Leviathan's Lunar Feature of the Week

Leviathan's Lunar Feature of the Week #5:  Billy Crater & the Ocean of Storms
Article by Peter Grego

Oceanus Procellarum (Ocean of Storms) is the largest expanse of lava on the Moon, with an area of more than two million square kilometres, a little smaller than Earth's Mediterranean Sea. The idea that Procellarum was created by a single mammoth impact that carved out a basin more than 3,200 km across has now fallen into disfavour - such an impact would probably have torn the Moon to pieces. The western maria, including the plains of Procellarum, were formed around 3.2 billion years ago and were the last large maria to appear on the Moon's surface.

Procellarum's southern area is low on spectacle but high in terms of geological interest. An irregular bay on the ocean's edge contains the craters Billy and Hansteen, two features useful in finding your way around this region. Billy is a dark floored crater 46 km in diameter, and is named in honour of the French Jesuit astronomer Jacques de Billy (1602-79) who was one of the first to reject the role of astrology in science, along with superstitious notions about the malevolent influence of comets.

Billy is an old impact crater whose rim rises to 1,300 metres above its dark flat floor. The floor is one of the darkest spots on the Moon's face, and can easily be seen any time when it is illuminated, even at full Moon. Billy contrasts with Hansteen, a light coloured 45 km crater with a hummocky floor. The relationship between Billy and Hansteen is somewhat similar to that between dark floored Herodotus and brilliant Aristarchus (35 km and 40 km respectively) some 1,130 km due north, also in Oceanus Procellarum.

Billy crater is famous for its very dark, lava-filled floor. Image by Damian Peach.

According to the American lunar expert Ralph Baldwin in "Measure of the Moon" (1963 - pre-Apollo), Billy, Hansteen and Herodotus are all age class 5, indicating very ancient craters that have been flooded with lava (or, he supposed, volcanic ash). Billy and Herodotus are impact craters pre-dating the later lava flows of Oceanus Procellarum, and therefore formed more than 3.2 billion years ago, their floors being subsequently filled with lava. Hansteen was formed after vulcanism had ceased in Procellarum and is younger than 3 billion years old. However, Aristarchus is far younger than Hansteen, having been formed by an asteroidal impact a mere 250 million years ago. Whilst Aristarchus is one of the brightest areas on the entire Moon, Billy is one of the darkest, and both features are easy to locate, even through a pair of steadily held small binoculars.To the north of Billy is the mountain massif of Mons Hansteen, a triangular outline 30 km broad, shaped like the Inner Hebridean Isle of Mull (only slightly smaller), with peaks rising to similar heights.

Billy Crater above and Mons Hansteen and Hansteen Crater below.

At the time of my observation the sunlight is illuminating Billy from the east, having risen just 17 degrees high. It is mid-lunar morning at Billy, and detail on the floor comprises subtle tonal variations that hint towards slight undulations in the level of the floor. These may be lava flows or traces of the original floor structure before it was buried by lava. Billy can be seen any time this month between the evening of 8 February and the morning of 22 February. At a suitable low illumination a small telescope will show the triangular shape of Mons Hansteen and indicate the roughness of Hansteen crater's floor. Larger instruments will reveal structure on the floors of Hansteen and Billy, the small hills to the north of Hansteen and the narrow rille that runs next to the outer western rim of Hansteen.

Leviathan's Lunar Feature of the Week #4:  Gassendi Crater - Fractured Floor Beauty

Gassendi crater is one of the most beautiful telescopic objects on the moon's visible surface, and structurally one of the most interesting and suggestive. It is a walled-plain, 55 miles in diameter, of a distinctly polygonal type, where the N.E. and S.E. sections are practically straight, while the intermediate E. section exhibits a slightly convex curvature, or bulging in towards the interior. The western side of the crater walls also appears very angular, which is most evident at an early stage of local sunrise. The wall on the N. is broken through and almost completely wrecked by the great ring- plain "Gassendi A". The bright western section of the border is in places very lofty, rising at one peak, N. of the well-known triangular depression upon it, to 9,000 feet, and at other peaks on the same side still higher. It is very low on the S., being only about 500 feet above the surface. The floor, however, on the N. stands 2,000 feet above the Mare Humorum. On the E. there is a peak towering 4,000 feet above the wall, which is here about 5,000 feet above the floor, and 8,000 feet above the Mare Nubium. 

