In this paper we study three domes that have been observed in the Rima Birt region. Two of them are bisected by the Rima itself. One of the bisected domes lies to the west of the dome located at –155-347 and shows the same appearance and dimensions as –155-347. The approximate location is –161-349 (long. -9° 53.56’, lat. -20° 25.27’) and it seems to need very specific lighting conditions for its observation due to the fact that it has been described by few observers until now and has appeared only two times in the reports of the ALPO Lunar Dome Survey of the last 30 years.
 
 

                                                          Introduction
 
 Lunar domes are gentle swells between 3 and 60 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. Many have
a central 'pit crater', which occurs upon magma withdrawal with collapse about the
vent.

The distribution of domes in the lunar surface favors the Western Hemisphere of the moon. The large cluster of 28 domes in the well-studied Hortensius-Millichius-Tobias Mayer region coupled to a greater expanse of maria accounts for the greater manifestation of domes in the western hemisphere.

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.

Geology of the Rima Birt Section

The Rima Birt section of the moon represents a vast lava field in the Mare Nubium
region. The rima itself occurs in a massive flooded crater, somewhat over 200 km
in diameter. While the outline of the western half of this crater can been seen
in the accompanying highland region, the eastern half can only be visualized by
the mare ridges, which mark the rim. It is likely that the faults created by this
impact produced the conduits for lavas in this region. Rima Birt itself is a
rille just over 50 km in length, with small craters at each end. From
photographs, both of these craters appear to occur in domes. The rille itself
is unique in that it has an offset in its mid-section. This rules against its
origin as a flow feature (lava tube, etc.). Rather, our sense is that it
represents extension and faulting due to dike intrusion. There is
a complex of three domes at the north end of Rima Birt, further
supporting our notion that all these features have a volcanic origin. There is also a
general 'darkening' about these domes which likely represents a late stage lava
flow with slightly different metal content (this produces the 'color' of lava on
the moon). However, another option could be that this represents lava
fountaining. The age of this dome complex is difficult to estimate, as we lack
high resolution satellite imagery of this region. However, a general estimate
would put it at 2-3 billion years of age. As a final note, it is of interest that
dome complexes often occur on gentle rises (Marius Hills, Mons Rumker), and we
would suspect that just such a rise might exist here.
 
 

 Observation of Lunar Domes

The observation of lunar domes is a challenging activity that requires dedication and good timing coupled to good observing conditions. All observations of domes must be carried out near the lunar terminator where the solar altitude does not exceed 8 degrees. The domes may come in many shapes and sizes but the most common ones are hemispherical in shape with a high profile, low non-pointed shadow and sometimes with a central crater on the top. The crater is a good indication of the volcanic origin of these structures but it is not necessarily present at all times. In 1964 John Westfall created a dome classification scheme that is widely used by the Association of Lunar and Planetary Observers Lunar Dome Survey. Basically the classification takes into account the size of the dome, the shape, location, surface detail, surroundings and profile. Each category is given a letter or number and their combination provides a clear encoded description of the dome in question.

Observations of The Birt Domes

On May 23 1999 at 22:00UT P.G. Salimbeni and P. Ricciardi of the Geologic Lunar Research group observed three domes to the north of the Rima Birt. This observation was carried out independently by both observers. The telescopes used were a 200mm SCT and a 100mm f/10 refractor. Figure 1 below shows the aspect of the region as drawn by Salimbeni.
 

 
 Fig 1- Piergiovanni Salimbeni
 

As depicted in the drawing, there are two bisected domes that follow closely in the same line as the Rima Birt itself. Forming a triangle with these two bisected domes lies a third dome to the east. The more difficult dome is the one observed on the northern tip of the Rima . The drawing was carried out under very favorable observing conditions with a seeing of II on the Antoniadi scale. Raffaello Lena did another observation of this region, on the 24th of May using a 100mm refractor at f/15 and 250x. The seeing was also estimated at II on the Antoniadi scale. The drawing below shows the region as observed by Lena (figure 2).
 

 
 
 
Fig 2. Raffaello Lena

 The two bisected domes can be readily observed to the north of the Rima Birt with the more northerly dome clearly bisected by the Rima. This dome is not as clear in Lena’s drawing as in that of Salimbeni but its bisected nature is clearly suggested in this drawing too. The dome seems to be a hemispherical one with a central cleft that bisects it right in the center and forms a continuous line with the Rima Birt. The central cleft needs good seeing conditions in order to be observed clearly. A CCD image obtained by Giorgio Mengoli on July 21, 1999 at 19:55UT also shows this dome (see figure 3). The CCD image was obtained using a 152-ED refractor and HX5 Starlight Express CCD (16 bit) coupled to a Celestron Ultima Series 2x barlow. Integration was 0.12sec with a 2x unsharp masking adaptive filter and De Gauss filter (sigma 0.8, coefficient 1.0), Zoom hardware (1.5x). The software used was Astroart and Pix Win 4.

 
 
Fig 3- Giorgio Mengoli

Previous observations of the dome at -161-349 (long. -9° 53.56’, lat. -20° 25.57’) were reported to the ALPO by one observer. Marvin Huddlestone detected the dome in an observation made on July 22, 1972 at 06:03 UT (see figure 4 ). Huddlestone reported a position of –162-348. Ours is the third observation of the dome reported to the ALPO and including CCD imaging. This dome has also been observed by Harold Hill and appears in his splendid book of lunar drawings. Although not specifically described as in the other three observations, it is definite that Hill also observed three domes during his study of this region.

 Fig 4-Marvin Huddlestone

This dome was also observed by Massimo Giuntoli on April 22, 1991 at 19:50UT using a 100mm refractor at f/10 and 166 to 250x. Giuntoli’s drawing does not include the dome’s position or its bisected nature. A hint of this dome is also present in a single frame sent to the GLR group by F. Badalotti. The image was made using a videocam and a 250mm f/10 SCT. It was obtained on the same night as Salimbeni’s observation on May 23, 1999 at 19:10 UT.

Fig 5- Massimo Giuntoli

Fig 6- Francesco Badalotti

Using both Salimbemi’s and Lena’s drawings plus Mengoli’s CCD image this dome may be classified according to the Westfall classification scheme as DW/2a/6f/9j. Huddlestone classifies it as DW/2a/4f/7j .The fact that there have been only two previous reported observations of this dome, strongly suggests that this may be a very difficult object requiring specific lighting conditions in order to be clearly defined. .G. Santacana observed this region several times from 1994 to 1997 finding domes only in the coordinates -155-347 and -156-353 using both a 200mm SCT and 8" Newtonian.

Using Mengoli’s CCD images Lena has been able to measure both the heights and the diameters of the three domes in the Birt region. The measurement consists of enlarging the images and measuring the size of known objects like the crater Birt and Rupes Recta in millimeters to produce a conversion factor to kilometers. Then measurements are made of the domes and converted to kilometers. For height measurement the same process was followed to measure the shadow lenght of the domes. Using the Lunar Toolkit program by Harry Jamieson, the heights of the three domes were then calculated. The table below summarizes diameter and heights of the three domes in the Birt region.

Table 1

Dome Diameter Height

-161-349 10km 200 meters

-155-347 12km 200 meters

-156-353 10km 250 meters

Due to the fact that foreshortening is not being considered in the calculations, they must be regarded as close approximations.

Conclusions

The appearance of the three domes so close together is a definite indication that at some time in its geologic history, the north end of the Rima Birt was an area with a high level of volcanic activity.