MITM15
Solar System observations have accounted for 4-6% of all JWST time allocated during the first three cycles, with almost every major body being viewed at least once in JWST’s major instrument modes, as well as >100 small bodies across the Solar System. This has generated a host of new discoveries, from the atmospheres and ionospheres of giant planets; to the distribution of ices on ocean moons; the hydration properties of small bodies; the chemical composition of comets; and the taxonomy of Trans-Neptunian Objects to tell the story of Solar System evolution. These exceptional new insights will set the scene for the next generation of planetary missions beyond Mars, both those en route to their destinations (e.g., Lucy, Psyche, JUICE, Europa Clipper and others), and those preparing for the next steps in our exploration of the Solar System.
This interdisciplinary session welcomes papers spanning the entire planetary science community, reporting new discoveries using JWST in any discipline.
Session assets
H3+ is one of the key components of the auroral emissions from the Solar System’s giant planets. Produced from auroral precipitation ionising neutral hydrogen molecules, H3+ can have lifetimes ~100 s in the Jovian ionosphere, leading to highly variable emission in active regions. H3+ radiates strongly in the near infrared with a temperature and density dependent spectrum, providing a significant source of cooling for the upper atmosphere of Jupiter.
We present spatially and temporally resolved JWST/NIRCam and JWST/NIRSpec observations of Jupiter’s north polar auroral H3+ emissions. In December 2023, NIRCam observed Jupiter using the F335M filter with a ~3 s time resolution and ~190 km spatial resolution (Figure 1). This filter, centred at 3.35 µm measures the broadband H3+ emission, and can be used to measure the time variability and spatial structure of the H3+ aurora across the entire polar region (Nichols+2025). Subsequently, in January 2024, NIRSpec acquired 3 – 5 µm spectra with a ~30 s time resolution and ~300 km spatial resolution (Figure 2). This spectral range includes the bright H3+ emissions around 3.5 µm and 4 µm, which we can use to probe the H3+ density and temperature in the Jovian ionosphere. The high sensitivity, spectral, spatial and temporal resolutions offered by these two JWST instruments offers an unprecedented window into the evolution and time variability of H3+ at Jupiter.
We have developed a custom Monte Carlo wrapper of the h3ppy Python package to fit and model the observed H3+ NIRSpec spectra and study the evolution of temperature and number density during transient events. We show that during transient brightening events column density rapidly increases (over ~30s) while the temperature decreases, suggesting that brightening is caused by the production of a cooler layer of H3+.
Figure 1: JWST/NIRCam observations of Jupiter’s north polar aurora (Nichols+2025). The ~3 second time resolution allows us to the measure rapid variation and morphological changes of the H3+ intensity over the entire auroral region.
Figure 2: Spatial coverage of JWST/NIRSpec observations of Jupiter’s north polar aurora. Each observed location has a full 3-5 µm spectra (R=2700), acquired with a time resolution of ~30 seconds, providing a comprehensive spatial, spectral and temporal observations of Jupiter’s infrared auroral emissions.
Figure 3: Transient brightening of Jupiter’s H3+ aurora observed by JWST/NIRSpec. The column density (purple) increases rapidly during the brightening event, while the temperature (orange) decreases.
How to cite: King, O., Nichols, J., Fletcher, L., Clarke, J., de Pater, I., Melin, H., Moore, L., and Tao, C.: JWST observations of Jupiter's time variable H3+ auroral emissions, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-813, https://doi.org/10.5194/epsc-dps2025-813, 2025.
The 1994 impact of comet Shoemaker Levy 9 on Jupiter triggered the injection of new chemical species into the planet’s stratosphere – including H₂O, CO, HCN and CS – that had never been detected before [1,2,3]. Recent studies have also proposed that similar cometary impacts may have delivered similar species to other outer planets [4,5,6].
The temporal and spatial evolution of these SL9-derived species has since been monitored in Jupiter. These molecules were originally injected at 44S, confined to a narrow layer at 0.1 mbar. Over the following years, these molecules have diffused vertically, reaching pressure levels of 3 mbar by 2017, and have also dispersed horizontally across the planet. Notably, recent ALMA observations revealed that CO is now evenly distributed across latitudes [7]. However, other molecules, such as HCN and CO₂, show unexpected spatial patterns that challenge current transport and chemical models. First, HCN is well mixed at mid-latitudes but it exhibits polar depletion [7]. Second, CO₂, a daughter molecule of the SL9-derived CO and H₂O, presented an enhanced abundance in the South Polar Region in 2000 as revealed by Cassini/CIRS observations [8].
