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The bolometric Bond albedo is a fundamental parameter of planets and moons. Here, combined observations from the Cassini spacecraft and the Hubble Space Telescope are used to determine the bolometric Bond albedo of Enceladus. We provide the full-disk reflectance of Enceladus across all phase angles (0° -180°) from 150 nm to 5131 nm, a spectral range that accounts for nearly all incoming solar power. Considering the distribution of the monochromatic Bond albedo over wavelength, we find a value of 0.76 ± 0.03 for Enceladus' bolometric Bond albedo. The corresponding optical characteristics (e.g., geometric albedo and phase function), which are closely related to Enceladus' surface properties, are also investigated. The wavelength-dependent nature of Enceladus' Bond albedo suggests that the bolometric Bond albedos of other icy moons, if they are mainly determined by the visible observations only, should be carefully considered. Our new measurements of bolometric Bond albedo can be used to better determine the radiant energy budget of Enceladus and further constrain the internal heat flow, a critical driving force for the water plumes on Enceladus.

Water vapour in the stratospheres of Uranus and Neptune has previously been shown to originate from external sources. These sources could include comet impacts [4], interplanetary dust particles [8], or rings and moons [1]. Stratospheric water was first detected on Uranus and Neptune by the Short-Wavelength Spectrometer (SWS) on the Infrared Space Observatory (ISO) [2], but the uncertainties were relatively large due to lack of constraint on the vertical water profiles and relatively low spectral resolution of the observations. Here we present new observational constraints on Uranus’ and Neptune’s externally sourced stratospheric water abundance using disc-averaged high spectral resolution observations of the 557 GHz water emission line from Herschel’s Heterodyne Instrument for the Far-Infrared (HIFI). On both planets the emission line is significantly broadened by disc-averaging of Doppler shifts from planetary rotation, which was carefully accounted for in our analysis [10]. Derived stratospheric column water abundances are 0.56+0.26-0.06 x 1014 cm-2 for Uranus and 1.9+0.2-0.3 x 1014 cm-2 for Neptune. These results imply Neptune has about four times as much stratospheric water as Uranus, and are consistent with previous determinations from ISO-SWS and Herschel-PACS, but with improved precision. For Uranus excellent observational fits are obtained by scaling photochemical model profiles [3,7] or with step-type profiles with water vapor limited to =1.0 mbar or +4.0-0.9 x 104 cm-2s-1 for Uranus and 12.7+1.3-2.0 x 104 cm-2s-1 for Neptune. This inferred water influx rates suggest that Uranus and Neptune may in fact have very similar IDP fluxes, unless there are significant water-loss processes that are not accounted for in current photochemical models [3,7]. This is unexpected as the IDP flux on Neptune is expected to be higher due to its closer proximity to the Kuiper belt. For example, the dynamical model of [8] predicts that the flux of IDP grains is around seven times higher on Neptune than on Uranus, but model uncertainties are large enough so as not to preclude a similar flux. The comet impact rates on Uranus and Neptune are also predicted to be quite similar [5,11], so both planets may experience similar external flux processes. Our new analysis suggests that Neptune’s approximately four times greater observed water column abundance is primarily caused by its warmer stratosphere preventing loss by condensation, rather than by a significantly more intense external source. Larger error bars on the Uranus estimates are due to greater uncertainty in the high-altitude temperature profile. To reconcile these water fluxes with other observed stratospheric oxygen species (CO and CO2) requires either a significant CO component in interplanetary dust particles (Uranus) or contributions from cometary impacts (Uranus, Neptune). In particular, the large CO abundance in Neptune’s stratosphere suggests that we just happen to be observing Neptune at a time shortly after a large comet impact [4,6,9]. Further details of our results and analysis are available in our recent publication [10]. Fig1: Herschel-HIFI line-to-continuum ratio spectra of the 557GHz water line for HRS and WBS spectrometers. The water line is clearly visible at high