The Great Red Spot, a gigantic storm of crimson clouds bigger than the Earth, has been raging on Jupiter's surface for centuries. We've known its size in two dimensions for a long time, but after a close flyover in July the Juno probe has finally given us an answer about how deep into the atmosphere the storm's roots run. In the process, the mission also uncovered two new radiation zones.
While astronomers have been monitoring the Great Red Spot since 1830, the storm is believed to have marked the gas giant's face for up to 350 years. As of April this year it measured around 10,000 miles (16,000 km) wide, making it about 1.3 times the diameter of Earth. Impressive as that sounds, it seems the spot is shrinking at an increasing rate: when Voyager 1 and 2 whipped past in 1979 on their grand tour of the Solar System, the storm was twice the size of Earth.
To get a closer look, Juno swung low during its first pass over the Spot in July 2017, and along with producing a collection of stunning photos, the spacecraft tuned all of its instruments towards the storm. In particular, the Microwave Radiometer was able to peer deep beneath the clouds and reveal how far down it goes.
"One of the most basic questions about Jupiter's Great Red Spot is: how deep are the roots?" says Scott Bolton, principal investigator on the Juno project. "Juno data indicate that the solar system's most famous storm is almost one-and-a-half Earths wide, and has roots that penetrate about 200 miles (320 km) into the planet's atmosphere."
As striking as images of the Great Red Spot are, videos and animation show a swirling soup that's even more mesmerizing. That motion comes courtesy of insane winds whipping around the edge of the Spot at speeds no earthly storm could ever hope to muster.
"Juno found that the Great Red Spot's roots go 50 to 100 times deeper than Earth's oceans and are warmer at the base than they are at the top," says Andy Ingersoll, co-investigator on the Juno project. "Winds are associated with differences in temperature, and the warmth of the spot's base explains the ferocious winds we see at the top of the atmosphere."
Other instruments detected two new radiation zones around the gas giant that had previously been uncharted. Juno's Stellar Reference Unit (SRU-1) detected extremely high noise signatures in images gathered by the team's radiation monitoring investigation, as the craft passed through Jupiter's electron radiation belt. The signatures are those of a high-energy heavy ion population, but the team isn't sure yet which particles are responsible or where they come from.
The second radiation zone is much closer to the planet's surface and lies near the equator. There, energetic hydrogen, oxygen and sulfur ions zip around at close to the speed of light.
"The closer you get to Jupiter, the weirder it gets," says Heidi Becker, radiation monitoring investigation lead on the Juno program. "We knew the radiation would probably surprise us, but we didn't think we'd find a new radiation zone that close to the planet. We only found it because Juno's unique orbit around Jupiter allows it to get really close to the cloud tops during science collection flybys, and we literally flew through it."
The findings were presented this week at the American Geophysical Union. Juno's next science pass is scheduled for December 16.
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