LPSC: Friday: Hayabusa
Posted by Emily Lakdawalla
20-03-2006 16:54 CST
So, finally, on Friday morning, came a session I'd been waiting all week for. I made sure to get up early and get to the room to make sure I'd be able to sit down; it'd been scheduled for the second-largest room in the conference center, but I knew that a lot of people were interested in seeing the Hayabusa team presenting their results. It turns out I needn't have worried -- by Friday, a lot of people had already left for home, and fewer and fewer people can muster themselves out of bed in time to make the start of an 8:30 session. (The extracurricular activities at LPSC can run rather late and be very, very fun.)
Still, the audience was rapt as Project Manager Jun'ichiro Kawaguchi stood up to give an introduction to the spacecraft and described the saga of the mission to date. The Hayabusa spacecraft and its story have been covered extensively on this website (with lots of input from Kawaguchi directly) so I didn't take any notes on his talks.
Akira Fujiwara stood up to outline the high points of Hayabusa's work at Itokawa (here's the abstract). Some high points: The mass of Itokawa was found to be 3.43 Â· 1010 Â± 5%. The density was 1.9 g/cm3 Â± 7%. "It should be noted this is a very low value compared to Eros, which was 2.6."
Fujiwara showed an image of the strange shape of Itokawa, and described to the audience how the Hayabusa scientists saw its shape as that of a sea otter, and in fact throughout the session the scientists referred to Itokawa's "head," "neck," "body," and "back." This seemed excessively cutesy at first, but I have to admit it was far easier to follow the scientists who referred to Itokawa by these animorphic references than the ones who used the place names like "Muses Sea" and "Little Woomera." My attempt at cartooning the otter is at right.
Fujiwara went on: "Otter's head and otter's body, each one is round rather than irregular shape. The rough terrain are composed of many boulders. The smooth terrain are composed of centimeter to millimeter uniform sized gravels. Larger boulders are more abundant on the western side. The maximum boulder, Yoshinodai, is 50 by 30 by 20 meters." Yoshinodai is the huge boulder that sticks out of the side of the rump end of the otter. "Around the back side of the neck region, you find standing boulders." Overall, Fujiwara said, Itokawa "has a faceted structure. Most facets are of impact origin -- I think so -- and some are part of interior fragments, exposed." I would have had a hard time understanding this if I hadn't talked with Olivier Barnouin-Jha earlier in the week. I'm pretty sure that what Fujiwara means is that Itokawa is not a solid body but is instead composed of several large blocks or fragments, rough in shape, and the "faceted" shape of the asteroid represents the different faces of these blocks.
Fujiwara showed some maps of gravitational potential and slope on the asteroid. (I don't know if I've seen these maps on the Internet before -- I'd appreciate if someone could point me to them, if so.) He said, "low slope regions coincide with smooth terrains" and concluded that "mass moved toward low slope regions by impact shaking." In general, "Itokawa is probably a rubble pile asteroid because it has a low density and [calculated] porosity of about 40%, a round shape rather than an irregular one, and a bouldery appearance." Itokawa's shape is plenty irregular, but if you compare it to Eros or Gaspra it's not quite as angular -- it's got that round head and round body. Fujiwara went on, "Itokawa may be a contact binary, because its shape is composed of head and body, because of high-slope regions not yet relaxed to each other. The landslide region is explained by an event associated with a low-velocity collision of the head and body, impact fragmentatin of the parent body, and coagulation among the flying fragments."
Next to talk was H. Demura on shape modeling of the asteroid (click for his abstract). He explained that the Hayabusa team are pursuing an understanding of the shape of Itokawa through three independent efforts. One is by limb profiling (he referred to a poster by M. Maruya et al.); the second was by stereogrammetry (his talk); and the third was with a combination of photoclinometry and input from the LIDAR instrument (presented in a poster by Gaskell et al.). For stereogrammetry, Demura began with many images from the AMICA instrument. They defined 308,205 control points on these images, which they converted to a polygon model with 4,285 facets.
Demura described a few of the interesting characteristics of the model; most interesting to him and others interested in Itokawa's shape is the constricted "neck" region, which Demura found to be "20 meters in depth and 60 to 120 meters in width." He flashed a map onto the screen in which he had outlined all sorts of different terrains and blocks, unfortunately too quickly for me to get any coherent notes from it; I'll have to look forward to a publication. He concluded, "Unique structure of head, neck, body implies it is composed of collided two parts; these two and pedestal blocks could be large 'rubbles.'"
After Demura finished, Guy Consalmagno stood up and asked, "It's fascinating that you can identify individual bits of rubble within the body. What are the typical sizes, and how many pieces are within the asteroid?" Demura answered, "This shape model shows body and head clearly, but other features are not so good. I think 50 facets, 100 meters in diameter." Consalmagno followed up by asking how they could be stuck together within such a tiny collective body. "I don't know the mechanics," Demura answered. "But some landslide features show materials flowed to the neck." After that, Don Yeomans stood up and said, "I think the mean collision velocity" between objects in the asteroid belt "is about 2 kilometers per second, so it's very difficult to understand how two things could approach each other so slowly and stick. But I guess Mother Nature figured out how a long time ago."