signal to noise on both planets, but the line is broadened due to Doppler shift combined with the disc-broadened nature of the HIFI spectra. (Figure from https://doi.org/10.3847/PSJ/ac650f, see reference [10]). Fig2: Fits to Uranus and Neptune HIFI-HRS spectra 557GHz water line. (a,b) Uranus can be fitted with step profiles with a step pressure less than ~0.6mbar or by scaling photochemical profiles. However, significant water vapour is unlikely at pressures above ~0.03mbar due to saturation. (c,d) Neptune can be fitted with step profiles with a step in the pressure range 0.3-0.6mbar or by scaling photochemical profiles. (Figure from https://doi.org/10.3847/PSJ/ac650f, see reference [10]). References [1] Cavalié+ 2019. https://ui.adsabs.harvard.edu/abs/2019A%26A...630A..87C/abstract [2] Feuchtgruber+ 1997. https://ui.adsabs.harvard.edu/abs/1997Natur.389..159F/abstract [3] Lara+ 2019. https://ui.adsabs.harvard.edu/abs/2019A%26A...621A.129L/abstract [4] Lellouch+ 2005. https://ui.adsabs.harvard.edu/abs/2005A%26A...430L..37L/abstract [5] Levison 2000. https://ui.adsabs.harvard.edu/abs/2000Icar..143..415L/abstract [6] Moreno+ 2017. https://ui.adsabs.harvard.edu/abs/2017A%26A...608L...5M/abstract [7] Moses+Poppe 2017. https://ui.adsabs.harvard.edu/abs/2017Icar..297...33M/abstract [8] Poppe 2016. https://ui.adsabs.harvard.edu/abs/2016Icar..264..369P/abstract [9] Teanby+ 2019. https://ui.adsabs.harvard.edu/abs/2019Icar..319...86T/abstract [10] Teanby+ 2022. https://ui.adsabs.harvard.edu/abs/2022PSJ.....3...96T/abstract [11] Zahnle 2003. https://ui.adsabs.harvard.edu/abs/2003Icar..163..263Z/abstract

In July 2017, we used the Texes high-resolution spectrometer at the NASA Infrared Telescope Facility to observe Titan in the 19.3-µm (519 cm-1) region where the ν4 band of deuterated acetylene (C2HD) is located. Six individual lines were clearly detected with a S/N ratio up to 10. Spectral intervals around 8.0 (745 cm-1) and 13.4 µm (1247 cm-1) were observed during the same run to constrain the disk-averaged temperature profile and acetylene (C2H2) abundance profile respectively. Telluric correction and flux calibration were obtained from observations of asteroid Hygiea. Constraints from Cassini/CIRS observations in 2017 around the sub-Earth latitude were also used to constrain the atmospheric model and check the flux calibration. The D/H ratio is derived from the C2HD/C2H2 abundance ratio. Preliminary results of this analysis indicate a D/H ratio in acetylene around 1.7×10-5 with an uncertainty of about 10%, in agreement with the previous, less precise, determination from Cassini/CIRS measurements. This ratio is slightly larger than that in methane (approximately 1.3×10-5), which suggests some fractionation at work in the photochemical production of acetylene from methane.

Icy moons of Jupiter and Saturn are privileged targets in currently developed space missions by several space agencies and in particular ESA, NASA and their partners. One of these missions is ESA’s first large mission in the Cosmic Vision Programme, JUICE [1], which is being developed to address questions regarding the Jupiter system and its satellites, with a focus on the largest moon, Ganymede. The overarching theme for JUICE is the emergence of habitable worlds around gas giants taking into account the requirements involving the presence of organic compounds, trace elements, water, energy sources and a relative stability of the environment over time.Among other, JUICE will determine the characteristics (composition and dynamics) of the exospheres of the icy moons [2], in particular Ganymede and Europa, with for instance coordinated observations among sets of instruments like UVS, PEP, RPWI, MAJIS, 3GM, J-MAG, JANUS and SWI. The JUICE mission is scheduled to be launched in spring 2023 and arrive at Jupiter in mid-2031 and is foreseen to last nominally for 3 and a half years. JUICE investigations will benefit from current observations by JUNO and will be synergistic to NASA’s Europa Clipper mission. I will describe the foreseen investigations of the tenuous atmospheres of the icy moons around Jupiter.Cassini explored the dense and organic-laden atmosphere of Titan during several flybys over 13 years [2,3] and also determined the characteristics of the Enceladus plumes. However, new questions have risen and several cold cases [4] remain that will constitute major science objectives for future space missions to the satellites around Saturn, like Dragonfly [5] or