Dr Carly Howett

Abstract:

The Europa Thermal Emission Imaging System (E-THEMIS) on the Europa Clipper spacecraft will investigate the temperature and physical properties of Europa using thermal infrared images in three wavelength bands at 7-14 µm, 14-28 µm and 28-70 μm [Christensen et al., 2024]. The specific objectives of the investigation are to 1) understand the formation of surface features, including sites of recent or current activity, in order to understand regional and global processes and evolution and 2) to identify safe sites for future landed missions. The E-THEMIS radiometric calibration includes removing the thermal emission from the instrument housing, optical elements, and filters using observations of space and an internal calibration flag [Christensen et al., 2024]. On February 28, 2025, the Clipper spacecraft performed a close flyby of Mars for a trajectory gravity assist. Twenty four hours prior to closest approach the spacecraft pointed the E-THEMIS instrument at Mars and performed a sequence that scanned E-THEMIS across the planet at a slew rate of 100 micro-radians per second. This rate is the same as what be used to image Europa during each flyby [Pappalardo et al., 2024]. This activity accomplished two primary objectives: 1) collect images of a well-characterized target (Mars) to validate the E-THEMIS calibration methodology and software prior to the first observations of Europa; and 2) rehearse the data collection procedure that will be used to obtain global observations of Europa.Mars makes an excellent thermal calibration target because it has been extensively studied and characterized by numerous thermal infrared instruments. The E-THEMIS observations were simulated using modeled surface temperatures generated using global maps of thermal inertia albedo made from the MGS TES data [Christensen et al., 2001], together with the krc thermal model [Kieffer, 2013]. The wavelength-dependent atmospheric absorption and emission was modeled using data from the UAE Emirates Mars Mission EMIRS thermal infrared spectrometer [Amiri et al., 2022; Edwards et al., 2021]. EMIRS collects global scans of hyperspectral data from 6-100 µm at 5 and 10 cm-1 spectral sampling at ~200 km spatial resolution [Edwards et al., 2021]. These spectra were resampled from wavenumber to wavelength and weighted by the three E-THEMIS spectral bandpasses to produce 3-band simulated E-THEMIS global images. EMIRS data were not collected simultaneously with the E-THEMIS imaging, but global observations were acquired at the same season and within 5° of latitude, 10° of longitude, and 0.3 H local time of the E-THEMIS data. Fig. 1 shows an example of the nearest EMIRS observation to the E-THEMIS observing conditions of sub-spacecraft latitude=20.3° N, longitude=163.0° E, local time=11.48 H, and Ls=50.5°. A transfer function from the krc-generated surface temperatures and the bandpass-weighted EMIRS data was created by averaging the ratio of forty-five EMIRS observations to the krc-generated surface temperatures. Simulated E-THEMIS observations were produced using the average of these ratios and the krc surface temperatures. The results are given in Fig. 2. The E-THEMIS data could not be transmitted to Earth until Clipper was more than 2 AU from Sun due to spacecraft thermal constraints. As a result the data were received on Earth on May 7, 2025, and the results and an assessment of the E-THEMIS calibration will be discussed.Fig. 1. Measured Mars temperatures. Comparison of temperature globes for surface temperature (krc model) and E-THEMIS-bandpass-weighted EMIRS data for Bands 1, 2, and 3. The EMIRS observations were acquired on Feb. 17, 2025, at a sub-spacecraft viewing geometry of 16.0° N latitude, 174.1° E longitude, 11.60 H local time, and 45.5° Ls. Fig. 2. Simulated E-THEMIS temperature images. The data for each E-THEMIS band were created using the krc model surface temperatures transferred to E-THEMIS wavelength bands using a transfer function derived from EMIRS observations. ReferencesAmiri, H., D. Brain, O. Sharaf, P. Withnell, M. McGrath, M. Alloghani, M. Al Awadhi, S. Al Dhafri, O. Al Hamadi, and H. Al Matroushi (2022), The emirates Mars mission, Space Science Reviews, 218(1), 4.Christensen, P. R., et al. (2001), The Mars Global Surveyor Thermal Emission Spectrometer experiment: Investigation description and surface science results, J. Geophys. Res., 106, 23,823-823,871.Christensen, P. R., J. R. Spencer, G. L. Mehall, M. Patel, S. Anwar, M. Brick, H. Bowles, Z. Farkas, T. Fisher, and D. Gjellum (2024), The Europa Thermal Emission Imaging System (E-THEMIS) Investigation for the Europa Clipper Mission, Space Science Reviews, 220(4), 1-65.Edwards, C. S., P. R. Christensen, G. L. Mehall, S. Anwar, E. A. Tunaiji, K. Badri, H. Bowles, S. Chase, Z. Farkas, and T. Fisher (2021), The Emirates Mars Mission (EMM) Emirates Mars InfraRed Spectrometer (EMIRS) Instrument, Space science reviews, 217, 1-50.Kieffer, H. H. (2013), Thermal model for analysis of Mars infrared mapping, J. Geophys. Res, 116, 451-470.Pappalardo, R. T., B. J. Buratti, H. Korth, D. A. Senske, D. L. Blaney, D. D. Blankenship, J. L. Burch, P. R. Christensen, S. Kempf, and M. G. Kivelson (2024), Science Overview of the Europa Clipper Mission, Space Science Reviews, 220(4), 1-58.

