Jupiter, king of the gods in Roman mythology, shrouded himself in clouds to hide his true nature. But his wife, the goddess Juno, was able to peer past this veil and reveal his mischievous behavior.
It is fitting, then, that the NASA spacecraft now circling Jupiter the planet is named after Juno. The gas giant obscures its inner workings with opulent swirls and bands of clouds, and the Juno orbiter is tasked with probing past this barrier to search for clues about the planet’s origin and evolution. What secrets lie beneath?
Published on October 28 in Science, Juno’s latest results gauge the depths of Jupiter’s violent storms, including its Great Red Spot—a vortex wider than the Earth that has been violently churning for hundreds of years. Gathered via a combination of microwave observations and gravity measurements, the findings will help scientists characterize the dynamics of Jupiter’s weather and how it has cooled down over time, providing insight into the still-mysterious births of gas giants within our solar system and beyond.
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“Jupiter’s beauty is not just skin deep,” says Scott Bolton, an astrophysicist at the Southwest Research Institute and principal investigator of the Juno mission. “And we are seeing, for the first time, the atmosphere in three dimensions.”
Gazing below Jupiter’s dense cloud layer is key to learning about the more general energetic processes transferring heat out of gas-giant planets across the universe, says Rakesh Yadav, a theoretical planetary scientist at Harvard University and co-author of one of the papers. “It tells us not just about the present condition of these planets, but also what might have happened in their pasts,” he says. In the specific case of Jupiter—the first and most massive planet to form around our sun—its genesis must have played a large role in the sculpting the rest of our solar system.
Jupiter 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 microwave transmissions at six distinct frequencies, each value corresponding to a different depth. “It’s almost like we’re sticking a thermometer in,” Bolton says. With the radiometer, the Juno team can create heat maps of Jupiter’s atmospheric layers and look for circular features at different altitudes in the atmosphere, which indicate the presence of a vortex. They discovered that the roots of the Great Red Spot extend to at least 240 kilometers below the cloud tops, as far as down the microwave radiometer could see.
To probe even further, the researchers turned to studying distortions in Jupiter’s gravity field, indicated by the way the planet pushes and pulls on Juno itself, ever so slightly changing the orbiting spacecraft’s speed and direction relative to Earth. Data collected from Jupiter is sent back to our home planet with a radio signal of a specific frequency, but these tiny deviations in Juno’s orbit will cause the frequency of that signal to change; it falls when Juno is moving away from the Earth, and rises when the craft is pushed back toward us. By measuring these tiny shifts as Juno flies over the Great Red Spot, scientists can make precise measurements of the local gravitational field and the depth to which the massive vortex extends.
The gravity data revealed that the Great Red Spot’s atmospheric “roots” extend no more than 500 kilometers below the cloud tops of Jupiter. And it likely doesn’t have a sharp cut off at this point, Bolton says. Rather, its vortex probably fades off gradually, though to confirm this would require measurements of frequency shifts currently too tiny for Juno to resolve.
Jonathan Lunine, a planetary scientist at Cornell University and co-investigator on the Juno mission, thinks that the deep roots of the Great Red Spot could explain the raging storm’s longevity. On Earth, he says, the energy that drives our weather primarily comes from the condensation of water vapor at the bases of clouds, which is what forms features like rain, wind and lightning. But the Great Red Spot bottoms out well past Jupiter’s cloud base. “So, it’s not just a meteorological feature,” he says. Instead, its circulation must be drawing energy from much deeper and denser layers of the atmosphere.
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, who is not involved in the mission. But the exact energetic processes involved in this remains unknown. In principle, further clarity could come from comparative studies of Jupiter’s atmospheric dynamics with those of gas giants orbiting other stars, even though the latter are far beyond the reach of any Juno-like direct investigations. In the meantime, Koskinen says, we must content ourselves with lavish observations of Jupiter, “the gas giant in our own backyard.”
As for the fate of the Great Red Spot—whether it will continue churning for many years to come, or eventually shrink and disappear altogether—that answer is still hidden behind Jupiter’s clouds. For now, anyway: NASA has extended the Juno mission for four more years, so scientists on the team look forward to the next phase of the experiment. Yadav, in particular, is excited for more data to use as an “observational anchor” for refining models of how Jupiter’s atmospheric dynamics have evolved over time. Bolton also says that Juno’s slowly shifting orbit is approaching an alignment with the planet’s north pole, where another giant tempest lies in wait. As the spacecraft flies over this extraterrestrial polar vortex, the mission team will be able to measure its stability, structure and depth as well, adding another data point to compare with the Great Red Spot.
“This is an exploration into the unknown parts of Jupiter,” Lunine says, a first chance to observe how weather works in the essentially bottomless atmospheres of gas giants. “And it’s exciting to be able to probe that with Juno.”