Raffaello Lena, Giorgio Mengoli, Francesco
Badalotti, Piergiovanni Salimbeni, Eric Douglass, Guido Santacana. Geologic
Lunar Researches Group.
Abstract
This study describes a previously unreported and unclassified dome in the lunar
Apennine region south of Archimedes and near the crater Huxley. Located
at -0.071+0.353
(longitude -4.35° latitude 20.67°), this structure does not appear in the
A.L.P.O. Lunar Dome Survey database and seems to require specific lighting
conditions to be visible. This may explain why it has eluded most observers
to
date, though an indistinct structure is recorded at these coordinates in “Atlas
of the Moon” by A. Rukl (1).
Introduction:
Geology and Geologic History of the General
Area
The geology of this lunar region is remarkable for its complexity.
The oldest
materials visible are those from the impact that produced the Imbrium Basin,
a
multi-ring structure formed 3.85 billion years ago (2). The outermost
ring is visible as the mountain range called Montes Apenninus. Multiple inner
rings also formed, though only the highest points of these are now visible
above the later forming lavas (below). Examples of such peaks to the north are
Montes Spitzbergen and Mons Piton.
During a subsequent period of several hundred million years, partial melts
formed deep under the Imbrium Basin, due to heat released by radioactive decay
of elements like potassium, uranium, and thorium (5). These melts tracked
up faults created by the shock wave from the Imbrium impact, and resulted in
the deposition of lava within the basin itself. The earliest (oldest) lavas
visible are probably those of the lighter albedo plains units inside the main
Imbrium ring. These occur between Archimedes and Montes Apenninus, though some
extend out nearly to the crater Timocharis (note that this visible section is
only a small part of the lateral extent of the original flows). Called the
Apennine Bench Formation, these plains are notable for their 'roughness,'
lighter color, tectonic features (grabens), and domes. While other explanations
for the Apennine Bench formation are possible, a volcanic origin is suggested
by the presence of domes on the Bench and the finding of appropriate types of
volcanic rock in the Apollo 15 specimen collection.
More recent lava flows in the Imbrium Basin were of different
mineral
composition and consequently, displayed different albedo and color. These were
much darker, covered most of the Apennine Bench lavas, and covered many of the
inner
rings of the Imbrium Basin. Only the topographical highs within the rings themselves
remained above these later lavas (or for that matter, above the earlier ones).
Note that all flows in this stage were 'sheet' flows, generally erupting from
fissures, with no formation
of volcanoes. As the final stages of volcanism came toward their end, the viscosity
of the
erupting lavas also changed, primarily as a decrease in temperature. In
addition, the lower rate of lava effusion produce shorter flows (9) that remained
near the vent, aiding in the construction of these domes (3). This type
of formation, on earth, produces basaltic shields, which is the form most lunar
domes take (4). Thus, in the final states of lunar volcanism, there was a switch
from extensive sheet flows to the construction of domes. Further, since
these lavas tracked up deep seated faults (created by the massive impacts that
produced the basins), it is not surprising that they tend to occur in clusters,
as is the case here.
Observations:
On July 9, 2000 at 21:00 UT ( Colongitude 8.97°, Solar Altitude
4.26°) Raffaello Lena observed two domes located to the south of the crater
Archimedes. This observation was carried out under good seeing conditions using
a 100mm f/15 refractor. Figure 1 shows the aspect of this region as observed
and sketched by Lena.
Fig.1-R. Lena July 9,2000 at 21:00UT, 100mm refractor
at 250x
Colongitude 8.97°, Solar Altitude 4.26° , Solar Azimuth 91.81°
The drawing shows one dome that follows closely on the same line as the Rima Bradley. The dome appears to be hemispherical with the presence of a small dark area on the top that could suggest a craterlet. This dome requires a specific solar altitude in order to be observed clearly and requires a narrow solar angle for viewing (probably range 2.2 -5.0 °).
The region of Archimedes has been well monitored from 1995 to
1999 during an observational program carried out by the GLR group. During this
period only the large dome located at -0.073+0.375 ( longitude -4.52°, latitude
22.02° ) was recorded. This more northerly dome is a well know structure of
about 13km in diameter by visual estimation and as reported in the ALPO Lunar
Dome Survey database (8).
The following illustration (Fig. 2) shows a video image of the region.
Fig.2- The Archimedes region. This image was made using
a video camera and a 250mm f/10 SCT. It was obtained on may 23, 1999 at
19:15 UT, Colongitude 11.9°, Solar Altitude 6.5° , Solar Azimuth 94.1°
It is probable that the dome located at -071+353 was not easily
seen because the rising sun was too high for the dome to show up well by the
time it emerged from the shadows of the Apennines. This dome can be well observed
under a setting sun, near the moon's last quarter.
Since this is period during lunation when comparatively fewer observers are
active,
this dome has generally eluded detection until now (though it is noted in Rukl).
A CCD image by Giorgio Mengoli obtained on March 13, 2000 at
18:10 UT shows both domes (see Fig. 3) in the field of view. The image was obtained
using a 152-ED refractor and HX516 Starlight Express CCD coupled to a Celestron
Ultima Series 1.83x Barlow. The integration was 0.12 sec and the software used
was Astroart and Pix Win 4.
Fig.3- Archimedes region image by Giorgio Mengoli. March
13,2000 at
18:10 UT. Colongitude 7.11°, Solar Altitude 2.16°, Solar Azimuth,
92.09°.
The Consolidated Lunar Atlas has an image of the dome at -071+353.
