by Jim Bell
Jupiter Bound. Voyager 1 (top) and Voyager 2 (bottom) flyby trajectories past Jupiter. (NASA/JPL)
All was going well as Voyager approached its first moon, Io. We saw weird splotches that weren’t craters, but what were they? Io didn’t look like we expected; what was down there? Everyone was excited. But then something went wrong.
Candy Hansen and her colleagues on the imaging team had learned from the Pioneer flybys of Jupiter that the radiation environment was harsh. Charley Kohlhase told me that the spacecraft engineers had taken extra measures to shield the most sensitive electronics from radiation by using nearly fifty pounds of extra shielding made out of tantalum. Tantalum is a so-called transition metal because it can easily give up electrons to other atoms, thereby transitioning to different energy levels. It is not far from tungsten, molybdenum, and zirconium on the periodic table, making it a relatively dense metal. It is much more strongly resistant to corrosion than lead or gold and particularly good at blocking the kinds of high-energy solar proton and galactic cosmic-ray radiation that is commonly encountered in space missions. Still, despite this high-tech shielding, the intense bombardment of high-energy particles from Jupiter’s enormous magnetic field was still having an effect on Voyager’s computers.
Specifically, the radiation was making the clock in the computer that controlled the scan platform get very slightly out of sync with the separate clock in the computer that controlled the camera’s exposure time—telling the camera when to open and close the shutter. Computers work on the principle of binary numbers, ones and zeros, stored in voltages (electrons) inside transistors and microchips in a specific pattern that represents their software. Changing that pattern dramatically—by shutting off the power, for example—is essentially catastrophic to the software. But changing that pattern in very minor ways—for example, flipping a one to a zero here or a few zeros to a one there—may not be catastrophic but instead can make the software behave differently from how it’s supposed to. This is what radiation (high-energy protons and atomic nuclei) can do: those particles can burrow down into a computer chip and strip off electrons, flipping a one to a zero, or cause new electrons to be added in, flipping a zero to a one. If this damage is localized to certain unlucky places in the software, the resulting change in the software could vary widely, from catastrophic and obvious to insidious and difficult to recognize. Software and hardware designers try to guard against the catastrophic by using radiation shielding, as well as multiple copies of the most critical software—the code that, if changed, could cause a catastrophe—in the computer. But it’s much harder to guard against minor, insidious changes to the code, like a very small change in the rate of a clock in one computer as compared to another.
In some of the early Voyager 1 images, particularly of Io, the out-of-sync computers caused the scan platform to start slewing to the next target while the camera’s shutter was still open. The resulting images were hopelessly smeared, and the team “lost some really nice satellite data,” laments Candy. “In addition to dealing with the anomaly it was stressful because there were so many ‘guest’ scientists in town and they were complaining bitterly, not understanding that we had not anticipated this situation.” Even if they had understood—and once the explanation was found, they all did—I suspect that they were still upset and frustrated simply because of the loss of precious, one-time-only measurements as the spacecraft sped through the Jupiter system.
After years of working closely with the specific quirks and personalities of the Voyager spacecraft and system, and especially in the lead-up to the big events like the planetary flybys, Candy had begun to think of each of the spacecraft almost like a child. “You coach them and coach them to perform at the big school pageant,” she says, “and then at the moment the performance is on you just sit in the audience, helpless, mouthing their lines and praying that they remember the tricky dance step. In that moment during the first Jupiter flyby, it was like the chandelier came crashing down in the middle of the performance.” Rather than panic, however, she and her colleagues realized that they had time to devise a work-around. They had discovered the problem early enough in the encounter. They learned from this early experience, compensated for the out-of-sync computers, quickly uploaded new, last-minute versions of the camera sequences (time for Candy to bake more sorry-for-the-extra-work cookies!), and were able to avoid such problems for the rest of the mission.
With the Internet not yet invented, the only way for anyone not on the team to see all the images was to watch the “live” feed from the DSN on the monitors in the science, press, or public areas scattered around JPL or on a few of the team members’ college campuses, such as at Caltech. Some of the coolest-looking photos would eventually get aired on the evening news or printed in the paper. Later, more would appear in magazines such as Scientific American, Astronomy, or Sky & Telescope, but only months after the images came in. The Voyager team needed to have near-instant access to the images, of course, to make sure the cameras were working properly, to make sure the spacecraft was on the right trajectory, and to try to do some “instant science” to share with the public and eagerly awaiting media at each day’s press briefing. “Back in those days—it’s amazing to think about now—we didn’t have laptops or Photoshop,” says Candy. “We had two small rooms, like broom closets, without windows, each with a display terminal hardwired to the image processing building where the telemetry came in and was turned into black-and-white images. We called those the browse rooms. You could call up the latest images and do simple contrast stretches. The rooms were shared by every scientist on the imaging team.” According to Candy, the browse rooms were huge bottlenecks for the imaging team, and they were occupied 24/7. “There was huge pressure to get press releases formulated, images processed, and captions written. The more senior members, of course, had higher priority, so all of us ‘youngsters’ would stay late into the night to have our turn.” Still, she recalls with a wistful smile and twinkle in her eye, “That was actually a lot of fun.”