We analyzed James Webb Space Telescope (JWST) Mid InfraRed Instrument (MIRI) medium-resolution spectroscopy observations from latitudes of 17S to 26S, and from 45S towards the south pole to retrieve the molecular abundances of CO₂, H₂O, and HCN (see Figure 1) by coupling a radiative transfer code with an inversion algorithm. This model was used to retrieve, in the first place, the temperature from the CH4 ν4 band in first place and, in the second place, the abundance of the species for the different latitudes.
Our results show a complex latitudinal structure for each molecule. Regarding HCN, comparisons with ALMA (2017) and Cassini/CIRS (2000) allow us to assess its temporal evolution. Using a simple transport model, we could suggest a possible origin for its polar depletion, that could be linked to the presence of stratospheric aerosols. These aerosols could trigger heterogeneous chemical reaction that can lead to the adsorption of HCN onto these aerosols. For the oxygenated molecules, this is the first time H₂O’s meridional distribution has been mapped with such resolution , and the first time CO₂ has been observed in Jupiter since 2000 with Cassini/CIRS. We observed the column density to be affected at the regions where the auroral oval is present. Interestingly, both H₂O and CO₂ show variations linked to Jupiter’s southern auroral oval, with CO₂ exhibiting strong depletion and H₂O showing enhancement in the same region. This potential anti-correlation is not well understood and may indicate the involvement of an unknown mechanism — possibly initiated by particle precipitation or complex meridional transport — that affects the oxygen exchange between these molecules.
While these findings mark a significant step forward in understanding post-impact chemistry and atmospheric dynamics, several questions remain unclear, particularly the chemical relationship between the oxygenated molecules. Upcoming JWST Cycle 4 observations (PI: Rodriguez-Ovalle) will provide a full meridional coverage of these molecules (see figure 2), and are expected to shed some light on the chemistry and transport behind H₂O and CO₂.
Figure 1. Top panel: Example of Jupiter spectral region (zonally averaged) encompassing CH4 lines and the H2O ν2 lines at 75S (red), 65S (blue) and 25S (black). The vertical lines display the spectral features of CH4 (black) and NH3 (orange). Bottom left panel: Same but for HCN ν2 band (zonally averaged) next to C2H2 lines. Bottom right panel: Same but for CO2 ν2 band. The grey regions mark the positions of the emission lines of each molecule. The spectra at 65S is shifted to match the continuum emission of the 75S spectrum. The offset values in nW.cm−2.sr−1 / cm−1 are indicated in the legend of each panel.
Figure 2: MIRI/MRS footprints on Jupiter for a single dither for channels 1 (blue) and 3 (red). The order of the observation is labeled from 1 to 10. The northern auroral oval is marked in fuchsia.
[1] Lellouch et al., 1995. Chemical and thermal response of Jupiter’s atmosphere following the impact of comet Shoemaker–Levy 9. Nature, 373:592–595, February 1995. ISSN 1476- 4687. doi: 10.1038/373592a0.
[2] Marten et al., 1995. The collision of comet Shoemaker-Levy 9 with Jupiter: Detection and evolution of HCN in the stratosphere of the planet. Geo- phys. Res. Lett., 22(12):1589–1592, June 1995. ISSN 0094-8276. doi: 10.1029/95GL00949.
[3] Bjoraker et al., 1996. Detection of Water after the Collision of Fragments G and K of Comet Shoemaker–Levy 9 with Jupiter. Icarus, 121(2):411–421, June 1996. ISSN 0019-1035. doi: 10.1006/icar.1996.0096.
[4] Cavalié et al., 2010. A cometary origin for CO in the stratosphere of Saturn? Astron. Astrophys., 510:A88, February 2010. ISSN 0004-6361. doi: 10.1051/0004-6361/200912909.
[5] Lellouch et al. 2005. A dual origin for Neptune's carbon monoxide? Astron. Astrophys. 430, 2, February 2005. ISSN: 0004-6361. doi: 10.1051/0004-6361:200400127
[6] Moreno et al., 2017. Detection of CS in Neptune’s atmosphere from ALMA observations. Astron. Astrophys. 608, L5. ISSN: 0004-6361. doi: 10.1051/0004-6361/201731472
[7] Cavalié et al., 2023. Evidence for auroral influence on Jupiter’s nitrogen and oxygen chemistry revealed by ALMA. Nat. Astron., 7:1048–1055, September 2023. ISSN 2397-3366. doi: 10.1038/s41550-023-02016-7.