Abstract:

Enceladus maintains its global, unconsolidated ocean around its rocky, porous core by tidal dissipation with Saturn and torque from its resonance with Dione [1].  The active South Polar Terrain (SPT) region is associated with intense concentrations of endogenic heat, but it is the significantly lower-power conductive heat flow that dominates global heat loss as it occurs over the entire surface.  If Enceladus’ global ocean is to be sustained over a significant fraction of its existence, heating rates would have to be balanced endogenic heat loss. Estimates of heating rates from models vary from 1.5-150 GW [2].  The large range results from uncertainty in both the structure of the bodies’ interiors and their evolution.  Ice shell thickness/shape models, which interpret gravity, libration and topographic data, produce global conductive heat loss estimates of around 18-35 GW [3,4,5].Endogenic heat loss from the SPT has been estimated using thermal observations from Cassini Composite Infrared Spectrometer (CIRS) to be between 5-19 GW [6,7,8], resulting in a conventional, combined heat loss estimate of around 50 GW [9].Detecting endogenic heat loss using thermal observations presents a significant challenge, principally relating to limited data coverage and uncertainty about the surface thermal properties but is possible under some circumstances [10].We use thermal observations CIRS to identify endogenic heat at the north pole of Enceladus in the form of conductive heat flow.   From this estimate we can infer global average heat loss.  We are then able to invoke the same mechanisms used to estimate the global average heat loss from ice shell thickness models [5, 9] to characterize the first north polar and global average ice shell thicknesses independently derived from thermal observations.Acknowledgments: This work was made possible through NASA’s support of Cassini Data Analysis Program Grant Number 80NSSC20K0477. References[1] Nimmo, F., Barr, A.C., Behounková, M. and McKinnon, W.B., 2018. The thermal and orbital evolution of Enceladus: observational constraints and models. Enceladus and the icy moons of Saturn, 475, pp.79-94.[2] Lainey, V., Casajus, L.G., Fuller, J., Zannoni, M., Tortora, P., Cooper, N., Murray, C., Modenini, D., Park, R.S., Robert, V. and Zhang, Q., 2020. Resonance locking in giant planets indicated by the rapid orbital expansion of Titan. Nature Astronomy, 4(11), pp.1053-1058.[3] Thomas, P.C., Tajeddine, R., Tiscareno, M.S., Burns, J.A., Joseph, J., Loredo, T.J., Helfenstein, P. and Porco, C., 2016. Enceladus’s measured physical libration requires a global subsurface ocean. Icarus, 264, pp.37-47.[4] Čadek, O., Tobie, G., Van Hoolst, T., Massé, M., Choblet, G., Lefèvre, A., Mitri, G., Baland, R.M., Běhounková, M., Bourgeois, O. and Trinh, A., 2016. Enceladus's internal ocean and ice shell constrained from Cassini gravity, shape, and libration data. Geophysical Research Letters, 43(11), pp.5653-5660.[5] Hemingway, D.J. and Mittal, T., 2019. Enceladus's ice shell structure as a window on internal heat production. Icarus, 332, pp.111-131.[6] Spencer, J.R., Pearl, J.C., Segura, M., Flasar, F.M., Mamoutkine, A., Romani, P., Buratti, B.J., Hendrix, A.R., Spilker, L.J. and Lopes, R.M.C., 2006. Cassini encounters Enceladus: Background and the discovery of a south polar hot spot. science, 311(5766), pp.1401-1405.[7] Howett, C.J.A., Spencer, J.R., Pearl, J. and Segura, M., 2011. High heat flow from Enceladus' south polar region measured using 10–600 cm− 1 Cassini/CIRS data. Journal of Geophysical Research: Planets, 116(E3).[8] Spencer, J.R., Nimmo, F., Ingersoll, A.P., Hurford, T.A., Kite, E.S., Rhoden, A.R., Schmidt, J. and Howett, C.J., 2018. Plume origins and plumbing: from ocean to surface. Enceladus and the icy moons of Saturn, 163.[9] Nimmo, F., Neveu, M. and Howett, C., 2023. Origin and evolution of Enceladus’s tidal dissipation. Space Science Reviews, 219(7), p.57[10] Miles, G., Howett., C., Spencer J., Vol. 16, EPSC2022-1190, 2022, https://doi.org/10.5194/epsc2022-119

