| NASA Ames Research Center | |
| School of Electrical and Computer Engineering | |
The modified Ben-Reuven lineshape used to model the microwave absorption of gaseous ammonia (NH3) in the Jovian atmosphere suggests that the absorption due to a solar abundance of ammonia (150 ppm) is not large enough to account for the actual absorption from 10 to 20 cm as inferred by recent spectral emission studies and radio occultation experiments. To determine whether there is an additional microwave absorbing constituent present in Jupiter's atmosphere, we have measured the microwave absorption of methane (CH4) and water vapor (H2O) under simulated Jovian conditions at 2.25 GHz (13.3 cm), 8.5 GHz (3.5 cm), and 21.7 GHz (1.38 cm). Methane absorption was measured at 153 K for total pressures as high as 6 atm both for pure methane and for a mixture of 40% CH4, 53.3% H2, and 6.7% He. The measurements of the opacity due to water vapor were made at 297 K under pressures as high as 6 atm for a 90% H2/10% He mixture with an H2O mixing ratio of 3.5 x 10-3. Because of large error bars, the measurements represent upper limits on the microwave opacity due to H2O and CH4. However, the results of these measurements are consistent with theoretical expressions for microwave opacity in a Jovian atmosphere at the specified frequencies. The upper limits on the microwave opacity due to water vapor suggest that it is unlikely that the presence of water vapor can account for this excess absorption. This supports the presence of an ammonia abundance which exceeds a solar abundance by a factor of 1.5 in the 2- to 6- bar levels in Jupiter's atmosphere.