H3+ characteristics in the Jupiter atmosphere as observed at limb with Juno/JIRAM

Abstract

NASA's Juno spacecraft has been orbiting Jupiter since August 2016, providing unprecedented insights into the giant planet's atmosphere. The Jupiter Infrared Auroral Mapper (JIRAM) experiment on board Juno has made spectroscopic observations of the trihydrogen cation (H<SUB>3</SUB><SUP>+</SUP>) emissions in both northern and southern auroral regions (Dinelli et al., 2017; Adriani et al., 2017; Mura et al., 2017) and at mid-to-low latitudes (this paper). Observations targeting the limb of the planet from 60° North to 60° South latitudes were acquired with JIRAM's spectrometer in August 2016 and March 2017. We use these observations to characterize, for the first time, the vertical distribution of the H<SUB>3</SUB><SUP>+</SUP> emissions as a function of latitude across Jupiter's dayside. H<SUB>3</SUB><SUP>+</SUP> emission features in the 3-4 μm spectral band were used to retrieve the H<SUB>3</SUB><SUP>+</SUP> volume mixing ratio (VMR) and atmospheric temperatures as a function of altitude. The H<SUB>3</SUB><SUP>+</SUP> density profile has a quasi-symmetric distribution with latitude, decreasing from 5 × 10<SUP>4</SUP> cm<SUP>-3</SUP> at 300 km to 2 × 10<SUP>3</SUP> cm<SUP>-3</SUP> at 650 km altitude above the 1-bar level (column densities of 3.5 × 10<SUP>12</SUP> cm<SUP>-2</SUP> to 1.4 × 10<SUP>11</SUP> cm<SUP>-2</SUP>, assuming a 700 km column depth). The H<SUB>3</SUB><SUP>+</SUP> VMR is higher in the Southern hemisphere than in the North with values at 500 km of 4 × 10<SUP>-4</SUP> ppmv at 40°N and 8 × 10<SUP>-4</SUP> ppmv at 40°S. Retrieved temperatures increase almost monotonically with increasing altitude, hovering around 400 K at 300 km and >900 K at about 700 km.