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By Emily Lakdawalla

Itokawa: Gravel slides in near-zero gravity

May. 11, 2007 | 16:14 PDT | 23:14 UTC

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Itokawa, the asteroid visited by Hayabusa, is really, really tiny. In fact, it makes a nice bridge between the scales of natural objects in the solar system and manmade objects in the solar system. Here are Itokawa and the International Space Station, both shown at the same scale of 2 meters per pixel:

Itokawa at a scale of 2 meters per pixel

When the images of Itokawa were returned to Earth, the distribution of gravels on its surface was truly puzzling. The first puzzle is how there can be gravels at all. Itokawa has nearly no gravity, so when something hits it, you expect basically everything to be blasted off from the surface at much higher than its puny escape velocity of something between 10 and 20 centimeters per second. (Take a moment to think about how slow 20 centimeters per second is. That's much slower than walking speed, though it's also about four times faster than the designed top speed of the rovers.)

But we'll leave that puzzle for another day. The puzzle to consider today is how such a tiny body as Itokawa can have some very smooth-looking areas and some very rough-looking areas. It's small, so when impact events spread material out, that material should come down on the asteroid all over the entire surface, producing a globally continuous and generally uniform "regolith," or surface layer of broken-up rock. But that's not at all what Hayabusa saw at Itokawa. The contrast between the rough and smooth areas is nicely summed up in this image:

Click to enlarge >

Smooth and rough areas on Itokawa
Hayabusa captured this view of Itokawa as it slowly approached the asteroid on November 9, 2005. The scale of the full-size image is 6 centimeters per pixel. The smooth area to the right is Muses Sea, where the spacecraft would eventually make its touchdown. On the left edge is a circular impact crater called Komaba. The white box indicates the area covered the closer view below. Credit: JAXA / ISAS / University of Tokyo

So, scientists asked, how did Itokawa's regolith get sorted into more gravelly and less gravelly areas? A team led by Hirdy Miyamoto of the University of Tokyo published a paper in Science Express two weeks ago indicating the answer. They found an interesting clue when they got close-up views of the rougher gravelly patches:

Click to enlarge >

Oriented gravels on Itokawa near Komaba crater
This is one of the close-up views of Itokawa taken by Hayabusa during its first descent toward the asteroid on November 12, 2005. The scale of the full-size image is 1.6 centimeters per pixel. The cobbles in this image are clearly "imbricated," oriented and stacked on each other in a direction that indicates that the gravels have moved in a direction from left to right. This is the same direction as local downhill, moving toward the local low of Muses Sea. Credit: JAXA / ISAS / University of Tokyo

If you stare at this image for a while, you might notice that pieces of gravel that have one longer and one shorter dimension are preferentially oriented so that the longer dimension is more up-and-down. In fact, Miyamoto's team measured the orientations of all gravel chunks in the image and found that they had a strongly preferred orientation.

On Earth, gravels get lined up in this fashion -- imbricated -- in stream beds, where the action of flowing water pushes and shoves rocks so that they are stacked and lined up and indicate the direction of water flow. There's no water flowing on Itokawa of course, but it appears that the gravels have somehow been flowing. The imbrication of Itokawa's gravels indicates that they are flowing downhill, toward "gravitational potential lows." What could make the gravels flow?

The answer, Miyamoto and coworkers argue, is vibration. Itokawa could have been set into vibrational motion by even very small impacts: "a centimeter-sized impactor can globally induce seismic acceleration on Itokawa as large as its surface gravity," the paper states. "Other possible reasons for vibrations include tidal effects, thermally induced mechanical fluctuations, or low-speed collisions between the Head and the Body."

Click to enlarge >

Itokawa: a sea otter?
Hayabusa's scientists see a sea otter in the shape of Itokawa, and refer to its parts as the "head," "neck," "body," and even the "shell" being clutched in the otter's paw. Credit: JAXA / Emily Lakdawalla

The other thing that appears to be happening besides vibration-induced flow of large gravels is the sorting of the much finer gravels into the gravitational potential lows. What's probably happening there is that the finer gravels move more readily than the coarse stuff, so they flow farther during vibrational events, eventually making it to these gravity lows and filling them up. In addition, those large gravels contain a lot of gaps; the shaking and settling of the little asteroid lets the finer stuff percolate down into the gaps between the bigger chunks, much as shaking a bucket of LEGO bricks will bring the largest bricks to the top. (The usual analogy is a can of nuts, but I don't eat many nuts; I am, however, very familiar with how deep into a bucket of LEGOs you have to dig to find those pesky little 1x1 tiles.)

All of this explains why there aren't really that many impact craters on Itokawa. Just one little impact with a fist-sized rock can set the whole thing to vibrating, and move gravel around; so it probably doesn't take long, geologically speaking, to obliterate evidence of craters. Even a tiny little lumpy thing like Itokawa is a dynamic place, rewarding close observation.

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