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Register a new userTitans Prolific Propane: The Cassini CIRS Perspective
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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 propanes 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.
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We have searched for the presence of simple P and S-bearing molecules in Titans atmosphere, by looking for the characteristic signatures of phosphine and hydrogen sulfide in infrared spectra obtained by Cassini CIRS. As a result we have placed the first upper limits on the stratospheric abundances, which are 1 ppb (PH3) and 330 ppb (H2S), at the 2-sigma significance level.
We report the detection of a spectral signature observed at 682 cm$^{-1}$ by the Cassini Composite Infrared Spectrometer (CIRS) in nadir and limb geometry observations of Titans southern stratospheric polar region in the middle of southern fall, while stratospheric temperatures are the coldest since the beginning of the Cassini mission. The 682 cm$^{-1}$ signature, which is only observed below an altitude of 300-km, is at least partly attributed to the benzene (C$_6$H$_6$) ice $ u_{4}$ C-H bending mode. While we first observed it in CIRS nadir spectra of the southern polar region in early 2013, we focus here on the study of nadir data acquired in May 2013, which have a more favorable observation geometry. We derived the C$_6$H$_6$ ice mass mixing ratio in 5{deg}S latitude bins from the south pole to 65{deg}S and infer the C$_6$H$_6$ cloud top altitude to be located deeper with increasing distance from the pole. We additionally analyzed limb data acquired in March 2015, which were the first limb dataset available after the May 2013 nadir observation, in order to infer a vertical profile of its mass mixing ratio in the 0.1 - 1 mbar region (250 - 170 km). We derive an upper limit of $sim$1.5 $mu$m for the equivalent radius of pure C$_6$H$_6$ ice particles from the shape of the observed emission band. Several other unidentified signatures are observed near 687 and 702 cm$^{-1}$ and possibly 695 cm$^{-1}$, which could also be due to ice spectral signatures as they are observed in the deep stratosphere at pressure levels similar to the C$_6$H$_6$ ice ones. We could not reproduce these signatures with pure nitrile ice (HCN, HC$_3$N,CH$_3$CN, C$_2$H$_5$CN and C$_2$N$_2$) spectra available in the literature except the 695 cm$^{-1}$ feature that could possibly be due to C$_2$H$_3$CN ice.
In this chapter we describe the remote sensing measurement of nitrogen-bearing species in Titans atmosphere by the Composite Infrared Spectrometer (CIRS) on the Cassini spacecraft. This instrument, which detects the thermal infrared spectrum from 10-1500 cm-1 (1000-7 microns) is sensitive to vibrational emissions of gases and condensates in Titans stratosphere and lower mesosphere, permitting the measurement of ambient temperature and the abundances of gases and particulates. Three N-bearing species are firmly detected: HCN, HC3N and C2N2, and their vertical and latitudinal distributions have been mapped. In addition, ices of HC3N and possibly C4N2 are also seen in the far-infrared spectrum at high latitudes during the northern winter. The HC(15)N isotopologue has been measured, permitting the inference of the 14N/15N ratio in this species, which differs markedly (lower) than in the bulk nitrogen reservoir (N2). We also describe the search in the CIRS spectrum, and inferred upper limits, for NH3 and CH3CN. CIRS is now observing seasonal transition on Titan and the gas abundance distributions are changing accordingly, acting as tracers of the changing atmospheric circulation. The prospects for further CIRS science in the remaining five years of the Cassini mission are discussed.
From 2004 to 2017, the Cassini spacecraft orbited Saturn, completing 127 close flybys of its largest moon, Titan. Cassinis Composite Infrared Spectrometer (CIRS), one of 12 instruments carried on board, profiled Titan in the thermal infrared (7-1000 microns) throughout the entire 13-year mission. CIRS observed on both targeted encounters (flybys) and more distant opportunities, collecting 8.4 million spectra from 837 individual Titan observations over 3633 hours. Observations of multiple types were made throughout the mission, building up a vast mosaic picture of Titans atmospheric state across spatial and temporal domains. This paper provides a guide to these observations, describing each type and chronicling its occurrences and global-seasonal coverage. The purpose is to provide a resource for future users of the CIRS data set, as well as those seeking to put existing CIRS publications into the overall context of the mission, and to facilitate future inter-comparison of CIRS results with those of other Cassini instruments, and ground-based observations.
Simulations of Titans atmospheric transmission and surface reflectivity have been developed in order to estimate how Titans atmosphere and surface properties could affect performances of the Cassini radar experiment. In this paper we present a selection of models for Titans haze, vertical rain distribution, and surface composition implemented in our simulations. We collected dielectric constant values for the Cassini radar wavelength ($sim 2.2$ cm) for materials of interest for Titan: liquid methane, liquid mixture of methane-ethane, water ice and light hydrocarbon ices. Due to the lack of permittivity values for Titans haze particles in the microwave range, we performed dielectric constant ($varepsilon_r$) measurements around 2.2 cm on tholins synthesized in laboratory. We obtained a real part of $varepsilon_r$ in the range of 2-2.5 and a loss tangent between $10^{-3}$ and $5.10^{-2}$. By combining aerosol distribution models (with hypothetical condensation at low altitudes) to surface models, we find the following results: (1) Aerosol-only atmospheres should cause no loss and are essentially transparent for Cassini radar, as expected by former analysis. (2) However, if clouds are present, some atmospheric models generate significant attenuation that can reach $-50 dB$, well below the sensitivity threshold of the receiver. In such cases, a $13.78 GHz$ radar would not be able to measure echoes coming from the surface. We thus warn about possible risks of misinterpretation if a textquotedblleft wet atmospheretextquotedblright $ $is not taken into account. (3) Rough surface scattering leads to a typical response of $sim -17 dB$. These results will have important implications on future Cassini radar data analysis.
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