Fig.1
Characteristics of a possible dome on the floor of Petavius
By Raffaello Lena, Brendan Shaw and KC Pau, GLR Group
Petavius is a large, interesting, Imbrian-age, crater located at 60.0° E, 25.3° S, on the southern edge of Mare Fecunditatis. It possesses a massive central mountain complex and several well-defined rilles. Wilhelms [1] considered it a floor-fractured crater, a crater that has undergone uplift and fracturing due to magmas pushing up from below. Much of the floor is rough, but there are smooth areas in the very north and south. Gaddis, using the multispectral Clementine imagery, has recently identified two pyroclastic deposits in Petavius [2]. The two dark areas are localised in the northernmost and southernmost portion of the crater, as seen in the Clementine visible-wavelength view of Petavius [3]. They can also be seen on the USGS lunar geology maps, where they are identified as unit CId (Copernican-Imbrian dark mantling material). Wilhelms and McCauley [4] say that this unit is amongst the darkest mapped on the Moon and mention a “pyroclastic origin suggested by subduing effect”. Clearly, there is evidence for limited volcanic activity on the floor of Petavius.
A recent LPOD item by Wood [5] focused on the presence of a possible dome located in the southern area of the crater floor. Our observations, carried out under low-lighting conditions, allowed us to examine this area in more detail (see figures 1-4 and Table 1). In addition, we received another, confirming, image from C. Zannelli, taken on September 30, 2004 at 21:44 UT with a 180 mm Maksutov- Newtonian.
Fig.1
In our opinion a possible dome lies at longitude 60.4° E, latitude 26.8° S. The solar altitude (H) and colongitude (C), were calculated using the Lunar Observer's Tool Kit software by Harry Jamieson.
 |
 |
| Fig.2 | Fig 3 |
Table 1
Date and times (UT) |
H °
|
C |
Observer (Figure) |
telescope |
September 30, 2004 23:05 UT |
4.78 |
114.04 |
B. Shaw (Fig.1) |
Newtonian 25 cm f/12 |
February 18,2003 17:02 UT |
4.55 |
115.22 |
K. C. Pau (Fig.2) |
Newtonian 25 cm f/6 |
October 1, 2004 00:10 UT |
4.12 |
114.5 |
A. Bianconi (Fig. 3) |
SC 30 cm f/10 |
November 21,2001 23:15 UT |
2.98 |
116.06 |
D. Joye (Fig. 4) |
SC 20 cm f/10 |
Fig.4
The shading on its antisolar slopes are not black, indicating that the slopes are of low inclination. Interestingly the swelling (about 11 km across), here named Petavius-1, seems to have a small off-centre pit. The exact nature of this summit pit is difficult to ascertain and the crater itself could be a random impact [6]. Thus this small off-centre pit remains undefined in this note. As a point of interest the dome-like feature, also visible in Lunar Orbiter image 184-H2 [7], appears at the edge of the pyroclastic deposit, not at its centre.
Fig.5
The study and classification of these volcanic structures on the moon is far from being complete – there is still work to be done! Our images provide some evidence about the morphology of “Petavius-1”, but more observations of this structure are needed to establish its height andits slope. Any such observations will be gratefully received by the GLR for our survey.
The GLR group has an ongoing project to find domes. Our activities are shown on our webpage, http://www.glrgroup.org
References.
[1] Wilhems, D. “The geologic history of the Moon”, USGS Prof. Paper 1348, 1987
[2] Gaddis, L. et al “Mafic characteristics of lunar pyroclastic deposits”, Lunar and Planetary Science XXXI. www.lpi.usra.edu/meetings/lpsc2000/pdf/1700.pdf
[3]Campbell, B. A et All. “Integrating radar, multispectral, and landing site data for analysis of the lunar surface”, Workshop on New Views of the Moon. www.lpi.usra.edu/meetings/moon98/pdf/6032.pdf
[4] Wilhems, D.and McCauley J., “Geologic map of the near side of the Moon. USGS Map I-703, 1971.
[5] Wood, C.LPOD- http://www.lpod.org/LPOD-2004-09-27.htm
[6] http://www.lpi.usra.edu/research/lunar_orbiter/images/img/iv_053_h1.jpg
[7] http://www.lpi.usra.edu/research/lunar_orbiter/images/img/iv_184_h2.jpg
