Dr Carly Howett

Abstract:

The presence of cryovolcanic activity in the form of geyser-like plumes at Jupiter’s moon Europa is a much-debated topic. As an active plume could allow direct sampling by a passing spacecraft of a potentially habitable interior environment, the detection and analysis of ongoing plume activity would be of the highest scientific value. In the past decade, several studies have interpreted different remote and in situ observations as providing evidence for large gaseous plumes at different locations on Europa. However, definitive proof is elusive, and visible imaging data taken during spacecraft flybys do not reveal clear indications of ongoing activity. After arrival at Jupiter in 2030, the NASA Europa Clipper spacecraft will systematically search for and constrain plume activity at Europa utilizing a variety of investigations and methods during, before, and after close flybys. Given the lack of a confirmed plume detection to date, the Europa Clipper science team has adopted a global plume search strategy, not focusing on any specific geographical area or any specific type of observation. This global search strategy assigns enhanced value to data obtained early in the mission, which allows time for further observations and characterization of any observed plume at later times. Here we describe the current state of knowledge on plume activity, the Europa Clipper search strategy, and the role of various instruments on the Europa Clipper payload in this search.

Abstract:

Activity in small bodies, defined here as the episodic or continuous release of material, was long thought to be exclusively a behavior of comets, but it has since been discovered in some centaurs, main-belt asteroids, and near-Earth asteroids. To date, however, no activity has been discovered on Jovian trojan asteroids, the target of NASA’s Lucy Discovery Program mission. Although Lucy was originally conceived without studies of or searches for trojan activity, it was realized in 2016–2017 that the spacecraft and scientific payload aboard Lucy could provide unique and meaningful constraints or detections on activity in these trojans. Here we describe how the Lucy mission will search for such activity using (i) its terminal tracking navigation camera to search for wide-field coma scattered light, (ii) its Lucy Long Range Reconnaissance Imager narrow-angle camera to also search for scattered light from any coma or jets, and (iii) its Multispectral Visible Imaging Camera imager to search for CN emission (a common activity tracer species in comets). Sensitivity estimates for each of those measurements are discussed below.

Abstract:

Lucy is the first mission to Jupiter Trojan asteroids, primitive bodies preserving crucial evidence of Solar System formation and evolution [1]. En route to its primary science encounters with the L4 swarm Trojans (2027-2028) and L5 swarm (2033), the spacecraft executed a flyby of asteroids (152830) Dinkinesh on November 1, 2023 and (52246) Donaldjohanson (DJ) on April 20, 2025. These Main Belt asteroid flybys function as operational rehearsals for the mission's Trojan targets. This work examines the performance of L'Ralph, a core Lucy science instrument, during these encounters, including data collection, instrument behavior, and analysis of the acquired datasets.L'Ralph integrates two complementary imaging systems spanning visible to near-infrared wavelengths (0.35-4 μm) [2]. The instrument has two focal plane assemblies: the Multi-spectral Visible Imaging Camera (MVIC) operating at 350-950 nm and the Linear Etalon Imaging Spectral Array (LEISA) covering 0.97-3.95 μm. LEISA delivers hyperspectral mapping capabilities with variable spectral resolving power (50-160, Δλ

Abstract:

Introduction: Enceladus Thermal Mapper (ETM) is an Oxford-built high-heritage instrument that is being developed for outer solar system operations. ETM is based upon the design of Lunar Thermal Mapper (LTM, launched on Lunar Trailblazer, Fig. 1). It has a strong heritage story, including MIRMIS (on Comet Interceptor), Compact Modular Sounder (on TechDemoSat-1) and filters shared with Lunar Diviner (on Lunar Reconnaissance Orbiter). ETM is a miniaturized thermal infrared multispectral imager, with space for 15 spectral channels (bandpasses) that can be tailored to the mission requirements. It consists of a five-mirror telescope and optical system and an uncooled microbolometer detector array. Real-time calibration is achieved using a motorized mirror to point to an onboard blackbody target and empty space. ETM has an IFOV of 35 mm, so assuming a 100 30 km orbit it will have a spatial resolution of 40 to 70 m/pixel and a swath width of 14 - 27 km. ETM Updates: Through UK Space Agency funding we have developed three areas of ETM: its filter profile, radiation tolerance and sensitivity to Enceladus-like surfaces. Filters: ETM is a push broom thermal mapper, which works by the detector being swept over a surface. Each of the detector’s 15 channels is made up 16 rows, which are coadded to increase the signal to noise. A recently completed preliminary study has updated ETM’s bandpasses to include filters between 6.25 mm and 200 mm to enable it to detect Enceladus’ polar winter (170 K). Depending on the mission goals not all channels need to be utilised to achieve this, making some available for additional studies (e.g. searching for salt). Radiation: The radiation environments of Enceladus are vastly different to those of the Moon. Recent radiation testing and analysis showed that the majority of ETM’s existing design is already highly radiation tolerant. With some additional shielding and one component change all parts can reach the radiation hardness required to operate in the Saturn-system. The additional shielding may be provided by the spacecraft structure, depending on the adopted design. Sensitivity: ETM’s sensitivity to cryogenic surfaces is currently predicted through a well-characterised model. However, as part of the LTM calibration campaign we plan to directly measure its sensitivity to

Abstract:

