Whitepeak Observatory, Tacoma, WA

Alphonsus; Nectarian... or Pre-Nectarian?


by M. Clark June 2006

An irregular shaped crater of a form typical to that caused by an oblique impact is located on the flank of the crater Alphonsus and designated Alphonsus B. An investigation into the possible origin of this crater resulted in realization that it's supposed direction of impact (impact axis) is oriented almost exactly radial to the center of the Nectaris basin; and that further two craters, of similar size and morphology, were also located and their impact axis found similarly aligned radially to the Nectaris basin. This raised the possibility that these represent secondary impacts to the Nectaris basin forming event. This relationship, if true, has the consequence of making a somewhat earlier relative formation period correct for the crater Alphonsus. In accordance to the principle of superposition, the location of one of these Nectaris basin secondaries on the rim of the crater Alphonsus would necessarily limit it's age to that era preceding the Nectaris impact event, the Pre-nectarian.

The event which delineates the beginning of the Nectarian era of lunar geologic history is the impact which formed the Nectaris basin. The area is shown in the graphic above. Also indicated are the locations of the three oblique impact craters examined, Alphonsus B, Albufeda D and Nicolai Z. Also indicated is Vallis Rheita, which is commonly attributed as a chain of secondaries radial to and formed as a result of the Nectaris basin forming impact. This valley gives some idea of the relative distance at which secondary craters of the 15-25km size range can be expected. Note also that all these possible secondaries lie outside the outermost ring of the multi-ring Nectaris basin, within the outlying zone of secondary impacts.

The above graphic sums up the radial relationship between two of these possible secondary oblique impact craters and the Nectaris basin. As can be seen, both craters exhibit very similar "beak-shaped" crater rims and are further oriented on their longer axis directly to the center of the basin. Further they are aligned in proper accordance to the purported direction of impact were they to have originated from the Nectaris basin forming impact.

In the above graphic the relationship between morphology and direction from which the impactor arrived is expressed. Experimental impacts (Gault & Wedekind (1978) performed in the laboratory illustrate that such morphologies are possible and very specific as to direction of impactor arrival. This directional alignment is consistent with an impactor origin from the Nectaris basin formation event.

Since crater morphology, particularly rim morphology, can be shaped by other mechanisms than an oblique impactor, a closer examination of the individual craters is in order. One of the mechanisms that modifies the rims shapes of craters is mass wasting, commonly expressed through the form called 'scalloping". This often can be seen to result in crater rims which vary from circularity to various polygonal shapes instead. Obviously, if the nature of these craters is to be determined as caused primarily from an oblique impact, any evidence of mass-wasting as an alternate rim-altering mechanism must be eliminated. First we'll examine Albufeda D:

The above image presents Albufeda D in a territorial setting. The crater is situated on an elevated landform with lobate edges, seen to the left of the crater proper. I found this particular crater associated with the Nectaris basin event by CJ Byrne (Lunar Orbiter Photographic Atlas of the Moon 2005). Referring to Albufeda D in comments about LOPAM image IV-096-H2 he writes: "Between Burnham and Albufeda there is another irregular rim, surrounded by a flow with a lobate edge. These features may have been produced by the molten electa from the Nectaris basin."

The above large scale image is provided with dimensional data and alignment to the center of the Nectaris basin. Arrows point to the presence of a defined rim still extant defining the 'beak-like' portion of the rim. Mass wasting (landslides) on such a scale as would be required to form such a large irregularity in the rim morphology of an originally circular crater would surely leave none of the original rim elevation intact in this area. We can thus most likely eliminate mass wasting as a principle formative mechanism responsible for this portion of the craters rim outline. Other areas of the crater rim do show modification due to the actions of mass wasting (however irrelevant to the present examination). This can be seen along the north edge near where the upper dimensional arrow points and as minor scalloping of the right side of the crater rim. The differences in morphology of rim modification by mass wasting and by direction of impact can so be seen to be quite plainly different. The state of preservation of this crater can be seen to be consistent with a early Nectarian age.

The next purported Nectaris basin secondary we'll examine is Nicolai Z shown above in a large scale image with it's linear relationship to the Nectaris basin and dimensionality indicated as before. Although it's western rim (left) does not appear to have a pronounced rim remaining intact, there is no evidence upon it's floor of the effects of the scale of mass wasting that would, again, be required to modify it's rim so radically. Instead the floor follows the shape of the rim, having also an anomalous extension beyond circularity. Also, a notable similarity in morphology can be seen when comparing this crater to Albufeda D: in size, in ratio of the long to short dimensions of both and in outline. (the large slide on the north wall of the latter crater excepted.)

Finally we'll examine Alphonsus B the crater that started this little investigation:

Shown above is an Apollo image taken of Alphonsus and enlarged to show the area of interest. (The corkscrew object is a boom attached to the orbiter). Indicated by dashed lines is the gross rim area of Alphonsus and Alphonsus B is also labelled. In relation to the latter craters superposition upon the former, the area of Alphonsus B which overlays Alponsus is indicated by diagonal hashmarks. Two things are evident from this image: 1) that Alphonsus B overlays Alphonsus and therefore was created after Alphonsus was formed. 2) that Alphonsus B also has a well preserved rim including on the irregular "beak-like" portion. This morphology, as before, argues against this irregularity as having been the result of mass wasting.

