Lunar Volcanoes

An Introduction to Lunar Domes

By Nigel Longshaw


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Characteristics

Lunar Domes are more like the popular idea of a volcano than any other lunar surface features. These domes generally fall into two categories ranging in size from one to twenty kilometers across.

The first type generally consists of low, relatively flat topped smooth circular structures, which often have summit craters and slopes of not much more than one to three degrees. Some domes of this type are punctured by lines of coalesced or partially coalesced craterlets, resembling the fissure pits of Threngslaborgir in Iceland, which consists of a series of vents and lava flows arranged linearly across the terrain. Some of the cones have coalesced, and flows of lava have spilled over the vents. This type of structure is observed on the moon on a group of domed structures north of the crater Prinz.


left. Threngslaborgir fissure. A series of cones along a surface fissure some 2 miles long.
right. A series of coalescent craters on a group of domes north of the crater Prinz.

It is considered that this type of volcanic dome formed as a result of relatively low rates of eruption, and has a strong resemblance to terrestial shield volcanoes, examples of which are found in Hawaii in the basalt domes of Mauna Loa, Kilauea and others, all of which are associated with basaltic magmatism, as are most of the larger shield volcanoes.


Hawaiian volcano Mauna Loa, the structure is 250 miles across at the base.

Summit craters generally exhibit a rimless structure with steep inner slopes, their size usually being dependant upon the main dome.

The second types of domes are generally high standing islands of terrain remaining unflooded by younger surface deposits. This type is usually more hill-like and often shows evidence of surface detail, some features within this second group do not exhibit summit crater pits, suggesting they may have a different origin, perhaps the equivalent of terrestial lava fountains, cinder cones, or maybe localised uplifts caused by underground magma chambers.

Domes generally represent centers from which their has been a quiet extrusion of successive flows of liquid lava (the chemical compositionof lunar basalts is such that they become almost as fluid as water), or where sub-surface laccolithic magma (viscous magma, forming bun shaped masses) have arched up the surface rocks.


Viscous magma (dotted) injecting rocks which they have been able to updome.

Bearing in mind these vehicles for dome formation we must conclude that these features should form where magma has been able to reach the moon's surface, or very near to it.

These facts are confirmed when we consider the distribution of lunar domes, the majority are found on or near the borders of the Maria, or on the smooth, dark floors of some creaters. The distribution of the confirmed domes seems to point to two facts, one, that domes are generally found in groups rather than in single units, and two, on a large scale view there may be some evidence of chains as is found with large scale views of crater distribution.

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History

Domes were formally thought to be quite rare, although certain areas of major dome distribution had received attention by several early lunarians, for example S.R.B. Cook's observations of the Milichius/Tobias Mayer area in 1935 and Schlumberger's drawing of the Hortensius domes published in "The Moon", by W. Goodacre.

It has only been since the war that various exhaustive searches have been made, and observations have been recorded and catalogued, the more notable by Moore and Cattermole, W.L. Rae and the joint project by the B.A.A. and A.L.P.O. in the mid sixties. The apparent lack of early observation could be attributed to the difficulty in the detection of domes. As discussed, these features have very low slopes and therefore a lack of appreciable shadow, it is this anomaly which means the dome must be on, or very close to the terminator, to be successfully observed, as the sun rises over the lunar surface the delicate shadow is soon lost leaving a very low contrast object.

We must also take into consideration that some time ago the moon was one of, if not, the most observed astronomical object. With all this attention from amateurs certain features must been subject to changing degrees of interest, it became almost 'fashionable' to make observations of certain objects. Many authors of the older astronomical books seemed to constantly refer to 'mountain' when describing lunar features. They included descriptions of the mountainous terrain, lunar ring mountains and mountain ranges. If these early observers of the moon were so impressed by the mountainous grangeur of the moon they were unlikely to take notice of features which were difficult to observe. Gruithuisen and Schroter both saw domes in various locations, but thought they were clouds because they saw them disappear soon after sun rise.

Hundreds of domes are now known to exist, some appear as very tiny features visible on orbiter photographs, in fact lunar orbiter photography shows that domes are among the commonest features on the moon!

