Raffaello Lena, Cristian Fattinnanzi, Piergiovanni Salimbeni
Geologic Lunar Research group (GLR)
Abstract
This study describes a dome which is not reported in the ALPO list. Positioned at longitude 5° 40' E and latitude 41° 27' N ( Xi +0.074 and Eta +0.662), this dome lies near the crater Cassini.
Introduction and general geology
The study of the lunar domes require extreme patience and specific conditions of lunar solar altitude and good seeing (1-3). The distribution of domes in the lunar surface favours the Western emisphere of the moon. The large cluster of 28 domes in the well-studied Hortensius-Milichius-Tobias Mayer region coupled to a greater expanse of maria accounts for the greater manifestation of domes in the western hemisphere (1, 4).
There are, probably, two basic modes of formation:
(a) domes that form as constructional forms. These formed at the end of a volcanic era, where the rate of extrusion is lower, and perhaps the temperature of eruption is lower. The pit crater is likely the central vent which has been enlarged by collapse (lack of support and meteorite erosion);
(b) domes that form as subsurface intrusions. This would be similar to a laccolith on the Earth, so one would not expect to find pits in them.
This paper reports our results on the presence
of an unlisted dome near Cassini. This dome is of low height and angle of inclination,
making it difficult to identify when distant from the terminator.
Observations:
The images reported in fig. 1-6 are oriented with north at the top and west (IAU) at the left. For each observations considered here, the local altitude of the Sun, H, and its colongitude , C, were calculated with the Lunar Observer's Tool kit software.
On September 28, 2002 at 4:08 UT ( C 168.53°, H 5.29° over the structure located at +0.074+0.662) Raffaello Lena observed, under a setting sun, a dome-like feature located near the crater Cassini (fig.1).
This observation was carried out under good
seeing conditions (II on the Antoniadi scale) using a 100mm refractor at f/15
and 250 magnifications.
Figure 1
A CCD image by Morio Higashida obtained on October 1, 1999 at 20:37 UT ( C 172.44°,
H 2.28° ) shows the dome (see fig.2) in the field of view. It has a low profile.
The image was obtained using a Newton 200 mm f/8.
Figure 2
Previous observations of the dome was reported to the GLR group by one observer. Gerard Therin (see figure 3 ) detected the dome but we were not able to know the exact date and time on his image. Therin imaged the dome using a SCT 200 mm f/10 (Kodak TP 2415).
Figure 3
The Consolidated Lunar Atlas has an image of the dome (figure 4).
It is detectable on the photograph made on 6 september 1966 at 10:43 UT (C 167.3°, H 6.19°).
Piergiovanni Salimbeni and Cristian Fattinnanzi did another observation of this region using a SCT 200mm of diameter at f/10 and a Newtonian 200 mm f/6 respectively. A dome like feature is visible in the image reported in fig 5. The image was made by Fattinnanzi on 19 february 2002 at 17:25 UT. In this image (figure 5) the dome +0.074 +0.662 is marked with the left arrow. At the right there is a structure that is not a dome, as it has an irregular shape and it is visible also under an high angle of illumination .
Figure 5
Dimensions and classifications:
The estimated position of the dome at +0.074
+0.662 (longitude 5.67º E and latitude 41.45 ºN) was confirmed by Harry Jamieson,
using measurements taken from plate C2-a of the Orthographic Lunar Atlas (Table
I).
Table 1
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5.67º 41.45 º |
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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 could have eluded detection until now.
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 and a good seeing in order to be clearly defined.
The high angle of illumination of the Lunar Orbiter imagery cannot be useful for a identification of +0.074 +0.662.
More observations of this dome structure are needed to establish its height and other surface features.
At present, we report an unlisted dome near
crater Cassini at +0.074 +0.662.
This report also demonstrates that the study and classification of these volcanic structures on the moon is far from being complete.
Acknowledgement: Many thanks to Morio Higashida and Gerard Therin for providing us with additional images.
References
1) Pg Salimbeni, R. Lena, G. Mengoli, E. Douglass, G. Santacana, JALPO, 2000, n. 2 Vol 42.
2) R. Lena, G. Mengoli, F. Badalotti, Pg Salimbeni, E. Douglass, G. Santacana, JALPO, 2001, n. 4 Vol 43.
3) R. Lena, Pg Salimbeni, C. Fattinnanzi, G. Mengoli, Selenology, 2002, n. 3 Vol 21.
4) B. Spudis, P. Volcanism on the Moon; in Encyclopedia of Volcanoes, ed. H. Sigurdsson. San Diego: Academic Press, 2000; p. 705.