an orbiter in the Saturn system or a dedicated Enceladus mission…In particular, Titan’s atmosphere has not yet revealed all its secrets, in particular for the chemical composition, which should be much more complex than what was detected by Cassini-Huygens. Future in situ measurements will be extremely useful in unveiling this unique complex world. In the meantime, ground-based observations with large telescopes like ALMA, elsewhere in Chile or the ones in Hawaii can help complement the past discoveries.References:[1] Coustenis, A., Witasse, O., Erd, C., 2021. The JUICE mission: expectations and challenges. Fall issue of The Bridge on space exploration, Sept. 2021, Vol. 51, issue #3, pp. 41-50.[2] Coustenis, A., Tokano, T., Burger, M. H., Cassidy, T. A., Lopes, R. M., Lorenz, R. D., Retherford, K. D., Schubert G., 2010. Atmospheres/exospheres characteristics of icy satellites. Space Sci. Rev., 153, 155-184. https://www.nae.edu/260902/The-JUICE-Mission-Challenges-and-Expectations[3] Coustenis, A., 2021. “The Atmosphere of Titan”. In Read, P. (Ed.), Oxford Research Encyclopedia of Planetary Science. Oxford University Press (August 31). doi: https://doi.org/10.1093/acrefore/9780190647926.013.120[4] Nixon, C. A., Lorenz, R. D., Achterberg, R. K., et al. (2018). Titan's cold case files - Outstanding questions after Cassini-Huygens. Planetary and Space Science, 155, 50-72.[5] Barnes, J. et al. (2021). Science Goals and Objectives for the Dragonfly Titan Rotorcraft Relocatable Lander. The Plan. Sci. J., VoL. 2, Issue 4, id.130, 18 pp.

Titan’s atmosphere and surface (a complex system) evolve with season, as Titan follows Saturn in its orbit around the Sun for 30 years with an inclination of about 27°. We performed an analysis of spectra acquired by Cassini/CIRS at high resolution covering the range from 600 to 1500 cm-1 since the beginning and until the last flyby of Titan in 2017 and describe the temperature and chemical composition variations ([1-3]. By applying our radiative transfer code (ARTT) to the high-resolution CIRS spectra we study the stratospheric evolution over almost two Titan seasons [1,2], corresponding to the Cassini mission duration. CIRS nadir and limb spectral together show variations in temperature and chemical composition in the stratosphere during the Cassini mission, before and after the Northern Spring Equinox (NSE) and also during one Titan year. Since the 2010 equinox we have thus reported on monitoring of Titan’s stratosphere near the poles and in particular on the observed strong temperature decrease and compositional enhancement above Titan’s southern polar latitudes since 2012 and until 2014 of several trace species, such as complex hydrocarbons and nitriles, which were previously observed only at high northern latitudes. This effect followed the transition of Titan’s seasons from northern winter in 2002 to northern summer in 2017, while at that latter time the southern hemisphere was entering winter. Our data show a continued decrease of the abundances which we first reported to have started in 2015. The 2017 data we have acquired and analyzed here are important because they are the only ones recorded since 2014 close to the south pole in the far-infrared nadir mode at high resolution. A large temperature increase in the southern polar stratosphere (by 10-50 K in the 0.5 mbar-0.05 mbar pressure range) is found and a change in the temperature profile’s shape. The 2017 observations also show a related significant decrease in most of the abundances which must have started sometime between 2014 and 2017 [3]. In our work, we show that the equatorial latitudes remain rather constant throughout the Cassini mission. We have thus shown that the south pole of Titan is now losing its strong enhancement, while the north pole also slowly continues its decrease in gaseous opacities. It would have been interesting to see when this might happen, but the Cassini mission ended in September 2017. Perhaps future ground-based measurements and the Dragonfly mission can pursue this investigation and monitor Titan’s atmosphere to characterize the seasonal events. Our results set constraints on GCM and photochemical models. References: [1] Coustenis et al., 2016, Icarus 270, 409-420 [2] Coustenis et al., 2018, Astroph. J., Lett., 854, no2 [3] Coustenis et al., 2020. Titan’s neutral atmosphere seasonal variations up to the end of the Cassini mission. Icarus 344, 113413. https://doi.org/10.1016/j.icarus.2019.113413.

