Whitepeak Observatory, Tacoma, WA

Sightseeing on the Southwest Limb


While examining the western limb of the gibbous Moonit's natural to be drawn to this fascinating area...it has so many interesting and classic examples of lunar geologic features that I couln't pass up making a graphic to share. The seeing was, unfortunately, horrid (>3"arc)the night i made the main graphic image but fortunately I had some other 'old stock' waning Moon images to help illustrate the "Southlimb story" that was waxing this week...

There are, first of all, three different examples of dark albedo features and three different mechanisms for their formation are found in this area--and a good detective story as well!

First, in the crater Shickard, a Pre-Nectarian ringed plain 227km in diameter (similiar in size to Petavius but much older, more degraded) we see a striped floor, with two dark areas (indicated by "a" in the graphic) adjoining the walls and broken through the center of the floor by a lighter stripe of material. Lunar scientists I. Antonenko and J. W. Head, Dept. Geol. Sci., Brown Univ. (paper) puzzled this feature out as follows: 1) crater formed by impact pre-Nectarin era, 2) the floor was flooded with mare lava through vents within the floor during the Imbrium period, 3) the impact forming the Orientale basin dusted a layer of ejecta over the crater's floor covering the smooth lava floor with lighter colored material, and last, 4) another late Imbruium mare lava eruption occurred which partially covered the eastern and western portions of the floor, leaving the dark albedo mare lava patches we can still see so plainly today. The white stripe through the middle is still-exposed Orientale basin ejecta.

So how did Antonenko and Head determine this sequence? It was done primarily through examining the crater halos on the lighter area of the floor and by crater counting.

Within the lighter stripe of exposed Orientale ejecta (as seen in the Clementine image in the graphic) they found dark halo craters--not of volcanic origin, but instead formed by impacts excavating through the lighter Orientale material and into the older, buried and darker underlying mare. This underlying mare-type lava is called "cryptomare" as in "hidden" mare, as the only visual evidence of it's existance is found in these halos of dark ejecta.

The ages of emplacement of the three layers; cryptomare, Orientale ejecta and the mare lava flows emplaced last were determined though crater counts. The new, dark mare lava flows we still see today have the fewest craters in their surfaces so they are youngest; the Orientale ejecta forming the lighter bisecting stripe of material is heavily impacted indicating a still older age of emplacement..and the principle of superposition (newer overlies older) indicates that the material underneath the Orientale material is older yet--the ancient cryptomare.

Schickard certainly provides an excellent example of how lunar scientists deduce the geologic history of the Moon from afar!

Here's a closer look--the arrows point to the 'smoking guns'--one of the larger of the dark halo craters seen in the Clementine image inset above and penetrating the Orientale ejecta of the similiarly layered crater, Wargentin (h), as well.

The second type of dark albedo material is represented at "b" in the graphic located on the southern reaches of Mare Humorum opposite from Gassendi. These areas are hard to miss when the illumination is right! These dark albedo patches are the result of blanketing of the surface with volcanic ash deposited from vents erupting in the same area. This is a similiar mechanism to that which creates the dark halo volcanic craters such as are on the floor of Alphonsus, but on a much larger scale, representing a much greater volume of pyroclasics and ash ejecta.

The third darkening mechanism is represented by Tycho's halo ("c" in graphic). Tycho is obviously not a volcanic dark halo crater nor is this dark halo the result of penetration into dark mare material--this darkening is caused by rock melted by the extreme heat of impact and ejected throughout the "halo" area surrounding the crater, and then consolodating into many pools of lava and glassy melt scattered throughout the ejecta immediately surrounding the newly formed crater. So why don't all the rayed craters have a dark halo? The answer lies in their ages--the dark halo is the first feature to weather from impacts of kicrometeroites, solar wind etc and is soon lightened to the point of non-visibility. Tycho is the youngest large crater on the Moon and so it has the most pristine ejecta blanket as well.

So in one field of view we have three different dark features with as many different ways of forming them!

Other treats and curiosities lie nearby...

Schiller (d) is the finest example of a extremely oblique impact or perhaps a string of closely following impacts. Tycho is also an example of an oblique impact, though not of as acute an angle as those impactor(s) which created Schiller.

One can tell which direction an impactor arrived by examining the shape of the crater's rays. Take a look at Tychos ray pattern in the graphic below.. You'll notice it is of a "fan" shape, just as the experimental impact results perdict. The shape of the fan is indicative of the direction of the impact, as indicated by the arrow...

(You can really see Tycho's 'halo' in the CLA thumb above.. and note the beautiful parallel rays (f) radiating from Tycho as well.

Next is of course Mare Humorum (e) and it's primary ring wall as defined by it's lave flooded interior. Humorum is the forth oldest Nectarian basin, older than Crisium, younger than Nectaris. The dark lunar "swamp", Pallus Epidemarium, and Mare Nubium, another ancient basin & maria, lie to the east.

Speaking of basins, a very ancient and obscure basin lies in this area, the Schiller-Zucchious basin (i). Formed during the Pre-Nectarian period, it's age falls between the older Nubium basin to the North and the much younger Humorum basin. The dark areas indicate the infilling of the old basin and are mare-type lavas. The remnants of two rings can be seen in the surrounding topography as indicated in the graphic--Schiller lies right atop the second ring wall.

The "Bear Claw" or crater Capuanus (j) is an interesting and possibly basin related feature---the three "claws" are what makes this feature so unusul and eyecatching. Where did these three parallel ridges come from? Nothing is firm but one specualtion is that they are in part remnant of one or both basin rings, one of Humorum's and one outer ring of Porcellarum. Both pass right through this crater *and* they also lie along the same line as the ridges lie.

Last but not least is "The Helmet" (g). This albedo feature is a combination of ejecta from the Humorum basin and a mixture of volcanic materials. This material is of what is called the Vitello group and is the result of ejecta "catching" on pre-existing isolated rugged areas of topography.

So ends our little tour of the Southwest limb. Hope you enjoyed yourself! :)

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