LPOD May 6, 2008

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Lunar Orbiter IV 115H image from Lunar & Planetary Institute, annotations by Jim Mosher

Stellar observers have multitudes of targets to test their visual acuity: stars of accurately measured magnitudes. Similarly, lunar observers and imagers have lots of tiny craters and rilles, but their dimensions generally are not accurately known. LPOD previously published a diameter sequence for small craters in Serenitatis, and Mardi Clark has created four more sequences with many more craters. Recently Jim Mosher added two pages to the Moon Wiki expanding the list of known diameter targets to Plato and the narrow rille within the Alpine Valley. Rilles are so thin that it is difficult to make accurate measures of their widths without using spacecraft images, and many rilles even have poorly estimated lengths, and few have any depth measurements at all! So Jim has used his LTVT program to determine all three measures for the Alpine Valley Rille. Surprisingly, he finds that the rille is less than 1 km wide nearly everywhere along its 142 km length. I say surprisingly because it is being successfully imaged almost regularly. This means that more than a handful of lunar imagers are achieving sub-kilometer resolution. That is impressive! It would be wonderful if all the known rilles had their dimensions measured on spacecraft images using LTVT...

Chuck Wood

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(1) Chuck--Jim's measurements are very interesting and useful for describing the Alpine Valley. Is the Alpine Valley rille considered a collapsed lava tube? Was the valley once a channel for lava flowing between Mare Imbrium and Mare Frigoris?


(2) Bill - All sinuous rilles are thought to be lava channels and/or collapsed lava tubes. The Valley is floored by lava but it does not connect to Mare Frigoris. And there is such a narrow opening at the Imbrium end it doesn't seem that much lava could have come from there. The lava probably rose up a fracture associated with the Valley, flooding from below.