ALS LUNAR DOME SECTION
Guido Santacana and Eric Douglass
American Lunar Society
With this issue of Selenology we open a new section in the ALS that will devote its efforts to the observation and study of lunar domes. The main objective of this section is to encourage and promote the active and systematic observation of lunar domes by amateurs worldwide. It will accept observations from interested observers who would like to participate in this very challenging area of lunar study. Observations of lunar domes in the form of images, drawings and/or notes may be sent directly to the section coordinator by e-mail or postal service. The observations received as well as a selected dome or dome field in need of observation will be included in Selenology and/or the ALS updates.
A sample format for reporting dome observations is included with this article. A list of areas or domes in need of observation is also included with this article. We highly recommend that the observer obtain two tools for these projects: (1) a good lunar atlas like Rukl's, and (2) the Lunar Toolkit Program obtainable from the Association of Lunar and Planetary Observers' (ALPO). This program has a complete database of lunar domes prepared from almost 40 years of observations by ALPO members under the direction of Harry Jamieson who is the Lunar Dome Survey coordinator for ALPO. It tells you the domes that you can observe from your locality at any specific date during the lunar cycle. To obtain the program you can contact Mr. Jamieson at firstname.lastname@example.org .
Introduction to the Geology of Lunar Domes
Lunar domes are gentle swells between 3 and 20 km across, and at most a few
hundred meters in height. Most have very low angle of inclinations, only a few
degrees at most. This makes domes similar to earth's shield volcanoes. They are the best evidence of volcanic activity in the moon. Many have a central 'pit crater', which occurs upon magma withdrawal with collapse about the vent. Two typical and well observed domes are depicted in figure 1.
Kies Dome Milichius Dome
The Kies dome above was observed on May 25, 1999 at 1:37 UT using an 8" SCT at 444x under very good atmospheric conditions. The dome lies near the flooded crater Kies and has the typical spherical form and a central pit crater. The low solar altitude for this observation produced excellent contrast, which is a must for effective dome observation.
Domes probably formed in the latter stages of volcanism on the moon. Early stage lavas were very fluid, due to their high heat, massive volumes, and mineralogy. This latter feature was particularly important, as lunar lavas are mafic in composition (low silica content, high metal oxide content) which tend to be very fluid (low viscosity). This is as opposed to felsic lavas (high silica content), which on earth produce steep-sided, rhyolitic domes with short lava runs. Because of this, the original lavas on the moon flowed from eruptive fissures and did not produce 'volcanoes'. An example of this kind of volcanic activity on earth is the 'Great Crack' fissure eruption of 1823 in the Hawaiian volcanoes. The vast majority of these eruptive fissures were covered over by their lavas, and so cannot be seen. Across time, the erupting lavas cooled, decreased in flow rate, and began to crystallize. This changed the characteristics of the lava, decreasing its fluidity so that it began to 'pile up' around its vent, forming low shield-like volcanoes. This is the source of our lunar domes.
The distribution of domes in the lunar surface favors the Western Hemisphere of the moon with almost twice the number of domes of the eastern hemisphere. Most of the domes lie in the region of the maria. The large cluster of 28 domes in the Hortensius-Millichius-Tobias Mayer region and the vast collection of domes in the Marius Hills region, coupled to a greater expanse of maria accounts for the greater manifestation of domes in the western hemisphere. Indeed, by 1970 it had been determined that of the 149 domes confirmed at the time, 45% were members of such dome clusters. Most observed domes are found in the maria. Without a doubt, the vast majority of domes are in the maria, as this is the place where lunar lavas tracked up deep-seated faults created by the basin impacts. However, a few are expected to occur in highland regions, as a variety of 'cryptomare' (lavas covered over by thin veneers of highland material) do occur. Such domes would be unusually difficult to identify due to contrast issues and locally hilly geography. In the maria most domes seem to be located in the outer regions with the exception of Mare Tranquilitatis where a great proportion of domes lie in the central region. The fact that lunar domes tend to lie along the borders of the lunar maria, strongly suggest that the lunar domes are products of ancient volcanic activity which has displayed itself along lines of crustal weakness.
