 |
 |
||
 |
|
||
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
||||||||
 |
Planetary News: Cassini-Huygens (2004)Cassini Photographs Saturn's Moon Phoebe with Astonishing ResultsBy Emily Lakdawalla Cassini’s first close encounter at the Saturn system went perfectly. On Friday, June 11, 2004, the Cassini-Huygens spacecraft passed within 2,000 kilometers (1,200 miles) of the surface of Phoebe. The new images of Phoebe being released every day are stunningly sharp, packed with detail, and sometimes, very surprising.
Phoebe is the most distant large moon of Saturn and is an oddball among Saturn’s 31 known moons. Most of Saturn’s moons orbit the planet in the same plane and in the same direction as Saturn’s rings, but Phoebe travels in the opposite direction and in a steeply inclined orbit. This unusual orbit, in combination with its dark surface, has caused scientists to suggest that Phoebe originally formed elsewhere in the Solar System and was later captured by Saturn’s gravity. As Cassini's images of Phoebe were played back to Earth by the spacecraft they were disseminated across the world to the home institutions of members of Cassini’s imaging team. For Amanda Hendrix, located with the Cassini project management at the Jet Propulsion Laboratory, watching the new images come in was like being a spectator at a fireworks show. “It was like the fourth of July,” she said, “because every time a new image came down there would be ‘oohs’ and ‘aahs.’ It was really fun!” Elizabeth Turtle, a science planner at the Lunar and Planetary Laboratory at the University of Arizona, reports that she spent the day at her desk, repeatedly clicking her mouse to refresh her computer screen and see new images. “It was spectacular. Just sitting here in my office this weekend and seeing the images come back was brilliant.” First Impressions of the Cassini Images of PhoebeThe images from the encounter had mostly been received by 2 p.m. on Saturday afternoon, shortly after the encounter had ended. Imaging team scientists wasted no time in examining the images. “I think there must have been 200 emails over the weekend exchanged between all the [team members] talking about what we were seeing in the images,” Turtle said. There has not yet been time to reduce the images—to calibrate them or correct their brightness for the influence of the angle of solar illumination—so it’s premature for anybody to be drawing conclusions about what they mean. But it’s not too early to make observations.
The first and most obvious observation is that Phoebe is covered with impact craters. “We just had no idea how cratered this thing was going to be,” Hendrix said. The craters appear to cover every bit of the surface at every possible scale. Such saturation with craters indicates that Phoebe’s surface is probably old, geologically speaking. But only probably. Scientists have to map out where the craters lie on the surface, methodically counting each crater by size and location. The locations and sizes of the craters will help them tease out the geologic history of Phoebe—whether all the craters happened very early in its history, as is true of many asteroids, or whether there has been more than one episode of “resurfacing” in Phoebe’s history. In addition to the craters, there are enormous gouges with steep walls—gouges that, in some cases, cover nearly an entire hemisphere. Could something besides cratering have shaped the surface of Phoebe?
Another observation is that there are lighter places and darker places on the surface—but not where scientists expected them to be. “People expected to see bright patches that weren't necessarily associated with the [locations of] craters,” Hendrix said. But the bright material seems to be “associated with patches on the craters at the rims and walls.” However, the fact that these observations were made on raw images that had not yet been calibrated makes this correlation somewhat questionable. “A lot of the bright stuff that we see is bright because it’s close to the sub-solar point,” Hendrix cautioned. (In other words, the Sun was shining most directly on those surfaces.) “So right now it’s hard to figure out how bright the bright stuff really is.” In fact, the bright stuff might not be bright at all—it might just be less black than the rest of Phoebe. According to Carolyn Porco, the Team Leader for Cassini’s camera system, the spacecraft is so steady that it can capture very long exposures of very dark surfaces and make them appear “as bright as day,” so the relatively bright stuff could indeed be quite dark. Still, the science team appears to think that the bright material could be exposed ice. Finding exposed ice on Phoebe would be exciting because it would indicate that Phoebe came from the outer solar system—possibly even the Kuiper Belt—and not from the asteroid belt. Because of the densely packed craters, Turtle says, “At first glance it looks like an asteroid, but it’s not. And as we looked at the images more and more we found features that are clearly different from the asteroids.” The most exciting feature seen in the images were what appear to be layers in some of the crater walls. “When I saw that, I just freaked out,” said Turtle. “The inside of Phoebe could have layer upon layer” of bright and dark material, Hendrix thinks. But Turtle is more cautious. “It looks like layers, but it doesn’t have to be layers. There’s dark, rigid material and then there’s lighter-colored debris on top of it. We can’t say for sure [whether it is layered] but it is really intriguing. And the fact that there’s dark, apparently more resistant rock around the rim is fascinating.” It Takes a Very Steady SpacecraftThe astonishing images of Phoebe could not have been so sharp were it not for the remarkable steadiness of the spacecraft. Because the science instruments are bolted onto the spacecraft and not to a mobile scan platform, the entire spacecraft has to be pointed precisely at a target in order for images to be captured. Even the tiniest error in pointing can cause the target to lie off-center in the camera frame, or to fall out of the image entirely, a problem that was repeatedly encountered by the Galileo spacecraft during the latter part of that mission. But “The navigation was spot-on,” reported Hendrix. “From the very first images that came out, Phoebe was absolutely centered in the field of view as we had hoped it would be.” Cassini didn’t only have to worry about centering the target. During the encounter, Cassini was traveling past Phoebe at a relative speed of 6 kilometers per second. And Phoebe is a dark target, requiring long camera exposures. That means that in order to prevent the image from being blurry, Cassini actually had to rotate while the camera shutter was open in order to maintain her pointing at Phoebe. “The spacecraft was basically told ‘stay pointed at this particular region on the body, regardless of the fact that you are moving,’” said Porco. “It is incredible. This is a capability that we built into Voyager after it was launched—and after we learned how to do it!—but in Cassini it’s done in a much more sophisticated way.” Cassini actually updates her pointing on the fly, an autonomous capability akin to the Mars Exploration Rovers’ ability to autonomously navigate around rocks. Cassini examines the images as she captures them, determines whether the target is where she expected it to be, and adjusts her pointing accordingly. The autonomous tracking was tested successfully during a flyby of the asteroid Masursky on January 23, 2000. Improvements on Voyager
