Big Ohm Boulder

In LRO's 336th orbit, still early during the commissioning phase, at 140 km in altitude (90 km higher than the present first year survey), LROC's Narrow Angle Camera took an excellent look at the complex interior of 64 km-km wide farside crater Ohm (246.5°E, 18.4°N). The structures of Ohm's interior reminds me of it's 20 percent larger, younger nearside sister Tycho, high intact walls with slumped terraced interior, very similar melt fill further in with a less distinct central peak. The 74 km long & 7.1 km wide stereo NAC strip is a rich cross section. Because it's central peak failed to coalesce as well as did Tycho's, on the rebound from what was likely a more oblique impact, Ohm's interior is filled with massive boulders, part of a mix of deep materials that may be part of the island crust that originally formed over the Moon's primeval magma ocean. The boulder above is among the most massive of the chunks, casting a over a quarter kilometer of shadow in the low sunset [NASA/GSFC/Arizona State University].

Like Tycho, Ohm has a respectable radiant ray system which would likely be quite distinctive if it's locus were visible from Earth. Its rays form the apex of a kind of chevron when seen in small scale from a great distance over the Moon's western hemisphere. Ohm's rays remind one more of those that formed when the progenitor of Proclus slammed into the western wall of Mare Crisium. Also likely to have been an oblique impact, the rays of Proclus spread east over Crisium in a fan whose sides projected north and southeast, less than 180 degress apart.

Heading North over the Far Side Japan's Kaguya took this full sun HDTV shot of Ohm in 2008. The bright sides of the north and eastward fan of the bulk of Ohm's rays system can be seen, along with the other differing aspects of Tycho's smaller and probably older sister. The central "peak" is more a system of parts of a peak. A comparison showing how the ray system of Ohm and Proclus are similar appears below [JAXA/NHK/SELENE].

There's a world of difference, literally, half a Moon apart, between Ohm (above) and the more familiar Proclus, the bright jewel of Crisium and the rugged Palis Somni and the sparkling marker of the young crescent Moons of Earthside evenings. Both impacts lose their distinctive shine when seen only in the context of blind topography. But in albedo, in this case as mapped by Clementine in 1994, each has left a mark more telling of their youth and optical immaturity, perhaps, then any permanent etching apparent to the human eye [NASA/DOD/USGS].