Titan's Prolific Propane: The Cassini CIRS Perspective

In this paper we select large spectral averages of data from the Cassini Composite Infrared Spectrometer (CIRS) obtained in limb-viewing mode at low latitudes (30S--30N), greatly increasing the path length and hence signal-to-noise ratio for optically thin trace species such as propane. By modeling and subtracting the emissions of other gas species, we demonstrate that at least six infrared bands of propane are detected by CIRS, including two not previously identified in Titan spectra. Using a new line list for the range 1300-1400cm -1, along with an existing GEISA list, we retrieve propane abundances from two bands at 748 and 1376 cm-1. At 748 cm-1 we retrieve 4.2 +/- 0.5 x 10(-7) (1-sigma error) at 2 mbar, in good agreement with previous studies, although lack of hotbands in the present spectral atlas remains a problem. We also determine 5.7 +/- 0.8 x 10(-7) at 2 mbar from the 1376 cm-1 band - a value that is probably affected by systematic errors including continuum gradients due to haze and also an imperfect model of the n6 band of ethane. This study clearly shows for the first time the ubiquity of propane's emission bands across the thermal infrared spectrum of Titan, and points to an urgent need for further laboratory spectroscopy work, both to provide the line positions and intensities needed to model these bands, and also to further characterize haze spectral opacity. The present lack of accurate modeling capability for propane is an impediment not only for the measurement of propane itself, but also for the search for the emissions of new molecules in many spectral regions.Comment: 7 Figures, 3 Tables. Typeset in Latex with elsart.cls. In press for Planetary and Space Scienc

TNOs are cool: a survey of the transneptunian region

Over one thousand objects have so far been discovered orbiting beyond Neptune. These trans-Neptunian objects (TNOs) represent the primitive remnants of the planetesimal disk from which the planets formed and are perhaps analogous to the unseen dust parent-bodies in debris disks observed around other main-sequence stars. The dynamical and physical properties of these bodies provide unique and important constraints on formation and evolution models of the Solar System. While the dynamical architecture in this region (also known as the Kuiper Belt) is becoming relatively clear, the physical properties of the objects are still largely unexplored. In particular, fundamental parameters such as size, albedo, density and thermal properties are difficult to measure. Measurements of thermal emission, which peaks at far-IR wavelengths, offer the best means available to determine the physical properties. While Spitzer has provided some results, notably revealing a large albedo diversity in this population, the increased sensitivity of Herschel and its superior wavelength coverage should permit profound advances in the field. Within our accepted project we propose to perform radiometric measurements of 139 objects, including 25 known multiple systems. When combined with measurements of the dust population beyond Neptune (e.g. from the New Horizons mission to Pluto), our results will provide a benchmark for understanding the Solar debris disk, and extra-solar ones as well

Saturn Atmospheric Structure and Dynamics

2 Saturn inhabits a dynamical regime of rapidly rotating, internally heated atmospheres similar to Jupiter. Zonal winds have remained fairly steady since the time of Voyager except in the equatorial zone and slightly stronger winds occur at deeper levels. Eddies supply energy to the jets at a rate somewhat less than on Jupiter and mix potential vorticity near westward jets. Convective clouds exist preferentially in cyclonic shear regions as on Jupiter but also near jets, including major outbreaks near 35°S associated with Saturn electrostatic discharges, and in sporadic giant equatorial storms perhaps generated from frequent events at depth. The implied meridional circulation at and below the visible cloud tops consists of upwelling (downwelling) at cyclonic (anti-cyclonic) shear latitudes. Thermal winds decay upward above the clouds, implying a reversal of the circulation there. Warm-core vortices with associated cyclonic circulations exist at both poles, including surrounding thick high clouds at the south pole. Disequilibrium gas concentrations in the tropical upper troposphere imply rising motion there. The radiative-convective boundary and tropopause occur at higher pressure in the southern (summer) hemisphere due to greater penetration of solar heating there. A temperature “knee ” of warm air below the tropopause, perhaps due to haze heating, is stronger in the summer hemisphere as well. Saturn’s south polar stratosphere is warmer than predicted by radiative models and enhanced in ethane, suggesting subsidence-driven adiabatic warming there. Recent modeling advances suggest that shallow weather laye

