Uranus and Neptune

The two icy giant planets, Uranus and Neptune, offer a wide population of icy moons and rings of great interest for the planetary geologist. Among moons, Miranda has a very rough surface, peculiar for such a small moon. Triton is characterized by the presence of active geysers on the south polar region and by a tenuous atmosphere made of nitrogen and methane. All moons show the presence of very volatile ices (nitrogen, ammonia, methane, water) thanks to their low surface temperatures

The Rings of Saturn

One could become an expert on Saturn's iconic rings pretty easily in the early 1970s, as very little was known about them beyond the distinction between the A, B, and C rings, and the Cassini Division or "gap" between rings A and B (Alexander, 1962; Bobrov, 1970). Water ice was discovered spectroscopically on the ring particle surfaces, and radar and microwave emission observations proved that the particles must be centimeters to meters in size, consisting primarily, not just superficially, of water ice (Pollack, 1975). While a 2:1 orbital resonance with Mimas had long been suspected of having something to do with the Cassini Division, computers of the time were unable to model the subtle dynamical effects that we now know to dominate ring structure. This innocent state of affairs was exploded by the Voyager 1 and 2 encounters in 1980 and 1981. Spectacular images revealed filigree structure and odd regional color variations, and exquisitely detailed radial profiles of fluctuating particle abundance were obtained from the first stellar and radio occultations, having resolution almost at the scale of single particles. Voyager-era understanding was reviewed by Cuzzi et al. (1984) and Esposito et al. (1984). While the Voyager data kept ring scientists busy for decades, planning which led to the monumentally successful NASA-ESA-ASI Cassini mission, which arrived in 2004, had been under way even before Voyager got to Saturn. A review of pre-Cassini knowledge of Saturn's Rings can be found in Orton et al. (2009). This chapter will build on recent topical and process-specific reviews that treat the gamut of ring phenomena and its underlying physics in considerable detail (Colwell et al., 2009; Cuzzi et al., 2009; Horányi et al., 2009; Schmidt et al., 2009; Esposito, 2010; Tiscareno, 2013b; Esposito, 2014). We will follow and extend the general organization of Cuzzi et al. (2010), the most recent general discussion of Saturn's rings. For brevity and the benefit of the reader, we will frequently refer to the above review articles instead of directly to the primary literature they discuss. We will focus on new work since 2010, within a general context, and will connect our high-level discussions with more detailed chapters in this volume

Chromophores from photolyzed ammonia reacting with acetylene: Application to Jupiter's Great Red Spot

The high altitude of Jupiter's Great Red Spot (GRS) may enhance the upward flux of gaseous ammonia (NH3) into the high troposphere, where NH3 molecules can be photodissociated and initiate a chain of chemical reactions with downwelling acetylene molecules (C2H2). These reactions, experimentally studied earlier by (Ferris and Ishikawa [1987] Nature 326, 777-778) and (Ferris and Ishikawa [1988] J. Amer. Chem. Soc. 110, 4306-4312), produce chromophores that absorb in the visible and ultraviolet regions. In this work we photolyzed mixtures of NH3 and C2H2 using ultraviolet radiation with a wavelength of 214 nm and measured the spectral transmission of the deposited films in the visible region (400-740 nm). From these transmission data we estimated the imaginary indices of refraction. Assuming that ammonia grains at the top of the GRS clouds are coated with this material, we performed layered sphere and radiative transfer calculations to predict GRS reflection spectra. Comparison of those results with observed and previously unreported Cassini visible spectra and with true-color images of the GRS show that the unknown GRS chromophore is spectrally consistent with the coupled NH3-C2H2 photochemical products produced in our laboratory experiments. Using high-resolution mass spectrometry and infrared spectroscopy we infer that the chromophore-containing residue is composed of aliphatic azine, azo, and diazo compounds. <P /

Comet 67P/CG: surface temperature maps from Rosetta/VIRTIS during the pre-landing phase