The NASA Ice Giants Pre-Decadal Survey Mission Report (2017) recommended the high scientific importance of sending a mission with an orbiter and a probe to one of the Ice Giants, with preferential launch dates in the 2029-2034 timeframe. Such a mission concept is equally well supported by European scientists and Mousis et al (P&SS, 155, 12, 2018) give compelling scientific rationales for the exploration of these worlds with missions carrying in situ probes. In this presentation we will outline the conceptual design of the Advanced Ice Giants Net Flux Radiometer (IG-NFR) instrument, currently being designed by NASA Goddard Space Flight Center to make in situ observations of the upward and downward fluxes of solar and thermal radiation in the atmospheres of Uranus and Neptune. The IG-NFR is designed to: (i) accommodate seven filter bandpass channels in the spectral range 0.25-300 µm (ii) measure up and down radiation flux in a clear unobstructed 10° FOV for each channel; (iii) use thermopile detectors that can measure a change of flux of at least 0.5 W/m2 per decade of pressure; (iv) view five distinct view angles (±80°, ±45°, and 0°); (v) predict the detector response with changing temperature environment; (vi) use application-specific integrated circuit technology for the thermopile detector readout; (vii) be able to integrate radiance for 2s or longer, and (vii) sample each view angle including calibration targets. The IG-NFR system noise equivalent power at 298 K is 73 pW in a 1 Hz electrical bandwidth. We present initial simulations of the anticipated observations using two radiative transfer and retrieval tools, NEMESIS (Irwin et al., JQSRT, 109, 1136, 2008) and the Planetary Spectrum Generator (PSG, Villanueva et al., 2017, https://psg.gsfc.nasa.gov). For the NEMESIS modelling the radiative fluxes observable at varying pressure levels were calculated with a Matrix-Operator plane-parallel multiple-scattering model, using between 5 and 21 zenith angle quadrature points and up to 38 Fourier components for the azimuth decomposition. We also employed PSG to further validate our flux estimates, providing an important benchmarking and comparison test between both models. PSG solves the scattering radiative transfer employing the discrete ordinates method, with the scattering phase function described in terms of an expansion in terms of Legendre Polynomials. Molecular cross-sections are solved via the correlated-k method employing the latest HITRAN database (Gordon et al., 2017), which are completed with the latest collision-induced-absorption (CIA, Karman et al., 2019), and UV/optical cross-sections from the MPI database (Keller-Rudek et al., 2013). For the nominal case the Sun was assumed to be at an altitude of 10° above the horizon. The internal radiance field was calculated at each internal level for a standard reference Uranus atmosphere (e.g., Irwin et al., 2017) with the addition of a single cloud layer, based at 3 bar and composed of particles with a mean radius of 1.0 µm (and size variance 0.1) and assumed complex refractive index of 1.4 + 0.001i at all wavelengths. The opacity and fractional scale height of this cloud were fitted in both models to match the combined near-infrared observations of HST/WFC3, IRTF/SpeX and VLT/SINFONI analyzed by Irwin et al. (2017). The internal radiance fields were calculated from 0.4 to 300 µm using this atmospheric model. We will show how these simulations are being used to guide the choice of spectral filter bandwidths and centres to optimize the scientific return of such an instrument. We will show that observations with such an instrument can be used to constrain effectively the radiation energy budget in the atmospheres of the Ice Giants and can also be used to determine the pressures of cloud and haze layers and broadly constrain particle size. Such modelling also allows us to simulate the visible appearance of Uranus’ atmosphere during a descent and to perform detailed validations of the simulations by comparing the two radiative transfer models (NEMESIS and PSG).

