Water vapor produces a series of diagnostic emission lines in the near infrared between 2.60 and 2.75 microns. The Visual and Infrared Mapping Spectrometer (VIMS) onboard the Cassini spacecraft detected this emission signal from Enceladus' plume, and so VIMS observations provide information about the variability of the plume's water vapor content. Using a data set of 249 spectral cubes with relatively high signal-to-noise ratios, we confirmed the strength of this water-vapor emission feature corresponds to a line-of-sight column density of order 10^20 molecules/m^2, which is consistent with previous measurements from Cassini's Ultraviolet Imaging Spectrograph (UVIS). Comparing observations made at different times indicates that the water-vapor flux is unlikely to vary systematically with Enceladus' orbital phase, unlike the particle flux, which does vary with orbital phase. However, variations in the column density on longer and shorter timescales cannot be ruled out and merit further investigation.
We present an analysis of data about Saturn's atmosphere from Cassini's Visual and Infrared Mapping Spectrometer (VIMS), focusing on the meteorology of the features seen in the 5-micron spectral window. We present VIMS mosaics and discuss the morphology and general characteristics of the features backlit by Saturn's thermal emission. We have also constructed a zonal wind profile from VIMS feature tracking observation sequences using an automated cloud feature tracker. Comparison with previously constructed profiles from Voyager and Cassini imaging data reveals broad similarities, suggesting mi…
The spectral position of the 3.6 micron continuum peak measured on Cassini-VIMS I/F spectra is used as a marker to infer the temperature of the regolith particles covering the surfaces of Saturn's icy satellites. This feature is characterizing the crystalline water ice spectrum which is the dominant compositional endmember of the satellites' surfaces. Laboratory measurements indicate that the position of the 3.6 micron peak of pure water ice is temperature-dependent, shifting towards shorter wavelengths when the sample is cooled, from about 3.65 micron at T=123 K to about 3.55 micron at T=88 K…
Titan is one of the primary scientific objectives of the NASA ESA ASI Cassini Huygens mission. Scattering by haze particles in Titan's atmosphere and numerous methane absorptions dramatically veil Titan's surface in the visible range, though it can be studied more easily in some narrow infrared windows. The Visual and Infrared Mapping Spectrometer (VIMS) instrument onboard the Cassini spacecraft successfully imaged its surface in the atmospheric windows, taking hyperspectral images in the range 0.4 5.2 ?m. On 26 October (TA flyby) and 13 December 2004 (TB flyby), the Cassini Huygens mission fl…
We describe the prediction, design, execution and calibration of stellar and solar occultation observations of Saturn's rings by the Visual and Infrared Mapping Spectrometer (VIMS) instrument on the Cassini spacecraft. Particular attention is paid to the technique developed for onboard acquisition of the stellar target and to the geometric and photometric calibration of the data. Examples of both stellar and solar occultation data are presented, highlighting several aspects of the data as well as the different occultation geometries encountered during Cassini's 13 year orbital tour. Complete c…
We present Cassini VIMS observations of sun glitter -- wave-induced reflections from a liquid surface offset from a specular point -- on Kraken Mare. Sun glitter reveals rough sea surfaces around Kraken Mare, namely the coasts and narrow straits. The sun glitter observations indicate wave activity driven by the winds and tidal currents in Kraken Mare during northern summer. T104 Cassini VIMS observations show three sun glitter features in Bayta Fretum indicative of variegated wave fields. We cannot uniquely determine one source for the coastal Bayta waves, but we lean toward the interpretation…
Jupiter's tropospheric composition and cloud structure are studied using Cassini VIMS 4.5-5.1 μm thermal emission spectra from the 2000-2001 flyby. We make use of both nadir and limb darkening observations on the planet's nightside, and compare these with dayside observations. Although there is significant spatial variability in the 5-μm brightness temperatures, the shape of the spectra remain very similar across the planet, suggesting the presence of a spectrally-flat, spatially inhomogeneous cloud deck. We find that a simple cloud model consisting of a single, compact cloud is able to reprod…