It was seldom possible, with observations carried out from spaceborne facilities, to derive spatially-resolved thermal maps of small bodies, and even more rarely this result was achieved in the case of close observations of comets. The Visible InfraRed Thermal Imaging Spectrometer (VIRTIS) onboard the Rosetta Orbiter Coradini (2007) is able to obtain hyperspectral images of the observed targets in 864 wavelengths simultaneously, in the overall spectral range 0.25-5.1 μm, with the major goal of inferring and mapping the surface composition and temperature of comet 67P/Churyumov-Gerasimenko. VIRTIS spectra acquired on the dayside of the comet’s nucleus show the thermal emission of the surface at wavelengths ¿ 3.5 μm, which can be ex- ploited to derive and map the surface temperature at different spatial scales and under changing lighting conditions. To do this, we rely on a Bayesian approach that was previously adopted to derive surface temperature maps of the two asteroids 2678 Steins and 21 Lutetia, encountered by Rosetta during its long cruise phase towards the comet Coradini (2011); Keihm (2012), and of the large asteroid Vesta from the entire infrared dataset acquired by the VIR instrument onboard the Dawn spacecraft Tosi (2014). In this paper we summarize the main results concerning the thermal mapping of comet 67P, obtained by VIRTIS in the first months of observation at a reso- lution between 1000 and 1 m, and at a heliocentric distance between 3.6 and 3.4 AU. Comet 67P was shown to be everywhere rich in organic materials with little to no water ice visible on the surface Capaccioni (2015). In the range of heliocentric distances from 3.59 to 2.74 AU, daytime surface temperatures were overall comprised in the range between 180 and 220 K Tosi (2015), which is incompatible with large exposures of water ice and is consistent with a low-albedo, organics-rich surface. Maximum temperature values as high as 230 K were recorded in very few places Tosi (2015). In the above period, the highest values of surface temperature were obtained with observations carried out at small phase angles, implying that the observed surface has a large predominance of small incidence angles, and local solar times centered around the max- imum daily insolation. In all cases, direct correlation with topographic features was observed, i.e. largest temperature values were generally associated with the smallest values of illumination angles, while no evidence was found of thermal anomalies, i.e. places of the surface that are intrinsically warmer or cooler than surrounding terrains observed at the same local solar time and under similar solar illumination

Juno's Earth flyby: the Jovian infrared Auroral Mapper preliminary results

The Jovian InfraRed Auroral Mapper, JIRAM, is an image-spectrometer onboard the NASA Juno spacecraft flying to Jupiter. The instrument has been designed to study the aurora and the atmosphere of the planet in the spectral range 2-5 μm. The very first scientific observation taken with the instrument was at the Moon just before Juno's Earth fly-by occurred on October 9, 2013. The purpose was to check the instrument regular operation modes and to optimize the instrumental performances. The testing activity will be completed with pointing and a radiometric/spectral calibrations shortly after Jupiter Orbit Insertion. Then the reconstruction of some Moon infrared images, together with co-located spectra used to retrieve the lunar surface temperature, is a fundamental step in the instrument operation tuning. The main scope of this article is to serve as a reference to future users of the JIRAM datasets after public release with the NASA Planetary Data System

Nippur Sulcus from 2D to 3D: A Multidisciplinary approach in preparation for JUICE