In our recent publication [1] we reported new results concerning the seasonal atmospheric evolution near Titan’s poles and equator in terms of temperature and composition using nadir spectra acquired by the Cassini Composite Infrared Spectrometer (CIRS) at high spectral resolution during the last year of the Cassini mission in 2017 complementing previous investigations covering almost two Titan seasons. In previous papers [2,3], we reported on monitoring of Titan’s stratosphere near the poles after the mid-2009 northern spring equinox. In particular we have reported on the observed strong temperature decrease and compositional enhancement above Titan’s southern polar latitudes since 2012 and until 2014 of several trace species, such as complex hydrocarbons and nitriles, which were previously observed only at high northern latitudes. This effect accompanied the transition of Titan’s seasons from northern winter in 2002 to northern summer in 2017, while at that latter time, the southern hemisphere was entering winter. Our new data, acquired in 2017 and analyzed here, are important because they are the only ones recorded since 2014 close to the south pole in the mid-infrared nadir mode at high resolution. A large temperature increase in the southern polar stratosphere (by 10-50 K in the 0.1 to 0.01 mbar pressure range) is found associated with a change in the temperature profile’s shape. The 2017 observations also show a related significant decrease in most of the southern abundances which must have started sometime between 2014 and 2017 [1]. For the north, the spectra indicate a continuation of the decrease of the abundances which we first reported to have started in 2015 and small temperature variations [1]. We discuss comparisons with other results and with current photochemical and dynamical models which could be updated and improved by the new constraints set by the findings presented here. [1] Coustenis et al., 2019, Icarus 344, 1 July 2020, 113413 ; [2] Coustenis et al., 2016, Icarus 270, 409-420; [3] Coustenis et al., 2018, Astroph. J. Lett. 854, no2.

Using spectrally and spatially resolved observations of Titan made with the Atacama Large Millimeter/submillimeter Array (ALMA) in August 2016 and May 2017, we obtained global snapshot maps of emission lines from atmospheric CH3CN, HC3N and HNC gases. High-altitude wind fields (in the altitude range ~300-1000 km) were retrieved through Doppler mapping, confirming the existence of high-velocity superrotating zonal winds on Titan, with speeds in the range 180-390 m/s near the equator (speeds increasing with altitude). At the highest, thermospheric altitudes probed by HNC, a ~50% reduction in the equatorial zonal wind speed was measured over the 9-month period covered by these observations. This is consistent with a dramatic slowing of the recently-discovered thermospheric equatorial jet. The implications of this result for our understanding of Titan's global circulation, and the forcing mechanisms impacting the uppermost atmosphere, will be discussed. 1. Introduction Saturn's largest moon, Titan has an unusually thick, dense (1.45 bar) atmosphere comprised predominantly of molecular nitrogen and methane, subject to a complex atmospheric chemistry and seasonally-evolving pattern of winds and global circulation [1]. The presence of zonal winds on Titan was initially inferred from stellar occultation observations [2], and later confirmed by in-situ Doppler measurements during the descent of the Cassini-Huygens probe [3]. More recently, Lellouch et al. [4] took advantage of the extremently high sensitivity and resolution of the Atacama Large Millimeter/submillimeter Array (ALMA) to derive the first Doppler wind maps of Titan's thermosphere. An unexpectedly intense, superrotating equatorial jet was discovered at thermospheric altitudes (~1000 km), with a velocity of ~350 ms-1. Here we present new measurements of Titan's high-altitude wind field using ALMA observations from May 2017, compared with a re-analysis of the August 2016 data from Lellouch et al. [4]. 