A Wideband Self-Consistent Disk-Averaged Spectrum of Jupiter Near 30 GHz and Its Implications for NH$_{3}$ Saturation in the Upper Troposphere

TL;DR

This study presents new microwave observations of Jupiter near 30 GHz to analyze ammonia distribution in its upper troposphere, revealing sub-saturation of ammonia clouds and providing constraints on ammonia mole fractions.

Contribution

It offers the first detailed measurement of Jupiter's microwave spectrum near 30 GHz and models ammonia distribution, improving understanding of cloud formation and saturation in the upper troposphere.

Findings

Ammonia mole fraction below the NH3 ice cloud is approximately 2.4×10⁻⁴.

The NH3 cloud-forming region is globally sub-saturated by 55%.

Upper limit on ammonia mole fraction above the cloud layer is 2.4×10⁻⁷.

Abstract

We present a new set of measurements obtained with the Combined Array for Research in Millimeter-wave Astronomy (CARMA) of Jupiter's microwave thermal emission near the 1.3 cm ammonia (NH$_{3}$) absorption band. We use these observations to investigate the ammonia mole fraction in the upper troposphere, near $0.3 < P < 2$ bar, based on radiative transfer modeling. We find that the ammonia mole fraction must be $\sim2.4\times 10^{-4}$ below the NH$_{3}$ ice cloud, i.e., at $0.8 < P < 8$ bar, in agreement with results by de Pater et al. (2001, 2016a). We find the NH$_{3}$ cloud-forming region between $0.3 < P < 0.8$ bar to be globally sub-saturated by 55% on average, in accordance with the result in Gibson et al. (2005). Although our data are not very sensitive to the region above the cloud layer, we are able to set an upper limit of $2.4\times 10^{-7}$ on the mole fraction here, a factor of $\sim$10 above the saturated vapor curve.