Swirling ammonia over Jupiter

Print edition : July 08, 2016

The VLA radio map of the region around the Great Red Spot in Jupiter’s atmosphere shows complex upwellings and downwellings of ammonia gas (upper map) that shape the colourful cloud layers seen in the approximately true-colour Hubble map (lower map). Two radio wavelengths are shown in blue (2 cm) and gold (3 cm), probing depths of 30-90 km below the clouds. Photo: Picture courtesy: Michael H. Wong, Imke de Pater, Robert J. Sault. Optical: NASA, ESA, A.A. Simon, M.H. Wong

USING the upgraded Karl G. Jansky Very Large Array (VLA) in New Mexico, University of California Berkeley (UCB) researchers measured radio emissions from Jupiter’s atmosphere in wavelength bands where clouds are transparent. They were able to see as deep as 100 km below the cloud tops, a largely unexplored region where clouds form. The planet’s thermal radio emissions are partially absorbed by ammonia gas. On the basis of the amount of absorption, the researchers could determine how much ammonia is present and at what depth. The research was published in a recent issue of Science.

By studying these regions of the planet’s atmosphere, astronomers hope to learn how global circulation and cloud formation are driven by Jupiter’s powerful internal heat source. These studies will also shed light on similar processes occurring on other giant planets in the solar system and on newly discovered giant exoplanets around distant stars. “We in essence created a three-dimensional picture of ammonia gas in Jupiter’s atmosphere, which reveals upward and downward motions within the turbulent atmosphere,” said the principal author Imke de Pater, a UCB professor of astronomy.

The radio map shows ammonia-rich gases rising into and forming the upper cloud layers: an ammonium hydrosulphide cloud at a temperature near 200 kelvin (−73 °C) and an ammonia-ice cloud in the approximately 160 K cold air. These clouds are easily seen from the earth by optical telescopes. Conversely, the radio maps show ammonia-poor air sinking into the planet, similar to how dry air descends from above the cloud layers on the earth. The map also shows that hot spots (so-called because they appear bright in radio and thermal infrared images) are ammonia-poor regions that encircle Jupiter like a belt just north of the equator. Between these hot spots are ammonia-rich upwellings that bring ammonia from deeper in the planet. The final maps have the best spatial resolution ever achieved in a radio map: 1,300 km.