3/9/98: Schroter's Valley was just barely all in sunlight -- the region between the Cobra Head and Herodotus was still darkly shadowed by nearby high terrain. I could see Rimae Aristarchus, and associated rilles I, III, and VI. Rille III was most prominent northwest of Prinz -- I could not see any of III, IV, or V where they lie parallel, close to 27 N 46 W.
Schroter's Valley does not begin to be visible until the area where
it lies is well past the terminator. Just before sunrise on the valley
itself, Rupes toscanelli is obvious, but look for a small but prominent
trough which bisects a small hill north of R. toscanelli.
The trough is unnamed on Rukl, but is drawn just over the "50 W"
on chart 18.
Current theory states that the entire valley, including the head, are a collapsed lava tube -- a course lava found from the "Cobra Head" through the highlands and to its eventual terminus near the lowland mare material. After the lava drained, it is presumed the tube collapsed from the weight of the material above, leaving the trench as we see it. The "head" was formed due to a larger collapse into the chimney from which the lava emerged from a deeper source.
Such selenographic processes are not yet well enough understood to say with any certainty if that is indeed the process that took place, or from what level the lava originally began to rise. The curious nature of the "island" in which Schroter's Valley is found adds further fuel to the speculative fires; it's a curious area of isolated highland in the midst of large maria.
Possibly the interior matter is considerably brighter than the surrounding mare material; I don't know. That would explain the extraordinary contrast, and there's some small reason to believe the underlying material in that area could be intrinsically bright, with Aristarchus so nearby. But that would also imply a certain degree of youth -- unlikely in my understanding that rimae are largely old features. On the other hand, after forming there is some collapse of the sides of the walls, and perhaps some of this "mass wasting" took place relatively recently..? Jarred loose by the impact that formed Aristarchus?
And there was a lesson for me: just because the mapmaker draws it dark doesn't mean it will be when you see it. I should know better by now having seen this kind of thing before, but I don't. Ah, well.
The rille itself is a prototype "sinuous," bending throught three distinct arcs somewhat reminiscent of the profile of a face from the bridge of the nose to the chin. Most of this showed well in the telescope, save the thinnest part of the end, which eluded me (the rille goes a bit further past Marius B than I noted). It's graceful shape outlined against the dark floor was very attractive of itself, even without the excitement of the hunt. This was clearly the best sighting I've ever had of it, and I have to say it's much more appealing when the majority of it is visible -- rather than hunting little bits and dabs in shakier seeing or poorer light.
|Chart 17||Moon-Lite Atlas for chart 18||Chart 19|