Whitepeak Observatory, Tacoma, WA

An Examination of Humboldt Crater and Environs

I was lucky to have a clear night --clear and cold (registered 15 degrees at one point during the night!) to observe Upper Imbrium crater Humboldt near the nascient evening terminator --and the seeing was decent as the Moon was quite high in the nightsky! ('decent' for me is <2"arc seeeing)

The libration, noted in the image below, was very favorable for Humboldt plus it's position near the terminator just made the view even better. I was able to discern both Catena Humboldt (indicated on the image) and the darkened areas on the floor (one at the end of the chain of central peaks near the upper-left rim and the other stretching along the opposite, lower-right rimward-edge of the floor) plus, though not captured in the LPI image, there were visual glimpses of the rima upon the floor of Humboldt to be had as well--just a hint of one of the more promenent is *just* visible in the image running left to right on the near floor. It was much more clear in the eyepiece.

Humboldt is one of the largest craters (125 miles/207km diameter) with a well defined & still exposed central massif system; others this size, like Clavius, have buried central massif systems. The central uplift is considered to be (formulaicly) as representative of a uplift depth of 10% of the crater's diameter, so this central peak may have excavated material from over 20km deep into the Moon's crust! Humboldt's size also puts it in the classification of a 'basin transitional crater'...'basin' size begins at about 300km (Grimaldi) but the transitional size craters (~135km-270km) provide an explanation for the uplifted floors common amoung such craters; the uplift is a response to the negative gravity well created by the impact. To put it in Wilhelms words, "originally negative gravity anomolies resulting from mas loss during impact excavation were erased by the uplifts." A fine example of 'equal & opposite reactions' if I ever heard one!

Resultant from floor uplift, Humboldt is also a floor fractured crater very similiar to Petavius (visible in very flat light at the bottom of the image). The darkened areas on Humboldt's floor are the results of volcanic activity related to the formation of deep-reaching fractures from uplift of the floor post-impact which provided pathways for later intrusion of lava and pyroclastic material.

Other interesting aspects of this view is the evidence of two distant basin forming events (Nectaris and Crisium) in their secondaries emplaced here. Vallis Snellius, seen running out to the terminator from just to the left of Funerius, is a long Nectaris basin impact secondary chain. But there are features here also that resulted from the impact which created the Crisium basin too (marked 'a' in image); Vallis Palitzsch just above Petavius is one of these (notice the skewed orientation from the Nectaris basin secondary chains) and the other is the large four crater chain lying in the foreground between Barnard and Abel askew from but running towards Vallis Snellius.

Here is another, small shot of the Neper area...very spectacular view with the central peak beaming like a lighthouse, it's sillouette cast upon the far wall. Nice libration all around!

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