Abstract:

NASA’s Lucy mission [1] successfully completed a flyby encounter with the main-belt asteroid (52246) Donaldjohanson on April 20, 2025, collecting data as part of a full-scale operational test for Lucy’s future Trojan encounters.  Donaldjohanson was known to be a C-type asteroid and based on our ground-based observations, to have a Fe-bearing phyllosilicate 0.7 µm absorption. Similar absorptions in spectra of CI, CM, and CR carbonaceous chondrites are indicative of aqueously altered mafic silicates [2-4]. Donaldjohanson is also a member of the 155 Mya Erigone family [5], which is dominated by objects that have also been inferred to be aqueously altered based on their visible 0.7 µm absorptions [6].The Multi-spectral Visible Imaging Camera (MVIC), part of Lucy’s L’Ralph instrument [7-8], was specifically designed to include a filter covering the 0.7 µm absorption to detect evidence of aqueous alteration on the mission’s primary Trojan targets. The Donaldjohanson encounter is thus an excellent opportunity to compare the performance and calibration of MVIC to ground-based data. Here, we will report on both these validation efforts and our exploration of the spatial variability of the 0.7 µm phyllosilicate absorption across the imaged surface of Donaldjohanson to understand potential variability with surface features and photometry, and in relation to other Erigone family objects.References: [1] Levison et al. (2021) PSJ. [2] Cloutis et al. (2011a) Icarus. [3] Cloutis et al. (2011b) Icarus. [4] Cloutis et al. (2012) Icarus. [5] Marchi et al., (2025) PSJ. [6] Morate, D., et al. (2016) A&A. [5] Reuter et al. (2023), SSR. [6] Simon, A.A., et al. 2025 PSJ.Acknowledgments: The Lucy mission is funded through the NASA Discovery Program (Contract No. NNM16AA08C).

Abstract:

Introduction: On the 20th of April 2025, NASA’s Lucy mission [1] flew by the C-type main-belt asteroid (52246) Donaldjohanson (hereafter DJ). The encounter’s goal was to test the spacecraft and instruments during an observation sequence commensurate with those to be used on Lucy’s main targets – Jupiter’s Trojan asteroids. Data returned from the panchromatic Lucy LOng Range Reconnaissance Imager (L’LORRI, 450-850 nm, [2]) during this testing sequence reveal the asteroid to be bi-lobed and elongated shape (Fig. 1).DJ is a member of the Erigone collisional family, named after the parent body asteroid (163) Erigone (see references in [3]). Ground-based color observations (Fig. 2) show it to decrease in color towards shorter wavelengths, possibly due to the presence of hydrated materials [4].In this work, we present an analysis of color images taken by Lucy’s Multispectral Visible Imaging Camera (MVIC). MVIC consists of six time delay integration (TDI) charge-coupled devices (CCDs). TDI works by synchronizing the transfer rate of the image between CCD rows and the relative motion of the instrument allowing a high signal to noise image to be built up even for fast scans. It covers wavelengths between 375 nm and 950 nm using five color filters and a panchromatic one (see Table 1).Color Analysis: We focus our analysis on images acquired with the four wide band filters: violet, green, orange and near-IR. Our results will provide resolved color variations and contextualise DJ’s color with respect to ground-based observations of DJ, Erigone (Fig. 2), other members of the Erigone family, and the broader asteroid and small body populations.Filter Wavelength Violet 375-480 Green 480-520 Orange 520-625 Phyllosilicate 625-750 Near-IR 750-950 Panchromatic 350-950 Table 1 – MVIC filters [5]Figure 1 – (52246) Donaldjohanson as seen by the panchromatic Lucy L’LORRI instrument, taken on April 20, 2025 at 17:51 UTC.  Figure 2 – Ground-based normalized (at 0.55 µm) visible spectrum of DJ (blue) acquired with the Gran Telescopio Canarias compared to the Bus-DeMeo’s Cg-type (black) and the mean spectrum of the C-type members within the Erigone family (grey). Taken from [6]. Acknowledgments: The Lucy mission is funded through the NASA Discovery program on contract No. NNM16AA08C.References: [1] Levison et al. (2021) PSJ 2, 171. [2] Weaver et al. (2023), SSR 219, 82. [3] Marchi et al., (2025) PSJ 6, 59. [4] Vilas (1995) Icarus 115, 217-218. [5] Reuter et al. (2023), SSR 219, 69. [6] Souza-Feliciano et al. (2020), Icarus 338, 113463.

