A clever use of radio signals from planetary spacecraft is allowing researchers to pierce the swirling clouds that hide the interiors of Jupiter and Saturn, where crushing pressure transforms matter into states unknown on Earth. The effort, led by Luciano Iess of Sapienza University in Rome, turned signals from two NASA probes, Cassini at Saturn and Juno at Jupiter, into probes of gravitational variations that originate deep inside these gas giants.
What the researchers have found is fueling a high-stakes game of compare and contrast. The results, published last year in Nature for Jupiter and this week in Science for Saturn, show that "the two planets are more complex than we thought," says Ravit Helled, a planetary scientist at the University of Zurich in Switzerland. "Giant planets are not simple balls of hydrogen and helium."
In the 1980s, Iess helped pioneer a radio instrument for Cassini that delivered an exceptionally clear signal because it worked in the Ka band, which is relatively free of noise from interplanetary plasma. By monitoring fluctuations in the signal, the team planned to search for gravitational waves from the cosmos and test general relativity during the spacecraft's journey to Saturn, which began in 1997. Iess's group put a similar device on Juno, which launched in 2011, but this time the aim was to study Jupiter's interior.
Juno skims close to Jupiter's surface every 53 days, and with each pass hidden influences inside the planet exert a minute pull on the spacecraft, resulting in tiny Doppler shifts in its radio signals. Initially, Iess and his team thought measuring those shifts wouldn't be feasible at Saturn because of the gravitational influence of its rings. But that obstacle disappeared earlier this decade, after the Cassini team decided to end the mission by sending the craft on a series of orbits, dubbed the Grand Finale, that dipped below the rings and eliminated their effects. As a result, Iess and colleagues could use radio fluctuations to map the shape of gravity fields at both planets, allowing them to infer the density and movements of material deep inside.
One goal was to probe the roots of the powerful winds that whip clouds on the gas giants into distinct horizontal bands. Scientists assumed the winds would either be shallow, like winds on Earth, or very deep, penetrating tens of thousands of kilometers into the planets, where extreme pressure is expected to rip the electrons from hydrogen, turning it into a metallike conductor. The results for Jupiter were a puzzle: The 500-kilometer-per-hour winds aren't shallow, but they reach just 3000 kilometers into the planet, some 4% of its radius. Saturn then delivered a different mystery: Despite its smaller volume, its surface winds, which top out at 1800 kilometers per hour, go three times deeper, to at least 9000 kilometers. "Everybody was caught by surprise," Iess says.
Scientists think the explanation for both findings lies in the planets' deep magnetic fields. At pressures of about 100,000 times that of Earth's atmosphere—well short of those that create metallic hydrogen—hydrogen partially ionizes, turning it into a semiconductor. That allows the magnetic field to control the movement of the material, preventing it from crossing the field lines. "The magnetic field freezes the flow," and the planet becomes rigid, says Yohai Kaspi, a planetary scientist at the Weizmann Institute of Science in Rehovot, Israel, who worked with Iess. Jupiter has three times Saturn's mass, which causes a far more rapid increase in atmospheric pressure—about three times faster. "It's basically the same result," says Kaspi, but the rigidity sets in at a shallower depth.
The Juno and Cassini data yield only faint clues about greater depths. Scientists once believed the gas giants formed much like Earth, building up a rocky core before vacuuming gas from the protoplanetary disc. Such a stately process would have likely led to distinct layers, including a discrete core enriched in heavier elements. But Juno's measurements, interpreted through models, suggested Jupiter's core has only a fuzzy boundary, its heavy elements tapering off for up to half its radius. This suggests that rather than forming a rocky core and then adding gas, Jupiter might have taken shape from vaporized rock and gas right from the start, says Nadine Nettelmann, a planetary scientist at the University of Rostock in Germany.
The picture is still murkier for Saturn. Cassini data hint that its core could have a mass of some 15 to 18 times that of Earth, with a higher concentration of heavy elements than Jupiter's, which could suggest a clearer boundary. But that interpretation is tentative, says David Stevenson, a planetary scientist at the California Institute of Technology in Pasadena and a co-investigator on Juno. What's more, Cassini was tugged by something deep within Saturn that could not be explained by the winds, Iess says. "We call it the dark side of Saturn's gravity." Whatever is causing this tug, Stevenson adds, it's not found on Jupiter. "It is a major result. I don't think we understand it yet."
Because Cassini's mission ended with the Grand Finale, which culminated with the probe's destruction in Saturn's atmosphere, "There's not going to be a better measurement anytime soon," says Chris Mankovich, a planetary scientist at the University of California, Santa Cruz. But although the rings complicated the gravity measurements, they also offer an opportunity. For some unknown reason—perhaps its winds, perhaps the pull of its many moons—Saturn vibrates. The gravitational influence of those oscillations minutely warps the shape of its rings into a pattern like the spiraling arms of a galaxy. The result is a visible record of the vibrations, like the trace on a seismograph, which scientists can decipher to plumb the planet. Mankovich says it's clear that some of these vibrations reach the deep interior, and he has already used "ring seismology" to estimate how fast Saturn's interior rotates.
Cassini's last gift may be to show how fortunate scientists are to have the rings as probes. Data from the spacecraft's final orbits enabled Iess's team to show the rings are low in mass, which means they must be young, as little as 10 million years old—otherwise, encroaching interplanetary soot would have darkened them. They continue to rain material onto Saturn, the Cassini team has found, which could one day lead to their demise. But for now they stand brilliant against the gas giant, with more stories to tell.