Astronomers have produced the most detailed radio map yet of the atmosphere of Jupiter, revealing the massive movement of ammonia gas that underlies the planet’s colourful bands, spots and whirling clouds.
The results provide a better understanding of how Jupiter was formed and the global circulation and cloud formations that are driven by Jupiter's powerful internal heat source.
The research will also shed light on similar processes occurring on other giant planets in our solar system, and on newly discovered giant exoplanets around distant stars.
The study, published in the journal Science today, was conducted by an international team of researchers, led by the University of California, Berkeley and also including a scientist from the University of Melbourne.
The results come one month before the NASA unmanned spacecraft Juno arrives at Jupiter for its 18-month mission there.
The team used the upgraded Karl G Jansky Very Large Array (VLA) in the United States to measure radio emissions from Jupiter’s atmosphere in wavelength bands where clouds are transparent.
The observers were able to see as deep as 100km below the cloud tops, a largely unexplored region where clouds form. The team will be making further complementary VLA observations at the same time as Juno’s microwave instruments are probing for water on the giant planet.
The planet’s thermal radio emissions are partially absorbed by ammonia gas. Based on the amount of absorption, the researchers could determine how much ammonia is present and at what depth.
“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 principal author Imke de Pater, a UC Berkeley Professor of Astronomy.
The map bears a striking resemblance to visible-light images taken by amateur astronomers and the Hubble Space Telescope, she said.
The radio map shows ammonia-rich gases rising into and forming the upper cloud layers: an ammonium hydrosulfide cloud at a temperature near 200 Kelvin (minus 70 degrees Celsius) and an ammonia-ice cloud in the approximately 160 Kelvin cold air (minus 110 degrees Celsius).
These clouds are easily seen from 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 Earth.
The map also shows that hotspots – so-called because they appear bright in radio and thermal infrared images – are ammonia-poor regions that encircle the planet like a belt just north of the equator.
Between these hotspots are ammonia-rich upwellings that bring ammonia from deeper in the planet.
Key to the new observations was an upgrade to the VLA that improved sensitivity by a factor of 10 said Bryan Butler, a co-author and staff astronomer at the National Radio Astronomy Observatory in the United States.
“Reseachers can now see fine structure in some bands, much like what can be observed in the visible, especially near the Great Red Spot.and downwelling motions there,” Professor de Pater said.
The observations also resolve a puzzling discrepancy between the ammonia concentration detected by the Galileo probe when it plunged through the atmosphere in 1995 – 4.5 times the abundance observed in the sun – and VLA measurements from before 2004, which showed much less ammonia gas than measured by the probe.
“Jupiter’s rotation once every 10 hours usually blurs radio maps, because these maps take many hours to observe,” said University of Melbourne researcher and co-author Robert Sault.
“But we have developed a technique to prevent this and so avoid confusing together the upwelling and downwelling ammonia flows, which had led to the earlier underestimate.
UC Berkeley research astronomer Michael Wong said: “With radio, we can peer through the clouds and see that those hotspots are interleaved with plumes of ammonia rising from deep in the planet, tracing the vertical undulations of an equatorial wave system.”
The team observed over the entire frequency range between 4 and 18 gigahertz (1.7cm–7cm wavelength), which enabled them to carefully model the atmosphere, said David DeBoer, a research astronomer with UC Berkeley’s Radio Astronomy Laboratory.
This research was supported by Planetary Astronomy and Outer Planets Research Program awards from the National Aeronautics and Space Administration. NRAO is a National Science Foundation facility operated under cooperative agreement by Associated Universities, Inc.