In its decade of operation the Cassini mission has allowed us to look deep into Saturn's atmosphere and investigate the processes occurring below its enshrouding haze. We use Visual and Infrared Mapping Spectrometer (VIMS) 4.6-5.2 micron data from early in the mission to investigate the location and properties of Saturn's cloud structure between 0.6 and 5 bars. We average nightside spectra from 2006 over latitude circles and model the spectral limb darkening using the NEMESIS radiative transfer and retrieval tool. We present our best-fit deep cloud model for latitudes between -40 and 50 degree…
Albedo, spectral slopes, and water ice band depths maps for the five midsized saturnian satellites Mimas, Enceladus, Tethys, Dione, and Rhea have been derived from Cassini-Visual and Infrared Mapping Spectrometer (VIMS) data. The maps are systematically built from photometric corrected data by applying the Kaasalainen-Shkuratov model (Kaasalainen et al., 2001, Shkuratov et al., 2011}. In this work a quadratic function is used to fit phase curves built by filtering observations taken with incidence angle $i\le70^\circ$, emission angle $e\le70^\circ$, phase angle $10^\circ \le g \le 120^\circ$, …
We have analyzed the complete Visual and Infrared Mapping Spectrometer (VIMS) data archive of Titan. Our objective is to build global surface cartographic products, by combining all the data gathered during the 127 targeted flybys of Titan into synthetic global maps interpolated on a grid at 32 pixels per degree (~1.4 km/pixel at the equator), in seven infrared spectral atmospheric windows. Multispectral summary images have been computed for each single VIMS cube in order to rapidly identify their scientific content and assess their quality. These summary images are made available to the commu…
A Processing Workflow for Cassini VIMS Jupiter Cubes
Universidad del País Vasco/Euskal Herriko Unibertsitatea (UPV/EHU)（巴斯克大学）；University of Oxford（牛津大学）；Istituto di Astrofisica e Planetologia Spaziali–Istituto Nazionale di Astrofisica (IAPS-INAF)（空间天体物理学研究所-意大利国家天体物理研究所）；Max-Planck-Institut für Sonnensystemforschung (MPS)（马克斯·普朗克太阳系统研究所）
The Cassini flyby of Jupiter in 2000 provided spatially resolved spectra of Jupiter's atmosphere using the Visual and Infrared Mapping Spectrometer (VIMS). These spectra contain a strong absorption at wavelengths from about 2.9 $μ$m to 3.1 $μ$m, previously noticed in a 3-$μ$m spectrum obtained by the Infrared Space Observatory (ISO) in 1996. While Brooke et al. (1998, Icarus 136, 1-13) were able to fit the ISO spectrum very well using ammonia ice as the sole source of particulate absorption, Sromovsky and Fry (2010, Icarus 210, 211-229), using significantly revised NH$_3$ gas absorption models…
We used 0.85 - 5.1 micron 2006 observations by Cassini's Visual and Infrared Mapping Spectrometer (VIMS) to constrain the unusual vertical structure and compositions of cloud layers in Saturn's south polar region, the site of a powerful vortex circulation, shadow-casting cloud bands, and spectral evidence of ammonia ice clouds without the lightning usually associated with such features. We modeled spectral observations with a 4-layer model that includes (1) a stratospheric haze, (2) a top tropospheric layer of non-absorbing (possibly diphosphine) particles near 300 mbar, with a fraction of an …
Saturn's main rings exhibit variations in both their opacity and spectral properties on a broad range of spatial scales, and the correlations between these parameters can provide insights into the processes that shape the composition and dynamics of the rings. The Visual and Infrared Mapping Spectrometer (VIMS) instrument onboard the Cassini Spacecraft has obtained spectra of the rings between 0.35 and 5.2 microns with sufficient spatial resolution to discern variations on scales below 200 km. These relatively high-resolution spectral data reveal that both the depths of the near-infrared water…
Cassini/ISS imagery and Cassini/VIMS spectral imaging observations from 0.35 to 5.12 microns show that between 2012 and 2017 the region poleward of the Saturn's northern hexagon changed from dark blue/green to a moderately brighter gold color, except for the inner eye region (88.2 deg - 90 deg N), which remained relatively unchanged. These and even more dramatic near-IR changes can be reproduced by an aerosol model of four compact layers consisting of a stratospheric haze at an effective pressure near 50 mbar, a deeper haze of putative diphosphine particles typically near 300 mbar, an ammonia …
In the last few years Cassini-VIMS, the Visible and Infared Mapping Spectrometer, returned to us a comprehensive view of the Saturn's icy satellites and rings. After having analyzed the satellites' spectral properties (Filacchione et al. (2007a)) and their distribution across the satellites' hemispheres (Filacchione et al. (2010)), we proceed in this paper to investigate the radial variability of icy satellites (principal and minor) and main rings average spectral properties. This analysis is done by using 2,264 disk-integrated observations of the satellites and a 12x700 pixels-wide rings radi…
