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Previous studies have demonstrated an external source of CO on Jupiter, Saturn, and Neptune. However, it has not been possible to demonstrate this on Uranus because of its low CO abundance, low upper-tropospheric temperatures, and low stratospheric thermal gradient, which make detection very challenging. Here we use 17 Herschel/SPIRE observation sequences spanning 3 yr (2009-2012), which cover 14.6-51.8 cm(-1) with a combined integration time of 5 hr. These spectra were originally taken for routine calibration purposes, so were corrected for continuum offsets prior to analysis. The final stacked spectra had an extremely low noise level of 10-50 pW cm(-2) sr(-1)/cm(-1). Despite this, CO was not observed, but we were able to obtain stringent 3 sigma upper limits at the 0.1-0.2 bar level of 2.1 ppb for a uniform profile, and 9.4 ppb for a stratosphere-only profile-an order of magnitude improvement over previous studies. Comparison with observed CO fluorescence by Encrenaz et al. suggests the majority of Uranus' stratospheric CO has an external origin. It thus appears that external supply of oxygen species-via comets, micrometeorites, or dust-is an important process on all giant planets in our solar system.
- planets and satellites: atmospheres
- ROTOTRANSLATIONAL ABSORPTION-SPECTRA
- MILLIMETER-WAVE OBSERVATIONS
- STRATOSPHERIC TEMPERATURE
- SPIRE INSTRUMENT
- CLOUD STRUCTURE