[8] Lellouch et al., 2006. On the HCN and CO2 abundance and distribution in Jupiter’s stratosphere. Icarus, 184 (2):478–497, October 2006. ISSN 0019-1035. doi: 10.1016/j.icarus.2006.05.018.
How to cite: Rodriguez-Ovalle, P., Fouchet, T., Cavalié, T., Dobrijevic, M., Fletcher, L. N., Lefour, C., Lellouch, E., Sinclair, J., Benmahi, B., Toogood, S., Wong, M. H., and de Pater, I.: The evolution of comet-impact products in Jupiter’s atmosphere using JWST-MIRI observations , EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-447, https://doi.org/10.5194/epsc-dps2025-447, 2025.
Elongated cloud structures in Jupiter’s polar and subpolar regions have been observed in JunoCam images [1]. These clouds present a variety of sizes and colours, and are mainly observed near the terminator, suggesting that these clouds are located at high altitudes. These elongated and elevated clouds could be divided in three main groups depending on their albedo: (i) high albedo clouds; (ii) low albedo clouds; and (iii) high albedo clouds featuring large shadows. So far, their nature remains largely unexplained. Similarly, JWST/NIRCam images of Jupiter at wavelengths between 1.65 µm and 3.60 µm captured on July 22, 2022 as part of the Early Release Science program 1373 [2], also showed dark and bright filamentary structures on Jupiter’s polar and subpolar regions. Among the diverse observed elongated structures, we focus this study on three different phenomena (all indicated in Figure 1).
The first feature is a low-albedo, elongated feature located at the sub-polar region near 50°-60° latitude south in 3.35-µm and 3.6-µm images (sensing stratospheric hazes and tropospheric clouds). This feature displays a fast meridional movement with peak velocities of 30 m/s and does not follow the usual zonal movement of Jupiter’s troposphere and stratosphere. However, the most poleward extend of this feature (near -60° latitude) follows a zonal flow. This, together with its shape, suggest that this perturbation was originated near -60° latitude and rapidly propagated equatorward. Additionally, the observed fast meridional movement hints at this feature being located at high altitudes, above the zonal flow regime. Analysis of the aurora activity and Io’s footprint compared with the polar most latitude of the dark feature and its width open the possibility that this feature might be directly related to Io’s footprint where the flux of energetic particles in Jupiter’s upper stratosphere might locally modify the chemistry of the atmosphere (see Figure 2).
The second phenomena analysed in this study are low-albedo features present at the boundary of the polar hazes in the north polar region at 1.64 µm, 2.12 µm, 3.35 µm and 3.6 µm. Unlike in the south polar region, the northern high latitudes display elongated and straight dark filaments that move with the background, creating a boundary-like structure. Similar features have previously been observed in JunoCam images at the same latitudes, appearing as slightly bright, narrow, and elongated clouds. The origin of these features, unlike the dark filament in the southern hemisphere, does not seem to be related aurora or activity from the satellite’s footprints.
The third feature is a bright arc-like structure observed in the northern polar region at latitudes higher than 70° in 3.35-µm and 3.6-µm images. Due to the lack of image pairs at 3.6 µm that could be used to track motions, we are not able to characterise its dynamics. So far, the presence of this feature remains a mystery.
In this presentation, we will share an analysis of these rare structures in Jupiter’s polar and subpolar regions, characterising their sizes, distribution and dynamics, and we will also propose different candidates that could explain the nature of these features.
Figure 1. Images of Jupiter taken by NIRCAM onboard the JWST at 3.35 µm (left) and 3.60 µm (right), showing the diverse elongated featured analysed in this study.
Figure 2. Polar projections at 3.35 µm showing the potential relation of the dark filament in the south polar region and the bright arc in the north polar region with Io’s footprint (indicated in dark blue). The red solid lines represent the average position of the aurora, the light blue solid line represent Europa’s footprint and the green solid line represents Ganimede’s footprint.