2. Results We used Gaussian fits to the observed sub-mm emission lines to determine line-of-sight velocities for the CH3CN, HC3N and HNC gases as a function of spatial coordinate across Titan. The resulting Doppler maps are shown in Figure 1, where the color of each pixel indicates the line-of-sight radial velocity, dominated by west-to-east (zonal) flow. The 2016 and 2017 ALMA data were convolved to a common (~1600 km) beam FWHM. Due to their differing vertical abundance distributions, the observed gases preferentially emit from different altitudes on Titan. The CH3CN molecule emits most strongly in the upper stratosphere, whereas HC3N is sensitive to the mesophere/thermosphere, and HNC probes the highest, thermospheric altitudes (around 1000 km) [4]. In 2016, there was a clear trend for increasing equatorial zonal wind speed with altitude, which is less apparent in the 2017 data. The greatest temporal variation is for HNC, which showed a dramatic (~ factor of two) reduction in wind speed over the 9-month period between the two epochs: after correcting for the obliquity of Titan's polar axis, the mean, beam-averaged equatorial HNC wind speed was 294±10 ms-1 in 2016 and 159±7 ms-1 in 2017. Deconvolution with respect to the finite ALMA spatial resolution element is required in order to derive the true wind speeds as a function of latitude, which will be presented in a future article [5]. 3. Conclusion Capitalizing on the high sensitivity and resolution of ALMA, we have generated Doppler maps of three different molecules, from which Titan's zonal wind field was derived as a function of latitude, altitude and time, in the range ~300-1000 km (upper stratosphere to thermosphere). Rapid changes in wind speeds were observed over a 9-month period from 2016-2017, the most striking of which is a ~50% drop in the velocity of the recently-discovered thermospheric equatorial jet. In this presentation, we will focus on the detailed wind field variations as a function of altitude and will discuss their implications for our understanding of Titan's global circulation, and the dynamical coupling between the lower and upper atmosphere. References: [1] Horst, S. M. 2017, Journal of Geophysical Research (Planets), 122, 432 [2] Hubbard, W. B., Sicardy, B., Miles, R. et al. 1993, A&A, 269, 541 [3] Bird, M. K., Allison, M., Asmar, S. W. et al. 2005, Nature, 438, 800 [4] Lellouch, E., Gurwell, M. A., Moreno, R. et al. 2019, Nature Astronomy, 3, 614 [5] Cordiner, M. A, Garcia-Berrios, E., Cosentino, R. et al. 2020, in prep.

Results are presented from our ongoing studies of Titan using ALMA during the period 2012-2015, including a confirmation of the previous detection of vinyl cyanide (C2H3CN), as well as the first spatial map for this species on Titan. Simultaneous mapping of HC3N, CH3CN and C2H5CN reveal characteristic abundance patterns for each species that provide insight into their individual photochemical lifetimes, and help inform our understanding of Titan’s unique, time-variable atmospheric chemistry and global circulation. A time-sequence of HC3N maps covering 38 months reveals a dramatic change in the distribution of this gas consistent with high-altitude photochemical production followed by advection towards the southern (winter) pole, combined with rapid loss in the north after Titan’s 2009 seasonal equinox. The 2015 C2H3CN and C2H5CN maps show abundance peaks in Titan’s southern hemisphere, similar to those observed for the short-lived HC3N molecule. The longer-lived CH3CN, on the other hand, remains more concentrated in the north.

We present spectrally and spatially-resolved maps of HNC and HC$_3$N emission from Titan's atmosphere, obtained using the Atacama Large Millimeter/submillimeter Array (ALMA) on 2013 November 17. These maps show anisotropic spatial distributions for both molecules, with resolved emission peaks in Titan's northern and southern hemispheres. The HC$_3$N maps indicate enhanced concentrations of this molecule over the poles, consistent with previous studies of Titan's photochemistry and atmospheric circulation. Differences between the spectrally-integrated flux distributions of HNC and HC$_3$N show that these species are not co-spatial. The observed spectral line shapes are consistent with HNC being concentrated predominantly in the mesosphere and above (at altitudes $z\gtrsim 400$ km), whereas HC$_3$N is abundant at a broader range of altitudes ($z\approx70$-600 km). From spatial variations in the HC$_3$N line profile, the locations of the HC$_3$N emission peaks are shown to be variable as a function of altitude. The peaks in the integrated emission from HNC and the line core (upper-atmosphere) component of HC$_3$N (at $z\gtrsim300$ km) are found to be asymmetric with respect to Titan's polar axis, indicating that the mesosphere may be more longitudinally-variable than previously thought. The spatially-integrated HNC and HC$_3$N spectra are modeled using the NEMESIS planetary atmosphere code and the resulting best-fitting disk-averaged vertical mixing ratio (VMR) profiles are found to be in reasonable agreement with previous measurements for these species. Vertical column densities of the best-fitting gradient models for HNC and HC$_3$N are $1.9\times10^{13}$ cm$^{-2}$ and $2.3\times10^{14}$ cm$^{-2}$, respectively., Accepted for publication in ApJ Letters