On day 138 of 2010, the plume of dust and gas emerging from Enceladus' South Polar Terrain passed between the Sun and the Cassini spacecraft. This solar occultation enabled Cassini's Ultraviolet Imaging Spectrograph (UVIS) and the Visual and Infrared Mapping Spectrometer (VIMS) to obtain simultaneous measurements of the plume's gas and dust components along the same lines of sight. The UVIS measurements of the plume's gas content are described in Hansen et al. (2011, GRL 38:11202) , while this paper describes the VIMS data and the information they provide about the plume's particle content. To…
The Cassini Ultraviolet Imaging Spectrograph (UVIS) observed a plume of water vapor spewing out from the south polar regions of Enceladus in occultation geometry 7 times during the Cassini mission. Five of them yielded data resolved spatially that allowed fits to a set of individually modeled jets. We created a direct simulation Monte Carlo (DSMC) model to simulate individual water vapor jets with the aim of fitting them to water vapor abundance along the UVIS line of sight during occultation observations. Accurate location and attitude of spacecraft together with positions of Enceladus and Sa…
We present an analysis of the VIMS solar occultations dataset, which allows us to extract vertically resolved information on the characteristics of Titan's atmosphere between 100-700 km with a characteristic vertical resolution of 10 km. After a series of data treatment procedures, 4 occultations out of 10 are retained. This sample covers different seasons and latitudes of Titan. The transmittances show clearly the evolution of the haze and detect the detached layer at 310 km in Sept. 2011 at mid-northern latitudes. Through the inversion of the transmission spectra with a line-by-line radiativ…
Cloud and ammonia vertical profiles in the equatorial atmosphere of Jupiter determined from visible to near-IR observations made by VLT/MUSE, Cassini/VIMS, IRTF/SpeX and Juno/JIRAM
University of Oxford（牛津大学）；Universidad del País Vasco UPV/EHU（巴斯克大学）；Istituto Nazionale di Astrofisica（意大利国家天体物理研究所）
Between 2004 and 2017, spectral observations have been gathered by the Visual and Infrared Mapping Spectrometer (VIMS) on-board Cassini (Brown et al., 2004) during 23 Enceladus close encounters, in addition to more distant surveys. The objective of the present study is to produce a global hyperspectral mosaic of the complete VIMS data set of Enceladus in order to highlight spectral variations among the different geological units. This requires the selection of the best observations in terms of spatial resolution and illumination conditions. We have carried out a detailed investigation of the p…
The extraordinary activity at Enceladus' warm south pole indicates the presence of an internal global or local reservoir of liquid water beneath the surface. While Tyler (2009, 2011) has suggested that the geological activity and the large heat flow of Enceladus could result from tidal heating triggered by a large obliquity of at least 0.05°-0.1°, theoretical models of the Cassini state predict the obliquity to be two to three orders of magnitude smaller for an entirely solid and rigid Enceladus. We investigate the influence of an internal subsurface ocean and of tidal deformations of the soli…
A goal of Cassini's extended mission has been to examine the seasonal variations of Saturn's magnetosphere, moons, and rings. Recently we showed that the magnetospheric plasma between the main rings and Enceladus exhibited a time dependence that we attributed to a seasonally variable source of oxygen from the main rings (Elrod et al., 2012). Such a temporal variation was subsequently seen in the energetic ion composition (Christon et al., 2013). Here we include the most recent measurements by the Cassini Plasma Spectrometer (CAPS) in our analysis (Elrod et al., 2012) and modeling (Tseng et al.…
Our analysis of Cassini/VIMS near-infrared spectra of Saturn's Great Storm of 2010-2011 reveals a multi-component aerosol composition comprised primarily of ammonia ice, with a significant component of water ice. The most likely third component is ammonium hydrosulfide or some weakly absorbing material similar to what dominates visible clouds outside the storm region. Horizontally heterogeneous models favor ammonium hydrosulfide as the third component, while horizontally uniform models favor the weak absorber. Both models rely on water ice absorption to compensate for residual spectral gradien…
Fermentation heterotrophy can reproduce Enceladus's plume H2 and CH4 under limited serpentinization
University of Arizona（亚利桑那大学）；ETH Zurich（苏黎世联邦理工学院）；Queen Mary University of London（伦敦大学玛丽女王学院）；Natural History Museum, London（伦敦自然历史博物馆）；Aix Marseille Université（艾克斯-马赛大学）；Université de Toulon（土伦大学）；CNRS（法国国家科学研究中心）；IRD（法国国家科研中心）；ENS–PSL University（巴黎文理研究大学）；École Normale Supérieure（巴黎高等师范学院）；Muséum National d’Histoire Naturelle（法国国家自然历史博物馆）