References: [1] Hueso, R., Sánchez-Lavega, A., Fouchet, T. et al. An intense narrow equatorial jet in Jupiter’s lower stratosphere observed by JWST. Nat Astron 7, 1454–1462 (2023). [2] Orton, G.S., Rogers, J. et al. Jupiter’s High-Altitude Hazes as Observed by JunoCam. Geophysical Research Abstracts, Vol. 21, EGU2019-3188 (2019).
How to cite: Antunano, A., Rodriguez-Ovalle, P., Hueso, R., Fouchet, T., Sánchez-Lavega, A., de Pater, I., Orton, G. S., and Fletcher, L. N.: Filamentary structures in Jupiter’s polar regions, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1275, https://doi.org/10.5194/epsc-dps2025-1275, 2025.
Europa is a prime candidate for extraterrestrial life due to its subsurface ocean (Carr et al., 1998). Its surface offers a partial glimpse into oceanic processes while also acting as the interface between the moon’s internal geology and the extreme conditions of Jupiter’s magnetosphere. Shaped by both internal activity and external irradiation (Chyba and Phillips, 2002; Nordheim et al., 2022), Europa’s surface exhibits spatial and spectral variability that is key to interpreting its composition and assessing its astrobiological potential.
We present a novel data-driven analysis of JWST NIRSpec-IFU (Böker et al., 2022) observations of Europa’s leading hemisphere, using spectral decomposition to isolate dominant modes of variability.
The NIRSpec-IFU data form a three-dimensional cube with two spatial dimensions (nrow, ncol) and one spectral dimension. Each spatial pixel ("spaxel") records a high-resolution reflectance spectrum from ~1.0–5.2 μm. We flatten the cube into a two-dimensional matrix S∈Rm×n, where m = nrow×ncol, and n is the number of wavelength bins.
To analyze the spatial-spectral structure, we apply singular value decomposition (SVD) independently to seven spectral bands centered on diagnostic features: the 1.65 μm crystalline ice absorption feature, the 3.1 μm Fresnel peak of crystalline water ice (Hansen and McCord, 2004), the 3.5 μm hydrogen peroxide absorption (H2O2) (Trumbo et al., 2019; Wu et al., 2024), two carbon dioxide (CO₂) isotopologue absorptions near 4.25 μm and 4.38 μm (Trumbo and Brown, 2023; Villanueva et al., 2023), and two newly identified broadband continuum features. The SVD takes the form:
Sλ∈band=UΣV⊤
The columns of V represent the principal spectra v(i), where v(0) captures the average spectral shape across the hemisphere, and higher orders describe progressively weaker, spatially variable deviations from it. The columns of U are the spatial modes, u(i), which specify how much each v(i) contributes at each spaxel. Each local spectrum is thus approximated as a weighted sum of a global mean and a few spatially structured variations in shape.
We focus on the first two modes for each band, encoding the mean shape and the strongest perturbation. The spatial modes exhibiting variability, u(1), are fitted with spherical harmonics (
), yielding a sparse, smooth representation of hemispheric structure and enabling consistent comparison across bands. Figure 1 illustrates the first two principal spectra, v(0) and v(1), and the first spatial mode, u(1), for the seven analyzed bands. Figure 2 illustrates a projection of the fitted models for u(1) of the seven bands onto Europa’s leading hemisphere.
Figure 1. Spectral and spatial modes of seven spectral bands on Europa. Panels (a)–(e) correspond to the selected molecular bands: (a) 1.65 μm crystalline ice absorption band, (b) ~3.1 μm water ice Fresnel peak, (c) ~3.5 μm H₂O₂ absorption, (d) ~4.25 μm CO₂ doublet, and (e) ~4.38 μm ¹³CO₂ absorption. Two newly characterized broadband absorption features are labeled ζ and η. The zeroth- (mean) and first-order spectral modes, v(0) and v(1), are shown in gray and black, respectively, in the left panels. The sum and difference between these modes are shown in the middle panels (blue for sum, red for difference). The two right panels display the zeroth- and first-order spatial modes, u(0) and u(1), respectively.
Figure 2. Qualitative projections of best-fit spherical harmonic models for the first-order spatial modes, u(1), associated with five molecular bands (labeled a–e) and two broadband continuum features (labeled ζ and η), overplotted on a binary mask of Tara and Powys Regiones (black).