The dome is detectable on the photograph made on 19 January 1967 at 3:35 UT,
Colongitude 7.2°, Solar Altitude 2.23° and Solar Azimuth 92.45°.
Figure 4: Consolidated Lunar Atlas. Reversed
image South to the top and N to the bottom. The arrow shows the dome located
to -071+353
Finally, this dome was imaged by the Apollo 17 cameras (figure
5)
.
Figure 5: AS 17-2433 (m) from “Apollo
Over the Moon,” NASA SP 362. reversed image South to the bottom and N
to the top
Figure 5 reveals much finer detail in the dome than can be detected
by traditional earth-based imaging, including two enigmatic small prominences.
Two possible mechanisms for the origin of these prominences are (1) that they
are floor ridges which were embayed by the dome, and (2) that they are endogenic,
created as small cinder cones on top of the dome. The latter mechanism is suggested
by the difference in shape and size of
the prominences, but embayment is indicated by their alignment with ridges
to the north.
Our study of this elusive dome is the first to be added to the A.L.P.O. Lunar
Dome Survey and to include CCD imaging. This has made it possible to extract
additional information on both this dome and its more northerly neighbor. Using
unsharp masking and geometric deformation, we were able to obtain several elaborations
of these images (Figs 6-7-8), and reveal the presence of dark areas on the top
for both domes. Close inspection, particularly of Mengoli's frame (Fig 3,6,7,8)
shows this most distinctly.
Figure 6: elaboration of the dome located to -0.071+0.353
Figure 7: elaboration of the dome located to -0.073+0.375
Figure 8: Geometric deformation of the Archimedes
region
Using Mengoli's images, we were able to measure the diameter of both domes in this region. These were obtained by enlarging the images, counting the number of pixels in the object of interest, and then converting this into kilometers per pixel (Table 1). The estimated position of the newly reported dome at -071+353 was confirmed by Harry Jamieson, coordinator of the ALPO Lunar Dome Survey, using measurements taken from plate D3-a of the Orthographic Lunar Atlas.
TABLE 1
Archimedes Region Domes
Position
Diameter
Major/Minor Axes
-0.071+0.353
10.0 km
1.16
-0.073+0.375
12.9 km
1.14
The average slope angle of a dome can be considered to
equal the current solar
altitude, when it is ¼ covered by shadow (see ref. 7 below). The height (in
meters) is
then calculated by the formula:
Height = (diameter/2) * ( Tangent of
average slope angle)
Close inspection of the photo reproduced from the Consolidated
Lunar Atlas (Fig. 4), reveals that the shadow profile on -071+353 covered about
¼ of the dome's surface at the time this image was taken. From this, we estimate
its height at about 195 meters, which would correspond to a moderate slope angle
of 5 degrees. This estimated is comparable to the value measured in the frame
shown in Fig. 3. The height of this dome was also calculated from length of
its shadow
using Jamieson's Lunar Toolkit program. The resultant 198 meters, is comparable
to the value obtained from the Consolidated Lunar Atlas.
Using the Westfall Classification Scheme (see ref. 6 below),
we categorize the unlisted
dome as DW/2a/6f/7K 8n. The dome located at -073+375 has been classified as
DW/2a/6f/7J 8n .
Conclusion:
The lunar dome we have described here (-071+353) is located just south of the
Apennine Bench formation. It is probably not a part of that formation, however,
since its lava color appears much darker than the Bench; indeed, it is much
darker than other domes of that formation (especially those visible in the western
part of the Apennine Bench). Thus, we feel that the present dome is
likely unconnected to that early pulse of volcanism, though it had to traverse
the Apennine Bench layer to reach the surface. Our sense is that the present
dome represents that final pulse of volcanism, which was cooler and lower in
volume. As is observed, the color of the lava in this
region would be expected to be different from that of the surrounding earlier
flows. This dome occurs in a larger field, with another dome 30 km to the north,
and is probably very different in age from those other massifs just to the north
and east, since they are remnants of the (much older) Imbrium basin ring.
This dome at -071+353 is another clear example of the elusive nature of these volcanic structures on the moon and the need for more work in this challenging area. At the same time, this study shows that a combination of careful visual and CCD observations by different observers provide powerful tools in the study of lunar domes.
References
1) Rukl, Antonin; “Atlas of the Moon;” Kalmbach Books; 1990.
2) Wilhelms, D.; The Geologic History of the Moon; USGS
Professional Paper 1348; US Government Printing
Office; 1987
3) Frankel, C.; Volcanoes of the Solar System; Cambridge
University Press; 1996.
4) Spudis, P.; Volcanism on the Moon, in Encyclopedia of Volcanoes; ed. Sigurdsson,
H.;
Academic Press; 2000.
5) Taylor, S.; The Moon, in Encyclopedia of the Solar System; ed. Weissman,
et. al.;
Academic Press; 1999.
6) Westfall, John; “A Generic Classificaion of Lunar Domes” in J.A.L.P.O. Vol
18, no 1-2 (July 1964), p 15-20.
7) Ashbrook, Joseph; “Dimensions of the Lunar Dome Kies1” in J.A.L.P.O. Vol
15, NO 1-2 (Jan 1961), p 1-3.
8) Jamieson, H.D.; Phillips, J.H.; "Lunar Dome Catalog (April 30, 1992 Edition)",
J.A.L.P.O. 36, No. 3, pp. 123-129.
9) Greeley, R.; Planetary Landscapes; Allen and Unwin Press; 1987.