We investigate the chemical interaction between Saturn's corotating plasma and Enceladus' volcanic plumes. We evolve plasma as it passes through a prescribed H2O plume using a physical chemistry model adapted for water-group reactions. The flow field is assumed to be that of a plasma around an electrically-conducting obstacle centered on Enceladus and aligned with Saturn's magnetic field, consistent with Cassini magnetometer data. We explore the effects on the physical chemistry due to: (1) a small population of hot electrons; (2) a plasma flow decelerated in response to the pickup of fresh io…
Some of the major discoveries of the recent Cassini-Huygens mission have put Titan and Enceladus firmly on the Solar System map. The mission has revolutionised our view of Solar System satellites, arguably matching their scientific importance with that of their planet. While Cassini-Huygens has made big surprises in revealing Titan's organically rich environment and Enceladus' cryovolcanism, the mission's success naturally leads us to further probe these findings. We advocate the acknowledgement of Titan and Enceladus science as highly relevant to ESA's long-term roadmap, as logical follow-on …
Context. The origin of water in the stratospheres of Giant Planets has been an outstanding question ever since its first detection by ISO some 20 years ago. Water can originate from interplanetary dust particles, icy rings and satellites and large comet impacts. Analysis of Herschel Space Observatory observations have proven that the bulk of Jupiter's stratospheric water was delivered by the Shoemaker-Levy 9 impacts in 1994. In 2006, the Cassini mission detected water plumes at the South Pole of Enceladus, placing the moon as a serious candidate for Saturn's stratospheric water. Further eviden…
Over the last eight years, the Visual and Infrared Mapping Spectrometer (VIMS) aboard the Cassini orbiter has returned hyperspectral images in the 0.35-5.1 micron range of the icy satellites and rings of Saturn. These very different objects show significant variations in surface composition, roughness and regolith grain size as a result of their evolutionary histories, endogenic processes and interactions with exogenic particles. The distributions of surface water ice and chromophores, i.e. organic and non-icy materials, across the saturnian system, are traced using specific spectral indicator…
Observational data from the Cassini spacecraft are used to obtain a chemical model of ocean water on Enceladus. The model indicates that Enceladus' ocean is a Na-Cl-CO3 solution with an alkaline pH of ~11-12. The dominance of aqueous NaCl is a feature that Enceladus' ocean shares with terrestrial seawater, but the ubiquity of dissolved Na2CO3 suggests that soda lakes are more analogous to the Enceladus ocean. The high pH implies that the hydroxide ion should be relatively abundant, while divalent metals should be present at low concentrations owing to buffering by clays and carbonates on the o…
Features in the visible spectra of the Enceladus particle plume and E ring: Potential evidence of organic materials and/or missing sub-micron particles
University of Idaho（爱达荷大学）；Johns Hopkins University Applied Physics Lab（约翰斯·霍普金斯大学应用物理实验室）
Saturn's moon Enceladus offers a unique opportunity in the search for life and habitable environments beyond Earth, a key theme of the National Research Council's 2013-2022 Decadal Survey. A plume of water vapor and ice spews from Enceladus's south polar region. Cassini data suggest that this plume, sourced by a liquid reservoir beneath the moon's icy crust, contain organics, salts, and water-rock interaction derivatives. Thus, the ingredients for life as we know it-- liquid water, chemistry, and energy sources-- are available in Enceladus's subsurface ocean. We have only to sample the plumes …
Estimates of the total particulate mass of the plumes of Enceladus are important to constrain theories of particle formation and transport at the surface and interior of the satellite. We revisit the calculations of Ingersoll & Ewald (2011), who estimated the particulate mass of the Enceladus plumes from strongly forward scattered light in Cassini ISS images. We model the plume as a combination of spherical particles and irregular aggregates resulting from the coagulation of spherical monomers, the latter of which allows for plumes of lower particulate mass. Though a continuum of solutions are…
Since the discovery of an ice particle plume erupting from the south polar terrain on Saturn's moon Enceladus, the geophysical mechanisms driving its activity have been the focus of substantial scientific research. The pattern and deposition rate of plume material on Enceladus' surface is of interest because it provides valuable information about the dynamics of the ice particle ejection as well as the surface erosion. Surface deposition maps derived from numerical plume simulations by Kempf et al. (2010) have been used by various researchers to interpret data obtained by various Cassini instr…
