ISO Observations of C₂H₂ on Uranus and CH₃ on Saturn:
Implications for Vertical Mixing in the Voyager and ISO Epochs, and a Call for Improved Laboratory Data

Sushil.K. Atreya ¹ Th. Encrenaz ² H. Feuchtgruber ³ S. G. Edgington ⁴

¹ Department of Atmospheric, Oceanic and Space Sciences, The University of Michigan, USA
² Observatoire de Paris-Meudon, France
³ MPI-Garching, Germany
⁴ University of Michigan, USA

In October 1996 ISO/SWS recorded the spectra of Uranus in 7-16.5 micron range and detected unambiguously the (5 band signature of acetylene(C₂H₂) centered at 13.7 micron (Encrenaz et al., AA, 333, L43, 1998). Interpretation of the C2H2 density profile with photochemical models led to a value of for the eddy diffusion coefficient (Kh) at the Uranus homopause, which for these values of Kh corresponds respectively to 354 km (0.037 mb) and 390 km (0.02 mb) above the 1-bar level. The eddy coefficient is found to be consistent with the values obtained by Voyager ultraviolet spectrometer. It is this consistency, however, which is intriguing, since the ISO observations are disk averaged, whereas the Voyager data correspond to the equator and low latitudes. Moreover, the Voyager observations were done 10 years earlier when the south pole of Uranus was pointing nearly directly to the Sun, whereas the subsolar point was at approximately 45 deg latitude at the time of the ISO observations. The ISO-Voyager consistency in Kh indicates that whatever little manifestable internal energy Uranus possesses might still be adequate and responsible for controlling the behavior of vertical mixing in its visible atmosphere, and that the dynamical behavior of the upper atmosphere is fairly uniform over the planet.

SWS also detected the methyl radical (CH₃) for the first time in the outer solar system. In December 1997, CH₃ was detected in the (2 band of the Q-branch at 16.5 micron (Bézard et al. AA, 334, L41, 1998). The derived stratospheric abundance of CH₃ ( ) is about a factor of 10 lower than the predictions of methane photochemical models, however. The discrepancy could be explained by one of two means: either the value of eddy diffusion coefficient used in the models is too high by at least a factor of 100, or the rate constant for the self-reaction loss of CH₃ is too low by at least a factor of 10. It is argued that Kh could not possibly be reduced so drastically. Three independent groups have determined Kh using three unrelated techniques, and they all agree to within about a factor of two of (Atreya et al, in Saturn, U. Arizona Press, 1984, pp 239; Parkinson et al, Icarus,1998, in press). It is unlikely that the eddy mixing could have decreased so dramatically in the sixteen year time span between the Voyager and ISO observations, since the behavior of eddy mixing in the giant planet atmospheres is believed to result from their internal heat source, which should not have decreased much in this short duration. It also seems unlikely that the globally averaged vertical mixing (corresponding to the ISO observations (could be smaller than the equatorial/midlatitude values of Voyager. Experience shows that in planetary atmospheres eddy mixing increases from low to high latitudes, particularly in the region of external energy input such as that from magnetospheric charged particles. This would result in greater, not a lower, globally averaged vertical mixing compared to the equatorial values. We believe the source of discrepancy between the ISO derived CH3 and the model calculations lies in the poor knowledge of laboratory chemical kinetics data, which has serious implications for the interpretation of observations of the outer solar system atmospheres, particularly those of the giant planets and Titan.