Next up was S. Sasaki talking about the observations of the color of the surface of Itokawa using the spectral capabilities of the AMICA imager (here's his abstract). "We saw some brightness variations on Itokawa," he began. "Space weathering produces darkening of overall reflectance and a weakening of absorption bands. It occurs on a time scale of millions of years on airless bodies," and he showed that brightness variations attributable to different ages of surfaces were observable on Ida and Eros. But whether such variations would be visible on a body as small as Itokawa had been a question: "Itokawa is small asteroid, so we do not expect much regolith on surface. So whether it could be weathered or not is important question." The reason Itokawa wouldn't have much regolith is that regolith is the soil that is generated when an impact happens and the impact ejecta settles back on the surface. The Moon is covered with meters and meters of regolith. But for a body as small as Itokawa with almost no gravity, nearly all ejecta should theoretically escape into space instead of settling on the body, so you wouldn't expect there to be much dust.
Still, AMICA clearly saw the kinds of brightness and color variations that have, in the past, been attributed to processes that expose fresh, unweathered regolith. He reported brightness variations from place to place of up to 20 percent as seen in the global scale images, and up to 30 percent in the high-resolution images. He went on, "we have not only brightness variation, but also color variation. Bluer part correspond to brighter part, and redder part correspond to darker part." He showed a few exemplary places, and interpreted the geology: "Here is very steep slope. We could suppose that landslide exposed brighter materials. You can see bright patch and dark patch, and dark material is actually superposed on underlying bright material," just as you would expect if a landslide caused weathered regolith to slide and expose fresh, unweathered regolith. He also noted that the bouldery areas were some of the darkest, and therefore the most weathered.
"What are we learning from Itokawa-sensei?" Sasaki asked, which got a chuckle from the audience (especially because he said it as he showed a slide of a Japanese classroom in which the instructor's head had been replaced with a photo of Itokawa). "Brightness and color variations are probably due to differences in degree of space weathering; rough regolith-poor portions are darker. Removal of darker surface layer should have produced heterogeneity in brightness and color. Probably Itokawa is heterogeneous because it is small. What caused sharp brightness differences? Seismic shaking by impacts? Tidal distortion? Most of bright surfaces were exposed recently. We presume this was caused by a single to a small number of impact events."
Clark Chapman stood up and commented, "You have very few of these bright white spots. You must have more impacts in a million years than you have of these white spots. Either the white spots are due to something else, or there are fewer impacts than you would expect." Tom Ahrens stood up and also expressed surprise at the appearance: "I think it's kind of surprising you don't see small impact craters on boulders protruding from the asteroid. You see those on the Moon." Sasaki commented in response that they should wait to hear the upcoming cratering talks before they started counting craters: "surface is boulder-rich, so it's not easy to identify crater." A little later in the session, another commenter stood up to remark "We need to remember that Itokawa is in a short-lived orbit. Because it is in this kind of orbit, when exactly it left the Main Belt is extremely uncertain. It could have been only a million years ago. It could have formed only 1 or 2 million years ago, and spent most of its time since then outside the Main Belt. So I don't think it's at all surprising that you should find a low density of craters on the body."
Next up was H. Miyamoto describing the smooth terrains on the surface (here's his abstract). Using the shape models described above he found that 20% of Itokawa is covered by smooth terrain; the gravity and slope models reveal that "smooth terrains are always concentrated in local lows of gravity" by which he meant geoid lows, what would be equivalent to the lowest-elevation points on a planet. The phrase "low elevation" means little for a body as lumpy as Itokawa, but all bodies have a gravitational field and low points in that field toward which things will roll; on Earth it's ocean basins, on Itokawa the smoothest spots pretty much outline the equivalent places.
Miyamoto showed some absolutely gorgeous high-resolution views of the pebbly surface of the smooth places. He showed that some high res images overlap significantly, and they used these images, mapping pebbles from one image to another, to generate a stereo view. The stereo view revealed that "the area is completely flat." There was no deviation, it seemed, from planar flatness. Within the smooth areas are some places with more boulders. "The boulders are not randomly distributed, they are kind of making some clusters. This is typical, if you put in the laboratory, granular materials, and shake them. Boulders rise to top." He pointed out that some boulders appeared to have their long axes aligned in the same direction, and demonstrated that this direction was aligned with the local gradient in the gravity model, "so this probably indicates that there is the movement of granular materials."
After that the talks became more and more detailed in reference to high-resolution images and maps that I don't have, so I decided to quit the room and write up some of my earlier notes. But this session was an exciting one -- I look forward to more scientific results from Hayabusa. I do want to mention one more poster I saw on Thursday night, by Y. Yokota, on the "Opposition Effect on Itokawa." It was the studies of the opposition effect that yielded those incredible views of Hayabusa's own shadow on the surface of the asteroid. Yokota's poster explained that the opposition effect was thought to occur only where very fine particulates were present on the surface of a body or in fine, dusty planetary rings. Yet it was observed on Itokawa, which has a pebbly surface; the highest resolution images, with sub-centimeter resolution, seem to show pebbles that are all centimeter-scale or larger. Yokota theorized that either the visible pebbles were covered with a very fine dusting of fine particles, or perhaps that the so-called "pebbles" actually had a porous structure that behaved like a fine dust.
So, that's it for my LPSC notes. I hope you enjoyed them, technical though they occasionally were. And now back to our regularly scheduled programming...