Solar system

Abstract:

Thermal measurements provide key constraints on the physical properties of icy satellite surfaces. On Europa, previous analyses of the Galileo Photopolarimeter-Radiometer (PPR) dataset revealed heterogeneities in thermal inertia, but the limited spatial resolution prevented a detailed thermophysical characterization. We derive high-resolution maps of the Europa surface albedo and thermal inertia and infer the microphysical properties of its icy regolith from a reanalysis of the Galileo PPR dataset by discussing the processes controlling its thermophysical evolution. We used the KRC thermal model (K refers to the conductivity ąppa, R to the density ̊ho, and C to the specific heat C) to analyze the PPR brightness temperatures and retrieve the albedo and thermal inertia. These values were then interpreted using conductivity models of porous ice to constrain the grain size and porosity. We derived a mean Bond albedo of 0.64 ± 0.06 and a mean thermal inertia of 56 ± 17 J m -2 $ K^-1 s^-1/2 (1σ). The thermal inertia shows significant spatial variations, with a low-inertia equatorial band (39 ± 7) and higher values at mid-latitudes on the leading hemisphere (56 ± 11). The trailing-hemisphere equator also exhibits higher thermal inertia (63 ± 17), likely related to compositional differences. Conductivity models indicate 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 in the upper centimeters of the Europa surface. The thermal inertia distribution shows little correlation with geological units. Its agreement with modeled magnetospheric ion fluxes instead suggests that sputtering-driven sintering plays a fundamental role in shaping the thermophysical properties of Europa. The absence of a high-inertia equatorial band analogous to the PacMan anomaly on the icy moons of Saturn indicates inefficient electron-driven sintering, while temperature-gradient metamorphism might enhance grain growth at depth. The modeled surface temperatures range between ∼ 67 and 148 K at mid to low latitudes, with peak daytime values counteracting radiolytic amorphization while limiting volatile stability.

Abstract:

Abstract The Hubble Space Telescope collected imaging data spanning the full lifetime of Neptune's dark spot NDS‐2018, which is the sixth large, persistent dark spot seen in the planet's atmosphere. Neptune's dark spots are thought to be anticyclonic vortices, although internal flows have never been directly measured to confirm their rotation. Previous reports covered the formation and evolution of the mature NDS‐2018, while here we report that the contrast weakened over the 2021 to 2022 period, while the feature rapidly drifted equatorward from 14 deg N to 6–7 deg N planetographic latitude. The persistence of the spot so close to the equator is surprising, and represents a challenge for numerical models which find anticyclones to be disrupted within 15 deg of the equator. The changing contrast of the dark spot constrains changes in the aerosols over time, but the link between dynamical and microphysical properties of dark vortices is not known. Plain Language Summary The Hubble Space Telescope collected images spanning the full lifetime of Neptune's dark spot NDS‐2018, which is the sixth large, persistent dark spot seen in the planet's atmosphere. Neptune's dark spots are thought to be high‐pressure rotating storms, although internal flows have never been directly measured to confirm the direction of their rotation. Previous reports covered the formation and evolution of the mature NDS‐2018, while here we report that the contrast weakened over the 2021 to 2022 period, while the feature rapidly drifted equatorward from 14 deg N to 6–7 deg N planetographic latitude. The persistence of the spot so close to the equator is surprising, and represents a challenge for numerical models which find these storms to be disrupted within 15 deg of the equator. The changing contrast of the dark spot provides information on changes in the cloud particles over time, but it is not known what is the link between how the storm works and what are the properties of cloud particles in the dark spots. Key Points Dark spot NDS‐2018 was observed on Neptune over its full lifespan, between 3.7 and 4.9 years In 2022, NDS‐2018 remained within 8 deg of the equator as it faded away Secondary dark features had morphologies of discrete spots in 2019–2020 and extended arms in 2022

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)