Elsevier

Icarus

Volume 181, Issue 1, March 2006, Pages 266-271
Icarus

The first detection of propane on Saturn

https://doi.org/10.1016/j.icarus.2005.09.016Get rights and content

Abstract

We report the first detection of propane, C3H8, in Saturn's stratosphere. Observations taken on September 8, 2002 UT at NASA's IRTF using TEXES, show multiple emission lines due to the 748 cm−1 ν21 band of C3H8. Using a line-by-line radiative transfer code, we are able to fit the data by scaling the propane vertical mixing ratio profile from the photochemical model of Moses et al. [2000. Icarus 143, 244–298]. Multiplicative factors of 0.7 and 0.65 are required to fit the −20° and −80° planetocentric latitude spectra. The resultant profiles are characterized by a 5 mbar mixing ratio of 2.7±0.8×10−8 at −20° and 2.5−0.8+1.7×10−8 at −80° latitude. These results suggest that the time scale for meridional circulation lies between the net photochemical lifetimes of C2H2 and C3H8, 30600 years.

Introduction

Propane, C3H8, is a photochemical byproduct created in a long chain of chemical reactions that can be traced back to the initial photolytic destruction of CH4 in the upper stratosphere of Saturn. The photochemical models of Moses et al., 2000, Moses et al., 2005, Ollivier et al. (2000), and Moses and Greathouse (2005) predict that propane should be the most abundant C3 molecule in Saturn's stratosphere. With a predicted column abundance above 10 mbar only a factor of ∼5–40 less than that of the easily detected C2H2, it may seem surprising that such an important molecule has not been detected to date. Methylacetylene (CH3C2H), which is predicted to be less abundant than propane, has been seen on Saturn with the Infrared Space Observatory (ISO) (e.g., de Graauw et al., 1997) and with the Cassini Composite Infrared Spectrometer (CIRS) (e.g., Simon-Miller et al., 2004); however, only upper limits on the C3H8 abundance have been presented to date (see Moses et al., 2000, for C3H8 upper limits derived from ISO observations). The problem is not that the photochemical models are over-predicting the C3H8 abundance, but that propane is difficult to observe. The ν21 band of C3H8, centered at 748 cm−1, is due to the CH2 rocking fundamental vibration mode (Gayles Jr. and King, 1965, Giver et al., 1984, Gassler et al., 1989). This same CH2 rocking mode has been described in other works as the ν26 band (Shimanouchi, 1972, Hanel et al., 1981, Roe et al., 2003). Because propane has more transitions in its ro-vibrational spectrum than C2H2, emission from propane is spread over many weak emission lines rather than a few strong lines. The line-to-continuum ratios for C3H8 are ≈10%, rather than ⩾400%, as is the case for C2H2. To make matters even more difficult, the weak propane emission occurs in the same spectral region as the ν5 R-branch of C2H2, where even the weak lines of C2H2 are ⩾40 times stronger than the propane emission.

Therefore, it should not be surprising that an unambiguous detection using Voyager or ISO with resolving powers (R=ν˜Δν˜) of 170 and 1850, respectively, at 748 cm−1 was impossible. By making high spectral resolution observations of Saturn using TEXES, the Texas Echelon Cross Echelle Spectrograph (Lacy et al., 2002), we were able to spectrally isolate individual emission lines due to the ν21 band of C3H8. These measurements serve as constraints for current photochemical models, and when compared to the 1-D seasonal photochemical model of Moses and Greathouse (2005) they can constrain the timescale for meridional advection.

Section snippets

Observations

Spectra of Saturn covering 747.3–751.7 cm−1 were acquired at NASA's Infrared Telescope Facility atop Mauna Kea in Hawaii on September 8, 2002 UT. Using TEXES in high resolution cross-dispersed mode, we achieved a resolving power of R  80,000. Our analysis focuses on the spectral region between 748.5 and 749.1 cm−1, where the dominant emission feature is a blend of a C2H2 and a 13C12CH2 emission line seen at 748.88 cm−1 in Fig. 1. The remaining weak wiggles seen in the spectra of Fig. 1 are due

Modeling

The observations were modeled with a line-by-line radiative transfer code. The model atmosphere, divided into 75 layers separated equally in log(P) and spanning the pressures P of 1.81×10−7 bar, assumes local thermodynamic equilibrium at all pressure levels. Line positions, intensities and energies were taken from the GEISA databank (Jacquinet-Husson et al., 1999) for 13C12CH2, C2H2, and C3H8, which account for all the emission lines in these observations. We note the work of (Roe et al., 2003

Results

Scaling of the Moses et al. (2000) propane vertical mixing ratio profile by factors of 0.7 and 0.65 at 20±10° and 80°±10° planetocentric latitude produces the best fits to the two observed spectra. These factors result in a C3H8 mixing ratio at the contribution function peak, 5 mbar (Fig. 2), of 2.7±0.8×10−8 at −20° and 2.5−0.8+1.7×10−8 at −80° planetocentric latitude. The quoted errors include the uncertainties due to variations of temperature, saturnian airmass, and gravity over the

Discussion

The photochemical production and loss mechanisms for C3H8 and other C3Hx hydrocarbons on the giant planets are not well understood due to a lack of chemical kinetics laboratory data at relevant stratospheric temperatures and pressures. Propane is likely produced through three-body radical–radical combination reactions such as CH3 + C2H5 + M  C3H8 + M and H + C3H7 + M  C3H8 + M, where M represents any other atmospheric constituent (e.g., H2, He) (see Moses et al., 2005, Moses et al., 2000; Wilson and Atreya,

Conclusions

This is the first detection of propane in Saturn's stratosphere. We find the Moses et al. (2000) propane mixing ratio vertical profile scaled to a 5 mbar value 2.7×10−8 at −20° and 2.5×10−8 at −80° planetocentric latitude reproduces the observations. Within errors we detect no variation in the mixing ratio of propane with latitude; a result that disagrees with the 1-D time varying photochemical model predictions of Moses and Greathouse (2005), see Fig. 3, Fig. 4. This measurement, when combined

Acknowledgements

This work was supported by USRA Grant 8500-98-008, NSF Grant AST 0205518, and by the Lunar and Planetary Institute, which is operated by the Universities Space Research Association under NASA CAN-NCC5-679. This paper represents LPI Contribution 1269. M.J.R. acknowledges the support of NSF Grant AST-0307497. We thank Maarten Roos-Serote and an anonymous referee for reviewing the paper.

References (24)

  • J. Borysow et al.

    Collision-induced rototranslational absorption spectra of H2–He pairs at temperatures from 40 to 3000 K

    Astrophys. J.

    (1988)
  • T. de Graauw et al.

    First results of ISO-SWS observations of Saturn: Detection of CO2, CH3C2H, C4H2 and tropospheric H2O

    Astron. Astrophys.

    (1997)
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    Visiting Astronomer at the Infrared Telescope Facility, which is operated by the University of Hawaii under cooperative agreement NCC 5-538 with the National Aeronautics and Space Administration, Office of Space Science, Planetary Astronomy Program.

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