Summary. We focus on a portion of the Nippur Sulcus region on Ganymede, which is currently promising for a multisensor data analysis. We first combine data acquired by the Galileo/SSI framing camera and by the Galileo/NIMS imaging spectrometer. We then apply a self-similar clustering technique able to explain how the grooves visible on the surface are distributed inside the icy crust. Finally, we apply a simple model to obtain a synthetic topographic dataset based on the existing optical dataset, which is used to simulate radar echoes coming from the RIME instrument with the goal of evaluating the magnitude of clutter noise.Geological and compositional context. We first focus on optical images obtained by the Galileo/SSI framing camera on a portion of Nippur Sulcus bordering Galileo Regio to the east and Marius Regio to the west (167.4°E-193.5°E and 16.8°N-38.2°N) (Fig. 1). This region includes different terrain units, such as dark terrains, grooved terrains, and the palimpsest crater Epigeus [1].Fig. 1. Overview of the Nippur Sulcus region of inter-est used for our case study, from Ganymede's optical basemap (1 km/px). The violet contours highlight the regions for which high-resolution optical imagery is available, while the green contour highlights the region covered by hyperspectral, near-infrared (NIMS) data (see Fig. 2).The coverage obtained by the Galileo/NIMS imaging spectrometer [2] allows for the combination of geologic and compositional information, using specific spectral indices such as an IR slope, the 2-μm band depth, and a IR ratio, which can account for the distribution of contaminants, water ice, and grain size, respectively (Fig. 2).Fig. 2. NIMS Color composite map covering the Nip-pur Sulcus region at a spatial resolution of 6.7 km/px (R: IR slope 1.10-2.25 µm, G: 2-µm band depth, B: IR ratio between 3.6 µm and 1.82 µm). In the adopted color scheme, green marks high albedo, large (100s µm) regolith grains and stronger water ice band, while magenta is associated with dark terrain where the 2-µm water ice band is shallower and the grain size is smaller (10s of µm).Grooves' length and fractal analysis. On the selected region of interest within Nippur, we establish a link between geology and geophysics, namely between what is seen on the surface at visible to infrared wavelengths, and Ganymede's shallow subsurface, which is still largely unknown today. To do so, we analyse the grooves' length and spatial distribution to estimate the potential thickness of the icy crust above the deep ocean required to develop the grooves. A fractal approach, namely a self-similar clustering method, allows us to determine the maximum depth at which grooves can penetrate the icy subsurface [3, 4]. As a result, the grooves mapped in the Nippur Sulcus region in principle could penetrate the ice crust up to 130-145 km (Fig. 3).Fig. 3. A self-similar clustering method allows us to determine the maximum depth up to which grooves can penetrate the icy subsurface. To do this, we calcu-late an integral correlation coefficient within certain length ranges. In this plot, the x axis is the logarithm of these threshold lengths, while the y axis represents the logarithm of the correlation coefficient (on the left) and what is called the local slope (on the right, which is the point-by-point measurement of the slope of the tangent to the curve). As a result, the grooves mapped in the Nippur Sulcus region in principle could pene-trate the ice crust up to 130-145 km, which is in agreement with independent estimates of the average thickness of the icy shell.Simulation of radar surface scattering at RIME frequencies. The Radar for Icy Moon Exploration (RIME) is one of the payload instruments aboard the JUICE spacecraft [5]. Operating at a central frequency of 9 MHz, and transmitting either a 3-MHz or a 1-MHz bandwidth (corresponding to a range resolution in water ice of about 30 m and 90 m respectively), RIME is tasked to explore the subsurface of Ganymede, Europa and Callisto down to a depth of several kilometres. However, clutter noise from rougher terrains is a limiting factor in the detectability of deeper, weaker reflections from the subsurface. The application of a coherent model of surface scattering to a roughness model of Nippur Sulcus demonstrates that clutter decays much slower than on Mars for a Ganymede-like topography (Fig. 4), which creates a challenge for the scientific interpretation of future RIME data.Fig. 4. Radargram of simulated surface echoes pro-duced for a subset of Nippur Sulcus, for a 3-MHz bandwith pulse. The time delay of echoes has been converted to the depth in water ice from which a radar echo would have the same time delay. The radargrams have been cropped to show only the part of the echoes that is unaffected by the finite size of the pseudo-topographic model.Conclusions. The case of Nippur Sulcus is well suited to test what could be closely observed in the future by different instruments onboard the JUICE mission, combining remote sensing data probing the surface and theoretical modeling aimed at the geophysical study of the satellite's subsurface. The merits and issues brought to light by our study should be taken into account to achieve a broader understanding of the physical processes linking Ganymede's surface and subsurface. Acknowledgements: We acknowledge support from the research project: "Ganymede from 2D to 3D: A multidisciplinary approach in preparation for JUICE", selected in 2019 in the framework of an "INAF Mainstream" call.References: [1] Collins G. C., et al., Global geologic map of Ganymede: U.S. Geological Survey Scientific Investigations Map 3237, pamphlet 4 p., 1 sheet, scale 1:15,000,000 (2013). [2] Carlson R. W, et al., Space Sci. Rev. 60, 457-502 (1992). [3] Lucchetti A., Pozzobon R., Mazzarini F., Cremonese G., Massironi M. (2017). Icarus 297, 252-264. [4] Lucchetti A., Rossi C., Mazzarini F., Pajola M., Pozzobon R., Massironi M., Cremonese G. (2021). Planet. Space Sci. 195, article id. 105140. [5] Bruzzone L., Croci R. (2019). Proceedings of the 2019 IEEE 5th International Workshop on Metrology for AeroSpace, Turin (Italy), 19-21 June 2019

