Seasonal changes in Saturn’s stratosphere inferred from Cassini/CIRS limb observations

International audienceWe present temperature and hydrocarbons abundances (C 2 H 6 , C 2 H 2 , C 3 H 8) retrieved from Cassini/CIRS limb spectra, acquired during northern spring in 2010 (L S = 12 •) and 2012 (L S = 31 •). We compare them to the previous limb measurements performed by Guerlet et al. (2009) during northern winter. The latitudinal coverage (from 79 • N to 70 • S) and the sensitivity of our observations to a broad range of pressure levels (from 20 hPa to 0.003 hPa) allow us to probe the meridional and vertical structure of Saturn's stratosphere during northern spring. Our results show that in the northern hemisphere, the lower stratosphere (1 hPa) has experienced the strongest warming from northern winter to spring (11 ± 1.1 0.9 K), while the southern hemisphere exhibits weak variations of temperature at the same pressure level. We investigate the radiative contribution in the thermal seasonal evolution by comparing these results to the radiative-convective model of Guerlet et al. (2014). We show that radiative heating and cooling by atmospheric minor constituents is not always sufficient to reproduce the measured variations of temperature (depending on the pressure level). The measurements of the hydrocarbons abundances and their comparison with the predictions of the 1D photochemical model of Moses and Greathouse (2005) give insights into large scale atmospheric dynamics. At 1 hPa, C 2 H 6 , C 2 H 2 , and C 3 H 8 abundances are remarkably constant from northern winter to spring. At the same pressure level, C 2 H 6 and C 3 H 8 exhibit homogeneous meridional distributions unpredicted by this photochemical model, unlike C 2 H 2. This is consistent with the existence of a meridional circulation at 1 hPa, as suggested by previous studies

The distribution of methane in Titan’s stratosphere from Cassini/CIRS observations

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Analysis of Cassini/CIRS limb spectra of Titan acquired during the nominal mission. I. Hydrocarbons, nitriles and CO2 vertical mixing ratio profiles

International audienceObservations of the Composite InfraRed Spectrometer (CIRS) during the entire nominal Cassini mission (2004-2008) provide us with an accurate global view of composition and temperature in the middle atmosphere of Titan (between 100 and 500 km). We investigated limb spectra acquired at 0.5 cm- 1 resolution at nine different latitudes between 56°S and 80°N, with a better sampling in the northern hemisphere where molecular abundances and temperature present strong latitudinal variations. From this limb data acquired between February 2005 and May 2008, we retrieved the vertical mixing ratio profiles of C2H2, C2H4, C2H6, C3H8, CH3C2H, C4H2, C6H6, HCN, HC3N and CO2. We present here for the first time, the latitudinal variations of the C2H6, C3H8, CO2, C2H4 and C6H6 vertical mixing ratios profiles. Some molecules, such as C2H6 or C3H8 present little variations above their condensation level. The other molecules (except CO2) show a significant enhancement of their mixing ratios poleward of 50°N. C2H4 is the only molecule whose mixing ratio decreases with height at latitudes below 46°N. Regions depleted in C2H2, HCN and C4H2 are observed around 400 km (0.01 mbar) and 55°N. We also inferred a region enriched in CO2 located between 30 and 40°N in the 2-0.7 mbar pressure range. At 80°N, almost all molecules studied here present a local minimum of their mixing ratio profiles near 300 km (~0.07 mbar), which is in contradiction with Global Circulation Models that predict constant-with-height vertical profiles due to subsidence at the north pole. © 2009 Elsevier Inc. All rights reserved

Titan trace gaseous composition from CIRS at the end of the Cassini-Huygens prime mission

This paper reports on the results from an extensive study of all nadir-looking spectra acquired by Cassini/CIRS during the 44 flybys performed in the course of the nominal mission (2004-2008). With respect to the previous study (Coustenis, A., and 24 colleagues [2007]. Icarus 189, 35-62, on flybys TB-T10) we present here a significantly richer dataset with, in particular, more data at high northern and southern latitudes so that the abundances inferred here at these regions are more reliable. Our enhanced high-resolution dataset allows us to infer more precisely the chemical composition of Titan all over the disk. We also include improved spectroscopic data for some molecules and updated temperature profiles. The latitudinal distributions of all of the gaseous species are inferred. We furthermore test vertical distributions essentially for acetylene (C2H2) from CIRS limb-inferred data and from current General Circulation Models for Titan and compare our results on all the gaseous abundances with predictions from 1-D photochemical-radiative models to check the reliability of the chemical reactions and pathways. © 2009 Elsevier Inc