The first-order principal spectra, v(1), extracted from each band isolate the dominant sources of reflectance variability across Europa’s leading hemisphere. Each mode captures a clear, physically interpretable spectral deviation from the mean profile. In the 1.65 μm water ice band, the principal spectrum reflects variations in band depth and symmetry, consistent with disordered ice with lesser bulk crystallinity. The 3.1 μm Fresnel peak's principal spectrum reflects a transition between amorphous and crystalline phases at the surface (Hansen and McCord, 2004). In the hydrogen peroxide band, v(1) reveals broadening of the 3.5 μm feature, consistent with mixed ice phases (Giguerre and Harvey, 1959). Decomposition of the CO₂ bands shows that variability in the 4.25 μm doublet reflects shifts between disordered and crystalline CO₂ profiles, while variations in the 4.38 μm ¹³CO₂ feature primarily track abundance changes (Trumbo and Brown, 2023; Villanueva et al., 2023). The two broadband continuum features also exhibit simultaneous broadening, likely related to disordered and impure ice texture (Hansen and McCord, 2004).
Our results reveal a change in ice texture concentrated in southern Tara Regio, traced by the 1.65 μm crystalline ice band, the 3.1 μm Fresnel peak, and two broadened continuum features. These co-located signals point to porous, impure ice. The broadening of the 3.5 μm H₂O₂ band and the localized CO₂ enrichment further support this texture anomaly. Our modeling shows that persistent surface crystallinity in this region likely arises from porosity rather than a thermal anomaly (Thelen et al., 2024). Since porous ice enhances volatile retention (Baragiola et al., 2003), these findings suggest that apparent surface "freshness" may thus reflect structural preservation rather than recent exposure, essential for interpreting Europa’s spectral and habitability signatures.
References
Baragiola, R. A. (2003). Planet. Space Sci., 51, 953–961.
Böker, T. et al. (2022). Astron. Astrophys., 661, A82.
Carr, M. H. et al. (1998). Nature, 391, 363–365.
Chyba, C. F. et al. (2002). Orig. Life Evol. Biosph., 32, 47–67.
Hansen, G. B. et al. (2004). J. Geophys. Res. Planets, 109, E1.
Giguerre, P. and Harvey, K. (1959). Journal of Molecular Spectroscopy, 3, 36-45.
Nordheim, T. A. et al. (2022). Planet. Sci. J., 3, 5.
Trumbo, S. K. et al. (2023). Science, 381, 1308–1311.
Trumbo, S. K. et al. (2019). Astron. J., 158, 127.
Villanueva, G. L. et al. (2023). Science, 381, 1305–1308.
Wu, P. et al. (2024). Planet. Sci. J., 5, 220.
Thelen, A. et al. (2024). Planet. Sci. J. 3, 56.
How to cite: Yoffe, G. and Shahaf, S.: Spectral decomposition reveals surface processes on Europa, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1254, https://doi.org/10.5194/epsc-dps2025-1254, 2025.
Trans-Neptunian Objects (TNOs), which are the small bodies beyond the orbit of Neptune, are regarded to be the most primitive members of the Solar System, and as such, provides valuable insights into both history and the current state of the outer Solar System.
Their size distribution (SD), which can be inferred from their observed magnitude distributions, has remained relatively unaltered since the formation of the Solar System. This physical property is crucial for testing theoretical models of planet formation because it reflects the outcomes of accretion, collisional processes, and dynamical evolution over the history of the Solar System [1]. Therefore, comparing the observed size distribution with those predicted by models helps to constrain the proposed physical processes and underlying initial conditions that shaped the current Solar System. However, the relative faintness and distance of TNOs limits ground-based searches to only about m(r)~27 magnitude [2], while the lack of observations on the SD of TNOs smaller than m(r)~28 (D~20km) leaves theoretical models poorly constrained [3,4].
Using images obtained from our JWST Cycle 1 program #1568 we searched for ultra-faint TNOs to further constrain planet formation models. With this program’s NIRCam images, and simultaneous HST imaging, we detect and characterize TNOs as faint as m(r)~29.8 mag and as small as ~7 km (assuming 15% albedo) in diameter to explore never-before probed regions of the TNO size distribution. This is by far the deepest Solar System survey to date, with at least a visible magnitude deeper than the landmark survey by Bernstein et al. (2004) that used the Hubble Space Telescope (HST).