The Structure and Dynamics of Titan's Middle Atmosphere and Troposphere

Titan, after Venus, is the second example in the solar system of an atmosphere with a global cyclostrophic circulation. The origin and maintenance of these superrotating atmospheres is not well understood, but Titan has a strong seasonal modulation in the middle atmosphere, and the seasonal changes in the winds may offer clues. The pole in winter and early spring is characterized by temperatures 20-30 K cooler at 140-170 km than those at low latitudes, and strong circumpolar winds as high as 190 m/s at 200- 250 km. At these levels the polar region is characterized by enhanced concentrations of several organic gases, and also detectable condensates. All this suggests that the polar vortex provides a mixing barrier between winter polar and lower-latitude air masses, analogous to the polar ozone holes on Earth. Because the concentrations of organic gases increase with altitude in the middle atmosphere, the observed enhancements suggest subsidence over the winter pole. Consistent with this are the observed temperatures approximately 200 K at the winter-polar stratopause (280 km), making it the warmest part of the atmosphere. The warm stratopause likely results from adiabatic heating associated with the subsidence. Recent observations in late northern winter and early spring indicate that the warm anomaly at the winter-polar stratopause is weakening;. In contrast to the middle atmosphere, latitude contrasts in tropospheric temperatures are muted. During the northern winter season, they were approximately 5 K at the tropopause and 3 K or less near the surface, being coldest at high northern latitudes. This is understandable in terms of the long radiative relaxation times in the troposphere, compared to times that are much shorter than a season in the upper stratosphere and higher. Curiously, the transition between the small meridional contrast (and presumably seasonal variations) in temperatures observed in the troposphere and the large variations observed at higher altitudes occurs abruptly above 80 km. Here the temperatures in the lower stratosphere, generally increasing with altitude, exhibit a sudden drop with increasing altitude at high northern latitudes, producing the contrast between low and high northern winter latitudes in the upper stratosphere described above. While the radiative relaxation time associated with infrared gaseous coolants decreases with altitude in the stratosphere, the abrupt transition suggests the presence of an optically thick condensate at thermal-infrared wavelengths. Near the surface, temperature lapse rates are adiabatic over the lowest 2 km, with the suggestion of a nocturnal stable inversion over the lowest 200 m in radio-occultation soundings near the morning terminator. At mid and high latitudes in both winter and summer hemispheres, the profiles are more statically stable (i.e., subadiabatic). This is most pronounced in the winter hemisphere

Titan's Winter Polar Vortex

Titan's atmosphere has provided an interesting study in contrasts and similarities with Earth's. While both have N$_2$ as the dominant constituent and comparable surface pressures $\sim1$ bar, Titan's next most abundant molecule is CH$_4$, not O$_2$, and the dissociative breakup of CH$_4$ and N$_2$ by sunlight and electron impact leads to a suite of hydrocarbons and nitriles, and ultimately the photochemical smog that enshrouds the moon. In addition, with a 15.95-day period, Titan is a slow rotator compared to Earth. While the mean zonal terrestrial winds are geostrophic, Titan's are mostly cyclostrophic, whipping around the moon in as little as 1 day. Despite the different dynamical regime, Titan's winter stratosphere exhibits several characteristics that should be familiar to terrestrial meteorologists. The cold winter pole near the 1 -mbar level is circumscribed by strong winds (up to 190 m/s) that act as a barrier to mixing with airmasses at lower latitudes. There is evidence of enhancement of several organic species over the winter pole, indicating subsidence. The adiabatic heating associated with this subsidence gives rise to a warm anomaly at the 0.01-mbar level, raising the stratopause two scale heights above its location at equatorial latitudes. Condensate ices have been detected in Titan's lower stratosphere within the winter polar vortex from infrared spectra. Although not always unambiguously identified, their spatial distribution exhibits a sharp gradient, decreasing precipitously across the vortex away from the winter pole. The interesting question of whether there is important heterogeneous chemistry occurring within the polar vortex, analogous to that occurring in the terrestrial polar stratospheric clouds in the ozone holes, has not been addressed. The breakup of Titan's winter polar vortex has not yet been observed. On Earth, the polar vortex is nonlinearly disrupted by interaction with large-amplitude planetary waves. Large-scale waves have not been identified in Titan's atmosphere, so the decay of its polar vortex may be more gradual than on Earth. Observations from an extended Cassini mission into late northern spring should provide critical data indicating whether the vortex goes away with a bang or just fades away

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

Titan's neutral atmosphere seasonal variations up to the end of the Cassini mission

In this paper we report 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 recent previous papers (Coustenis et al., 2016, 2018), we have 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 mbar–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. 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. 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. © 2019 Elsevier Inc

Titan's neutral atmosphere seasonal variations up to the end of the Cassini mission

International audienceIn this paper we report 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 recent previous papers (Coustenis et al., 2016, 2018), we have 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 mbar-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. 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. 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

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

International audienc

Titan Surface Temperatures as Measured by Cassini CIRS

Thermal radiation from the surface of Titan reaches space through a spectral window of low opacity at 19-microns wavelength. This radiance gives a measure of the brightness temperature of the surface. Composite Infrared Spectrometer' (CIRS) observations from Cassini during its first four years at Saturn have permitted latitude mapping of zonally averaged surface temperatures. The measurements are corrected for atmospheric opacity using the dependence of radiance on emission angle. With the more complete latitude coverage and much larger dataset of CIRS we have improved upon the original results from Voyager IRIS. CIRS measures the equatorial surface brightness temperature to be 93.7+/-0.6 K, the same as the temperature measured at the Huygens landing site. The surface brightness temperature decreases by 2 K toward the south pole and by 3 K toward the north pole. The drop in surface temperature between equator and north pole implies a 50% decrease in methane saturation vapor pressure and relative humidity; this may help explain the large northern lakes. The H2 mole fraction is derived as a by-product of our analysis and agrees with previous results. Evidence of seasonal variation in surface and atmospheric temperatures is emerging from CIRS measurements over the Cassini mission