Intermediate Scale

A close up view of a wide groove on the West side of Eros. (130230097)
A closer look at the surface of Eros allows more features to become visible. With the introduction of higher resolution images from the NEAR spacecraft, prominent groove and ridge systems started to appear. In addition, thousands of small and medium size craters can be seen blanketing the entire surface.

Believed to have been formed from the stress of the formation of Eros, grooves can be seen clearly in many orbital images around Himeros and on the eastern end of Eros. The term groove is given to long, narrow channels on the surface. In some cases these features can be referred to as troughs, but for the most part, the terms are synonymous. Grooves on Eros are generally 75-100 meters wide and can be as long as 2 km. An example of one such groove can be seen in the image to the right. This is the Calisto Fossae groove. This wider than average groove (500-700 am) is made up of two parallel grooves with a prominent ridge that stretches for several kilometers and maintains an average depth of around a hundred meters. Grooves can also be found in large concentration in the region northwest of Himeros. The largest groove in this region can be found here inside the Himeros along the Western wall. This groove is 100 meters wide and stretches 2 km in a backward "C" shape (Prockter et al. 2002; Cheng et al. 2002).

(Top) A northern view of Rahe Dorsum - 129892867 (Bottom) Formation of a shallow angle fault

Located at the end of the groove in Himeros is one end of Rahe Dorsum ridge system. Spanning over 18 km is the ridge system, Rahe Dorsum, which starts inside Himeros then wraps around the north side of Eros to the eastern side of the Psyche crater. This ridge system generally has a width of about 300 meters and maintains a height of only a few tens of meters. The ridge is believed to be formed due to a shallow angle fault in which one side rises to form a hanging wall. This is apparent on the asteroid because one side of the ridge is "flat" while the other side is very steep (as seen in lower illustration to the left). It has been suggested that Rahe Dorsum was formed as a result of the last major impact event, Shoemaker crater. This is believed to be due to the fact that Rahe Dorsum overlaps Himeros, and grooves that crosscut the ridge on the western side of Eros also overlap part of Psyche's wall. This conclusion is uncertain because different parts could have formed at different times.

Coincidence or not, both the Rahe Dorsum ridge system and the Calisto Fassae groove exist on the same plane. This could be caused by the asteroid's bending after a large impact or a near miss with another large body. In this case the surface on the concave side would be pressed together (forming ridges), and the concave side would be stretched (forming grooves). The only problem with the theory are the two features' relative ages. Calisto Fossae appears worn and has several large impact craters that suggests a relatively old age. On the other hand, Rahe Dorsum is a prominent feature with no craters larger than 200 meters suggesting that it is a fairly new feature in comparison to other features on the surface of Eros, including Calisto Fossae. One answer to the issue is that Rahe Dorsum is still forming and growing from additional impacts. For example, after a large impact, like the one that formed Shoemaker crater, the thrust fault that Rahe Dorsum is on reactivated. When this happened, the ridge was "rebuilt" giving an impression of a young age (Prockter et al. 2002; Thomas 2002).

Square Craters (132151540)
In addition to grooves and ridges, craters also fit in the category of intermediate scale features. Scattered over the surface of Eros is a combination of small and medium sized craters. Currently there are 44 medium size craters that range from 1 to 5 km in diameter along with thousands of smaller craters that are less than 1 km in diameter. Throughout the years crater counts have been used to figure out relative age on the Moon and other terrestrial bodies. In this method, craters are counted in a region and compared to another region. The results can give a relative age. Unfortunately this process is not as reliable here as on the Moon or any other large body because Eros is so small that the sample areas would also be too small. If one section of Eros had 20 craters and another had 17, there is not a significant difference to say that the second area was younger. This method also assumes that one spot on the surface has the same odds as another in being impacted, but due to Eros' shape and spin axis, this may not be true. In addition steep areas along the crater walls show evidence of resurfacing. Resurfacing may cause craters to be wiped out and, therefore, change the crater count. Overall in large samples, like the ones taken on the moon, this process would be more reliable, but the practice on a small body of an asteroid, like Eros, this process of measuring relative age is defective (Veverka 2001; Thomas 2002).