Detection of the Methyl Radical on Neptune

Abstract

We report the Ðrst detection of the methyl radical in the upper atmosphere of Neptune. Obser-(CH 3 ) vations with the Short-Wavelength Spectrometer of the Infrared Space Observatory (ISO) satellite at a resolving power of 2200 revealed several emission features from the Q-branch of around 16.50 l 2 CH 3 km. The column density of methyl radicals above the 0.2 mbar level is molecules cm~2. 1.6~0 .91.2 ] 1013 Results are compared with predictions of photochemical models. The abundance is mostly sensitive CH 3 to the eddy mixing proÐle and to the poorly known methyl recombination rate at low pressure. Using Slagle et al.Ïs expression for this rate, the present observations imply an eddy mixing coefficient at the methane homopause in the range of 2È9 ] 106 cm2 s~1. Recombination rates higher than used in current photochemical models would lead to larger eddy mixing coefficients.

Improved constraints on Neptune's atmosphere from submillimetre-wavelength observations

“…We nevertheless considered that deviations of typically ±2 K around this profile could exist in the troposphere, such as can be seen in models presented by the latter authors. For Neptune, the nominal thermal model is based on Lindal (1992); Bézard et al (1991Bézard et al ( , 1999, and also agrees in the 1-µbar region with determinations by Roques et al (1994) from stellar occultation observations. We also considered the model profiles of Uranus and Neptune used by Feuchtgruber et al (1999) in the analysis of infrared ISO spectra.…”

“…3. The temperature structure is close to the nominal diskaveraged profile determined from ISO measurements taken in 1997 (see Bézard et al 1999). We note, however, that our nominal profile is 8 K warmer in the pressure range between 0.05 and 1 mbar, and also 3 K colder below 20 mbar, than the ISO thermal profile.…”

“…In the lower stratosphere, the profile appears to be 8 K warmer between 0.05 and 1 mbar, and 3 K colder below the 20 mbar level, than the disk-averaged profile proposed by Bézard et al (1999Bézard et al ( , 2000 to interpret ISO detections of the CH 3 radical and C 6 H 6 on Neptune. Finally, we note that our modeling applies well for regions into the upper stratosphere (i.e.…”

“…A simple first case (A) considers that K is inversely proportional to the atmospheric number density to some power (typically 0.6); the second case (B) exhibits a strong variation at around 1 mbar, and presumes a slow, and rapid, mixing in the lower and upper stratospheric regions respectively. In regard to the CH 3 radical present in the upper atmosphere Bézard et al (1999) found that the case B profile was not appropriate because it yielded too intense a CH 3 feature, in disagreement with the ISO observations. As a conservative approach we thus presume that a case A profile is more applicable, with a value of ∼10 5 cm 2 s −1 for pressures around 0.1 mbar.…”

“…For the stratosphere, at pressure levels above 0.1 mbar, different temperature lapse rates have been adopted by Yelle et al (1993), Bishop et al (1998) and Bézard et al (1999). As first suggested by the former authors, a uniform temperature model was used by Bishop et al (1998) in the pressure range 0.3-0.001 mbar with a nominal value of 168 ± 20 K, while Bézard et al (1999) indicated a quasiisothermal (165−180 K) profile in the (0.02-0.0003 mbar) region with a value of 170 ± 3 K near 3 µbar. We thus assumed a constant temperature above 0.01 mbar, the lower pressure limit (<1 µbar) not being significant since the contribution of the atmosphere above this level is practically negligible and, hence, cannot be measured even with the highest spectral resolution of our data.…”