Solar system

Abstract:

Jupiter’s Oval BA undergoes recurrent colour changes whose physical origin remains uncertain. Radiative transfer retrievals indicate that these changes occur in the upper chromophore haze of the vortex annulus, around and above the 0.2–bar level, and are primarily associated with a decrease in optical depth, with no significant change in particle size or haze altitude. We apply a one-dimensional microphysical model to this haze layer, constrained by the retrieved aerosol properties of the red annulus in 2016 and the whiter annulus in 2020, and use it to reproduce the observed colour-change timescale of approximately 0.5 years. Our results indicate that this transition is best reproduced by changes in tropospheric vertical transport within a subsiding annulus, corresponding to preferred downwelling velocities of order 10−4–10−3 m s−1 at chromophore-bearing pressures. These small vertical velocities may help explain why no clear dynamical signature has yet been identified.

Abstract:

Cassini observed Jupiter during its 2000–2001 gravity-assist flyby, acquiring a valuable set of Visible and Infrared Mapping Spectrometer (VIMS) observations over a wide range of viewing geometries and observing conditions. These data comprise visible and infrared spectral cubes covering the VIS channel from 0.35 to 1.05 µm and the IR channel from 0.9 to 5.1 µm. We present a calibrated catalog of these Jupiter VIMS observations,  excluding satellite-targeted observations, designed to provide a homogeneous and validated set of spectral products for future Jovian studies.Starting from the raw archive cubes, we developed a processing workflow that combines ISIS/SPICE-based geometry recovery with dedicated radiometric calibration procedures for both VIMS channels. The final products are delivered as multi-extension FITS files containing calibrated I/F spectral cubes, wavelength and FWHM vectors, and geometry backplanes for incidence, emission, and phase angles, planetocentric latitude, positive-east longitude, pixel resolution, and azimuth angle. The workflow also addresses several issues affecting the original data set, including saturation, VIS pointing-related offsets between radiometric cubes and geometric backplanes, channel-dependent dark-signal artifacts, and a subset of IR approach-phase for which the standard ISIS calibration pipeline can produce over-scaled reflected spectra.The resulting catalog provides a uniform, documented, validated, and publicly available set of Cassini/VIMS Jupiter products. By combining calibrated VIS and IR cubes with wavelength information and geometry backplanes, these products facilitate a wide range of applications in the study of Jupiter and its atmosphere.

Abstract:

JWST-NIRSpec observations are providing our first detailed view of the Ice Giants in the 5-micron spectral window [1]. Observations of Jupiter and Saturn (see Figure) at 5 microns have historically revealed important insights into their tropospheric composition, along with striking images of clouds silhouetted against the glowing thermal emission from deeper atmospheric layers [2]. Comparable observations of the colder Ice Giants, however, have long remained beyond observational reach—until now.In this talk, we present JWST-NIRSpec 5-micron observations of the Ice Giants and discuss their implications. We examine what these observations reveal about the tropospheric temperatures, clouds, and composition of Uranus and Neptune, as well as what they may imply for current radiative transfer models at these wavelengths.In particular, we find that the JWST data may suggest sub-adiabatic temperature gradients near the cloud layers, resulting in colder temperatures in the deeper atmosphere. However, these conclusions depend sensitively on the scattering properties of Ice Giant clouds and on accurate modelling of the gaseous opacity, both of which, we argue, remain uncertain under these conditions.Figure: Saturn at 5 microns (left, in red) from Cassini, with hazes shown in green (Image credits: NASA/JPL-Caltech/University of Arizona), compared to Uranus at 5 microns from JWST-NIRSpec [1].[1] Roman, M.T., et al., The Infrared Spectrum of Uranus Revealed with JWST (submitted)[2] Wong, M.H., Bjoraker, G.L., Goullaud, C., Stephens, A.W., Luszcz-Cook, S.H.,1047Atreya, S.K., Pater, I., Brown, S.T.: Deep clouds on jupiter. Remote Sensing104815(3), 702 (2023) 

Abstract:

During its flyby of Titan in 1980, Voyager 1 unveiled an atmosphere thick with an opaque, orange haze that completely obscured the surface, and provide  the first close-up images of its vertical layering (Smith et al. 1981, Hanel et al 1981). Subsequent observations from the Cassini-Huygens mission (Flasar et al. 2004) expanded this view, revealing a world dominated by photochemical hydrocarbons and nitriles with lakes of methane  (Stofan et al. 2007, Mastrogiuseppe et al. 2019), tholin-like organic sand (Lorenz et al. 2006), and seasonally evolving ice clouds (Jennings et al. 2012, West et al. 2016) as seen in Figure 1. While recent observations and laboratory studies (Anderson et al. 2011, Chatain et al. 2020) have provided greater insights into the physical properties of these aerosols and how they form, their composition and complexity remain a mystery.Figure 1: Image taken by Cassini Imaging Science Subsystem (Credit: NASA/JPL-Caltech/Space Science Institute) showing a large ice cloud possibly made of HCN at 300km in Titan’s south pole, 2012 (West et. 2016, Vinatier et al. 2018)In the upper atmosphere, nitrogen and methane are photodissociated by UV radiation and energetic particles to produce ions which act as embryos for the growth of large organic molecules (Vuitton et al. 2024). As these molecules sink through the atmosphere, they recombine, coagulate, and accumulate into larger aerosol particles, onto which trace species can condense, forming both photochemical hazes and stratospheric ice clouds (Vuitton et al. 2024). Cassini CIRS (Composite Infrared Spectrometer) far infra-red spectra (FP1) exhibit four types of hazes: Haze 0, Haze A, Haze B (otherwise known as the “Haystack”) and Haze C (de Kok et al. 2007). While Haze 0 is present throughout the CIRS spectral range and affects the spectral continuum, Haze A, B and C appear as broad features in the far-infrared range. Relatively little is known about these hazes, making their features difficult to fit and estimate. In addition, their signatures overlap with H2O, C4H2, C2N2 and CH4 rotational lines, complicating retrievals of these gases (Sylvestre et al.2017).  Among these hazes, Haze B exhibits the strongest spectral signature. Anderson et al. 2018 suggested that the composition must be a mixture of more than one chemical compound due to its magnitude, breadth and opacity. Haze B is observed to only be present at the winter poles, forming and dissipating with the changing seasons, most likely caused by reduced sunlight and temperature (Jennings et al. 2012a,b , Anderson et al 2018).To enable accurate fitting of water features, Ford et al. (in review) retrieved an effective spectral cross-section of Cassini CIRS FP1 spectra between 147-257cm-1 of 156 FIRNADCMP 0.5cm-1 observations (Figure 2) across different latitudes and times (see also Ford et al. 2025). This was achieved by scaling gaussian basis functions to fit Haze A, B and C (and any other unknown aerosols) in the spectra baselines using the NEMESIS radiative transfer code (Irwin et al. 2008). Previous aerosol cross-sections did not account for latitude or time variation and therefore this technique empirically and agnostically modelled the changing baseline.  In this study, we decomposed those 156 effective spectral cross-sections and extrapolated unique Haze B cross-sections for latitudes of -89° to 88° from June 2004 to April 2017.  We used principal component analysis to determine the number of complete hazes present in the spectral range, finding that Haze B accounts for 98% of the observed variation. We then applied non-negative matrix factorisation to seperate the spectra into two unique and stable components: one representing Haze B, and a second representing a mixture of other haze contributions, along with numerical and spectral noise.Figure 2: Effective spectral cross-section of all 156 Cassini CIRS FIRNADCMP 0.5cm-1 observations before decomposition. Each colour represents a different observation. The plot shows the variation extent of the Haze B feature at ~220cm-1. Using the maximum Haze B cross-section of each observation as proxy, we find a large increase at the winter poles with the increase during northern spring being nearly twice that of northern winter, consistent with previous investigations. We also find Haze B extends to 60°, different to 70° proposed by Anderson et al. 2018. The results also give an insight into the time of formation/dispersion of Haze B at the poles. We see that Haze B at the south pole forms around 2013, reaching its peak during the summer solstice. The final dissipation of Haze B at the north pole cannot be determined due to limited data. ReferencesAnderson, C.M. et al. (2011), Icarus,  212.2, 762-778. DOI: 10.1016/j.pss.2010.10.009Anderson, C.M. et al. (2018), Organic Ices in Titan’s Stratosphere in Space Science Reviews, 214.8, 125  DOI: 10.1007/s11214-018-0559-5Chatain, A.  et al. (2020), Icarus, 345, 113741. DOI: 10.1016/j.icarus/2020.113741Coustenis, A. et al. (1999), Planet Space Science, 47, 1305-1329. DOI:  10.1016/S0032-0633(99)00053-7de Kok, R. et al. (2007), Icarus, 191, 223. DOI:10.1016/j.icarus.2007.04.003Flasar, F.M. et al. (2004), Space Science Reviews, 115, 169–297. DOI: 10.1007/s11214-004-1454-9Ford, J.S. et al. (2025),  EGU General Assembly 2025, Vienna, Austria, EGU25-3741. DOI: 10.5194/egusphere-egu25-3741Ford, J.S. et al. In review. Titan’s Stratospheric Water: Latitudinal and Seasonal Variation from Cassini CIRS Data and Implications for External Oxygen Sources. PSJHanel, R.A. et al. (1981), Science, 212, 192–200. DOI: 10.1126/science.212.4491.192Irwin, P.G.J et al. (2008), JQSRT, 109, 1136. DOI: 10.1016/j.jqsrt.2007.11.006Jennings, D.E. et al. (2012), The Astrophysical Journal Letters, 754, L3. DOI: 10.1088/2041-8205/754/1/L3Khanna, R.K. (2005), Icarus, 178,165-170. doi:10.1016/j.icarus.2005.03.011Lorenz, R. et al. (2006), Science, 312, 724–727. DOI: 10.1126/science.1123257Mastrogiuseppe, M. et al. (2019), Nature Astronomy, 3, 535, DOI: 10.1038/s41550-019-0714-2Samuelson, R.E. Clouds and aerosols of Titan’s atmosphere, in The Atmospheres of Saturn and Titan. Proc.Int. Workshop, vol. ESA SP-241 (1985)Smith, B.A. et al. (1981), Science, 212, 163–191. DOI: 10.1126/science.212.4491.163Stofan, E.R. et al. (2007), Nature, 445, 61–64. DOI: 10.1038/nature05438Sylvestre, M. et al. (2017), A&A, 609:A64. DOI: 10.1051/0004-6361/201630255Vinatier, S. et al. (2018),  Icarus, 310, 89-104. DOI: 10.1016/j.icarus.2017.12.040Vuitton, V. et al. (2024), Chapter 6:Titan’s Atmospheric Structure, Composition, Haze, and Dynamics in Titan after Cassini–Huygens, COSPAR Scientific Symposium Series.West, R.A. et al, (2016), Icarus, 270, 399-408. DOI: 10.1016/j.icarus.2014.11.038 

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.