These images examine the two longest valleys visible on the Moon, Vallis Snellius and Vallis Rheita and touch on a shorter, but still interesting sculpture feature, Vallis Palitzsch also in the vicinity of crater Petavius.
This general arera of the Moon features sculpture from two large basin fomation events, the Nectaris basin and the Crisium basin. Of the nine Nectarian era basins on the Moon, four, Nectaris, Humboldtianum, Crisium and Serenitatis are visible on the near side. Beginning the Nectarian epoch, next to the Lower Imbrium the shortest lunar epoch in actual time (3.92by through 3.85by), is the impact which formed the Nectaris basin subsequently infilled with mare lava and now visible as Mare Nectaris. Other Nectarian basins formed in the order they were previously listed with Serenitatis being the last basin formed during this epoch. Knowing the length in time and order of the basin formations during the Nectarian period helps one to understand why Nectarian craters can overlie one another in the first place and in the second helps one correlate the generic Nectarian crater to an appropriate basin formation event--if indeed the crater or feature in question *is* a secondary feature to a basin impact and not an independant event! We'll see this differentiation actually applied in the discussion of Vallis Palitzsch below.
First the longest valley on the Moon at about 600km, Vallis Snellius:
Note that although both Vallis Snellius and Rima Hase are related to the Nectaris basin impact, one was formed immediately, concurrent with the impact (the sculpture valley) and the rima sometime later as a result of the release of stresses accumulated within the crust as a result of the same impact--thus their varient ages.
Now let's look more closely at Vallis Rheita, a more defined but slightly shorter (about 450km) lunar sculpture valley which, like Vallis Snellius, also formed as a result of the impact which created the Nectaris basin. Although the caption names Young D as an Imbrium era impact, it could be a later Nectarian era impact as well-- though I don't think this is likely given the relatively well preserved form of the crater which argues for a later formation date during the Imbrium more likely.
Another interesting possibility illustrated by this image is the existance of an almost completely obliterated very ancient (Pre-nectarian) large crater, traces only of it's circular wall may be evidenced by the arc running between Young C and Fraunhofer. The elevation overlay of the VMA seems to indicate a general depression roughly bound to this arc indicating the extent of this most ghostly of craters--if it is indeed a crater at all, and not a chance alignment of features brought out by a unique angle of illumination.
Speaking of the VMA, one thing I discovered in researching these areas is that there are a few errors or misimpressions in the crater information database of the VMA. The first one I found was a pre-Imbrium age quoted for crater Young--i took this for "Lower Imbrium" at first but technically this term covers everything prior to the Imbrium period as a whole; however there are two other periods preceeding the Imbrium and I feel they should be used if known. Upon looking into it further I found a more accurate Pre-Nectarian classification of this feature in Byrne's Lunar Orbiter Photographic Atlas. Also, an error was found regarding crater Palitzsch itself which is also classed as an Imbrium crater--clearly incorrect as it is virtually obliterated by the formation of the valley which bears it's name, itself a Nectarian era scuplture feature, a result of the Nectarian era Crisium basin formation. Any large database contains errors like this; Clementine, Rukl's, all of them. This is why it is a good idea to "get a second opinion" when dating or sizing any particular feature--check another source, several if possible, for best accuracy-- and for best understanding.
Hope you enjoyed this little examination of these two most remarkable features on our Moon! I learned a lot by putting it together which is, of course, why I do this in the first place! Comments, corrections & additions as always are warmly welcomed. E-me at photonovore (at) gmail (dot) com
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