Whitepeak Observatory, Tacoma, WA

Observer's Guide: Littrow to the Lake of Death

The region we're going to examine in some detail is illustrated above--much as it will appear on the 8th. I've divided this region into four areas of special interest: Littrow, the craters G. Bond and Römer, Posidonius proper and Lacus Mortis.

Geologically, this region can be best described as an area where the gross topography has been influenced by several major basin impacts: Tranquillitatis (oldest), Crisium, Serenitatis and Imbrium (youngest). All have cast their basin ejecta across this region, blanketing the terra in varying thicknesses. These giant basin impacts also sculpted the terrain, by their ejecta and through the formation of their concentric uplift rings, to varying degrees and according to their distance, size and age. Underlying, superimposed atop and (in the case of the pre-imbrium features) scarred by basin impact sculpture, are the pre-imbrium and imbrium era craters. In some cases the post basin age craters have obliterated the basin ring structure upon which they impacted (as Macrobius did to an outer Crisium ring).

Next came the volcanic era of maria formation which flooded the maria and the embayed areas reaching inland from the maria shores during successive episodes of volumous eruptions. Lacus Somniorium, an area of lava entombed pre-nectarian craters, is an example of maria-adjacent lava flooded plains. These ancient buried ghosts sleeping under the "Lake of Dreams" may be detectable if the light is right! Be sure and look for them!

Rilles and wrinkle ridges: Prior to 3.5billion years ago, midway through the Upper Imbrian Epoch when most of the maria were filled, the Moon was in an expansive phase globally which induced formation of expansion features (arculate faults) in response to subsidence of the maria (sinking from the great weight of the pooled lava) rather than compressive features (dorsa or wrinkle ridges). The latter were primarily formed [i]after[/i] 3.5BY when the Moon entered a global contraction (shrinking) phase. In Mare Serenitatis, for example, this can be seen in the distribution of arculate rilles and wrinkle ridges. The former are found primarily in the older lava deposits located on the borders of the maria and back in the adjoining terrae. But the biggest wrinkle ridges are primarily found in the younger lavas in the central areas of the maria (Serpentine ridge is a good example); the rilles are absent from these areas as by the time these lavas were emplaced, the arculate rille forming era of the Moon was pretty much concluded. (For more on the history of the emplacement of lave within Mare Serenitatis, see Chap. 8 of Wood's book, "Modern Moon".)

Now let's look more closely at the southernmost of the sub-areas of interest in this region, the Littrow area.

This was, of course, the area visited by the final Apollo mission, Apollo 17. It was chosen for geological investigation for two primary purposes: to sample an example of what was thought to be remnant basin ring structure expressed by mountainous terra of the Littrow range, and to sample the dark mantling material found in the Littrow valley and the surrounding area (this is where the famous "orange soil" was first found nearby Shorty crater).

Observationally, the historic Taurus-Littrow landing site is pretty easy to spot and the areas where the astronauts visited in their lunar rover represent some interesting features, some of which that one can actually see in a telescope. But first let's get our bearings...

Here we have an overview of the Littrow area. There are many features of interest here. The upper right image is the overview which represents the level of detail in average seeing through a 6" telescope. Find the circular "bay" le Monnier on the eastern shore of Mare Serenitatis and the rest is pretty easy.

The inset on the bottom right identifies the various Littrow rilles by the roman numeral designations of the various branches/segments. There are some reference crater diameters with which to assess your seeing and resolution floor.

One feature demands a special note: Catena Littrow. I was always curious as to where this feature really was and what it could be seen as (an actual crater chain or an unresolverd rille-like line). I found that it does not lie in the location presented in Rukls, which points to a N-S trending linear feature. In fact the Catena Littrow is an E-W trending feature located as is shown in the Lunar orbiter image inset in the lower left. This is another of the class of features identified and named in the space age and not known during the visual observational age.

The upper right inset is a enlargement of the Apollo 17 landing and excursion area. There are actually several largish landmarks use by the Apollo astronauts which you can catch in a small telescope. The massifs, South and North, are plainly visible. The "Twin Peaks", designated Hill F and A during the mission, are also detectable, as is "Family Mountain" or Hill E.

But the most interesting feature you can see here (ideally during the waning phase when the shadows of the North massif don't interfere) is the "Tycho landslide". This is visible as a lighter triangular patch of light colored material spilled upon the darker plain of the valley floor. This area is notable in that the dating of the samples from this landslide (~110million years) seem to confirm that this landslide resulted from the jolt of the distant Tycho impact (2300km away!) shaking loose a landslide from the southern face of the North Massif!

Above, a couple of views of the Taurus-Littrow valley from the Apollo 17 image archive. The left image is from the LEM while still on approach for landing--"out the porthole" literally! The second is on the surface with the famous Tycho Landslide spreading out just short of the horizon as a lighter colored horizontal streak radiating away from the N. Massif.

Proceeding north, the next area of interest is the G. Bond/Römer area, illustrated below...