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Telescopic Observation of Lunar Domes

For observers who are unfamiliar with lunar observation, or lunarians who have not observed these features before, it would be beneficial to acquaint themselves with some of the 'Classic' domes such as...
The above is by no means an exhaustive or complete listing, but will give prospective observers a starting point.

The following Notes and drawings are only a small representative selection of these objects, observed by myself, and are included to assist in the recognition of these features, and give a basis for comparison for further observation. (Drawings taken directly from my own observing records).

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Dome South West of Gambart C

Gambart C Image

Quite a large dome (the diameter of Gambart C being in the order of 12km), its position making it an easy object to locate. A little irregular in outline, with a spur at it's base to the west, and a small hill to the south, observed as a detached feature by myself although some charts show the hill encroaching onto the southern slope of the dome. W.L. Rae records four domes in the immediate vicinity and two between Gambart and Gambart A.

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Dome West of Kepler

Kepler Image

A small dome about the size of Kepler F (the craterlet to the west of Kepler). The dome gives the impression of quite a low feature, possibly plateau like, no evidence of a summit crater was observed, despite very good conditions.

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Darwin

Darwin Image

There has been in the past, and to the present, much debate regarding the interior features of this crater and it would seem that there is still opportunity for useful studies to be made using medium to large apertures. On the evening of 1994 March 25 the "large rounded hill" (as first describled by R. Barker in the 1930's), was very obvious. Being considered by many to conform to the general classification of domed structures this feature is possibly the largest of its type on the moon. Darwin itself measuring some 80 x 89 miles, and the dome occupying most of the northern floor it should be visible in quite small apertures given favourable conditions. The region is very complex and holds a great deal of opportunity for the amateur with any telescope to observe the diversity of geological features.

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Suspect Domes North of Milichius A

Milichius A Image

The larger formation at the north of the drawing is well documented, it is plotted on the Hamlyn Atlas of the Moon and recorded by Harold Hill in his portfolio of lunar drawings. Under these good seeing conditions the summit craterlet was readily visible. The features to the south and south west of the dome were not as readily identifiable. These features have very low relief and may be coalesced to form a multi-domed elevation. The sun was relatively high over the lunar surface at the time of the observation and the area would benifit from further observation under lower angles of illumination.

Index

Dome to the West of Milichius

Milichius Image

A very well documented dome, and again easily located due to its position between Milichius and Milichius A. There is a summit craterlet, but it was not observed on this evening, possibly due to poor seeing conditions. The flanks of this dome are relatively steep, the dome casting quite a dark shadow, even when others in the area are quite difficult to pick out due to their lack of appreciable relief.

On this particular evening, even considering lighting levels were not particularly favourable, many domes were visible along the terminator. Two were visible on the floor of the crater Capuanus. Although very difficult to detect, the dome west of Kies and serveral in the Milichius / Hortensius region, including the group nestled between the peaks to the south of Tobias Mayer. Try for a minute to imagine the view from a craft in a low level oribit as it passed over this region with its imposing pinnacles and soft, rounded domes clustered together in an area some thirty miles in diameter.

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Conclusions

As discussed earlier, many domes have now been recorded, and many more await confirmation, but due to the nature of the objects a number could have been overlooked. The Sun's altitude above the lunar surface is one of the governing factors in the observation of dome structures, and it is probable that some domes have escaped detection. Even images returned from lunar orbiters are inconclusive, (a vast number of them were taken during periods when the Sun was at a relatively high elevation above the surface).

The study and charting of lunar domes could be carried out by those with moderate apertures, and there is always the possibility of picking up a feature which has not yet been charted or confirming a suspect dome. In addition to observing areas where domes are know to exist (in the Mare regions for example), it may be useful to study the highland regions. Dome structures noted in these areas may give some clues to possible associated or post cratering volcanic activity. Dome hunting in these areas is obviously difficult mainly because the terrain contains many hills, some of which show dome like structure, although a hill will have steeper slopes, more angular outlines and a less rounded top. If an object suspected of being a dome casts a pointed shadow then it can be rejected.

Once familiarity with the nature of the objects has been achived it would be worthwhile to spend some time scanning the terminator, in an attempt to catch the fleeting moments of good seeing and favourable illumination when the illusive dome may present itself.

Nigel Longshaw

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Bibliography

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Last updated 22 January, 2005