We examine Saturn's atmosphere with observations from ground-based telescopes and Hubble Space Telescope (HST). We present a detailed analysis of observations acquired during 2018. A system of polar storms that appeared in the planet in March 2018 and remained active with a complex phenomenology at least until Sept. is analyzed elsewhere (Sanchez-Lavega et al., in press , 2019). Many of the cloud features in 2018 are long-lived and can be identified in images in 2017, and in some cases, for up to a decade using also Cassini ISS images. Without considering the polar storms, the most interesting…
Spacecraft observations suggest that the plumes of Saturn's moon Enceladus draw water from a subsurface ocean, but the sustainability of conduits linking ocean and surface is not understood. Observations show sustained (though tidally modulated) fissure eruptions throughout each orbit, and since the 2005 discovery of the plumes. Peak plume flux lags peak tidal extension by $\sim$1 radian, suggestive of resonance. Here we show that a model of the tiger stripes as tidally-flexed slots that puncture the ice shell can simultaneously explain the persistence of the eruptions through the tidal cycle,…
Water plumes erupting from the `tiger stripe' features on the south pole of Enceladus are thought to connect to a global subsurface ocean. Proposed origins for the initial stress necessary to form the `tiger stripes' include a giant impact, which would require true polar wander to explain the location of the plumes if the impact did not occur at the South Pole, or tensile stresses, which would require volumetric expansion associated with partial freezing of the subsurface ocean. In addition to the requirement of a fine-tuned impact, true polar wander, or partial freezing of the subsurface ocea…
Enceladus, an icy moon of Saturn, possesses an internal water ocean and jets expelling ocean material into space. Cassini investigations indicated that the subsurface ocean could be a habitable environment having a complex interaction with the rocky core. Further investigation of the composition of the plume formed by the jets is necessary to fully understand the ocean, its potential habitability, and what it tells us about Enceladus' origin. Moonraker has been proposed as an ESA M-class mission designed to orbit Saturn and perform multiple flybys of Enceladus, focusing on traversals of the pl…
The Cassini/Huygens mission provided new insights on the chemistry of the upper atmosphere of Titan. The presence of large molecules and ions (>100 s of amu) detected by Cassini at high altitude was not expected, and questions the original assumptions regarding the aerosol formation pathways. From recent laboratory studies, it has been shown that the inclusion of trace amounts of aromatic species drastically impacts the chemistry of aerosol formation and induces observable changes in the properties of the aerosols. In the present work we focus on the effect of one of the simplest nitrogenous a…
The detection of O2+ and O+ ions over Saturn's main rings by the Cassini INMS and CAPS instruments at Saturn orbit insertion (SOI) in 2004 confirmed the existence of the ring atmosphere and ionosphere. The source mechanism was suggested to be primarily photolytic decomposition of water ice producing neutral O2 and H2 (Johnson et al., 2006). Therefore, we predicted that there would be seasonal variations in the ring atmosphere and ionosphere due to the orientation of the ring plane to the sun (Tseng et al., 2010). The atoms and molecules scattered out of the ring atmosphere by ion-molecule coll…
The Cassini mission to the Saturn system discovered a plume of ice grains and water vapor erupting from cracks on the icy surface of the satellite Enceladus. This moon has a global ocean in contact with a rocky core beneath its icy exterior, making it a promising location to search for evidence of extraterrestrial life in the solar system. The previous detection of H$_2$ in the plume indicates that there is free energy available for methanogenesis, the metabolic reaction of H$_2$ with CO$_2$ to form methane and water. Additional metabolic pathways could provide sources of energy in Enceladus' …
The spatial distribution of the tropospheric methane on Titan was measured using near-infrared spectroscopy. Ground-based observations at 1.5$μ{\rm m}$ (H-band) were performed during the same night using instruments with adaptive optics at both the W. M. Keck Observatory and at the Paranal Observatory on 17 July 2014 UT. The integral field observations with SINFONI on the VLT covered the entire H-band at moderate resolving power, $R=λ/Δλ\approx1,500$, while the Keck observations were performed with NIRSPAO near 1.55254$μ{\rm m}$ at higher resolution, $R\approx25,000$. The moderate resolution o…