Analyzing every image and other measurement as they were sent back over that critical three-day encounter period was the job of the members of the Voyager science and engineering teams. Some of the images were taken for purely engineering purposes, such as calibration or navigation. Many of the images that the Voyagers took when they were far from their planetary targets were taken through what’s known as the Clear filter—a filter that let in the maximum amount of red, green, blue, and infrared light—so that faint stars could be photographed and the cameras used like a high-tech sextant to make sure the ship was sailing in the right direction. The Voyagers are what those in the business call a 3-axis stabilized spacecraft. That is, they do not use a rotisserie-like spinning motion (spin-stabilized) like some other spacecraft to maintain their orientation. Instead, using thrusters, special sun sensors, and special star trackers, the spacecraft is held in a fixed orientation relative to the stars. Often, spacecraft will use the sun and one or two bright stars, such as Canopus or Sirius, to establish and hold a fixed orientation (or “attitude”) while cruising through space. One nice advantage of this kind of attitude configuration is that cameras and other instruments can set very long exposure times for faint objects, without having to worry about the spacecraft’s motion blurring the data. However, sometimes the spacecraft needs to be pointed at an area of space with no bright stars. In that case, the science cameras can often be used to take photos of faint stars instead (there are always fainter and fainter stars to be found, anywhere in the sky). Whether imaging bright or dim stars for this purpose, this process is called optical navigation, or sometimes just opnav. Once that spacecraft-sun-star orientation is known—and it’s the job of the navigation team to know it—spacecraft- or camera-operations people can point the equipment to any other known target in the sky with confidence.
Still, most of the photos taken were for scientific purposes rather than for celestial navigation, especially
around the time of closest approach to Voyager’s various planetary and moon targets. Science images were often taken through color filters so that the red, green, and blue images could later be combined into a color image simulating what we would have seen with our own eyes if we were riding along on Voyager. Sometimes the team created “false color” images by using additional violet or infrared filters to produce more garish enhancements of what would have otherwise been only subtle colors to our eyes. In addition to their artistic or aesthetic value, color images also helped provide some information on the composition of the moons or the cloud layers in the atmospheres of the giant planets. Many times the colors or tones of features in the Voyager images required special image processing to bring them into view, however. This was often the job of staff scientists and engineers who worked in JPL’s Image Processing Lab, where they had a browse room of their own.
Indeed, it was the astute pair of eyes of engineering and spacecraft navigation team member Linda Morabito that made the first, and perhaps one of the most important, of Voyager 1’s discoveries at Jupiter’s innermost moon, Io. It had been known since Galileo’s time that the three largest innermost moons of Jupiter travel in a very special kind of orbital dance called a resonance. Specifically, for every single orbit of Ganymede around Jupiter, Europa orbits exactly twice and Io orbits exactly four times. That is, like the hour, minute, and second hands of a clock, those three worlds occasionally line up with one another, and thus their gravitational attractions nudge them each away from what would otherwise be perfectly circular orbits. Each of them, then, is sometimes slightly closer to or slightly farther from Jupiter than usual.
The mathematics of the orbital resonance of Io, Europa, and Ganymede had been worked out in detail around 1800 by the French astronomer Pierre-Simon Laplace (indeed, the resonance is named after him). But the implications of the Laplace resonance weren’t fully appreciated until just before the Voyagers arrived at Jupiter. In fact, in a scientific publication intentionally timed to appear in print just three days before Voyager 1’s flyby, a team of three celestial mechanics experts led by Stan Peale of UC Santa Barbara published a prediction in Science magazine that the resonance that was slightly changing the inner Galilean satellites’ distances from Jupiter would result in a gentle squeezing and relaxing of their interiors. Over time, the squeezing should heat the insides of those moons, with the strongest heating—perhaps all the way to the melting point—happening at Io, the moon closest-in to Jupiter. In one of the most famous modern examples of theorists making a testable prediction that eventually was dramatically proven to be correct, the authors ended their prescient paper by saying, “Voyager images of Io may reveal evidence for a planetary structure and history dramatically different from any previously observed.”
And holy cow, were they right! Io was revealed to be like no planet ever seen before, or since. Measured to be only slightly larger than our moon, its surface is an alien reddish-brown in color and covered in lots of circular or semicircular splotches of yellow, white, and black. The splotches don’t look like they are impact craters, however; in fact, no craters like those peppering the surfaces of our moon and Mars could be identified in the Io images. Because impact crater scars build up slowly and systematically on a planetary surface over time, the lack of any craters on Io suggests that its surface is constantly being refreshed, wiped clean by some process, and thus is very young. Io’s mass could be estimated by measuring the tiny tweak that its gravity gave Voyager 1 as it passed only 13,000 miles above the surface; combined with an estimate of its volume, this led to an estimate of Io’s density as 3.5 grams per cubic centimeter. This number suggested a very rocky, rather than an expected icy, composition. Io was proving to be strange indeed!
The kicker, though, came from Voyager 1 images taken three days after its closest approach to Jupiter, looking back toward Io. Linda Morabito had been tasked with helping to verify the post-flyby trajectory of Voyager 1 using opnav images of the positions of faint stars seen in the backgrounds of images, in this case images of Io. What she found in one of those images, which no one had noticed previously, was stunning.
