The Interstellar Age (15 page)

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.

“We’re talking about a moon whose geology changes the same way the weather changes on our planet,” points out Rich Terrile. “It’s something right out of science fiction. And yet, it’s right here, orbiting Jupiter.” The little moon is turning itself inside out trying to get rid of all that internal heat.

My JPL planetary science colleague Rosaly Lopes recalls, “I was a student at the time
Voyager
discovered Io’s volcanoes, and I thought that an incredible discovery.
Voyager
detected a dozen active volcanoes, and that blew our minds at the time.” Rosaly later earned a spot in the 2006 edition of
The Guinness Book of World Records
as the discoverer of the most active volcanoes anywhere—a total of seventy-one on Io.

Voyager
’s images at Jupiter’s second-closest big moon, Europa, were also full of surprises, although what was found to lie
under
the surface is what gained the most attention, then and since. Europa was only rather poorly photographed by the
Pioneer
missions earlier in the 1970s, and
Voyager 1
was able to see its bright icy surface only from a long distance away since its trajectory was optimized for
close passes by Io, Ganymede, and Callisto. Four months later,
Voyager 2
passed through the Jupiter system and got close-up views of Europa. Oh my goodness, was it worth the wait! The details seen in
Voyager 2
’s photos were truly new and exciting. Indeed, perhaps the most common first impression among the imaging team when seeing those first close-up photos was “Wow—that’s
flat
!” And it sure is. Europa is about the same size as our moon, but unlike the 3 to 5 miles of rugged elevation difference found among the mountains and valleys of our moon, the largest “mountains” and deepest “canyons” on Europa are only around 30 to 50 feet tall or deep. That is to say, if Europa were the size of a bowling ball, the tallest bump on its surface would be less than the thickness of a piece of thread! Another surprise was (again) the relative lack of impact craters—the scars left on ancient planetary surfaces by asteroid and comet impacts over the eons. This implied that some process must be resurfacing Europa, erasing the craters that must surely have built up over time. To everyone’s amazement,
Voyager
had discovered one of the flattest and youngest surfaces (though not as young as nearby Io) in the solar system.

But why is it so flat? A clue came from the crazy-quilt series of lines that crisscross Europa’s surface—dark cracks that separate the icy crust into a jumble of curvy or triangular sections, almost like the pieces of a big jigsaw puzzle. In some places, the puzzle pieces seem to have rotated relative to one another, and in other places to have spread apart, letting reddish-brown material ooze up in the intervening spaces, maybe similar to the way fresh, new volcanic lava oozes up between tectonic plates that make up the Earth’s mid-ocean ridges. Indeed, one of the most common and obvious reactions to Europa’s cracked and platelike surface was to compare it to
melting sea ice—a thin layer of frozen water floating on top of liquid water, sloshing around. On Earth, waves and the warmth of summer break up polar sea ice into millions of little icy “plates.” If that is essentially what is happening on Europa as well, the implications would be enormous. Life on Earth may have begun in the ocean. Does Europa have one?

Frustratingly, the
Voyager
view of Europa was only fleeting—two quick flybys, and one at a pretty far distance away. The known presence of strong heating from tidal forces (heavily in evidence at Io), the super-flat surface, the sea ice–like plates with distinctively colored material appearing to ooze up from the depths below—all these pieces of evidence pointed toward the possibility of a subsurface ocean. I like to imagine what would have happened if
Voyager
had been outfitted with a high-speed submarine probe that could have penetrated Europa’s thin ice shell and plunged into the watery depths below: Turning on its headlights, the sub relays real-time video as it dives deeper and deeper. Finally approaching the seafloor, the water becomes murky, and the probe’s thermal sensor detects a hot spot up ahead. Jupiter’s tidal energy is flexing Europa, heating its icy and rocky interior, and the chemical sensor identifies hot, sulfur-rich water and gases leaking out of the crust here just like at many mid-ocean ridges on Earth. With the outside pressure rising and beginning to stress the sub’s hull, the onboard mass spectrometer starts an analysis for organic compounds in the hot waters. Switching to wide-angle mode, the probe begins to scan for any signs of motion. The pressure is getting critically high and the sub’s signal is beginning to weaken as we’re now more than 60 miles below the surface. The video is getting ratty. There’s a flash of some kind—then static—then another flash, and then a strange, curved
silhouette of—something? But then the signal is lost and we all just stare, dumbstruck, at the static. What had we just seen?

“Launch a class-five probe, Number One!”
Star Trek
’s Captain Picard would have ordered, had the USS
Enterprise
encountered this ice-covered water world. “And fit it for higher-pressure submarine operations!”

But as Europa receded from view, all the
Voyager
team could do was make the best of the limited data in hand and dream about the day when they’d be able to go back, at least virtually, and take a longer, deeper look at this enigmatic world. US Geological Survey planetary geologist and
Voyager
imaging team satellites subteam lead Larry Soderblom remembers the sort of deer-in-the-headlights feeling that he and many others on the imaging team had after taking only those two brief passes by Europa. Although he is an expert geologist, his Earthbound experience left him stumped time and again when trying to interpret the strange new landscapes that were revealed in the
Voyager
images.

“Although we only had a glimpse of Europa, the fact that there were so few impact craters on its surface left no doubt that its surface was geologically young—maybe only 100 million years old,” he explains, being sure to note that 100 million years truly is “young” to geologists. “Europa formed about four and a half billion years ago along with the rest of the solar system, so 100 million years is only about 2 percent of its lifetime. Surely, we all thought, its surface must still be changing today. But what causes those changes? We all wished we’d had a chance to take a closer look.”