Journal Club

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Journal Club: Peering beneath Jupiter’s uppermost clouds

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 colorful cloud layers seen in the approximately true -color Hubble map (lower map). Two radio wavelengths are shown in blue (2 cm) and gold (3 cm), probing depths of 30- 90 kilometers below the clouds. Credit: Michael H. Wong, Imke de Pater (UC Berkeley), Robert J. Sault (Univ. Melbourne). Optical:  NASA, ESA, A.A. Simon (GSFC), M.H. Wong (UC Berkeley), and G.S. Orton (JPL-Caltech)

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 colorful cloud layers seen in the approximately true
-color Hubble map (lower map). Two radio wavelengths are shown in blue (2 cm) and gold (3 cm), probing depths of 30-
90 kilometers below the clouds. Credit: Michael H. Wong, Imke de Pater (UC Berkeley), Robert J. Sault (Univ. Melbourne). Optical: NASA, ESA, A.A. Simon (GSFC), M.H. Wong (UC Berkeley), and G.S. Orton (JPL-Caltech)

Astronomers have trained telescopes on Jupiter since Galileo, but much about what lies under the giant world’s uppermost clouds was a mystery. Now the most detailed radio map yet of Jupiter’s atmosphere reveals it is dynamically active beneath these clouds, findings that could shed light on the behavior of other giant planets in the solar system and beyond. The scientists detailed their findings in the June 3 issue of the journal Science.

Analyzing radio wavelengths can help scientists peer at altitudes in Jupiter’s atmosphere below its thick uppermost ammonia ice clouds. To generate the new radio map, scientists relied on the Karl G. Jansky Very Large Array (VLA) in New Mexico, which recently received an upgrade “that made it a factor of 10 more sensitive than it was before,” says study lead author Imke de Pater, a planetary scientist at the University of California, Berkeley. “That helped us make incredibly detailed maps.”

In addition, previous radio maps of Jupiter were limited to examining only a limited number of radio wavelength bands from the planet. In contrast, the new map analyzes the full range of radio wavelength bands that Jupiter generates, from 1.7 to 7 centimeters, de Pater says.

The researchers peered as deep as 100 kilometers below Jupiter’s uppermost clouds, down to a largely unexplored region where its main clouds form. They generated a three-dimensional picture of ammonia concentrations, achieving a spatial resolution of 1,300 kilometers, two to five times better than previous radio maps of Jupiter.

The scientists discovered that plumes of ammonia-rich gases rise to form the upper cloud layers visible from Earth — ammonium hydrosulfide clouds at a temperature near 200 Kelvin, and ammonia-ice clouds at roughly 160 Kelvin. These upwellings of ammonia swell up in wave patterns, a sign of motion deep within the atmosphere. Conversely, ammonia-poor air sinks in downwellings. Near the Great Red Spot, the researchers detected many intricate features, suggesting complex upwelling and downwelling.

“We had no inkling of what was going on in Jupiter’s atmosphere at depths down to 100 kilometers before,” says Glenn Orton, a planetary scientist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., who did not take part in this research.

De Pater and her colleagues also found that hotspots, which get their name from how they appear bright in radio and thermal infrared images, are ammonia-poor regions devoid of clouds and condensable gases. These hotspots encircle Jupiter like a belt just north of the equator.

The new map also helps solve a mysterious discrepancy related to ammonia. Jupiter and the sun coalesced from the same cloud of gas and dust, and one might expect them to have similar levels of elements such as nitrogen. Ammonia, made as it is of nitrogen and hydrogen, can in theory serve as a proxy of how much nitrogen Jupiter might possess. Oddly, when NASA’s Galileo probe dove into Jupiter’s atmosphere in 1995, it found that Jupiter was 4.5 times more abundant in nitrogen than the sun, but VLA data gathered before 2004 suggested Jupiter had much less ammonia globally on Jupiter than Galileo detected. The new VLA map suggests this discrepancy is due to the low resolution of earlier radio maps, which failed to capture how much ammonia concentrations could vary from spot to spot in Jupiter, de Pater says.

By studying Jupiter’s atmosphere, scientists could develop better models of how it and the atmospheres of other gas giants both in the solar system and beyond it might behave. “The dynamics are incredibly important,” de Pater says. “The abundances of, in our case, ammonia gas, can be completely different in some places than others. The regular models now typically just explain global averages.”

The new map will greatly complement the findings from NASA’s Juno spacecraft, which is schedule to arrive at Jupiter on July 4, de Pater says. “Juno will observe Jupiter at longer wavelengths and go deeper down in the atmosphere, but won’t have as complete a picture as we have, so VLA will help put Juno’s data into perspective,” de Pater says.

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