Guido Santacana, Eric Douglass, Daniel del Valle,
Raffaello Lena and Piergiovanni Salimbeni
In this report we discuss observations of some lunar domes that have the peculiarity of being bisected by a rille or cleft. Three domes clearly show this type of structure. Two of the domes are located in the Rimae Birt region while another one belongs to the area near the crater Hevelius. Most of the reported observations belong to the Rima Birt domes.
Observations of the Rimae Birt domes date back to the 1970’s. Marvin
Huddlestone, an ALPO Lunar Dome Survey observer, reported three domes in
the region. His drawing clearly shows two of the domes (-156-353 and –162-348)
bisected by the rille (figure 1).
Figure 1. Observation by Marvin Huddlestone of the Birt Domes.
July 22, 1972 at 06:03 UT
The following notes are from observations of these domes by Santacana in 1996:
Figure 2. Observation of the Birt domes by Piergiovanni Salimbeni and
P. Ricciardi. May 23,1999 at 22:00 UT.20cm SCT and 10cm refractor.
In figures 1, 2 it is clear that the two domes follow closely the same line as the Rima Birt and must be geologically related in terms of their origin. The observations of the Birt domes must be carried out at a low solar altitude in order to observe both domes and the shadow of the rille that bisects them. Sometimes conditions only permit observation of the larger dome. Thus, they are a good observational challenge for the dome observer.
The second bisected dome for which some observations are available is
a large dome near the crater Hevelius that is partially bisected by a cleft.
Figure 3 shows a drawing of this dome from an observation by Santacana.
This dome has an estimated diameter of 20km with a complex structure. There
are several features on its surface including a craterlet and some elevated
portions. The cleft can be seen in the drawing to bisect it diagonally
and partially and does not seem to extend beyond the structure of the dome.
It seems to have formed with the dome itself. As seen in the drawing this
observation was carried out at a very low solar altitude. The cleft disappears
as soon as the solar altitude increases above 4 degrees.
Figure 3. Dome Near Hevelius. Observation by
Guido Santacana on January 30, 1999
01:45 UT. 20cm SCT at 444x. Seeing=4
Transparency= 4. Position –897+144. North
is to the right.
A fourth observation reported by Daniel del Valle (figure 4) shows a
plateau dome described as the "Valentine Dome". The feature is described
as elliptical, heart shaped with a sharp and narrow eastern edge. The western
slope is heavily shaded and partly in the terminator’s shadow at this solar
altitude. There are what appear to be small hills and mounds on the surface.
One of the most interesting features reported by del Valle is that a suspected
rille traverses this dome but he was not able to observe it. The rille
was reported by the late Alika Herring whom we all know as one of the top
lunar observers ever apart from his ability to produce some of the best
telescope mirrors ever. I urge observers to observe this dome as much as
possible under conditions of low solar altitude in order to identify the
rille reported by Herring.
Figure 4. Observation by Daniel del Valle. "Valentine
Dome". Date: December 2, 2000 at 22:49 UT
20cm SCT at 271x. Coordinates: Lat. 31° N. Long. 10° E.
Discussion: General Geology
Domes are the surface expressions of volcanism. However, volcanic materials began deep within the lunar mantle as melts from the heat of radioactive decay. These melts tracked up through the crust to reach the surface. However, this ‘tracking’ was not randomly accomplished. Rather, the melts especially rose through areas of weakness, such as places where faults lay in the crust. These especially occurred about basins, where the crust was deeply faulted. As the melts preferentially rose through crustal faults, it should come as not surprise that they are associated with surface expressions of faults. The most common expression of such a volcanic melt tracking through a fault is a dike. A dike is a linear intrusion of lava into a planet’s crust. If the dike remains deep, then no surface expression is produced. However, if the dike comes close to the surface, it begins to stretch the crustal rock above it, so producing ‘upbowing’ of the bedrock. This kind of structure would be called a laccolith. If this upbowing produces stresses that are greater than the strength of the layer above it, then the layer will split, producing a rille. Finally, if the dike rises to the surface, then the lava in it will pour out on the surface as a fissure eruption. Because of the tendency of fissure eruptions to produce fluid lavas, which do not ‘pile up’ into domes, the presence of bisected domes is an uncommon occurrence.