Gassendi Crater is found on the boundary between Mare Humorum and Mare Nubium, in the moon's eastern/central region.

A very notable feature in connection with this formation is the little bright plain bounding it on the N.E., and separated from it by merely a narrow strip of wall. This enclosure is flanked on the N.W. by Gassendi A, and on the S.E. and N.E. by a coarse winding ridge, running from the E. wall and terminating at a large irregular dusky depression. Gaudibert has detected a crater near the S.W. edge of this bright plain, which includes also some oval mounds. The interior of Gassendi is without question unrivalled for the variety of its details, and, after Plato, has perhaps received more attention from observers than any other object. The bright central mountain, or rather mountains, for it consists of a number of grouped masses crowned by peaks, of which the loftiest is about 4,000 feet, is one of the finest on the moon. It was carefully studied with a 6 1/4 inch Cooke-achromatic by the late Professor Phillips, the geologist, who compared it to the dolomitic or trachytic mountains of the earth. The buttresses and spurs which it throws out give its base a digitated outline, easily seen under suitable illumination. There are between 30 and 40 clefts in the interior, the majority being confined to the S.E. quarter of the floor. Those most easily seen pertain to the group which radiates from the central mountain towards the S.E. wall. They are all more or less difficult objects, requiring exceptionally favourable weather and high powers. A fine mountain range, the Percy Mountains, is connected with the W. flank of Gassendi, extending in a S.W. direction towards

Gassendi crater is a beautiful crater that sits on the inner ring of mountains that surround Mare Humorum. The miriade of fractures on the crater's floor are best seen as the terminator sweeps over this region. Photos by Andre van der Hoeven, Netherlands.

This pair of images emphasizes how different imaging is from visual observing. Most observers can only barely glimpse the rille infestation of Gassendi and the linear rille to the west, but André captures them clearly.  Just think what it was like in the decades leading up to the Apollo missions, when observers had a difficult time increasing understanding of the Moon because they were never completely sure of what they were actually seeing there through the eyepiece. Now with images like this, the reality is there waiting to be interpreted and put into a story. 

The synoptic view is the place to start. Gassendi sits on an inner ring of the Humorus Basin - the ridge at center left that used to be called the Percy Mountains. Two outer rings are hinted at by the isolated collection of hills to the left and then beyond that an eroded scarp. Gassendi was tilted toward the center of Humorum as it subsided due to the increasing load of mare lavas. The lavas covered the crater's ejecta and rose up fractures under the crater to raise the floor, creating the rilles by fracturing, and some of the magma leaked onto the lowest part of the floor. At some point the large crater, Gassendi A, smashed into Gassendi's north rim, dislodging a massive landslide of rim material onto the big crater's floor, perhaps covering some of the rilles.

Sketch and Details by Carlos E. Hernandez. Observation of Gassendi using a 9-inch (23-cm) F/13.5 Maksutov-Cassegrain at 359x under steady (6-7/10) seeing conditions and transparent skies (4-5/6)

Leviathan's Lunar Feature of the Week #4:    Wargentin - A Walled Plain

Wargentin, the large filled-in crater to the left of center in this image is accompanied by slightly small crater Nasmyth and large Phocylides with its attendant smaller craters. Composite image by Damian Peach.

Wargentin is an unusual lunar impact crater which has been filled to its rim by a basaltic lava flow, forming a raised plateau. When the lava flow occurred, it erupted from within the crater walls and proceeded to accumulate until overrunning the lowest portion of the rim. Some blockage then prevented the lava flow from returning to equilibrium. Since the time when this occurred, some ejecta has been deposited across the top, giving the surface a higher albedo than is typical for deposits of basalt.
 
