Spatially-Resolved Millimeter-Wavelength Maps of Neptune

TL;DR

This paper presents the first spatially-resolved millimeter-wave maps of Neptune, revealing latitudinal brightness temperature gradients and variations in atmospheric composition and temperature at different depths.

Contribution

It provides the first high-resolution millimeter maps of Neptune, analyzing latitudinal atmospheric variations and their possible causes, advancing understanding of Neptune's atmospheric structure.

Findings

Latitudinal brightness temperature gradient of 2-3 K from 40°N to south pole.

Decreases in gas opacity of about 30% near the south pole at altitudes below 1 bar.

Evidence of 2-3% latitudinal variations in CO line consistent with temperature variations.

Abstract

We present maps of Neptune in and near the CO (2-1) rotation line at 230.538 GHz. These data, taken with the Combined Array for Research in Millimeter-wave Astronomy (CARMA) represent the first published spatially-resolved maps in the millimeter. At large (~5 GHz) offsets from the CO line center, the majority of the emission originates from depths of 1.1-4.7 bar. We observe a latitudinal gradient in the brightness temperature at these frequencies, increasing by 2-3 K from 40 degrees N to the south pole. This corresponds to a decrease in the gas opacity of about 30% near the south pole at altitudes below 1 bar, or a decrease of order a factor of 50 in the gas opacity at pressures greater than 4 bar. We look at three potential causes of the observed gradient: variations in the tropospheric methane abundance, variations in the H2S abundance, and deviations from equilibrium in the ortho/para ratio of hydrogen. At smaller offsets (0-213 MHz) from the center of the CO line, lower atmospheric pressures are probed, with contributions from mbar levels down to several bars. We find evidence of latitudinal variations at the 2-3% level in the CO line, which are consistent with the variations in zonal-mean temperature near the tropopause found by Conrath et al. (1998) and Orton et al. (2007).