Introduction: Mimas is a small moon of Saturn with a heavily cratered surface and high eccentricity, suggesting an inactive past. It was, thus, surprising when Cassini measurements of Mimas’ libration and tracking of its pericenter precession revealed that Mimas maintains an ocean under an ice shell 20-30 km thick [5,12]. Subsequent investigations into how an ocean-bearing Mimas could have avoided developing tidally-driven fractures [7], its tidal heating budget  [8], constraints on shell thickness from the formation of the Herschel impact basin [3], and thermal-orbital evolution models [5,9] all point to a young ocean that has emerged within the past 10-15 Myr. These results suggest that Mimas may possess the youngest ocean in the Solar System, making it an important target for understanding the early stages of ocean development – such as for Enceladus’ ocean – and the habitability of ocean worlds through time. Uranus’ moon, Miranda, may also have developed an ocean relatively late in its history (e.g., [2]); understanding the evolutionary and geophysical processes at Mimas may help prepare us for a future mission to the Uranian system.Here, we expand upon past work [8,9], in which we relied on globally-averaged tidal heating and constant surface temperature, to develop a map of plausible ice shell thicknesses and surface heat flows on Mimas assuming a present-day ocean. Our goals are to determine the variability in heat flow and ice shell thickness that result from spatial differences in surface temperature and strength of the tide and to quantify requirements that would enable ocean detection via heat flow measurements. Our results also provide estimates of tidal power, which affect the circularization timescale and ocean lifetime within thermal-orbital evolution models.Methods: We utilize the numerical toolkit MATH [13] to compute tidal heating within Mimas’ ice shell and identify the equilibrium ice shell thickness and surface heat flux at a suite of locations across Mimas. These calculations depend on the surface temperature and the basal heat flux. Here, we develop a surface temperature map based on models of solar insolation, and informed by Cassini measurements (e.g., [4]), to obtain robust temperatures. We then vary the basal heat flux across a range that encompasses minimal heating from only radiogenic decay to high heat fluxes associated with dissipation in Mimas’ rocky interior. We use the inferred ice shell thickness of 20-30 km [5,12] to determine the basal heating cases that provide consistent results. From these maps, we can deduce the precision needed to use heat flow measurements to differentiate between a fully frozen Mimas, which likely produces endogenic heat flows of ~1 to several mW/m2 (e.g., [9]), and an ocean-bearing Mimas.The ice shell thickness maps can also be used to compute the tidal dissipation associated with Mimas’ present-day orbit and interior structure. We will input these values into the numerical toolkit PISTES [11] to assess the extent to which Mimas’ ice shell evolution can occur over a longer timescale and/or begin at a higher eccentricity than in past models. These results are particularly important for understanding how Mimas came to possess an ocean. While we expect that Mimas’ ocean emerged due to a recent eccentricity-pumping event that increased its eccentricity to the point of melting, the cause and details are not well-understood. A gap in Saturn’s rings, known as the Cassini Division, appears to record Mimas’ phase of inward migration and increasing eccentricity [1,6]. However, models of this process require Mimas to reach a much higher eccentricity than the thermal-orbital evolution models predict; Mimas’ entire ice shell would have melted in that case, which is inconsistent with its geologic record (see discussion in [5]). In addition, the timescales for Mimas’ subsequent outward migration are in conflict. These discrepancies motivate further investigation into Mimas’ thermal-orbital evolution to determine whether the initial conditions and lifetime of the ocean can be extended.Anticipated results: In Figure 1, we show maps of surface heat flow and ice shell thickness for Europa, assuming different basal heating values [10], which we created using the same tools and approach we are now applying to Mimas. We will present similar maps of ice shell thickness and heat flow across Mimas at its present-day eccentricity that are consistent with the inferred average ice shell thickness. We will also present the precision required for future heat flow measurements to detect the ocean and constrain the thickness of the ice shell, which we will compare to our recent Europa results. Finally, we will present revised thermal-orbital evolution models that account for differences in tidal dissipation between the globally-averaged and spatially-variable models of Mimas and discuss the implications of our findings on the development and age of Mimas’ ocean.Figure 1: We show equilibrium ice shell thicknesses (left) and surface heat flows (right) for Europa assuming different values of the basal heat flux (rows) and applying surface temperatures from model fits to Galileo data (see [10]). Variations in tidal strength exert a strong control on the pattern of heat flow while surface temperature creates deviations in the shell thickness map from the purely tidal pattern. We are conducting a similar investigation of Mimas to better understand the current state of its ocean and ice shell, develop measurement requirements, and explore implications for the ocean’s evolution.References: [1] Baillié, K., et al. (2019) MNRAS 486, p. 2933-2946. [2] Beddingfield, C.B. et al. (2022) PSJ 3, 174. [3] Denton, C.A., and A.R. Rhoden (2022) GRL 49, e2022GL100516. [4] Howett, C. J. A., et al. (2020) Icarus 348. [5] Lainey, V., et al. (2024) Nature 626, p. 280 – 282. [6] Noyelles, B., et al. (2019) MNRAS 486, p. 2947–2963. [7] Rhoden, A.R., et al. (2017) JGR – Planets 122, p. 400-410. [8] Rhoden, A. R., & Walker, M. E. (2022) Icarus 376. [9] Rhoden, A. R., et al. (2024a) EPSL 635. [10] Rhoden et al. (2024b) AGU, Abs P23E-3117 [11] Rudolph, M. L., et al. (2022) GRL 49.[12] Tajeddine, R., et al. (2014) Science 346, p. 322–324.  [13] Walker, M. E., & Rhoden, A. R. (2022) PSJ 3, 149.