Discussion: Application of Geology to Specific Areas
Rima Birt Region: This rille did not develop as a flow feature,
such as a lava channel, which is especially appreciated when the rille
is examined in detail, and found to have a discontinuity (figure 5, marker
‘A’)(from: "The Moon as Viewed by Lunar Orbiter" NASA SP 200). Rather,
it likely marks the place where a dike of lava intruded into the subsurface
bedrock. Even more interesting is the second rille (figure 6, marked by
the dark line)(from: "Consolidated Lunar Atlas" by Kuiper et al., digital
version by E. Douglass), which is another surface expression of (probably)
the same dike. This rille is so close to Rima Birt that most smaller telescopes
cannot separate the two. A lunar orbiter image of this second rille is
provided in figure 7 (from: "Lunar Orbiter Photographic Atlas of the Moon"
NASA SP 206). From these images, we can reconstruct a geologic history
of this region. A lava melt tracked up a fault in this region, upbowing
the surface, and fracturing it into at least two rilles. Parts of this
intrusion reached the surface, and so produced the domes at the southern
reach of Rima Birt. Further, it is likely that other parts of the dike
also reached the surface, so producing the crater-like features marked
with the "l" in figure 6.
Figure 5. From "The Moon as Viewed by Lunar Orbiter"
NASA SP200.
Figure 6. Second Rille at the Rima Birt From "Consolidated Lunar Atlas"
second rille from "Lunar By Kuiper et al digital version by E. Douglass
.
Figure 7. Lunar Orbiter image of the second rille from "Lunar Orbiter
Photographic Atlas " SP 206.
Hevelius Region: This region of the moon marks a boundary between
the highlands and the lower flood lavas. When these lavas inundated the
region, they entered via dike intrusions, which reached the surface. As
these extended the surface, it would not be surprising to see the fracturing
of surface rock. Before lunar missions made it to the moon, this would
have been the explanation for what appeared to be bisected domes like this
one (figure 8) (from: "A New Photographic Atlas of the Moon" by Zdenek
Kopal). However, lunar orbiter missions revealed that some of these ‘domes’
were actually islands of highland regions that were just high enough to
escape inundation by the lava flows, as is the case here (figure 9) (from:
"A New Photographic Atlas of the Moon" by Zdenek Kopal). The rille here,
then, is from fracturing of the highland region. This image reveals, then,
the provisional nature of lunar dome classification: domes are not definite
until high-resolution spacecraft imagery confirms the geology of the region.
Figure 8. "Bisected Dome" from " A New Photographic
Atlas of the Moon" by Znedek Kopal.
Figure 9. From " A New Photographic Atlas of the Moon" by Znedek Kopal
Valentine Dome Region: This dome occurs at the edge of the Serenity basin. It is notable for being a complex dome, with small protrusions. The fact of the rille is difficult to ascertain (the authors examined 5 good images of this dome without verifying a central rille; cf. figure 10)(from: "Consolidated Lunar Atlas" by Kuiper et al., digital edition by E. Douglass). Still, a central rille would not be an unusual feature of domes, for the reasons listed above. Thus, if a rille can be confirmed, it is likely to represent the surface expression of a dike, where lava reached the surface to form the dome itself.
Figure 10. Valentine Dome for "Consolidated Lunar Atlas" by Kuiper et
al digital
version by Eric Douglass.
Conclusion: Bisected domes are interesting geological features of the moon and the data presented above shows that they are not common features. In fact they may be mistakenly labeled and confused with islands of highland regions that were not covered by lava. There are more domes or dome-like structures showing partial or total bisection by rilles or clefts in need of identification, positioning, imaging and drawings. Only careful study of the surface features of domes under low solar altitude conditions will identify these highly interesting and rare features. Observers are urged to report these observations to the ALS dome project coordinator.