January 11, 2014
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left: south up image by George Tarsoudis, Greece; center and right: LRO-QuickMap south up enlargements of Helmholtz D
While admiring George's excellent image of the Boussingault crater-within-crater landscape my gaze shifted to the nearby bright rim crest crater Helmholtz D.
This 45 km wide crater has a nice central peak but what caught my attention was the apparently steep upper walls and the broad flat terrace on side closest to
Boussingault. Shifting to the the higher resolution of the LRO QuickMap I saw that this was a fairly fresh crater, with crisp rim crests and impact melt ponds on
part of the floor. The broad flat plateau is about 900 m below the nearby rim crest, and may be essentially a large terrace. The opposite crater wall shows a hint
of 3 or 4 terraces that are sharply truncated to the right by bright wall rock. This suggests that the wall collapse to make the terraces was followed at some latter
time by downslope movement of wall material that created the bright slope. Then I noticed the rounded boulders on the floor between the central peaks and the
wide-terraced wall. Zooming in on QuickMap I discovered that these are most peculiar hills. There is a cluster of more than a dozen rounded hills about 1 km
wide and 30-50 m high. More occur around the peak toward the left of the peak on this south up image. The rounded shape and the apparently piling of one
around another looks very much like volcanic extrusions - domes. In particular, the rounded hill apparently cut into 3 pieces looks very much like a dome with
three closely spaced vents (or else its a baked dinner roll). These hills do not look like mounds of debris that rolled downslope. They are partially surrounded by
impact melts so conceivably they could be some sort of impact melt eruptions that produced domes. Or they could be volcanic domes that colonized the impact
crater floor. Neither of these explanations is very appealing, but either could be right. Although other crater floors sometimes have a few unusual hills, Helmholtz
D is rather unique in having so many suspicious structures. Are there more like it? What caused the rounded hills?
Chuck Wood
Technical Details
Jan 8, 2014. SkyWatcher 14 inch @f/4.5, camera QHY 5L-II, filter Red, barlow 3X.
Related Links
21st Century Atlas charts 14 & L4.
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