Storms
on Jupiter Are Disturbing the Planet’s Colorful Belts
By Robert Sanders, UC Berkeley Media
Relations
Storm clouds rooted deep in Jupiter’s
atmosphere are affecting the planet’s white zones and colorful belts, creating
disturbances in their flow and even changing their color.
Thanks to coordinated observations of
the planet in January 2017 by six ground-based optical and radio telescopes and
NASA’s Hubble Space Telescope, a University of California, Berkeley, astronomer
and her colleagues have been able to track the effects of these storms —
visible as bright plumes above the planet’s ammonia ice clouds — on the belts
in which they appear.
The observations will ultimately help
planetary scientists understand the complex atmospheric dynamics on Jupiter,
which, with its Great Red Spot and colorful, layer cake-like bands, make it one
of the most beautiful and changeable of the giant gas planets in the solar
system.
One such plume was noticed by amateur
astronomer Phil Miles in Australia a few days before the first observations by
the Atacama Large Millimeter/Submillimeter Array (ALMA) in Chile, and photos
captured a week later by Hubble showed that the plume had spawned a second
plume and left a downstream disturbance in the band of clouds, the South Equatorial
Belt. The rising plumes then interacted with Jupiter’s powerful winds, which
stretched the clouds east and west from their point of origin.
Three months earlier, four bright spots
were seen slightly north of the North Equatorial Belt. Though those plumes had
disappeared by 2017, the belt had since widened northward, and its northern
edge had changed color from white to orangish brown.
“If these plumes are vigorous and
continue to have convective events, they may disturb one of these entire bands
over time, though it may take a few months,” said study leader Imke de Pater, a
UC Berkeley professor emerita of astronomy. “With these observations, we see
one plume in progress and the aftereffects of the others.”
The analysis of the plumes supports the
theory that they originate about 80 kilometers below the cloud tops at a place
dominated by clouds of liquid water. A paper describing the results has been
accepted for publication in the Astronomical Journal and is now online.
Into the Stratosphere
Jupiter’s atmosphere is mostly hydrogen
and helium, with trace amounts of methane, ammonia, hydrogen sulfide and water.
The top-most cloud layer is made up of ammonia ice and comprises the brown
belts and white zones we see with the naked eye. Below this outer cloud layer
sits a layer of solid ammonium hydrosulfide particles. Deeper still, at around
80 kilometers below the upper cloud deck, is a layer of liquid water droplets.
The storm clouds de Pater and her team
studied appear in the belts and zones as bright plumes and behave much like the
cumulonimbus clouds that precede thunderstorms on Earth. Jupiter’s storm
clouds, like those on Earth, are often accompanied by lightning.
Optical observations cannot see below
the ammonia clouds, however, so de Pater and her team have been probing deeper
with radio telescopes, including ALMA and also the Very Large Array (VLA) in
New Mexico, which is operated by the National Science Foundation-funded
National Radio Astronomy Observatory.
ALMA array’s first observations of
Jupiter were between Jan. 3 and 5 of 2017, a few days after one of these bright
plumes was seen by amateur astronomers in the planet’s South Equatorial Belt. A
week later, Hubble, the VLA, the Gemini, Keck and Subaru observatories in
Hawaii and the Very Large Telescope (VLT) in Chile captured images in the
visible, radio and mid-infrared ranges.
De Pater combined the ALMA radio
observations with the other data, focused specifically on the newly brewed
storm as it punched through the upper deck clouds of ammonia ice.
The data showed that these storm clouds
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,” de Pater said.
The observations are consistent with one
theory, called moist convection, about how these plumes form. According to this
theory, convection brings a mix of ammonia and water vapor high enough — about
80 kilometers below the cloud tops — for the water to condense into liquid
droplets. The condensing water releases heat that expands the cloud and buoys
it quickly upward through other cloud layers, ultimately breaking through the
ammonia ice clouds at the top of the atmosphere.
The plume’s momentum carries the
supercooled ammonia cloud above the existing ammonia-ice clouds until the
ammonia freezes, creating a bright, white plume that stands out against the
colorful bands encircling Jupiter.
“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,” said 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.”
Hubble took images a week after ALMA and
captured two separate bright spots, which suggests that the plumes originate
from the same source and are carried eastward by the high altitude jet stream,
leading to the large disturbances seen in the belt.
Coincidentally, three months before,
bright plumes had been observed north of the Northern Equatorial Belt. The
January 2017 observations showed that that belt had expanded in width, and the
band where the plumes had first been seen turned from white to orange. De Pater
suspects that the northward expansion of the North Equatorial Belt is a result
of gas from the ammonia-depleted plumes falling back into the deeper
atmosphere.
De Pater’s colleague and co-author
Robert Sault of the University of Melbourne in Australia used special computer
software to analyze the ALMA data to obtain radio maps of the surface that are
comparable to visible-light photos taken by Hubble.
“Jupiter’s rotation once every 10 hours
usually blurs radio maps, because these maps take many hours to observe,” Sault
said. “In addition, because of Jupiter’s large size, we had to ‘scan’ the
planet, so we could make a large mosaic in the end. We developed a technique to
construct a full map of the planet.”
Among the co-authors of the paper with
de Pater and Sault are graduate students Chris Moeckel and Charles Goullaud and
research astronomers Michael Wong and David DeBoer, all of UC Berkeley, and
Bryan Butler of the National Radio Astronomy Observatory. Each was involved in
obtaining and analyzing the Hubble, Gemini, ALMA and VLA data. The VLT data
were contributed by Leigh Fletcher and Padraig Donnelly of the University of
Leicester in the United Kingdom, while Glenn Orton and James Sinclair of the
Jet Propulsion Laboratory in California and Yasuma Kasaba of Tokyo University
in Japan supplied the SUBARU data. Gordon Bjoraker of the NASA Goddard Space
Flight Center in Maryland and Máté Ádámkovics of Clemson University in South
Carolina analyzed the Keck data.
The work was supported by a NASA
Planetary Astronomy award (NNX14AJ43G) and a Solar System Observations award
(80NSSC18K1001).
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