Program #1568 is a 3-epoch pencil beam sky survey conducted using NIRCam filters with effective wavelengths of ~1.5µm (F150W2) and 3.2µm (F322W2), centered on a region of the sky near 13h RA, -10° Dec. The observations are near the ecliptic plane, where the sky density of cold classical TNOs is maximal. The observations were taken from Jan 24 - Feb 4, 2023, at solar elongation of ~100 degrees, where the TNOs are near their turnaround points and are least likely to move off of the NIRCAM field of view. Figure 1 shows the observation layout of this program. A deep combined background image is subtracted from individual exposures, which are then digitally tracked and stacked at different rates of motion to search for TNOs.
The probability with which a TNO will be detected as a good track during a single epoch, whether it falls on a detector during both dithers, is quantified using the implanted artificial moving objects. It is well fit by the functional form 
, with the bright-end efficiency p0=0.96, the magnitude of half that efficiency at m0=28.92 (F150W2), and transition width w = 0.61 mag (see Figure 2).
We present our preliminary set of candidate sources detected with a total SNR of 15. By constraining their orbital parameters, we measure the faint end of the luminosity function for both the dynamically cold and hot components, and present their implications to the TNO SD down to diameters of 7 km. In future work, we will conduct a detailed analysis to determine which functional form of the size distribution best characterizes the observed population. This will offer deeper insights into the physical mechanisms governing the formation and evolution of the cold and hot populations, as well as the Kuiper Belt as a whole.
Figures:
Figure 1. Observation footprints of the survey, consisting of 10×2 mosaic tiles. Each tile was observed with eight short-wavelength detectors (small squares with a 64″×64″ FOV) and two long-wavelength detectors, each equivalent in size to four short-wavelength detectors, covering 129″ × 129″. Two exposures were taken at each of the 20 mosaic tiles. Each of the two exposures consisted of three 215s integrations, and were acquired at dither positions <1" apart. The total exposure time per tile was thus 1290 sec.
Figure 2. Recovery efficiency for implanted sources in the JWST survey as a function of r-band magnitude, assuming a nominal color r-F150W2=~ 1.2 mag. The dashed curve represents the efficiency from a single-epoch observation, while the solid black curve shows the cube of the upper completeness function, which is the expected probability of detection in all three epochs, and agrees with the implant results. The vertical lines represent the (range of) 50% completeness levels in several previous and upcoming TNO surveys: DES [6], OSSOS [7], LSST [8], DEEP [4] and [5]. This plot was inspired from Figure 10 of [4].
References:
[1] Fraser, W. C., Brown, M. E., Morbidelli, A., Parker, A., & Batygin, K. 2014, Astrophys J, 782, 100 [2] Fraser, W. C., & Kavelaars, J. J. 2009, Astron J, 137, 72
[3] Kavelaars, J. J., Petit, J.-M., Gladman, B., et al. 2021, Astrophys J Lett, 920, L28
[4] Napier, K. J., Lin, H. W., Gerdes, D. W., et al. 2024, Planet Sci J, 5 (IOP Publishing), 50
[5] Bernstein, G. M., Trilling, D. E., Allen, R. L., Brown, M. E., & Holman, M. 2004, 128
[6] Bernardinelli, P. H., Bernstein, G. M., Sako, M., Yanny, B., Aguena, M., et al. 2022, ApJS, 258, 41.
[7] Bannister, M. T., Gladman, B. J., Kavelaars, J. J., et al. 2018, ApJS, 236, 18
[8] Ivezić, Ž., Kahn, S. M., Tyson, J. A., et al. 2019, ApJ, 873, 111
How to cite: Eduardo, M., Morgan, A., Fraser, W., Trilling, D., Bernstein, G., Holman, M., Stansberry, J., HIlbert, B., Grundy, W., and Napier, K.: The Luminosity Function Of Ultra-Faint Trans-Neptunian Objects Detected By James Webb Space Telescope, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1132, https://doi.org/10.5194/epsc-dps2025-1132, 2025.
The population of main belt asteroids includes both primitive Solar System bodies as well as the products of early planetesimal melting and differentiation. The surface compositions of these objects shed light on their origins and evolutionary histories. The mid-infrared (mid-IR; ~5-30 microns) spectral range covers emissivity features diagnostic of silicate composition, including the olivine/pyroxene ratio and the Mg/Fe content of the minerals. In addition, mid-IR spectroscopy enables constraints on regolith properties such as particle size and regolith porosity.
