Something is going on under Jupiter’s ammonia-ice clouds. While the Hubble Space Telescope can produce exquisite images of Jupiter’s ever-changing colorful belts and erupting giant storms (such as the Great Red Spot) it can’t peek beneath its swirling ammonia ice clouds to see what’s causing such turbulence.
Cue radio astronomy, which detects electromagnetic radiation emitted by astronomical objects. Using longer wavelengths of light than optical telescopes can detect, radio telescopes can collect data to produce images of things we would otherwise not be able to see. In Jupiter’s case, astronomers have used data from the Atacama Large Millimeter/submillimeter Array (ALMA) in northern Chile along with the Hubble Space Telescope, the Very Large Array, the Gemini, Keck and Subaru observatories in Hawaii, and the Very Large Telescope (VLT), also in Chile, to see what’s going on below the clouds.
Able to ‘see’ 31 miles/50 kilometers below Jupiter’s clouds, data collected by those telescopes after eruptions in January 2017 reveals that dramatic storm clouds are disturbing Jupiter’s belts, and even changing their color. In “First ALMA Millimeter Wavelength Maps of Jupiter, with a Multi-Wavelength Study of Convection” by I. de Pater, et al. published in the Astronomical Journal, plumes were recorded drifting from the planet’s South Equatorial Belt before interacting with its powerful winds, while four bright spots were seen to disappear from the North Equatorial Belt, which then widened and changed color from white to brown. The analysis of the plumes supports the theory that they originate about 80 kilometers below the clouds in a region governed by clouds of liquid water.
What is Jupiter’s atmosphere like?
It’s not nice. Hydrogen and helium makes up most of Jupiter’s atmosphere along with traces of methane, ammonia, hydrogen sulfide and water. The clouds visible to optical telescopes are of ammonia ice, and it’s those that create the planet’s brown belts and white zones. It’s here that storms, often with lightning, take place–visible as bright plumes–that disrupt the equatorial belts, the effects of which can be visible for months or years. Those plumes behave much like the cumulonimbus clouds that precede thunderstorms on Earth.
Why was radio astronomy so important at Jupiter?
“ALMA enabled us to make a three-dimensional map of the distribution of ammonia gas below the clouds,” says study leader Imke de Pater, a UC Berkeley professor emerita of astronomy. “Our ALMA observations are the first to show that high concentrations of ammonia gas are brought up during an energetic eruption. “The combination of observations simultaneously at many different wavelengths enabled us to examine the eruption in detail. This led us to confirm the current theory that energetic plumes are triggered by moist convection at the base of water clouds, which are located deep in the atmosphere.”
So what happened after the January 2017 storm?
The newly brewed storm clouds punched through the upper deck clouds of ammonia ice, reached as high as the tropopause–the coldest part of the atmosphere–where they spread out much like the anvil-shaped cumulonimbus clouds that generate lightning and thunder on Earth. “Our ALMA observations are the first to show that high concentrations of ammonia gas are brought up during an energetic eruption,” says de Pater. The observations are consistent with the ‘moist convection’ theory that states that a mix of ammonia and water vapor about 80 kilometers below the cloud tops makes the water condense into liquid droplets. That process releases heat that expands the cloud and sends it upwards, where it busts through the ammonia ice clouds at the top of the atmosphere. This supercooled ammonia cloud then freezes, creating a bright, white plume that stands out against the colorful bands around Jupiter.
Why is radio astronomy so difficult at Jupiter?
“Jupiter’s rotation once every 10 hours usually blurs radio maps, because these maps take many hours to observe,” says co-author Robert Sault of the University of Melbourne in Australia. He used special computer software to analyze the ALMA data to obtain radio maps of the surface to compare with the visible UV light images taken by Hubble. “Because of Jupiter’s large size we had to ‘scan’ the planet, so we could make a large mosaic in the end,” says Sault. “We developed a technique to construct a full map of the planet.”
“We were really lucky with these data, because they were taken just a few days after amateur astronomers found a bright plume in the South Equatorial Belt,” says de Pater. “With ALMA, we observed the whole planet and saw that plume, and since ALMA probes below the cloud layers, we could actually see what was going on below the ammonia clouds.” The work was supported by a NASA Planetary Astronomy award and a Solar System Observations award.