Asteroid probe shows Eros' surface the result of impacts
Posted: April 23, 2001

This image was taken in the early hours of October 26 during NEAR Shoemaker's low-altitude flyover of Eros. Many rocks of all sizes and shapes litter the scene. Locally, fine debris or regolith buries the rocks. Photo: JHU/APL
NEAR mission science team members have concluded that the majority of the small features that make up the surface of asteroid Eros more likely came from an unrelenting bombardment from space debris than internal processes. Details of the research from NASA's Near Earth Asteroid Rendezvous (NEAR) mission were published last week in Science and are based on the NEAR Shoemaker spacecraft's Oct. 25-26, 2000, low-altitude flyover of asteroid Eros that brought the spacecraft to within about 3 miles of the surface of the asteroid.

"We think that impacts to the asteroid's surface have probably been the single-most dominant process in shaping the surface texture of the asteroid," says NEAR Project Scientist Dr. Andrew Cheng of the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., which managed the mission for NASA. "We saw surface details such as regolith [surface dust and debris], craters and fields of small boulders in incredible detail. We also saw things that confound us, but we now have a more in-depth picture of Eros that will help us to decipher the asteroid's history."

During the flyover, simultaneous observations were taken by the spacecraft's multispectral imager and laser rangefinder over two tracks approximately 1 mile and 2.5 miles long that showed objects the size of a doghouse at three to four times better resolution than previously obtained. The data revealed an inordinate number of small boulders, a saturation of large craters and a dearth of small ones, crater "ponds," and unknown erosion processes.

A vast number of large craters, 1,630 to 3,280 feet (500 to 1,000 meters) in diameter, have been imaged, but there is a surprising scarcity of boulders large enough to make such impacts. There is more than 100 times the number of 10- to 12-foot (3- to 4-meter) boulders than there are impact craters in this region. Some angular or slab-like features were imaged that could indicate they are composed of stronger material than rounded objects. Some boulder clusters are thought to be fragments of a larger projectile that hit the asteroid.

The flyover also yielded evidence of an unusually low number of smaller craters. "There could be some unknown process, possibly something like seismic shaking following impacts, which is more likely on a small body such as Eros," says Dr. Joseph Veverka of Cornell University, Ithaca, N.Y., who heads the imaging team. "Other possibilities are processes that could erode or erase smaller craters such as micro-cratering [the pummeling of the surface by smaller objects] or thermal creep [the erosion of surface material through normal seasonal heating and cooling of the asteroid] that is eroding the smaller craters."

"We do know there is a substantial amount of regolith from erosion and impacts that is covering blocks [boulders] and craters possibly to a depth of several meters. So it could be that many smaller craters do exist but they're buried under the regolith," says Veverka. "A thick covering of fine dust that prevents us from seeing what lies beneath might also be part of the answer to why the asteroid has little color variation. It is possible that parts of Eros are covered in regolith as deep as a 10-story building."

Eros at close approach. Also taken on October 26, this view taken just 7 kilometers (4.3 miles) away. Most of the scene is covered in rocks of all sizes and shapes, but the floors of some craters are smooth, suggesting accumulation of fine regolith. For scale, the large boulder just below and to the right of the center of the picture is about 15 meters (50 feet) across. The smallest visible rocks are about 1.4 meters (5 feet) across. Photo: JHU/APL
The data also revealed ponds - flat surfaces at the bottom of craters - formed by regolith deposits. These ponds are intriguing science team members because of their extremely smooth surfaces. "The smoothness indicates that there is an efficient process on Eros which is able to sort out the finest component of the regolith from the coarser, more blocky portion and concentrate this fine material into some low-lying areas such as crater bottoms," Veverka says.

Moreover, the laser altimeter found that ponded deposits are not only smooth but also extremely horizontal - level relative to local gravity - as if formed by fluid-like motions. "It is astonishing that the total dry regolith of an asteroid like Eros can apparently be mobilized like a fluid," says Cheng. "There is no water on Eros, and there has not been any water, for billions of years. However, seismic shaking caused by impacts may be able to produce fluidized movement of regolith."

"Aprons" of debris at the base of some of the larger boulders indicate another phenomenon the researchers are studying: efficient erosion or disintegration of ejecta boulders (boulders forced out of a crater as the result of an impact) after they have landed on the surface. But scientists say they need to study higher resolution images to more definitively interpret the various forms of regolith that the low-altitude images have provided. "What causes this efficient disintegration remains a mystery," Veverka says. "But one we hope to solve over the coming months by studying the wealth of data that the NEAR mission has provided."