We present James Webb Space Telescope (JWST) spectra of 14 main belt asteroids observed over 5-25 microns with the MIRI/MRS instrument, complementing existing visible and near-infrared (near-IR; ~0.7-5 microns) spectra. Our targets include five members of the spinel-rich “Barbarian” L-type class (Figure 1) and nine members of the nominally metal-rich M-type asteroids. All objects show distinctive emissivity features in their spectra; these include the commonly studied ~8-12 micron emissivity “plateau”, but also include features across the 12-24 micron region that can be key to compositional determination, as well as features at 5-8 microns.
We investigate the composition of these objects using laboratory databases of minerals and meteorites across a range of particle sizes and regolith porosities. Mid-IR spectra reveal the presence of silicate minerals that are sometimes undetectable from near-IR spectra alone. For our targets, each wavelength regime is most sensitive to specific surface constituents, and combined data across the visible through thermal wavelengths is needed to provide a more comprehensive picture of the full surface composition. We will present the surface compositions and closest meteorite analogs to the objects that were observed with JWST, and discuss what our findings reveal about the asteroid origins.
Only laboratory spectra of very small particles (<30 microns) with high regolith porosities (>80%, and sometimes even >95%), as simulated with mixtures of IR-transparent potassium bromide (KBr), are able to match the general shape of the asteroid spectra. The spectral comparison therefore indicates that the surfaces of the asteroids in our study, which are tens of kilometers in size, are overlain with a very fine, porous regolith layer. This characteristic emphasizes the need for laboratory spectral databases of meteorites and minerals that include KBr to simulate regolith porosity, and/or the development of modeling approaches that can simulate these very high regolith porosities, which standard Mie theory + Hapke modeling approaches do not reproduce well.
Figure 1: JWST emissivity spectra of five L-type asteroids (colored points), with best-fit laboratory mixtures overlain (black curves). In all cases, the best-fit lab mixtures indicate surfaces composed predominantly of small-particle (<30 micron), porous (~85-97% porosity), crystalline Mg-rich (Fo80-100) olivine. In four out of five cases, the laboratory mixtures shown here are dominated by CV3 chondrite material, as measured in the lab by Izawa et al. (2021) and Dausend et al. (2025).
References
Dausend, M.D., Martin, A.C., Emery, J.P. 2025. PSJ, 6, 54.
Izawa, M.R.M., King, P.L. Vernazza, P., Berger, J.A., McCutcheon, W.A. 2021. Icarus 359, 114328.
How to cite: de Kleer, K., Ehlmann, B. L., Wong, I., Tissot, F. L. H., Martin, A., Lane, M. D., Jacobson, S. A., and King, O.: Surface texture and mineralogy of main belt asteroids from JWST mid-infrared spectroscopy, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1159, https://doi.org/10.5194/epsc-dps2025-1159, 2025.
JWST’s infrared exploration of the Saturn system has been designed to extend the legacy of the Cassini-Huygens mission, using the exquisite sensitivity, spectral coverage, and spectral resolution of the integral field units (IFUs) to reveal new insights into the atmosphere, ionosphere, rings and moons. Guaranteed-time programme GTO1247 used MIRI/MRS (4.9-28.5 µm) to explore Saturn’s seasonal circulation patterns during northern summer [1], and a combination of NIRSpec/IFU (1.8-5.3 µm) and MIRI/MRS to diagnose the icy composition of Saturn’s rings and small moons [2]. GTO1251 explored Titan’s atmosphere[3], GTO1250 revealed the extent of Enceladus’ watery plumes [4], while guest-observer programme GO3716 explored Saturn’s irregular satellites [5].
NIRCam Imaging (June 2023): NIRCam observations of Saturn at 3.23 µm (GTO1247) reveal structures never-before-seen in Saturn’s upper atmosphere (Fig. 1). Observations in this strong methane absorption band, overlain by methane fluorescence, show diffuse patterns across the disc, with dark polar domains in the north and south in Fig. 2, a faint auroral oval in the north, and unexpected structure (patches of brightness and darkness) extending from the poles towards mid-latitudes suggesting high-altitude wave activity. Conversely, images at 2.12 µm reveal bright reflection from polar hazes, with the northern hexagon (78oN) and ribbon wave (48oN) clearly visible, and small cyclones and anticyclones observed in both hemispheres. Brightness gradients at 2.12 µm reveal the edges of Saturn’s bands (Fig. 2), coinciding with zonal jets measured by Cassini. Intriguingly, zonally-averaged gradients at 3.23 µm also show peaks at the jet locations, suggesting that the winds provide zonal organisation to the 3.23 µm structure and that both stratospheric aerosols (sculpted by zonal winds) and CH4 fluorescence (likely independent of the winds) are contributing to the unusual appearance at 3.23 µm.