A little digression about Voyager’s digital images and the team’s image-processing methods seems warranted here, to put the circumstances surrounding Morabito’s discovery in context. The Voyager cameras took images at a resolution of 800 x 800 pixels, where each pixel could have a value between 0 (no signal) to 255 (maximum signal). According to Voyager imaging team member Torrence Johnson of JPL, almost all the Voyager images that were being streamed to the science team on the TV monitors and printed out for team members in the science workroom were being displayed in black and white, where black meant signal levels near 0, white meant signal levels near 255, and various shades of gray corresponded to values in between. That is, they were being displayed using the full dynamic range of the cameras, taking advantage of the hard work done earlier by the science and sequencing teams to make sure they got the exposure levels right. These were great for normal pictures of Jupiter and Io and other moons, but they weren’t very good for Morabito’s search for background stars in the Io images. That was because the background stars were really dim compared to Io, maybe at signal levels near only 5 or 10, and so unless something different was done, they’d come out on-screen and in printouts looking basically black. Thus, in order to see the stars, either the navigation team had to request images with extra-long exposure times to make the stars brighter (and, perhaps, saturate the pixels of anything else in the scene, like a planet or moon) or the navigation team technicians would have to do what image-processing people call stretching the images, that is, changing the display so that black is still near zero but white is set to a much lower level, like 10 or 20—making the stars show up. Of course, everything else in an Io image with values above 10 or 20 would also show up as white, making Io itself look washed out. But that would be OK; it was the stars they were after.
When Linda Morabito viewed the Io images taken for navigation purposes, displaying and stretching them on the Image Processing Lab’s workstation, the stars popped out as expected, but so did two other unexpected things: a bright circular blob along the day/night boundary on Io, and a fainter umbrella-shaped crescent sticking up a few hundred miles above the edge of Io’s limb. The cloudlike feature above the limb looks remarkably like another moon passing behind Io, but Morabito and other Voyager navigation engineers knew that there weren’t any other moons in the right place at the right time to explain that feature. There also weren’t any camera smudges or other artifacts that would look like that. After ruling out those and other ideas, she and her nav team colleagues were left with only one hypothesis that seemed to fit the data: the crescent and bright blob were eruption plumes from active volcanoes on Io.
Voyager 1 Io Volcano Discovery Image. Image C1648109, revealing the first evidence ever found for extraterrestrial volcanism. Displayed at left as originally seen in the rolling displays broadcast on the Voyager science team monitors and at right in the harshly stretched format first used by the Voyager navigation team. The black dots are reseau marks embedded in the camera and used to correct slight image distortion. (NASA/JPL/Jim Bell)
JPL director Bruce Murray was skeptical, recalls teammate Torrence Johnson, as Murray had spent considerable effort dismissing similar claims of active volcanism on the Martian volcano Olympus Mons, claims based on fuzzy, cloudlike features in the Mariner mission images. Johnson recalled that imaging team lead Brad Smith tasked a select subset of the team, headed by Cornell planetary scientist Joe Veverka, to examine all the relevant Io images in greater detail, to try to confirm Morabito’s hypothesis. Not only did Veverka’s subteam confirm the volcanic plume nature of two features in that image, they quickly identified seven additional plumes in earlier Voyager images of Io, using similar image-stretching techniques as Morabito and the navigation team.
“The reason no one noticed the plumes earlier,” Torrence
Johnson says, “was because the real-time images we were seeing on the monitors during the encounter had been processed on the fly to cut off the top and bottom 5 percent of the pixels. Effectively, we had an anti-volcanic plume filter on the science monitors!” In each new plume discovery, the plume was located near or above dark surface depressions, in places where Voyager’s thermal infrared instruments were finding strange signals as well.
“I still remember the first Io data,” recalls Linda Spilker, who was responsible for planning the infrared spectrometer observations. “The spectra had an unexpected slope because we were actually seeing both the Io background temperature and the much hotter volcano temperatures, but we didn’t know it at first. We kept checking and rechecking the calibration until finally, with the discovery of the volcanoes in the images, we knew our data were right.” The very high temperatures were consistent with molten or cooling lava. Supporting lab experiments showed that molten volcanic rocks containing large amounts of sulfur at different temperatures could reproduce the palette of white, yellow, red, orange, and black hues seen on Io’s surface in the color images.
Io was a volcanically active world! Peale and his colleagues were right in their prediction published just days before the flyby—all that flexing from the combined effects of the satellite resonances and the strong tidal pull of Jupiter did heat up and melt the inside of Io. It was the first major discovery of the mission, and the first discovery of active volcanoes beyond the Earth. Four months later, when Voyager 2 flew through the Jupiter system and photographed Io, the surface had changed significantly, including the formation of new plumes. Images taken by three more spacecraft, including the Galileo Jupiter orbiter, that have studied Io in the decades since have shown even more changes in the moon’s tortured volcanic surface. Not only is Io volcanically active, it is hyperactive, harboring the most intense and voluminous volcanic eruptions in the solar system.