The rim of Wargentin is somewhat worn and is overlain by a few small craters. The outer wall climbs to a height of 0.3 km above the surrounding terrain. A spoked pattern of wrinkle ridges can be discerned on the surface, radiating from the center of the crater.

Wargentin is located on the approaches to the southwest lunar, so that it appears elongated when viewed from the Earth. It is connected along the southeast rim to the slightly smaller crater Nasmyth, which is in turn overlaid by the larger Phocylides. To the northeast is the much larger walled plain Schickard.

Guide to the Wargentin Crater region. Photo by Math Heijen.

Sketch by Nigel Longshaw

Wargentin is one of my highlight objects to observe in the nights leading up to the full moon and new moon as the terminator approaches the area and shadows grow long in the region. The crater's filled 1400ft rim gives it the appears of a mesa or wide bluff through small telescopes and at times the whole crater can look like a raised 'coin' laying on the moon's surface in comparison to the others nearby. The sinuous wrinkle ridge running across the centre of its longer axis reminds me of a snake's forked tongue. Medium and high power observations bring out more detail in its flooded interior, showing very small crater chains in a few zones. The crater looks very elliptical to the observer here on earth, but that, of course, is due to foreshortening because of the craters postion near the moon's southeastern limb. Personally I have found that using a red filter has sometimes helped to improve detail in this region when seeing conditions on the night of observing are poor. Give it a try and let me know what you think!

Craters Wargentin, Nasmyth and Phocylides are located near the southeastern limb of the moon's face.

Leviathan's Moon Observer Feature of the Week:  
#3 Reiner Gamma - Magnetic Anomaly

Reiner Gamma (γ) is an albedo feature that is located on the Oceanus Procellarum, to the west of the crater Reiner on the Moon. The center of the formation is located at selenographic coordinates WikiMiniAtlas7°30′N 59°00′W  /  7.5°N  59.0°W / 7.5;  -59.0. It has an overall dimension of about 70 kilometres. The feature has a higher albedo than the relatively dark mare surface, with a diffuse appearance and a distinctive swirling, concentric oval shape. Related albedo features continue across the surface to the east and southwest, forming loop-like patterns over the mare.

Reiner Gamma is not associated with any particular irregularities in the surface, and so the cause was a mystery until similar features were discovered in Mare Ingenii and Mare Marginis on photographs taken by orbiting spacecraft. The feature on Mare Ingenii is located at the lunar opposite point from the center of Mare Imbrium. Likewise the feature on Mare Marginis is opposite the midpoint of Mare Orientale. Thus it is believed that the feature resulted from seismic energies generated by the impacts that created these maria. Unfortunately there is no such lunar mare formation located precisely on the opposite surface of the Moon, although the large crater Tsiolkovskiy lies within one crater diameter. The central feature of Reiner Gamma does bear a resemblance to the dipolar formation created by iron filings on a surface with a bar magnet on the underside. Low-orbiting spacecraft have observed a relatively strong
magnetic field associated with each of these albedo markings. Some have speculated that this magnetic field and the patterns were created by cometary impacts. However,  the true cause remains uncertain.

Lunar Reconnaissance Orbiter (LRO) closeup of Reiner Gamma swirl feature.

Reiner Gamma's magnetic field strength is approximately 15 nT, measured from an altitude of 28 km. This is one of the strongest localized magnetic anomalies on the Moon. The surface field strength of this feature is sufficient to form a mini-magnetosphere that spans 360 km at the surface, forming a 300 km thick region of enhanced plasma where the solar wind flows around the field. As the particles in the solar wind are known to darken the lunar surface, the magnetic field at this site may account for the survival of this albedo feature.

In early lunar maps by Francesco Grimaldi, this feature was incorrectly identified as a crater. His colleague Giovanni Riccioli then named it Galilaeus, after Galileo Galilei. The name was later transferred northwest to the current crater Galilaei.

