On Jupiter, a storm’s been brewing for more than 300 years. Known as the Great Red Spot, this swirling high-pressure region is clearly visible from space, spanning a region in Jupiter’s atmosphere more than 10,000 miles (16,000 kilometers) wide.
The first record of the Great Red Spot is a drawing made in 1831 by German amateur astronomer Samuel Heinrich Schwabe of the “Hollow” in which the spot sits. The Great Red Spot itself has been continuously observed since 1878 when it was described by American astronomer Carr Walter Pritchett.
High-resolution spacecraft pictures revealed that the feature’s pinkish cloud layer can be overlain from time to time by high-altitude white clouds, producing the gray impression seen from Earth. In the late 19th century the length of the Great Red Spot was about 48,000 km (30,000 miles), and since then the Great Red Spot has been shrinking. The Voyager spacecraft measured the spot’s length at 23,000 km (14,500 miles) in 1979.
But there’s even more to the churning tempest than meets the eye; according to two new studies published Oct. 28 in the journal Science. NASA’s Juno spacecraft has shown that the behemoth storm extends as much as 310 miles (500 kilometers) beneath Jupiter’s cloud tops.
It began as a simple idea: to turn Juno’s Gravity Science instrument on the 10,000-mile-wide (16,000 km) storm, in order to see if it left behind any fingerprints on Jupiter’s gravitational field.
It may be the same storm as the so-called “Permanent Spot” that was discovered in 1665 by Italian astronomer Gian Domenico Cassini and last seen in 1713.
Using gravity measurements to investigate atmospheric phenomena isn’t new; it’s been done on Earth, for instance, by the twin satellites of NASA’s GRACE (“Gravity Recovery and Climate Experiment”) mission. Gravity instruments can also see deeper into the atmosphere than other instruments tend to see.
Both new studies relied on observations from NASA’s Juno probe, which entered Jupiter’s orbit in 2016 and has since completed 36 passes of the nearly 87,000-mile-wide (140,000 km) gas giant. In one study, scientists examined the Great Red Spot using the probe’s microwave radiometer — a tool that detects microwaves emitted from inside the planet.
But even after the idea was approved, Parisi and her colleagues had no idea if they would actually find anything. The Great Red Spot might be a colossus, but it’s a drop in the bucket of Jupiter’s total mass.
Jupiter Great Red Spot emits much of its internal heat in the form of microwaves, and because the planet’s temperature increases with depth, the frequencies of these emissions are higher closer to Jupiter’s surface, and lower further in. Juno is equipped with a microwave radiometer, a device that tunes in to Jupiter’s Great Red Spot microwave transmissions at six distinct frequencies, each value corresponding to a different depth.
But the researchers did indeed pick up fluctuations in Jupiter’s Great Red Spot gravitational field, enough for them to get a handle on the storm’s depth: 310 miles (500 km), taller than the distance from sea level to the International Space Station. And the Great Red Spot storm seems to be fed by jets that reach down far deeper — as much as 1,900 miles (3,000 km), the team found.
The second study found the spot may be even bigger than that. That paper’s authors examined the Great Red Spot using Juno’s gravity detection tools. Synthesizing data from 12 flights that passed by the spot — including two direct overhead flights — the researchers calculated where the storm was concentrating the most atmospheric mass over the planet, allowing them to estimate its depth.
The instrument also observed two other storms, and while all three had roots past the cloud base, neither went down as far as the Great Red Spot. “This suggests that the driving mechanism of the Great Red Spot is different from the other vortices,” says University of Arizona planetary scientist Tommi Koskinen.
They found that the Spot, along with several other storms on Jupiter, stretches far down, with precipitation and drafts at unprecedented depths. They found signatures of these phenomena below Jupiter’s cloud level, beneath which the ammonia and water in the atmosphere are expected to condense.