Observation of Lunar Domes
The observation of lunar domes is a challenging activity that requires dedication and good timing coupled to good observing conditions. Most domes cannot be observed when far from the terminator. As their distance from the terminator increases the dome profile begins to blend with the local terrain and for all practical purposes the dome disappears from view. For the reasons mentioned, most authors recommend that observations of domes be carried out near the lunar terminator, where the solar altitude does not exceed 4-5 degrees. Other observers use 8 degrees of solar altitude as the minimum but we have found that at more than 4-5 degrees of solar altitude the smaller domes, low profile domes and surface details of larger domes become less conspicuous. There are only a few domes that can withstand high solar altitudes without disappearing into their local areas. One specific group is the Schlumberger Domes which are six very prominent domes located north of the crater Hortensius (see figure 2, below).
Lunar domes may come in many shapes and sizes but the most common ones are hemispherical in shape with a low profile, low non-pointed shadow, and sometimes a central craterlet (see figure 3, below).
The pit crater is a good indication of the volcanic origin of these structures but it is not always present. Domes without a pit crater are still volcanic in origin, but appear to have covered their central vent with lava. Another possibility is that these represent intrusion of lava with bending of overlying surface structures. However, such structures on earth have a variety of forms, as they grade into dikes, and the lack of such structures on the moon suggests a different mode of origin. Larger and more complex domes may show diverse surface features as seen in the complex dome structure near the crater Hevelius shown in figure 4 below.
Other domes will show either a complete or partial bisection by a cleft or rille. Typical domes showing this kind of feature may be found near the Straight Wall in the Rima Birt region.
Classification of Lunar Domes
In 1964 Dr. John Westfall created a dome classification scheme that is widely used by the Association of Lunar and Planetary Observers Lunar Dome Survey. Basically the
Figure 2 Figure 3 Figure 4
classification takes into account the size of the dome, the shape, location, surface detail, surroundings and profile. It also assumes that the dome is a discrete feature and not a part of another structure, the ratio of the major/minor axes does not exceed 2:1, the average slope angle does not exceed 5 degrees and no secondary feature occupies more than ¼ of the surface area of the dome. However, in a few domes, the slope angles may exceed this number, suggesting differentiated magmism with increasingly viscous lavas (the Marius Hills domes reach "7 to 20 degrees" according to The Lunar Sourcebook). Each category is given a letter or number and their combination provides a clear encoded description of the dome in question. The Westfall classification for lunar domes is as follows:
D - Dome
D - Dome complex
U - Uplands
W - Maria
UW - Uncertain if uplands or maria
Plan; Major Axis is:
1. Less than 5 kilometers
2. 5 to 20 kilometers
3. 20 to 35 kilometers
4. Over 35 kilometers
a. Circular ( major/minor axes 1.00-1.25)
b. Elliptical (major/minor axes 1.26-2.00)
e. Too ill defined to classify.
5. Gentle (under 2 degrees)
6. Moderate (2 to 5 degrees)
g. Flat summit
h. Sharp summit
i. Multiple summit
g'. Flat summit--Asymmetric
h'. Sharp summit--Asymmetric
i'. Complex summit
7. Depression (pit, craterlet, saucer)
8. Elevation (hill, ridge, or peak)
9. Cleft or valley
0. No observable detail
m. On margin
n. Transversal (crosses dome)
p. More than one such feature
An example of the use of this classification would be DW/2b/6f/7j9m8p. This describes a dome on a mare with a diameter of 5 to 20 kilometers, elliptical in shape, average slope angle of 2 to 5 degrees and hemispherical cross section. The surface contains a central depression, a cleft that cuts across its margin and several elevations.
With practice the use of this classification scheme becomes easier and is a must in every serious dome observation since it establishes the standard by which all domes are compared and studied.