The best time to observe Reiner Gamma is late on the 13th day after new moon or early on the 14th day. Small telescopes will require at least medium power eyepieces (100X - 250X) to show much in the way of details in this object.

Location of Reiner Gamma feature on eastern side of the lunar face, in the mid-southern portion of Oceanus Procellarum. CLICK TO ENLARGE.

Leviathan's Moon Observer Feature of the Week: 
#2 Ptolemaeus and the Chain Links

Visual lunar observers of the past were a romantic bunch. In addition to imagining that the large dark patches were seas, bays, lakes and marshes, and producing a fabulous system of names, they used to call craters like Tycho, Eratosthenes and Copernicus ‘ring mountains’. Even larger lunar craters in excess of 70 km with generally low rounded walls and flattish flooded or partially flooded floors were once called ‘walled plains’. The most impressive collection of ‘walled plains’ on the Moon’s near-side dominates the southern central regions. Among them is a striking north-south ‘chain’ of large craters which comprises the magnificent Ptolemaeus, Albategnius, Alphonsus, Arzachel, Purbach, Regiomontanus and Walter. Of these, Ptolemaeus is the main attraction, particularly when it’s lit up by a low morning or evening Sun.  

Ptolemaeus and the chain links by Peter Grego

At 153 km across, Ptolemaeus demands the observer’s attention by virtue of its sheer size, even though it is not the most geologically varied
feature on the Moon. It has a distinctly polygonal outline, and its floor, bounded by uncomplicated walls, appears relatively featureless in small
telescopes (even when illuminated by a low Sun), save for the 9 km diameter crater Ammonius in the northeastern sector of its floor. Through a 150 mm telescope, observable detail on the crater’s floor includes Ptolemaeus B, a 15 km diameter ghost ring adjoining Ammonius in the north, along with lots of dimple craters and a smattering of tiny craterlets on the verge of telescopic resolution. The region around Ptolemaeus has been sculpted into a fantastic northwest-southeast pattern by the immense forces unleashed by the asteroidal impact that formed the nearby Imbrium basin around 3.8 billion years ago. 

This month you can view Ptolemaeus and its ‘chain’ beginning on the
evening of 23 March when it’s lit by a lunar morning Sun.

Leviathan's Moon Observer Feature of the Week: 
#1 Clavius Crater and its Underlings

There are few lunar observers who have not devoted more or less attention to this beautiful formation, one of the most striking of telescopic objects. However familiar we may consider ourselves to be with its features, there  is always something fresh to note and to admire as often as we examine its apparently inexhaustible details. It is 142 miles from side to side, and includes an area of at least 16,000 square miles within its irregular circumvallation, which is only comparatively slightly elevated above the bright plateau on the E., though it stands at least 12,000 feet above the depressed floor.

 At a point on the S.E. a peak rises nearly 17,000 feet above the interior, while on the W. the cliffs are almost as lofty. There are two remarkable ring-plains, each about 25 miles in diameter, associated, one with the N., and the other with the S. wall, the floors of both abounding in detail. The latter, however, is the most noteworthy on account of the curious corrugations visible soon after sunrise on the outer N. slope of its wall, resembling the ribbed flanks of some of the Java volcanoes. There are five large craters on the floor of Clavius, following a curve convex to the N., and diminishing in size from E. to W. The most easterly stands nearly midway between the two large ring-plains on the walls, the second (about two- thirds its area) is associated with a complex group of hills and smaller craters. Both these objects have central mountains. In addition to this prominent chain, there are innumerable craters of a smaller type on the floor, but they are more plentiful on the S. half than elsewhere. On the S.W. wall are three very large depressions. On the broad massive N.W. border, the bright summit ridge and the many transverse valleys running down from it to the floor, are especially interesting features. There are very clear indications of "faulting" on a vast scale where this broad section of the wall abuts on the N. side of the formation.