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Radio Waves Hint at Drama Deep Below The Clouds of Jupiter
published during a waxing crescent moon.
06/08/2016

Radio waves Hint

The top image shows data from two radio wavelength maps captured by VLA data — blue (2 cm) and gold (3 cm). Variations in the opacity of these maps reveal the upwelling and downwelling movement of ammonia gas below Jupiter’s cloudy outer atmosphere. The lower image, of the same area, is a true color image from the Hubble Space Telescope. 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)

On December 7th, 1995, a small probe aboard NASA’s Galileo spacecraft hurled itself into Jupiter’s atmosphere, descending nearly an hour before incinerating under the high heat and pressure. During its brief journey, the probe reported a surprising finding—higher than expected concentrations of ammonia gas.

The reason, many scientists concluded, was that even deeper circulation patterns must affect the distribution of gaseous ammonia, a small but significant component of Jupiter’s atmosphere. Unfortunately, what lies below Jupiter’s visible surface has remained pretty much unobservable for as long as we’ve known the planet existed. Now, a study published in Science uses radio waves to figure out, for the first time, what kind of vertical air movement is going on up to 100 km below the planet’s clouds.

Radio waves Hint

Credits: M.H. Wong, I. De Pater, R.J. Sault; Visible: NASA, ESA, A.A. Simon, M.H. Wong, G.S. Orton.

In a big picture kind of way, Jupiter is really pretty simple. Though traces of ammonia mixed with other gases and powerful winds make for colorful jets and swirls on the surface, the planet is almost entirely hydrogen and helium, which transitions from gas to metallic liquid the deeper you get. Despite this simplicity, the inner-workings of the atmosphere below the cloud level are hard to study because the clouds block everything below them.

Imke de Pater, an astronomer at the University of California, Berkeley, and her colleagues used data from the newly revamped Very Large Array radio telescope in New Mexico to map the location of ammonia gas at different depths and latitudes within the atmosphere.  Ammonia gas is an important trace component of Jupiter’s atmosphere and is the most significant source of clouds on its surface. Deep within the atmosphere, this ammonia gas rises to the cooler, lower pressure outer atmosphere where it condenses into solid crystals, forming clouds.

Radio waves Hint

The movement of Jupiter’s clouds, with a detail of the Great Red spot. Credits: NASA/ESA/Goddard/UCBerkeley/JPL-Caltech/STScI.

The team’s basic plan was to analyze radio frequencies emitted naturally by the heat of the planet that are also unaffected by the clouds and weather of the planet’s outermost atmosphere. Only really long wavelengths are able to do this, and these require a really sensitive tool to pick up any signal at all.

Recent upgrades to the VLA telescope have increased its sensitivity by a factor of ten, making it possible to do just that. Since clouds do not interfere with the signal, the only thing left that has much of an effect are pockets of ammonia gas (which has different properties than the ammonia ice making up the clouds). The team compiled data for the strength of multiple radio wavelength signals at different latitudes and depths, attributing most fluctuations to concentrations of ammonia. By mapping the location of these areas of high and low concentration, the team was able to construct one of the first global views of vertical circulation patterns below Jupiter’s clouds.

Radio waves Hint

A full-disk radio image of Jupiter from the VLA at three wavelengths: 2 cm in blue, 3 cm in gold, and 6 cm in red. The pink glow surrounding the planet comes from radiation produced by spiraling electrons trapped in Jupiter’s magnetic field Credit: Imke de Pater, Michael H. Wong (UC Berkeley), Robert J. Sault (Univ. Melbourne).

These data, while presenting only a snapshot in time, nevertheless tell a lot about the movement of gases in the Jupiter’s atmosphere because the ammonia gas serves as a tracer for movement in the atmosphere as a whole. “Where we see regions with low ammonia abundances, we interpret that as areas where the air is going down. And if you see a lot of ammonia gas, we interpret that as air which is rising up,” de Pater told NOW.SPACE. That’s because when the ammonia gas rises and condenses into solid crystals, the air that remains below will have less ammonia, and that air inevitably sinks to replace the other rising air.

The grand picture de Pater and her team created shows large scale plumes (some bigger than Jupiter’s famous Red Spot) of rising and sinking ammonia gas across Jupiter’s equator. While there are still plenty of unanswered questions, this study provides the first glimpse into gas movement in Jupiter’s deeper atmosphere and lends credence to the notion, first hinted at by the Galileo probe in 1995, that ammonia gas is unevenly distributed due to complex circulation patterns.

This map, de Pater said, will also provide important context for another Jupiter mission—NASA’s upcoming Juno mission, set to reach the massive planet on July 4 of this year. While Juno’s radiometer will have the capability of peering even deeper into Jupiter’s atmosphere, it won’t be able to provide a global context for the areas it investigates, she said. But using this new map as a guide will allow scientists studying data from Juno to understand the larger scale picture of the regions Juno explores.

“We can then tell them, ‘Well, yeah, you see a large ammonia abundance at that position because right now, that is where we see one of these large plumes.'” she said.

That kind of information certainly would have solved some of the mysteries Galileo raised when its probe dove directly into an anomalously high concentration area of ammonia gas. Still, as great as solving old mysteries is, it will be just as exciting to see what new mysteries Juno brings to light when it arrives at Jupiter next month.