Water vapor in Titan's stratosphere from Cassini CIRS far-infrared spectra

Here we report the measurement of water vapor in Titan's stratosphere using the Cassini Composite Infrared Spectrometer (CIRS, Flasar, F.M. et al. [2004]. Space Sci. Rev. 115, 169-297). CIRS senses water emissions in the far infrared spectral region near 50μm, which we have modeled using two independent radiative transfer codes (NEMESIS (Irwin, P.G.J. et al. [2008]. J. Quant. Spectrosc. Radiat. Trans. 109, 1136-1150) and ART (Coustenis, A. et al. [2007]. Icarus 189, 35-62; Coustenis, A. et al. [2010]. Icarus 207, 461-476). From the analysis of nadir spectra we have derived a mixing ratio of 0.14±0.05ppb at an altitude of 97km, which corresponds to an integrated (from 0 to 600km) surface normalized column abundance of 3.7±1.3×10 14molecules/cm 2. In the latitude range 80°S to 30°N we see no evidence for latitudinal variations in these abundances within the error bars. Using limb observations, we obtained mixing ratios of 0.13±0.04ppb at an altitude of 115km and 0.45±0.15ppb at an altitude of 230km, confirming that the water abundance has a positive vertical gradient as predicted by photochemical models (e.g. Lara, L.M., Lellouch, F., Lopez-Moreno, J.J., Rodrigo, R. [1996]. J. Geophys. Res. 101(23), 261; Wilson, E.H., Atreya, S.K. [2004]. J. Geophys. Res. 109, E6; Hörst, S.M., Vuitton, V., Yelle, R.V. [2008]. J. Geophys. Res., 113, E10). We have also fitted our data using scaling factors of ~0.1-0.6 to these photochemical model profiles, indicating that the models over-predict the water abundance in Titan's lower stratosphere. © 2012 Elsevier Inc.

EVOLUTION OF THE FAR-INFRARED CLOUD AT TITAN'S SOUTH POLE

A condensate cloud on Titan identified by its 220 cm-1 far-infrared signature continues to undergo seasonal changes at both the north and south poles. In the north, the cloud, which extends from 55 N to the pole, has been gradually decreasing in emission intensity since the beginning of the Cassini mission with a half-life of 3.8 years. The cloud in the south did not appear until 2012 but its intensity has increased rapidly, doubling every year. The shape of the cloud at the south pole is very different from that in the north. Mapping in 2013 December showed that the condensate emission was confined to a ring with a maximum at 80 S. The ring was centered 4° from Titan's pole. The pattern of emission from stratospheric trace gases like nitriles and complex hydrocarbons (mapped in 2014 January) was also offset by 4°, but had a central peak at the pole and a secondary maximum in a ring at about 70 S with a minimum at 80 S. The shape of the gas emission distribution can be explained by abundances that are high at the atmospheric pole and diminish toward the equator, combined with correspondingly increasing temperatures. We discuss possible causes for the condensate ring. The present rapid build up of the condensate cloud at the south pole is likely to transition to a gradual decline from 2015 to 2016. © 2015. The American Astronomical Society. All rights reserved

Kronos: exploring the depths of Saturn with probes and remote sensing through an international mission

Kronos is a mission aimed to measure in situ the chemical and isotopic compositions of the Saturnian atmosphere with two probes and also by remote sensing, in order to understand the origin, formation, and evolution of giant planets in general, including extrasolar planets. The abundances of noble gases, hydrogen, carbon, nitrogen, oxygen, sulfur and their compounds, as well as of the D/H, 4He/3He, 22Ne/21Ne/20Ne, 36Ar/38Ar, 13C/12C, 15N/14N, 18O/(17O)/16O, 136Xe/134Xe/132Xe/130Xe/129Xe isotopic ratios will be measured by mass spectrometry on two probes entering the atmosphere of Saturn at two different locations near mid-latitudes, down to a pressure of 10 Bar. The global composition of Saturn will be investigated through these measurements, together with microwave radiometry determination of H2O and NH3 and their 3D variations. The dynamics of Saturn’s atmosphere will be investigated from: (1) measurements of pressure, temperature, vertical distribution of clouds and wind speed along the probes’ descent trajectories, and (2) determination of deep winds, differential rotation and convection with combined probe, gravity and radiometric measurements. Besides these primary goals, Kronos will also measure the intensities and characteristics of Saturn’s magnetic field inside the D ring as well as Saturn’s gravitational field, in order to constrain the abundance of heavy elements in Saturn’s interior and in its central core. Depending on the preferred architecture (flyby versus orbiter), Kronos will be in a position to measure the properties of Saturn’s innermost magnetosphere and to investigate the ring structure in order to understand how these tiny structures could have formed and survived up to the present times

Kronos: Exploring the depths of Saturn with probes and remote sensing through an international mission

Kronos is a mission aimed to measure in situ the chemical and isotopic compositions of the Saturnian atmosphere with two probes and also by remote sensing, in order to understand the origin, formation, and evolution of giant planets in general, including extrasolar planets. The abundances of noble gases, hydrogen, carbon, nitrogen, oxygen, sulfur and their compounds, as well as of the D/H, 4He/3He, 22Ne/21Ne/20Ne, 36Ar/38Ar, 13C/12C, 15N/14N, 18O/(17O)/16O, 136Xe/134Xe/132Xe/130Xe/129Xe isotopic ratios will be measured by mass spectrometry on two probes entering the atmosphere of Saturn at two different locations near mid-latitudes, down to a pressure of 10 Bar. The global composition of Saturn will be investigated through these measurements, together with microwave radiometry determination of H2O and NH3 and their 3D variations. The dynamics of Saturn's atmosphere will be investigated from: (1) measurements of pressure, temperature, vertical distribution of clouds and wind speed along the probes' descent trajectories, and (2) determination of deep winds, differential rotation and convection with combined probe, gravity and radiometric measurements. Besides these primary goals, Kronos will also measure the intensities and characteristics of Saturn's magnetic field inside the D ring as well as Saturn's gravitational field, in order to constrain the abundance of heavy elements in Saturn's interior and in its central core. Depending on the preferred architecture (flyby versus orbiter), Kronos will be in a position to measure the properties of Saturn's innermost magnetosphere and to investigate the ring structure in order to understand how these tiny structures could have formed and survived up to the present times. © Springer Science + Business Media B.V. 2008