A Mercury surface radiometric model for SIMBIO-SYS instrument suite on board of BepiColombo mission

The BepiColombo mission represents the cornerstone n.5 of the European Space Agency (ESA) and it is composed of two satellites: the Mercury Planetary Orbiter (MPO) realized by ESA and the Mercury Magnetospheric Orbiter (MMO) provided by the Japan Aerospace Exploration Agency (JAXA). The payload of the MPO is composed by 11 instruments. About half of the entire MPO data volume will be provided by the Spectrometer and Imagers for MPO BepiColombo Integrated Observatory System" (SIMBIO-SYS) instrument suite. The SIMBIO-SYS suite includes three imaging systems, two with stereo and high spatial resolution capabilities, which are the Stereoscopic Imaging Channel (STC) and High Resolution Imaging Channel (HRIC), and a hyper-spectral imager in the Vis-NIR range, named Visible and near Infrared Hyper-spectral Imager (VIHI). In order to test and predict the instrument performances, a radiometric model is needed. It consists in a tool that permits to know what fraction of the incoming light is measured by the detector. The obtained signal depends on the detector properties (such as quantum efficiency and dark current) and the instrument transmission characteristics (transmission of lenses and filter strips, mirrors reflectivity). The radiometric model allows to correlate the radiance of the source and the signal measured by each instrument. We used the Hapke model to obtain the Mercury reflectance, and we included it in the radiometric model applied to the STC, HRIC and VIHI channels. The radiometric model here presented is a useful tool to predict the instruments performance: it permits to calculate the expected optical response of the instrument (the position in latitude and longitude of the filter footprints, the on-ground px dimensions, the on-ground speed, the smearing and the illumination angles of the observed points), and the detector behavior (the expected signal and the integration time to reach a specific SNR). In this work we derive the input flux and the integration times for the three channels of SIMBIO-SYS, using the radiometric model to obtain the source radiance for each Mercury surface area observed

Emitted Power Of Jupiter Based On Cassini CIRS And VIMS Observations

The emitted power of Jupiter and its meridional distribution are determined from observations by the Composite Infrared Spectrometer (CIRS) and Visual and Infrared Spectrometer (VIMS) onboard Cassini during its flyby en route to Saturn in late 2000 and early 2001. Jupiter's global- average emitted power and effective temperature are measured to be 14.10+/-0.03 W/sq m and 125.57+/-0.07 K, respectively. On a global scale, Jupiter's 5-micron thermal emission contributes approx. 0.7+/-0.1 % to the total emitted power at the global scale, but it can reach approx. 1.9+/-0.6% at 15degN. The meridional distribution of emitted power shows a significant asymmetry between the two hemispheres with the emitted power in the northern hemisphere 3.0+/-0.3% larger than that in the southern hemisphere. Such an asymmetry shown in the Cassini epoch (2000-01) is not present during the Voyager epoch (1979). In addition, the global-average emitted power increased approx. 3.8+/-1.0% between the two epochs. The temporal variation of Jupiter's total emitted power is mainly due to the warming of atmospheric layers around the pressure level of 200 mbar. The temporal variation of emitted power was also discovered on Saturn (Li et al., 2010). Therefore, we suggest that the varying emitted power is a common phenomenon on the giant planets