Ancient Craters of Southern Rhea (APOD 13 May 2008)

martello · Post by **martello** » Wed May 14, 2008 2:14 pm

In looking at picture of the day - Rhea from May 13, it seems that the size of the craters are much more uniform than our Moon.

Rhea seems to have two basic sizes - large & a size that is roughly 3/8's of the large but there are no extra large craters. I wonder if this is because the collisions originate from objects in Saturns rings & these objects have somehow been ground down to uniform sizes by other objects in the rings?

orin stepanek · Post by **orin stepanek** » Wed May 14, 2008 2:35 pm

http://apod.nasa.gov/apod/ap080513.html
Rhea's composition is largely ice. I believe any large impacts would temporarily liquefy the surface which would limit any extra large craters. I think that is why they seem as shallow as they are.

Also this side always faces Saturn so the opposing side probably gets most of the impacts.
Orin

BMAONE23 · Post by **BMAONE23** » Wed May 14, 2008 5:06 pm

Perhaps the Icy crystalline structure would account for the craters appearing hexagonal or triangular.

neufer · Post by **neufer** » Thu May 15, 2008 12:24 am

orin stepanek wrote:http://apod.nasa.gov/apod/ap080513.html
Rhea's composition is largely ice. I believe any large impacts would temporarily liquefy the surface which would limit any extra large craters. I think that is why they seem as shallow as they are. Also this side always faces Saturn so the opposing side probably gets most of the impacts. Orin

It is leading & trailing hemispheres that differ:
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http://en.wikipedia.org/wiki/Rhea_%28moon%29

<<Rhea features resemble those of Dione, with dissimilar leading and trailing hemispheres, suggesting similar composition and histories.

Rhea is heavily cratered and has bright wispy markings on its surface. Its surface can be divided into two geologically different areas based on crater density; the first area contains craters which are larger than 40 km in diameter, whereas the second area, in parts of the polar and equatorial regions, has craters under that size. This suggests that a major resurfacing event occurred some time during its formation.

The leading hemisphere is heavily cratered and uniformly bright. As on Callisto, the craters lack the high relief features seen on the Moon and Mercury. On the trailing hemisphere there is a network of bright swaths on a dark background and few visible craters. It has been thought that these bright swaths may be material ejected from ice volcanoes early in Rhea's history when it was still liquid inside. However, recent observations of Dione, which has an even darker trailing hemisphere and similar but more prominent bright streaks, show that the streaks are in fact ice cliffs, and it is plausible to assume that the bright streaks on the Rhean surface are also ice cliffs.>>
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neufer · Post by **neufer** » Thu May 15, 2008 12:41 am

BMAONE23 wrote:Perhaps the Icy crystalline structure would account for the craters appearing hexagonal or triangular.

It is hard to imagine crystalline structure maintaining its integrity on the scale of kilometers.

http://en.wikipedia.org/wiki/Giant's_Causeway

BMAONE23 · Post by **BMAONE23** » Thu May 15, 2008 5:20 pm

Interesting though, how those Basaltic columns have the same shape as many of Rhea's craters