NEAR Science Update

Science Update March 31, 2000

As NEAR Shoemaker continues orbital mapping from its 200 km orbit, more and more details are emerging about the geologic features on Eros's surface. Three examples of what we are seeing were released as images of the day for March 17, March 20 and March 21. In all cases we are looking at the north polar region of Eros, but under different viewing and illumination conditions. By comparing sets of images as these, looking for features that are viewed in multiple images, we can infer whether portions of a feature look dark because of true brightness or reflectivity variations, as opposed to looking dark because of oblique illumination and/or viewing. In this way we sort out the detailed shapes of the features and any reflectivity variations that may be present, so we can classify the features and map their distributions across the asteroid. Mapping the features is a challenging task, especially on an irregularly shaped object like Eros, but it is critical to piecing together the story of how Eros came to be what it is. It is by mapping that we can study the spatial relationships - the degrees to which linear features line up and the ways in which features meet and/or cross one another - which provide clues to the nature of the underlying geologic processes. We shall return to the topic of mapping later.
In the image of the day for March 17 we can see three boulders at the top of the image - these are 80 m boulders sitting on the eastern rim of the large depression we have been calling the 'saddle'. We know these are boulders, protruding above the surface, because of where the shadows fall (toward the bottom of the image, whereas for craters the shadowed side is toward the top). At the lower left of this image, where the Empire State Building is outlined for scale, is the 5.5 km crater we have been calling the 'paw' (the reason for that name is not evident in these images). The 'paw' is the same crater with the prominent rim that appears in the top of the March 20 image (toward the left) and that I wrote about on February 8 and February 15. Now that we are oriented, we can examine the large crater on the far left of the March 17 image which has light and dark areas within its walls. This same large crater appears in all three images. It is in the center-left, below the paw, in the March 20 image, and it is on the horizon, at the lower left, in the March 21 image. We note especially that the pattern of light and dark areas is reproduced consistently in the three images. This is convincing evidence that there are true brightness variations in the materials lining the walls of this crater. We have seen consistently that the "bright spots" mentioned previously (see, for example, the February 15 update) are areas of higher reflectivity in the walls of craters.
click images to expand

March 17

March 20

March 21

Another important feature that appears in all three images is a long ridge that extends much of the way around the asteroid at its waist. Again, we know this feature is a ridge, standing above the surface, because of the side that the shadow falls on. In the March 17 image, the portion of the ridge that we can see begins at the left edge of the image immediately above the crater we have just discussed, and it points toward the boulders in the saddle. We can then find the ridge in the March 21 image, left side, and in the March 20 image, cutting diagonally across the lower left corner. This same corner is where an unusually smooth area, that may be relatively young, can be seen. Both this smooth area and the ridge have important stories to tell about the geology of Eros, but our studies of these features have barely begun.

Our focus on lighting conditions and their importance for understanding the images may seem surprising, but the lighting geometry is much more important on airless bodies like Eros than it is on Earth. Our atmosphere scatters light and creates diffuse lighting (where light comes from many directions at once). Diffuse lighting is especially prevalent indoors, as most of us make an effort to achieve that condition by putting shades and diffusers on lamps. With diffuse lighting, the effects of shadows and oblique illumination are reduced, but there is very little diffuse light on the surface of Eros. Most of us who live on Earth have no experience of such lighting - the only people who have actually seen it in nature are the lucky few who walked on the Moon.
Andrew Cheng
NEAR Project Scientist

Science Update Archive

As NEAR Shoemaker continues orbital mapping from its 200 km orbit, more and more details are emerging about the geologic features on Eros's surface. Three examples of what we are seeing were released as images of the day for March 17, March 20 and March 21. In all cases we are looking at the north polar region of Eros, but under different viewing and illumination conditions. By comparing sets of images as these, looking for features that are viewed in multiple images, we can infer whether portions of a feature look dark because of true brightness or reflectivity variations, as opposed to looking dark because of oblique illumination and/or viewing. In this way we sort out the detailed shapes of the features and any reflectivity variations that may be present, so we can classify the features and map their distributions across the asteroid. Mapping the features is a challenging task, especially on an irregularly shaped object like Eros, but it is critical to piecing together the story of how Eros came to be what it is. It is by mapping that we can study the spatial relationships - the degrees to which linear features line up and the ways in which features meet and/or cross one another - which provide clues to the nature of the underlying geologic processes. We shall return to the topic of mapping later. In the image of the day for March 17 we can see three boulders at the top of the image - these are 80 m boulders sitting on the eastern rim of the large depression we have been calling the 'saddle'. We know these are boulders, protruding above the surface, because of where the shadows fall (toward the bottom of the image, whereas for craters the shadowed side is toward the top). At the lower left of this image, where the Empire State Building is outlined for scale, is the 5.5 km crater we have been calling the 'paw' (the reason for that name is not evident in these images). The 'paw' is the same crater with the prominent rim that appears in the top of the March 20 image (toward the left) and that I wrote about on February 8 and February 15. Now that we are oriented, we can examine the large crater on the far left of the March 17 image which has light and dark areas within its walls. This same large crater appears in all three images. It is in the center-left, below the paw, in the March 20 image, and it is on the horizon, at the lower left, in the March 21 image. We note especially that the pattern of light and dark areas is reproduced consistently in the three images. This is convincing evidence that there are true brightness variations in the materials lining the walls of this crater. We have seen consistently that the "bright spots" mentioned previously (see, for example, the February 15 update) are areas of higher reflectivity in the walls of craters.		click images to expand March 17 March 20 March 21
Another important feature that appears in all three images is a long ridge that extends much of the way around the asteroid at its waist. Again, we know this feature is a ridge, standing above the surface, because of the side that the shadow falls on. In the March 17 image, the portion of the ridge that we can see begins at the left edge of the image immediately above the crater we have just discussed, and it points toward the boulders in the saddle. We can then find the ridge in the March 21 image, left side, and in the March 20 image, cutting diagonally across the lower left corner. This same corner is where an unusually smooth area, that may be relatively young, can be seen. Both this smooth area and the ridge have important stories to tell about the geology of Eros, but our studies of these features have barely begun.
Our focus on lighting conditions and their importance for understanding the images may seem surprising, but the lighting geometry is much more important on airless bodies like Eros than it is on Earth. Our atmosphere scatters light and creates diffuse lighting (where light comes from many directions at once). Diffuse lighting is especially prevalent indoors, as most of us make an effort to achieve that condition by putting shades and diffusers on lamps. With diffuse lighting, the effects of shadows and oblique illumination are reduced, but there is very little diffuse light on the surface of Eros. Most of us who live on Earth have no experience of such lighting - the only people who have actually seen it in nature are the lucky few who walked on the Moon. Andrew Cheng NEAR Project Scientist