NEAR image of the day for 2000 Jun 15

Images taken by NEAR Shoemaker often show what has been described as Eros' layer of fragmental impact debris, that is, its "regolith." What is the evidence that Eros is covered by loose "dirt" and not just by bare rock? Observations made from Earth provide compelling clues. Eros's surface temperature, measured in far-infrared light, varies greatly from day to night. Since loose, fine debris heats up and cools down quickly while bare rock tends to keep a more constant temperature, the temperature variations indicate the presence of loose "dirt." Also, the way the asteroid scatters light, measured from its brightness at different geometries, more resembles "dirt" than rock.

The most compelling evidence is the small-scale character of the surface. This image was taken from NEAR Shoemaker on June 14, 2000, from an orbital altitude of 52 kilometers (32 miles). Three characteristics of the surface are evidence that it is covered by regolith and not by bare rock. First, boulders are everywhere. Big broken pieces of rock are usually accompanied by small broken pieces of rock -- regolith, by definition. Second, there are patches with few craters. Because impact craters form relentlessly, the lack of craters is an indication that something has wiped them out--most likely, loose material has filled them in. Third, other patches appear bright. Freshly uncovered regolith tends to appear bright -- old regolith has been turned darker by small impacts and the solar wind.

The region shown is in the southern hemisphere, at the southeastern edge of the 5-kilometer (3-mile) diameter crater. The whole scene is 1.9 kilometers (1.2 miles) across.

(Image 0136298274)