Figure 1 NIRCam images of Saturn at 3.23 µm (left, with rings saturated to show atmospheric detail) and 2.12 µm (right), acquired in GTO1247 in June 2023.
Figure 2 Polar projections of NIRCam data at 3.23 µm (top row) and 2.12 µm (bottom row), for northern and southern hemispheres. The auroral oval is evident at 3.23 µm (top left); and small vortices, the hexagon and ribbon can be seen at 2.12 µm (bottom left). Zonally-averaged brightnesses (and their poleward gradients) are shown on the right, compared to the locations of Saturn’s prograde jet peaks (vertical dashed lines).
NIRSpec Spectroscopy (November 2024): Missing from the JWST Saturn observations of Cycles 1 and 2 was NIRSpec/IFU spectroscopy, without which it would be challenging to distinguish ionospheric emissions from H3+ or CH4 fluorescence from reflection from stratospheric hazes. In Cycle 3, GO 5308 [6] acquired a full NIRSpec/IFU map of Saturn’s northern hemisphere poleward of 45oN, spanning 2.8-5.3 µm using the F290LP/G395H filter/grating. Saturn was observed continuously between 04:00-13:58 UT on 29 November 2024 to observe all longitudes, resulting in 26 spectral cubes each constructed from four dithers. Cubes were processed using a custom pipeline to reduce saturation and assign geometry [7], producing polar projections (Fig. 3) and zonally-averaged spectra (Fig. 4).
Results: Combining the 2023 NIRCam observations with the 2024 NIRSpec observations, we observe the following:
- Extratropical and Polar Belt/Zone structure: Zonal jets organise both reflectivity and thermal emission, even in the CH4 fluorescence band near 3.32 µm (sounding high altitude). Latitudinal banding at 5-µm is on a scale finer than the zonal jets, potentially due to different latitudinal distributions of at least two aerosol layers: an upper tropospheric haze p < 0.3 bars, and a deeper cloud layer at 1–2 bars. Latitudinal gradients in aerosol opacity, phosphine, ammonia, and water, will be compared to the jet locations to search for evidence of vertical mixing on the scale of Saturn’s belts and zones.
- Polar Domain and Hexagon: As previously reported by MIRI/MRS in 2022 [1], NIRSpec shows that the north pole remains dark at 5 µm and surrounded by a cloud-free band of bright emission, consistent with its appearance in Cassini/VIMS observations in 2016 [8]. This latitudinal contrast prevents us from seeing Saturn’s hexagon (78oN) in thermal emission, but reflected sunlight near 2-3 µm clearly reveals the six vertices of the hexagon with a westward translation between 2023 and 2024. These data are possibly our last view of the hexagon before it becomes hidden in autumn darkness after Saturn’s equinox (May 2025), and confirm that the hexagon has now existed for at least ~45 years (since discovery by Voyager).
- Discrete Atmospheric Features: NIRCam (Fig. 2) reveals the presence of a long-lived cyclone-anticyclone pair (one dark, one bright) near 60-65oN that were also evident in Hubble observations in 2022-23. The NIRSpec data at 3 µm (Fig. 3) reveal the same vortex pair near 60oN, 135oW in 2024. In the southern hemisphere, NIRCam reveals reflective ovals near 44oS and 55oS that also appear to have persisted for several years (observed by Hubble as the south emerged from winter darkness).
- Large-Scale Upper Atmospheric Structure: Both NIRCam and NIRSpec reveal significant longitudinal structure in the 3.2-3.3 µm range sensitive to high-altitude aerosols and CH4 fluorescence (Fig. 3-4). NIRSpec reveals at least four dark patches extending equatorward of the dark north polar domain, creating a wave-like appearance at mid-latitudes that has changed between 2023 and 2024. NIRCam reveals hints of similar structure around the South Pole. We will present spectral retrieval analyses to determine the likely origin (CH4 fluorescence, stratospheric hazes) for these unexpected structures.