This is an area of linear and arculate graben rilles related to the subsidence of Mare Serenitatis and thus structural and not volcanic in origin. This aplies to all major rilles in this entire region, with the qualified exception of those in Lacus Mortis & Posidonius which we'll look at later. Rima G. Bond is in two parts but has only one designation "I". Rimae Römer has two parts designated I and II, the second of which more or less bisects the 46km diameter pre-imbrium crater G.Bond C. Neither system is difficult to observe; the VMA suggests a 8" reflector, but both are plain in a 3-4" refractor. The upland portion of Rima Römer is probably the most difficult of these.

You'll notice that the terrae in this area has an appearance of being "swept" from the upper right to the lower left. This is sculpture created by the ejecta flow from the Crisium basin impact.

The next area of interest is the area immediately surrounding Posidonius and Posidonius itself...

The left image is an overview and also identifies the various clefts within Posidonius by their roman numeral designations. (Rille systems are identified this way in two publications, the LAC charts and the Lunar Quadrant maps from the U. of Arizona sold by Sky & telescope. the latter are very inexpensive and map rilles much more extensively than Rukl's does.)

The feature most easily seen inside Posidonius is the massive block that separated from the wall on the crater's right side in the image above. It continues in an arc that more or less follows the original rim, making a sort of inner, false rim. This has no desigantion that I am aware of. As seeing improves, the rille system exposes itself; I and II being the easiest to discern and the remainder increasingly difficult.

Posidonius is a late Imbrium era crater and so formed during the great Maria flooding timeperiod on the Moon. It was thus greatly affected by volcanic and subsidence processes, becoming flooded, uplifted, cracked and tilted towards Mare Serenitatis in the process. This tipping towards the maria may be what helped the original wall cleave away the eastern inner wall as described earlier. It's linear rilles are a result of the swelling uplift of it's floor and not directly related to the adjoining maria.

There is another discreet feature worth looking for here: the unnamed rille that crosses Rima G. Bond near it northerly terminus crater (Hall J, 8km). This is one of those features that is barely shown on the Lunar Orbiter iimge of this area (IV-079-H1) but is quite plain visually in more favorable lighting.

The Chacornac rille system is composed of three parts, marked I, II and III. I and II begin at the north rim of crater Chacornac and lay north of the small crater on it's floor (Chacornac A; 5km). II terminates at Chacornac's southern rim while I continues all the way past le Monnier to a terminal craterlet just south of le Monnier A (see lower right inset). Interestingly, it seems to connect with the northern end of a Littrow rille at this point, making it look almost as though it continues much further south...but actually it discontinues and another rille picks up along the same trend, Littrow II (see Littrow graphic, upper right frame).

Last (like death itself) is Lacus Mortis, the spookily named "Lake of Death", named by pioneering selenographer Giovanni Riccioli in 1651. Nearly all of the names of the large dark features on the Moon that we are familiar with today originated on Riccioli's map. Why "Lake of death"? i don't think anyone but Riccioli could answer that...

Lacus Mortis is a flooded, floor fractured crater of 155km diameter created in pre-imbrium times. The crater flooded in the late Imbrium period, probably from the east as well as from floor vents within it's interior. Much later, (about 3 billion years later actually) the prominent crater on it's floor, Burg, was formed in the Copernican era, sometime since about 800 million years ago.

The rilles within Lacus Mortis are of the linear variety, stress fractures caused by localized uplift of the floor. These differ from arculate rilles not only in shape but in their genesis. Arculate rilles were formed as a result of maria subsidence, the maria sinking under the weight of their own pooled lava emplacements. But the straight or linear rilles were usually formed as a result of localized uplift, usually in the floors of larger lava flooded craters.

Rimae Daniell are examples of arculate rilles. It is a four part system with the locations of three of it's parts indicated in the graphic above. Rima Daniell II begins at the hill Daniell Eta and runs as indicated. Rima Daniell I runs past hill Plano Zeta and west out to midway between Luther K (~3km, one of an equal sized pairing) and Luther Y, where it terminates. Rima Daniell IV runs along the edge of the terrae above and paralell to Rima I, ending opposite the Luther K pair. Rima III (not shown) is located a sort distance into the terrae south of IV and parallel to it.

It's been my experience that all these rilles are quite faint and dificult to discern if the seeing isn't above average. I and II are definately the easiest, III and IV quite difficult.

On to Lacus Mortis proper, the rilles here are very unusual, as one (Rima Burg II) is a combination of rille and scarp. This most prominet feature is usually seen as a wall defined by either highl;ight or stark shadow. There's a good reason for it's promenence; at 800meters, it's at least twice the height of the much more well known Straight wall (Rupes Recta)! (Byrne; LOPAM pg217) As it continues north, the scarp gradually fallas away until at it's terminus it becomes a rather ordinary linear rille, which abruptly turns and crosses over to connect with Rima Burg I. This rille, a wide graben, reaches from the rim to a small craterlet marked with an arrow just to the right of crater Burg. To the right of this is another linear feature which appears as another rille in the telescope, but is in reality a small crater chain. there are numerous other floor fracture rilles within the western floor of Lacus Mortis but these are the one's most likely to be seen and the only ones with actual designations.

Hope you enjoyed this little tour of this fascinating area of the Moon!

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