Humbolt Crater Area Observation
Moon enthusiasts know the shadow effect created at the line of the terminator is a an optimal point of reference to observe a more detailed lunar landscape. As seen below it gives depth to the image of Humboldt Crater and the surrounding lunar surface.
This observation near the nascient evening terminator was very favorable for Humboldt, it’s position near the terminator makes the view even better.
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 (formulaic) representative of an uplift depth of 10% of the crater’s diameter so that 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 among such craters; the uplift is a response to the negative gravity well created by the impact.
To put it in Wilhelm’s words, “originally negative gravity anomalies resulting from mass loss during impact excavation were erased by the uplifts.” A fine example of ‘equal & opposite reactions’ if I have ever heard one.
Resulting from floor uplift, Humboldt is also a fractured crater very similar 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 the uplift of the floor post-impact, which provided pathways for the later intrusion of lava and pyroclastic material.
Another interesting aspect 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.