetrating the ice shell. Cassini observations demonstrated that Enceladus’ plume contains water vapor, ice grains, salts, organics, phosphates, and other compounds of astrobiological interest. Consequently, future landed or plume-sampling missions to Enceladus will seek to determine whether ocean-sourced organic molecules, and potentially biosignatures, can be preserved and detected in plume material.However, once plume grains are erupted and deposited onto the surface, they are exposed to multiple space weathering agents. Solar ultraviolet radiation, Saturnian magnetospheric charged particles, and galactic cosmic rays can alter or destroy organic molecules contained within surface material. The degree of processing depends not only on radiation environment and molecular susceptibility, but also on the rate at which fresh plume fallout buries previously deposited material. Identifying locations and depths where organics may remain minimally processed is therefore key for mission planning and interpretation of future measurements.Here we model the degradation and preservation of plume-deposited organics on Enceladus, using the amino acids glycine and phenylalanine as representative organic molecules and potential biosignature proxies. We combine solar UV photodestruction calculations, Geant4-based charged-particle irradiation simulations, and estimates of local plume deposition rates to produce a coupled model of irradiation and burial across the surface. Solar UV photolysis rates are calculated using experimental rate coefficients adjusted for seasonal variation in photon flux at selected locations on Enceladus, including equatorial and southern-hemisphere sites at leading and trailing longitudes. Because even small abundances of non-water-ice contaminants can strongly influence UV attenuation, we also examine the effect of trace tholin-like material mixed into the ice. Charged-particle processing is estimated using Cassini-derived magnetospheric electron spectra and a numerical transport model for the galactic cosmic ray flux at Saturn.Our results show that preservation of plume-deposited organics is highly sensitive to burial rate and season. At low to moderate surface deposition rates, less than approximately 0.002 mm yr⁻¹, newly deposited surface material is strongly processed by solar UV radiation before burial can shield it from further exposure. In such regions, there may be no practical “safe sampling depth” at which pristine or minimally processed organics can be accessed, because material has already been altered before reaching depth. By contrast, in regions receiving the highest predicted plume deposition rates, especially during local winter when solar UV exposure is reduced, burial can occur rapidly enough to protect a significant fraction of the organic inventory. Under these conditions, the near-surface stratigraphy may consist of millimeter-scale layers that alternate between relatively well-preserved and heavily photoprocessed material, reflecting seasonal variations in irradiation during emplacement.Our findings have implications for the mission design and sampling strategy of future Enceladus missions. A landed mission seeking minimally processed ocean-derived organics should prioritize regions of high plume fallout, where burial is most effective, and should consider the seasonal context of deposition and sampling. The most favorable strategy may be to collect plume grains directly before surface deposition, e.g., from a funnel on the surface or plume fly-through, thereby avoiding prolonged radiolytic and photolytic alteration. If surface sampling is required, targeting the uppermost approximately 0.1 mm of material in high-deposition regions during local winter may maximize the probability of detecting less-processed organics. More broadly, our results demonstrate that preservation potential at Enceladus is governed by the competition between surface irradiation and plume-driven burial, and that this balance should be incorporated into future landing site selection, sampling-depth requirements, and biosignature-detection strategies.

Abstract:

IntroductionThe presence of endogenic hotspots provides a measure of the level of geologic activity of icy moons, since they are indicative of ongoing resurfacing processes. However, to avoid misinterpreting thermal abnormalities, it is first necessary to understand passive thermal emission that is governed by the physical structure of materials. Two important thermophysical properties in such analysis are bolometric Bond albedo and thermal inertia: if diurnal temperature variations can be accurately modeled by adjusting these two parameters, a passive rather than endogenic origin is possible. In this work, we aim to constrain the thermal inertia and Bond albedo across the surfaces of Europa, Ganymede, and Callisto using brightness temperature observations recorded by the Galileo Photopolarimeter-Radiometer (PPR) instrument [1]. By presenting estimated values and their uncertainties for these thermophysical properties, we prepare for future thermal measurements carried out by both Europa Clipper and JUICE.MethodsWe first take average diurnal temperatures of the surfaces of Europa, Ganymede, and Callisto using brightness temperatures recorded by PPR. In this analysis, we use 29 datasets for Europa, notably increased beyond previous efforts [2], comparable to [3].  With less observations available, only 7 datasets each were used for both Ganymede and Callisto. By translating the observed radiance into brightness temperatures, the variation in temperature with local time is determined for latitude and longitude bins across each of the three moons. These diurnal curves are compared to those predicted by a 1-D thermal model [4] to determine what thermal inertias and Bond albedos can fit the data within a reduced chi-squared cut-off of χ2red ≤1.0. This analysis is used to extensively quantify the uncertainty of the two thermophysical parameters derived from PPR data.ResultsEuropa: Ensuring closed upper and lower limits within our χ2red cut-off, we map albedo and thermal inertia for 33% and 24% of Europa's surface area into 6°x6° latitude/longitude bins (Fig. 1, left). We find a range of 0.375-0.75 for albedo and 20-110 J m-2 K-1 s-1/2 (MKS) for thermal inertia, agreeing with [2] and [3]. Our uncertainty analysis indicates well-constrained estimates for albedo, with the average higher and lower ranges overlapping within their uncertainties: Ahigh = 0.11 ± 0.06 and Alow = 0.18 ± 0.12. Uncertainties for thermal inertia remain poorly constrained, with average higher and lower ranges being ­Γhigh = 103(+153/-103) MKS and Γ­low = 17 ± 9 MKS.Ganymede: Due to less available surface coverage, Ganymede’s surface is divided into 18°x6° longitude/latitude bins in order to meet the diurnal fitting routine requirements (Fig. 1, right). Preliminary results indicate the Bond albedo remains nearly uniform, with an average of 0.42 ± 0.07 across the surface and agreeing well with previous work [6]. Specifically, our χ2red-deduced higher and lower uncertainty ranges of albedo are: Ahigh = 0.10 ± 0.06 and Alow = 0.14 ± 0.08. No apparent distinct surface variations in thermal inertia are as of yet discerned, with global values distributed across a range of 20-70 MKS. Upper limit thermal inertia estimates align with [6] within χ2red-deduced uncertainty ranges of ­Γhigh = 57 ± 35 MKS and Γ­low = 25 ± 11 MKS.Callisto: With less PPR coverage available, Callisto fits are performed across grouped hemispheric regions, where the Jovian and anti-Jovian hemispheres are analyzed separately. Each hemisphere is divided into 10° latitude strips. Preliminary results for thermal inertia and Bond albedo indicate an overall agreement with lower-bound estimations from previous literature [5,6]. Further constraints are expected to be obtained at the time of the conference.ConclusionThese results give an indication of the albedo and thermal inertia variation across Europa, Ganymede and Callisto. They aid in preparing for the arrival of Europa Clipper and JUICE to the Jupiter system by improving estimates for passive surface thermal properties and providing uncertainties of their values. This will enable future work to help discern what temperatures may lie above those expected from passive emission alone, providing a critical first step in the search for endogenic heating anomalies. Future work aims to refine the Ganymede and Callisto calculations using a two-component (ice and non-ice) analysis, and characterize the causes behind thermal inertia variations by modeling microphysical ice states for the three Galilean moons.Fig. 1: Thermal Inertia (top) and Bond albedo (bottom) map of Europa (left) and preliminary results for Ganymede (right).[1] Russell, E.E. et al., 1992. Space Sci Rev 60, 531-563.; [2] Rathbun, J.A. et al., 2010. Icarus 210, 763-769.; [3] Lange, L. et al., 2026. arXiv:2604.14374; [4] Lyster, D. et al., 2025. pp. EPSC-DPS2025-1479; [5] Meyer, C. et al., 2026. Planet. Sci. J. 7, 10; [6] Spencer, J.R. et al., 1989. Icarus 78(2), 337-354.

Abstract:

Accurately characterizing the endogenic thermal signatures of icy moons requires a detailed understanding of surface heating driven by insolation, surface properties and local terrain. Due to the lack of high-resolution thermal observations, Europa’s surface temperatures have historically been sufficiently modelled using smooth surface or one-dimensional approximations [1]. However, Europa’s complex terrains alter local illumination conditions and surface temperatures. Here, we use TEMPEST, an open-source, python based thermophysical model [2, 3] to investigate the significance of radiative self-heating and light scattering on interpretation of Europa’s thermal IR emission, with a particular focus on detectability of near-surface liquid-water reservoirs by high spatial resolution (≤100 m/pixel) observations soon to be taken by Europa Clipper’s E-THEMIS instrument [4].Passive heating by solar illumination was modelled under Europa-like conditions using TEMPEST, which solves a surface energy balance that includes solar flux, thermal emission, vertical heat conduction, and (optionally) scattering, radiative self-heating and macroscopic surface roughness. The 1D periodic conduction solver at its core is based on thermprojrs [5]. Comparing a smooth sphere (fig. 1a, c) to the topographic digital elevation model (DEM) (fig. 1b, d) reveals that while mean temperatures remain comparable, topography introduces substantial heterogeneity. For example, the DEM has a wider distribution of surface temperatures due to local slope and shadowing effects. Thermal radiance scales with Temperature4 (Stefan-Boltzmann law), so the small minority of hotter regions can dominate the thermal IR signal (fig. 2). We show that rough terrain emits significantly higher total radiance than a smooth sphere despite the similar mean kinetic temperature.Simulated topographic temperature maps show that light scattering and radiative exchange with local terrain cause increased surface temperature heterogeneity, with local temperature increases in deep fractures as high as 35 K (fig. 3). Temperature heterogeneity can systematically bias mean temperature measurements, and localised mutual radiative heating can lead to apparently anomalous warm regions, potentially mimicking endogenic heating, or obscuring indications of conductive heating from near-surface water reservoirs. To place these effects in the context of E-THEMIS detectability, we will use first-order conductive heat-flux calculations to estimate the surface expressions of idealised subsurface liquid-water reservoirs at different depths and spatial scales. These calculations will provide test cases for distinguishing plausible endogenic thermal anomalies from topographically induced radiance variations. This work shows that if topography is ignored, the excess radiance caused by it could be misinterpreted as a region of lower thermal inertia (in daytime), or even an endogenic heat source (hotspot). Accurately modelling terrain reduces the risk of false positives when searching for plume sources or active regions.Figure 1: Surface temperature maps for a section of an icy moon modelled as a smooth sphere (a) and using a digital elevation model (DEM) of Enceladus (b) using Europa thermal parameters. Histograms (c) and (d) show the distribution of surface temperatures within the above terrain samples.   Figure 2: The integrated surface radiance from each terrain sample. The T4 dependence of radiance leads to significantly higher emission from the DEM terrain.Figure 3: Simulated Europa surface temperatures without (a) and with (b) multiple scattering and radiative self-heating. Panel (c) shows the resulting temperature difference, with local increases up to 35 K in shadowed terrain. Enceladus DEM [6] provides an icy-terrain analogue for topographic heating effects.  Future work will repeat this analysis using Europa-specific topography where available. References:[1] Rathbun, J. A., Rodriguez, N. J., & Spencer, J. R. (2010). Galileo PPR observations of Europa: Hotspot detection limits and surface thermal properties. Icarus, 210(2), 763-769.[2] Lyster, D., Howett, C., & Penn, J. (2025). TEMPEST: A Modular Thermophysical Model for Airless Bodies with Support for Surface Roughness and Non-Periodic Heating. EPSC-DPS 2025, 1479.[3] Chivers, C.J., Hayne, P.O. and Schmidt, B.E. (2025). Prospects For Detecting Shallow Liquid Water Bodies At Europa Using E-Themis. LPSC 2025, 1226.[4] Christensen, P.R., Spencer, J.R., Mehall, G.L., Patel, M., Anwar, S., Brick, M., Bowles, H., Farkas, Z., Fisher, T., Gjellum, D. and Holmes, A. (2024). The Europa thermal emission imaging system (E-THEMIS) investigation for the Europa clipper mission. Space Science Reviews, 220(4), 38.[5] Spencer, J.R., Lebofsky, L.A., and Sykes, M.V. (1989). Systematic biases in radiometric diameter determinations. Icarus, 78(2), 337-354. [6] Park, R.S., Mastrodemos, N., Jacobson, R.A., Berne, A., Vaughan, A.T., Hemingway, D.J., Leonard, E.J., Castillo-Rogez, J.C., Cockell, C.S., Keane, J.T. and Konopliv, A.S. (2024). The global shape, gravity field, and libration of Enceladus. Journal of Geophysical Research: Planets, 129(1), e2023JE008054.

Abstract:

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.AcknowledgementsLL’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, 221–22

Abstract:

The main belt asteroid (52246) Donaldjohanson (DJ) is a likely member of the Erigone asteroid family. This implies that DJ is a fragment of a larger parent body that was destroyed in a collision about 155 million years ago. We report observations taken during a flyby of DJ by the Lucy spacecraft. We found that DJ is composed of two heavily cratered lobes, connected by a smoother neck, with overall dimensions 8.8 kilometers (km) by 4.4 km by 3.1 km. The crater density is consistent with the Erigone family's age, except for craters <0.4 km, which have been preferentially erased. DJ rotates slowly in a tumbling state, likely owing to spin-down by radiative forces. Surface spectra show iron-bearing phyllosilicates, indicating moderate aqueous evolution on the parent body.