The first step in determining the nature of this anomoly was to be sure that it wasn't just an artifact of some sort; did it exist on other's images taken at other times? Was it indicated on any maps of this crater? If so that would provide positive proof that this is a real feature of some sort or another and not an artifact of light, shadow or photography.
So...turns out it's real enough; it is present on numerous images of this crater such as this image taken by Oliver Pettenpaul-- and is also indicated in the same location on the Rukl chart as well. It also shows up on an Apollo image"
So it is actually there. But is it a ghost crater? Or is it instead just a chance alignment of otherwise unrelated features that we only *see* connected as a circular formation at lower resolutions?
To help find some hints to help settle this question I felt that a higher resolution look at the area would be helpful so I dug up this Ranger mission image:
At this high resolution and at this high sun angle the feature seems to dissappear! At first blush this would seem to argue strongly for the 'optical illusion' explanation of this circular anomoly. But let's look closer.
I've marked the supposed location (rim) of the possible ghost crater as well as the clear craterform present to the immediate north. Also marked are two features that argue towards this being the remnants of a crater formed prior to floor "flooding"-- in Alphonsus' case by Cayley formation material rather than lavas (though there exists some question about this--more later).
The first evidence for this feature being a ghost is the linear block aligned along the putative wall, indicated by an arrow. This is the sort of feature geologists call 'anomolous' (until it's explained) as there is no obvious explanation for it's presence. It's a linear ridge where there is no reason for a ridge, especially in this perpendicular orientation, to exist. Contrast this feature to the more easily explained but similar feature indicated by the arrow labelled 'slump block'--this is obviously a chunk of crater wall that fell down onto the floor--we can tell this by both it's location near to the wall and it's roughly parallel orientation to the wall as well. But for the former block no such easy explanation is evident--it's orientation is wrong as well as it's distant location to the wall--it's not a slump block. But, if seen as a remnant of a crater wall, *that* would explain it's presence quite tidily.
The second feature arguing for a crater form here is the generally curved and elevated area indicated by the small arrows which, at lower sun angles, serves to define the 'circularity' of this formation as seen visually. This arc is quite evident even in the Apollo image linked to earlier. That this elevated & curved area is coincident with the linear block examined earlier, continues to auger for a craterform explanation for this feature.
There is also circumstantial evidence to be found in the literature for the presence of ghosts generally in these particular central highland areas. For example Ptolemaeus, the neighbor to the north, has a number of subsumed craterforms evident on it's surface. As both Ptolemaeus and Alphonsus are ancient (pre-nectarian and nectarian age respectively) craters and thus subject to similar post-formation impact histories, what is said about Ptolemaeus should also apply to Alphonsus in this regard. Also, both these craters are blanketed with Cayley formation material--forming the light plains found in this part of the central highlands.
Lastly, anomolous blocky remnants of rims of craters, as well as basin rings for that matter, are not unknown nor unrecognized. Indeed, more than one basin ring is considered to be marked only by otherwise unexplained & isolated mountain peaks.
More on the Cayley formation-- it is most commonly considered to consist of a thick blanket of ejecta cast out during the latter stages of the formation of the Imbrium basin (immediately after the ballistic ejecta which formed the Imbrium sculpture so evident in this area). This is the material, not mare lavas, which fill craters in this area, like Ptolemaeus and Albategnius, and give their floors the flat mare-like appearance we note telescopicly. (Wilhelms) Some speculate that Cayley deposits may consist of later deposits from Orientale basin formation as well (Hodges, Chau). Still others posit a possibility of the presence of highland lavas that differ in mineralogy and thus in albedo from mare lavas (Wood) as having a role of unquantified extent in forming these particular crater plains. As far as I can tell the question is not settled as to the exact nature of this material, but it is agreed that there is an emplacement of *something* of a significant thickness blanketing these areas--more than sufficient to subsume craters formed prior to the emplacement of the overlying material.
Based on the reasoning & evidence above i feel it's a pretty sure bet this is indeed a ghost craterform on Alphonsus' floor. And so concludes another investigation into a question that probably no one but me has ever bothered to even ask! :lol: But this, to me, is what makes the Moon so interesting--as a body that is known enough and accessable in enough detail to both faciltate forming and answering questions regarding features of such relatively small scale as this-- represents a unique investigative opportunity for amateurs even when armed only with small telescopes and cheap cameras!
Comments and criticisms welcome as always.
Carol Lakomiak made the following comment: "Wouldn't the impact of the ghost have punched down that section of Al's inner wall and left a depression which would then have been covered by the Caley material?" (used by permission)
That's a very interesting observation--about the lack of apparant effect upon the pre-existing wall! Legitimate and spurred me to look at this closer this am.
Still, I agree the ghost crater *is* involved with the wall to a certain extent...but I don't think there would necessarily be a depression in Alphonsus' wall still visible for two reasons; 1) because the ghost is almost completely subsumed by the Cayley deposits...only remnants of the very uppermost rim are still evident and since there is usually little disturbance of the pre-existing wall beyond the rim of an in-crater impactor like this, thus no divot/depression etc would be expected--as any such would be buried, along with that portion of Alphonsus' wall and 2) the slope of the interior of the smaller crater would be closely congruent with the slope of the impacted wall in any case and the difference could easily be erased by the combined effect of mass wasting and burial by Cayley material.
Also--look again at the much more unambiguous crater form directly north and adjacent to the ghost--this crater has seen a large block of Alphonsus' wall slump into it--in fact much of it is destroyed due to later infall of the neighboring wall--the effect upon the larger wall in this case is not negative but positive. Post-impact modification like this isn't unknown.
I thought to look at more pristine examples of this type formation on the Moon to see how a crater like this could have appeared prior to burial by the Cayley deposits...
For example, take crater Licetus:
Here is a pre-existing crater which has a much more involved and larger crater impacted even deeper/higher into it's inner wall. Notice the lack of deformation of the wall of the original crater even though this is a much larger crater than the Alphonsus ghost--and how the new crater's wall layers it's own interior slope into the wall's pre-existing slope. Sort of a bowl within a bowl...Bury this formation so only remnants of the small crater are still visible and there would be no depression visible on the impacted wall either.
I found other examples of this pattern--Vlacq, Metius, Neander, Isadorus, Rheita, and many more all have small craters involved to one degree or another with their inner walls, all show little to no effect outside their wall-ward rims upon the original crater wall --and if one imagines them buried to their very rims with many dozens of meters worth of particulate material (Cayley), a great deal of blending of small differences in slope, elevation and definition generally is bound to occur, with many features buried entirely, thus erasing or blurring all but the most radical differences between pre-existing structures.
All this said, if there *were* evidence of an impact present in Alphonsus' wall at this location (congruent depression) that would certainly serve as addditional evidence of the crater nature of this circular formation. It's absence, however, though notable, isn't inexplicable to me. What *is* inexplicable in this case is that anomalous, oddly angled linear ridge/peak out on Alphonsus' floor---which also happens to lie congruent with the circular ridgeform--but aside from being a rim remnant of a pre-existing crater, i can't think of any other more likely explanation for it's presence there? That's what tipped it for me anyway.
Anyway, thanks to Carol for the observation!! --that is something that needed to be looked at and I didn't until she mentioned it. This feature could still be just an optical anomoly. I feel it *probably* is not, but I mean to imply no certainty (which is not even possible I think), just assess probability one way or the other.
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