First Ground-based Infrared Detections Of Volatile Species In The Centaur Comet 29P/Schwassmann-Wachmann 1 At Rh = 6.26 AU

We observed comet 29P/Schwassmann-Wachmann 1 (hereafter 29P) in UT 2012 February and May with CRIRES/VLT (in the Atacama desert, Chile) and NIRSPEC/Keck-2 (atop Mauna Kea, Hawaii) when the comet was placed at 6.26 AU from the Sun and 5.50 AU from Earth. We detected five CO emission lines on several nights, confirming the ubiquitous content and release of carbon monoxide from the comet’s nucleus. This is the first multiple line detection of any gaseous species in 29P, at any wavelength, and the first extraction of a rotational temperature based on the intensities of multiple spectral lines. We present the production rate and (remarkably low) rotational temperature for CO, and compare them with results extracted from previous observations at radio wavelengths. Along with CO we pursued detections of other volatiles, namely H2O, C2H6, CH4, HCN and CH3OH. Although their detections are not evident in 29P, we present very sensitive upper limits. These results establish a new record for detections by Infrared Spectroscopy of parent volatiles in comets at relatively large heliocentric distances, previously held by detection of CO in C/1995 O1 (Hale-Bopp) at 4.11 AU (with CSHELL/IRTF, DiSanti et al. 1999) and of CO in C/2006 W3 (Christensen) at 4.03 AU (with CRIRES/VLT, Bonev et al. 2012, in prep.). Until now considered to be a somewhat impossible task with IR ground-based facilities, these discoveries open up new opportunities for targeting volatile species in distant comets at low rotational temperatures. • M. A. DiSanti, M. J. Mumma, N. Dello Russo, K. Magee-Sauer, R. E. Novak, T. W. Rettig, Nature 399, 662-665 (1999) • Acknowledgements: L.P. thanks the NASA Postdoctoral Program. We gratefully acknowledge support from the NASA Astrobiology Institute (PI, Mumma), NSF (PI Bonev), the NASA Planetary Astronomy (PI Villanueva; PI Mumma; PI DiSanti) and Planetary Atmospheres (PI DiSanti, PI Villanueva) programs.