Above is a large-scale view of Alphonsus B. Again this indicates the presence of a raised rim all around the beak-like extension of the crater's rim. And again, although there is debris upon the floor in this area, there is debris and anomalous formations throughout the floor of this feature. As before, in no case is the amount of debris in the western portion of the floor sufficient to account for a mass wasting explanation fo this crater's shape. On the contrary, when comparing the significant wasting that has occurred on the eastern rim of this crater (blocky landslips are evident) it is even more apparent that another mechanism than mass wasting must be what accounts for the anomalous shape of the western rim.

An examination of the morphological differences between the two craters shown in the graphic above illustrates the range of preservation possible within craters formed prior to the Nectaris basin forming event. Rosenberger is a typical pre-nectarian age crater with subdued or absent central peak, heavily eroded walls with little or no evidence of terracing remaining, a very subdued rim and overall a thoroughly '"weatherbeaten" look. While both Vlacq and Rosenberger are considered to be Pre-nectarian, Vlacq is obviously the more recent of the two formations. In fact, Wilhelms referred to Vlacq at least once in his book (Geologic History of the Moon pp. 147) as "probably" a pre-nectarian crater. This would imply a state of preservation consistent with either a very late Pre-nectarian age or a very early Nectarian age. This places Vlacq as probably formed during a similar timeframe as Alphonsus, which is usually considered a very early Nectarian formation. Indeed there are striking similarities in morphology between Vlacq and Alphonsus; they are in the same size class (Vlacq 92km, Alphonsus 121km), they both have elongated central peak complexes exposed and possess complete rims with evidence of terracing still remaining. Differences include but are not limited to the presence of Imbrium basin sculpture in Alpohonsus and the longer central peak complex. This latter feature is thought to be related to deep radial crustal fractures resultant from the Imbrium basin forming event. Vlacq is missing these characteristics as it is much more removed from the range of such Imbrium sourced mechanisms than is Alphonsus.

Discussion:

To sum up, evidence has been presented to support an origin for three irregular crater forms surrounding the Nectaris basin as oblique radial secondaries to the Nectaris basin forming event. This is based upon 1) crater morphology indicating a Nectarian age, 2) the rim morphology indicating an origin from oblique impact; 3) an alignment of their impact directional axis, in all cases, almost precisely radial to the center of the Nectarin basin and 4) their common location within the zone of secondary impacts (beyond the outer rings) surrounding the Nectaris basin. One of these oblique Nectaris basin secondaries is situated on the flank of Alphonsus. Based on the principle of superposition, this location indicates that Alphonsus would have to have been formed *prior* to the Nectaris basin event instead of shortly thereafter, as commonly asserted at the present time. Further, comparison of the gross morphology of Alphonsus itself with the morphologies consistent with either a late Pre-nectarian age or an early Nectarian age crater (Vlacq) indicate that it's morphology alone does not preclude a late Pre-nectarian age for this crater.

I suspect that Alphonsus was assigned a Nectarian age by researchers because of the presence of a central peak, a feature not usually associated with Pre-nectarian craters and in consideration of it's superimposition upon Ptolemaeus, a plainly Pre-nectarian age crater. Any sculpture evident from the Nectaris basin impact which would have positively determined Alphonsus' age as Nectarian is in any case suppressed completely by the subsequent Imbrium event and it's ejecta-- which latter evidence at least proves beyond any doubt that Alphonsus pre-dates the Imbrium era.

So, is Alphonsus really datable as a Pre-nectarian crater simply by the presence of one anomalous crater, Alponsus B, lying on it's flank? Yes, but with an important proviso that being that an affirmative depends *entirely* on the origin of this small irregular crater. Oblique impacts do occur as primary impacts, certainly, and it is certainly also possible that this is one of them and that it has nothing to do with the Nectaris basin formation event and that Alphonsus, is, after all, properly classified. In fact this is what I would have assumed--had I not found first Albufeda D and then Nicloai Z, sharing almost identical form, age, morphology, presence in the secondary impact zone surrounding nectaris basin and virtually exact radiality vis a vis the morphologically indicated direction of their impactors pointing straight back to the Nectaris basin! In sum, these commonalities could be coincidental, but to in my view they argue, together, for at least a modest probability otherwise. Certainly, locating and identifying other similiar illregular craters possessing the same related characteristics within nectaris' secondary zones would only further support this premise. In any case such investigations could only be seen as profitable (and fun!) to undertake. The Moon's geological history is not perfectly understood or intrepreted on a 100% feature-by-feature basis-- even today, as any professional lunar geologist would certainly affirm. (Addendum: Dr. Charles Wood graciously reviewed this article, at my request, after it's intiial publication in the A.L.P.O.'s Lunar section publication, "The Lunar Observer". He expressed affirmation of the probability of the premise. Based on this confirmation I feel that Alphonsus can now be considered a Pre-Nectarian formation.)


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