Abstract:

On November 1, 2023, NASA’s Lucy spacecraft successfully imaged the Main-Belt asteroid (152830) Dinkinesh and its moon, Selam. Dinkinesh is an S- or Sq-type asteroid with multiple geologic features (i.e., craters, central ridge, and trough) [1].  The Dinkinesh system is complex, with satellite that itself is a contact binary [1]. Broadband visible (0.35-0.95 µm) and near-IR (0.97-3.95 µm) hyperspectral images collected by the L’Ralph instrument showed absorption features near 1-, 2-, and 3-µm [2, 3].  The vibrational absorption between 2.6 and 3.3 µm in asteroid spectra has generally been interpreted as OH and H2O (i.e., hydration). This ~3.0 µm band, has been a crucial tool of characterization to understand the degree of hydration on the surface of asteroids [4]. Detection of hydration or volatile-rich materials on S-type objects is surprising due to the expected high temperature at which these bodies formed in the main-belt and presence of anhydrous silicates. Ground-based facilities have provided crucial detections and insights about the 3.0 µm band on S-type asteroids [5,6], yet much remains unknown about its origin. Dinkinesh’s close approach by Lucy offers a fortuitous opportunity to better understand the hydration of these bodies and assess any spatial variation on the surface that might be related to geologic features. The Lucy L’Ralph Dinkinesh observations can help differentiate the source of hydration. For example, exogenous material (e.g., carbonaceous or cometary material) is expected to appear in discrete areas associated with specific surface features such as craters [7]. Alternatively, solar wind implantation on asteroids occurs when high H+ fluxes doses from the Sun interact with surface minerals, embedding hydrogen atoms and potentially leading to the formation of OH or H2O in the regolith [8]. We will report on the spectral analysis of Dinkinesh, with a focus on the shape model registration of hyperspectral images from the L’Ralph Multi-spectral Visible Imaging Camera (MVIC) and Linear Etalon Imaging Spectral Array (LEISA). We will present colors, spectral slopes, and band depth to look for possible spectral heterogeneities associated with geologic morphologies. Results: We registered the digital shape model of Dinkinesh to the L’Ralph instrument detectors. Figure 1 shows a preliminary example of the MVIC panchromatic filter frame during the close approach registered to the respective incidence angle backplane obtained using SpiceyPy [9]. Figure 2 shows an example of a LEISA-calibrated frame (e.g., I/F) registered to Dinkinesh’s shape model.  After registration, the 3 µm absorption feature is analyzed for each facet by computing the absorption strength (e.g., band depth) and looking for correlations with surface morphologies provided by stereophotogrammetry of L’LORRI images. Similarly, we obtained MVIC color maps and overlayed them on the shape model. Our preliminary analysis suggests a 3 µm detection across the entire imaged surface, showing variabilities in band depth. We will further explore such variability to find its possible relationship with surface morphologies, local color variations, and illumination geometry.Figure 1. MVIC panchromatic frame of Dinkinesh overlayed with the SpiceyPy incidence angle backplane.Figure 2. Left: Dinkinesh shape model with overlayed LEISA cross-track I/F frame 700 during close approach.  [1] Levison, H.F. et al. 2024. A contact binary satellite of the asteroid (152830)Dinkinesh. Nature 629, 1015–1020.[2] Simon, A. et al. 2025. Lucy L'Ralph In-flight Calibration and Results at (152830) Dinkinesh. Planet. Sci. J.  6, 7.[3] Kaplan, H., et al. 2024.  "Multi-spectral imaging observations of asteroid (152830) Dinkinesh by the Lucy Mission." Proceedings of the Lunar and Planetary Science Conference 2024,abstract #1474. Houston, TX: Lunar and Planetary Institute.[4] Rivkin, A. S. et al. 2018. Evidence for OH or H2O on thesurface of 433 Eros and 1036 Ganymed. Icarus 304, 74–82.[5] McGraw, L. E. et al. 2022. 3 μm Spectroscopic Survey of Near-Earth Asteroids. Planet. Sci. J. 3, 243.[6] McAdam, M. et al. 2024. Detection of Hydration on Nominally Anhydrous S-complex Main Belt Asteroids. Planet. Sci. J. 5, 254.[7] De Sanctis, M. C. et al. 2015. Mineralogy of Marcia, the youngest large crater of Vesta: Character and distribution of pyroxenes and hydrated material. Icarus 248, 392–406.[8] Hibbits, C. A., et al., 2011. Thermal stability of water and hydroxyl on the surface of the Moon from temperature-programmed desorption measurements of lunar analog materials. Icarus, 213, 64-72.[9] Annex, A. M., et al., 2020. SpiceyPy: a Pythonic Wrapper for the SPICE Toolkit. Journal of Open Source Software, 46, 2050.