Our Solar System: Jupiter
March 9, 1979 and Voyager 1 had just left the Jupiter system en route to its rendezvous with Saturn. Linda Morabito, the cognizant engineer for the navigation team, noticed something odd about one of the images of Jupiter’s moon Io: There was a ghost image protruding from the side of the moon. She was using surrounding stars and the circular disk of Io to determine Voyager’s path, but a circle wouldn’t fit. Was it another moon peeking out from behind Io? Over the next few hours Linda and her team determined that the feature emanated from Io and was centered over a strange heart-shaped feature on the surface. Finally it dawned on the team: They were witnessing a volcanic eruption – the first one on another world.
Voyager 1 was commanded to look back at Io as it sped away from Jupiter and confirmed the interpretation. A few months later, Voyager 2 passed through the system and its programming was updated to concentrate on the new discovery. Io turned out to have multiple volcanoes almost continually erupting!
Jupiter and its coterie of some 60 moons is almost a mini-solar system unto itself. But four of the moons are about the same size as our moon and are called the Galilean satellites because they were discovered 400 years ago by Galileo himself. You can recreate that excitement with even a small telescope: The four small dots aligned next to Jupiter can be seen to move around from night to night. The moons display an interesting sequence from incredibly active Io, to Europa with its young icy crust, Ganymede with patches of old and new icy crust, to Callisto, an old and dead dusty ice ball. The difference in their appearance is related to their distance from Jupiter. And the reason is that the closer they are to Jupiter’s massive gravity, the more they’re pushed and pulled like taffy by tidal forces. You can get the same effect by rapidly stretching a rubber band; in fact, sailors know that nylon dock lines must not be allowed to stretch over and over too quickly or they can melt. But Io isn’t the only Galilean satellite harboring a mystery: it was immediately obvious that Europa was also anomalous. There were virtually no impact craters, implying the surface was very young, and there were huge rift zones and areas of fragmented crust that looked a lot like frozen ice floes in our arctic. Models now indicate that a liquid-water ocean exists below a thin ice crust, another product of tidal heating.
The moons of Jupiter are just some of the strangeness found in this part of the solar system. Out here beyond the asteroid belt, the planets are all huge and made mostly of gases, primarily hydrogen and helium. In fact, Jupiter on its own contains twice the mass of all the other planets combined. It could contain over 300 Earths. Jupiter is also so massive that it’s still radiating more energy than it receives from the Sun. Because of the massive gravity the planet exerts, it’s difficult to probe very far into the atmosphere, so that much of what we know about its interior is based on models. And those models are extreme. The pressure down inside Jupiter is about 40 million times that at Earth’s surface. Extrapolating what we know about hydrogen to that kind of pressure implies that the gas may exist as a solid metal! Higher up in the atmosphere, we see bands of differing colors, visible even in small telescopes, that indicate circulation patterns. Many cyclones spin off of those bands and can be followed as they coalesce and eventually dissipate. One, though, has persisted at least for 400 years – the Great Red Spot was described by Galileo. It’s a cyclone that could swallow the Earth.
Pioneers 10 and 11 were the first spacecraft to do a drive-by of the Jupiter system. That was back in the early ’70s. But it was Voyager 1 and 2 that flew by in 1979 that really put Jupiter on the map. Launched in 1977, the Voyagers were sent on a Grand Tour of all the outer planets, a trip made possible by the fortuitous alignment of the outer planets allowing gravity-assist trajectories, literally sling-shot boosts as they flew by each planet. That alignment won’t occur again until 2150. Voyager 1 was diverted from the Grand Tour, however, so it could fly closely by Saturn’s largest moon Titan, which was found by the Pioneer spacecraft and terrestrial telescopes to have a thick atmosphere. (More on Saturn and Titan next time.) The two Voyagers are now leaving the solar system on a new, Interstellar Mission.
Starting in 1995, Jupiter and its moons were the subject of intense scrutiny by NASA’s Galileo spacecraft after it went into orbit around Jupiter. That mission, however, didn’t go without a hitch – it was ready to be launched from the Space Shuttle in 1986, but the explosion of Space Shuttle Challenger, with the loss of the crew, led to a re-design and a three-year delay, but finally it was on its way. After gravity assists from Venus and the Earth, Galileo was on its way to Jupiter and the main communication antenna was commanded to unfurl. Much like a huge gold-mesh umbrella, the antenna had to be folded for launch. However, because it had been stored in its launch configuration for several more years than planned, a couple of the ribs stuck to the central post and the antenna only partially unfolded. As there are no repairmen making house calls in deep space, a workaround to the decreased data-rate of the smaller antenna had to be found or thousands of images of the Jupiter system would never be sent to Earth. Luckily, a new image compression algorithm had been invented around that time, called jpeg. The software was uploaded to the spacecraft as it sped toward Jupiter and the mission was saved.
The Galileo mission cemented Io’s reputation as the most volcanically active body in the solar system and refined our understanding of the dynamics of Jupiter’s atmosphere. It also added to the mystery of Europa. Was there a way to learn more about what might be going on in its subsurface ocean? Could life exist there? As its fuel reserves dwindled, in 2003 Galileo was commanded to do a death spiral into Jupiter’s atmosphere. As with the Jupiter atmospheric probe it launched into the atmosphere at the beginning of the mission, Galileo radioed data back about pressure and temperature until it met its end.
Exploration of the Jupiter system continues with the Juno orbiter, which is concentrating on the atmosphere. But new spacecraft are on the drawing boards or proposed. JUICE (Jupiter Icy Moons Explorer) is a European Space Agency project planned to launch in 2022 and set to orbit Ganymede, which also may have a subsurface ocean, after orbiting Jupiter for three years. And NASA is developing the Europa Clipper, set to launch in 2024. One goal is to use multiple flybys of Europa with its long-wavelength radar to see through the icy crust and determine its thickness. If the crust is thin enough, ideas are being kicked around for a lander that would melt its way through the crust and launch a submarine into a new ocean. Europa Clipper will also use sophisticated remote sensing instruments to look for surface manifestations of what is going on in the ocean below. These missions are the beginning of a series of initiatives to explore the Ocean Worlds of the solar system (https://www.nasa.gov/specials/ocean-worlds).
Jupiter, along with its neighbor Saturn, recently gave us a nice show when they came together in the sky, forming a bright Christmas star. They’re both now low in the west at sunset as they slip behind the Sun.