It’s not really fair to compare the Voyager observations at Phoebe with Cassini’s. After all, the closest that either of the Voyager spacecraft got to Phoebe was a distant 2,200,000 km (1,400,000 miles) away, while Cassini passed 1000 times closer. Even so, Cassini’s Narrow Angle Camera is a real improvement on the Voyagers’. Porco listed the differences: “We have a larger range of brightnesses that we can capture in one picture; we’ll be able to see to finer contrast differences; for the same distance from a target we have 30% better resolving power; we have a 60% larger format [resulting in 60% more pixels in each image]; and we have more data compression modes,” which means that more images can be sent back to Earth. “I don’t say this easily!” Porco says. “Because I have such an emotional attachment to the whole Voyager project and those Voyager pictures. That goodbye picture of Saturn is still my favorite, even though now it’s looking more like a piece of artwork than a picture compared to what we’re seeing from Cassini. [Voyager] carries so much emotion for me that it’s hard for me to say that anything is going to surpass Voyager—but Cassini is going to surpass Voyager for the clarity and the content of its images.” What’s Coming NextThese images just represent the tip of the iceberg; there are many more to come. Furthermore, the Cassini spacecraft has eleven more instruments that were operating at various times during the encounter. Unlike the images from the camera system, the data that the other instruments produce must be very carefully calibrated and reduced before it’s safe to begin making observations.
Everybody is eagerly awaiting data from the Visible and Infrared Mapping Spectrometer, or VIMS. This instrument has a lower spatial resolution than the cameras, meaning that pictures created using VIMS will not be so detailed or sharp, but it has spectral resolution far surpassing the cameras and will produce a kind of data that has never before been available for the Saturnian moons: measurements of the color and composition of the surface, and how they change from point to point across the moon. “You can’t resolve Phoebe from the ground,” explains Porco, “so any information about its composition or its color [that we already have] is integrated over the whole body. [With VIMS] we’ll be able to see that one area has a slightly different color from another, so we’re looking at differences in composition across the surface.” But what VIMS measures—the way that Phoebe’s surface reflects different wavelengths of light—is strongly dependent on factors like the incidence angle of the Sun. And the incidence angle of the Sun varies erratically across the surface of Phoebe because that surface is so uneven and rough. Correcting for this effect will take some time. “The VIMS folks are working on analyzing their data,” Hendrix reports, “but they aren’t sharing anything yet. They don’t want to give the wrong impression of what they are seeing. But I’ll bet we’ll hear more from them [later] this week.” Everybody expressed hope that the hectic pace of receipt and release of images will slow down soon, so that the science teams can take the time necessary to perform more careful analysis of the data. Once they have done so, the instrument teams will get together to share their data with each other at a workshop to be held in Tucson, Arizona on June 21. Such workshops will allow scientists to turn conjectures based on one instrument’s tantalizing results into conclusions based on many separate lines of evidence. Apart from interpreting what the images mean, the scientists face another daunting task: assigning names to all of the newly observed craters, gouges, and other features on Phoebe's surface. The International Astronomical Union's Working Group for Planetary System Nomenclature is responsible for generating lists of names for features on planets. Each of Saturn's moons has already been assigned a naming scheme. For the moon Phoebe, the naming scheme was supposed to include people associated with the Greek goddess Phoebe (the goddess of wise counsel, daughter of Uranus and Gaia, sister of Saturn, and grandmother of Apollo and Artemis) as well as islands in the Greek archipelago. There's a snag with the naming of Phoebe's features. Toby Owen, who is an interdisciplinary scientist on Cassini and is also a member of the IAU Planetary Nomenclature Working Group, says "We've looked at the [Cassini] images, and there just aren't enough names on the list." In order to generate a long enough list of names for scientists to use to name all of the features visible on Phoebe, Owen says they will be turning to the legend of Jason and the Argonauts, which has "lots of place names," hopefully enough to cover Phoebe. But the science team won’t have much time to digest the Phoebe data and name its features. Less than three weeks from now, Cassini-Huygens will pass among Saturn’s rings, and Cassini will fire her main thrusters to place the two spacecraft into orbit around Saturn. The anticipation of this critical moment in the mission has the scientists on edge. “It’s really exciting but it’s getting kind of scary. It’s going to be a tense time but if everything goes well it’s going to be incredible,” said Turtle. The astonishing images from this brief encounter prove that—if everything continues to go well—the results from the four-year mission are going to be spectacular. “Can you imagine what Enceladus is going to look like?” asks Porco. “Or Dione? Or the surface of Titan—assuming we can see through the haze?” |
||||||||||||||||||
 |
 |
||||||||||||||||||
[ what is this? ]