The standard image of Jupiter is of a mysterious planet shrouded in colorful strips, spots and swirls. But what exactly is going on beneath the atmosphere's chaotic exterior is a question that has mystified astronomers for some time. Researchers have now peeled back the curtain by producing the most detailed radio map of Jupiter's atmosphere yet, revealing swathes of ammonia gas that drive its stormy weather and possibly, other giant planets just like it.
Confusion has previously plagued our understanding of Jupiter's atmosphere. In 1995, NASA's Galileo probe made the first detection of ammonia gas in another planet's atmosphere as it plummeted toward Jupiter, but doubt was cast on these observations when measurements taken by the Very Large Array (VLA) later revealed much lower concentrations. But upgrades to the VLA, along with a new method of interpreting its data, have now provided a way to account for previous inaccuracies.
"Jupiter's rotation once every 10 hours usually blurs radio maps, because these maps take many hours to observe," says study co-author Robert Sault, from the University of Melbourne. "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."
With the new and improved VLA, which has boosted its sensitivity by a factor of 10, an international team of scientists measured radio waves emitted from Jupiter that they know to be absorbed by ammonia. These were observed in wavelength bands where clouds are transparent, over the frequency range of four to eight GHz.
This allowed them to calculate the concentrations of ammonia at various locations around the planet's atmosphere, diving as deep as 100 km (62 mi) below the cloud tops to areas that had previously gone unexplored.
"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," says principal author Imke de Pater, a University of California, Berkeley professor of astronomy.
The detailed radio map reveals ammonia-rich gases rising to form the clouds you'll see when you point an optical telescope at Jupiter from Earth. These gather in upper cloud layers of ammonium-hydrosulfide at temperatures of almost -100° F (-73° C), and ammonia-ice clouds at around -170° F (-112.22° C).
In contrast, the map showed that air poor in ammonia was sinking back toward the surface of the planet. It also revealed that the planet's hotspots, those which appear bright in radio and thermal infrared images, are poor in ammonia. These wrap around the planet like a belt just north of its equator, with ammonia-rich upswells in between.
"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," says University of California, Berkeley research astronomer Michael Wong.
Studying these areas of Jupiter's atmosphere helps scientists learn more about how cloud formation is powered by the planet's internal heat source. These can assist with understanding how similar processes play out on other giant planets, not just in our solar system, but also in exoplanets that revolve around distant stars.
And the good news is that our understanding of Jupiter's atmosphere will continue to improve from here, and probably pretty quickly. Less than one month from now, NASA's deep-space probe Juno will enter the area. The unmanned craft will orbit the planet in an attempt to measure the amount of water and ammonia in its deep atmosphere, which will provide clues as to its structure, origin, and whether or not it has a solid core.
The team's research was published in the journal Science.
You can see how the radio map compares with optical maps of Jupiter's atmosphere in the short video below.
Source: University of Berkeley