Equipment and Observing Techniques
Even a small 3" refracting telescope will show the larger domes in the moon but for more serious work a refractor of no less than 4" or a reflector of no less than 6" is necessary. Another item that is almost indispensable is a good motor driven equatorial mount that can provide steady tracking. Due to the fact that domes are difficult objects to observe, it is usually necessary to use high powers at the telescope. This precludes the use of a hand driven telescope for prolonged observations. One of the authors (GES) uses a good and well collimated 8" SCT mounted in a sturdy equatorial mount with drives in both axes although the Right Ascension drive suffices for the purpose of maintaining the image in sight. Collimation of the optics must be stressed specially for reflectors and SCTs which are more affected by slight misalignment of the optics than refractors. A good clear image is paramount in order to provide the observer with the ability to capture an elusive dome specially when atmospheric conditions are not perfect, which will happen most of the time. The use of high power is recommended whenever possible. In good seeing conditions there is no reason why good optics cannot be pushed to perform to their maximum. GES commonly uses 444x with his 8" SCT for dome observing and drawing. By combining a short 2x barlow of good quality and a 9mm Plossl eyepiece, GES has been able to produce the observations that appear in this article. Glare reducing filters can also be utilized but are not a requisite. It is also important to mention that effective dome observing cannot be carried out with the moon too low near the horizon. An altitude of 45 degrees from the horizon is generally necessary to reduce atmospheric effects. A steady atmosphere is also a requisite although some of the larger domes can be observed with some turbulence.
Always plan ahead your dome observing session. If you use the highly recommended Lunar Toolkit Program from the ALPO, look for the domes observable from your area at the time that you plan to start observing. Don't try to observe many domes from different areas. It is better to observe a specific dome group from an area and search the area for any dome not listed. You will find that many listed domes have not been totally confirmed and are in need of more observations. Plot the positions of the domes in a lunar atlas map to guide you in the search. As soon as you localize the area that you wish to observe use medium powers to find domes near the terminator and when they are found increase the power as much as the seeing and optics will allow you. High power is important when observing surface features of domes. Draw a sketch of the dome showing its position relative to some known feature such as a crater or rille. Use a gray scale when drawing were zero is black and 10 is white. All other numbers in between are decreasing shades of gray as you go toward 10.
Usually the best way to go is to do a line sketch of the region and dome and then add the gray shades later. Mark each region with the gray shade that it should have in the final drawing. A drawing usually captures much more detail than any other system. We are born with the best imaging system already incorporated in our brain. The drawings in this article were performed using the simple techniques described above.
When reporting dome observations the following data must be included:
Date of Observation_______________UD
Time of Observation____________ UT
Colongitude_____________ (from the Lunar Toolkit Program)
Positions of Domes Observed_______________ (use longitude and latitude from Toolkit Program)
Classification of Domes Observed____________ (use the Westfall system)
Seeing should be reported in the ALPO scale were 0 is worst and 10 is excellent. This is basically a measure of atmospheric turbulence. The transparency is reported as the faintest star magnitude that you can see with you naked eye. Date and time of observation must be reported as Universal Date (UD) and Universal Time (UT). This means the date and time at Greenwich in the United Kingdom. If you are in Eastern Standard Time (EST) all you have to do is add five hours to your local time to obtain the universal time or UT (4 hours if you are on daylight savings time). As an example, if you are observing on July 31st 2000 at 9:00pm EST, you will report the observation date as August 1 2000 UD and time as 2:00 UT. If you notice, this is because at Greenwich in the United Kingdom it is already 2:00am in the morning of August 1 2000 and this is our reference point for UD and UT from anywhere in the world.
Include a sketch whenever possible. You may start by observing the domes that appear in this article specially the Kies and Gambart domes. This will familiarize you with the appearance of typical domes and will prepare you for your future observations.
We encourage old and new observers to dedicate observing runs to the search and observations of domes. You will not be disappointed and at the same time will learn to view the moon as an astronomical object in need of more study and were amateurs can do useful scientific contributions.
Westfall, John E. A Generic Classification of Lunar Domes. JALPO 18, Nos. 1-2,
2. Delano, K.J. The Distribution of Lunar Domes. JALPO 22, Nos. 1-2 (1970) 8-13
Phillips, J. The New Lunar Dome Survey: The Hortensius- Millichius-Tobias Mayer
Region JALPO 33, Nos. 4-6 (1989) 61-72.
Wilhelms, D.E. The Geologic History of the Moon. USGS Professional Paper 1384.
Heiken, G.H. et al. Lunar Sourcebook. Cambridge University Press. 1991.
Frankel, Charles. Volcanoes of the Solar System. Cambridge University Press. 1996.
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