Mons Rumker
By Raffaello Lena, Cristian Fattinnanzi , Alessandro Bares, Giorgio Mengoli and Rodrigo Viegas
Geologic Lunar Research group- GLR
Introduction
Aristarchus plateau, Marius Hills and Mons Rumker (both located in Oceanus Procellarum) are described as volcanic complexes in [1-3]. Rumker appears as a volcanic complex with a variety of smaller volcanic forms on its summit, described as "hilly plateau material" in [1]. Eric Douglass states "the origin is different from that of Aristarchus plateau. The latter is probably a fault block created by the Imbrium impact, while the former is an endogenous volcanic product. It is clear that the Rumker complex has a series of volcanic forms on its surface, but the larger geologic history is somewhat obscure" (personal communication).
It should be noted that the ALPO dome list reports for Rumker a dimension of 62 km and coordinates 57°54’ W and 40º37’ N. Interestingly, Rumker is identified without specific details as to the position and the heights of the prominences.
In this study we report some measurements
in order to ascertain the nature of the prominences.
Observations
The images reported in fig. 1-4 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.
Figures 1 and 2 were computer resampled to simulate Rumker ’s perspective as shown in the IV-163-H2 Lunar Orbiter IV photo. Each resampled image appears as a zenithal view.
Figure 1 reports the Rumker complex under
a low Sun. This image was obtained on january, 15, 2003 at 21:30 UT by C. Fattinnanzi
using a Newton 25 cm ( C 64.12° , H 3.81° over the structure).
Fig. 1 and 1a C. Fattinanzi
Figure 2 was obtained by A. Bares on march, 26, 2002 at 21:40 UT using a Mewlon
25 cm (C 66.42° , H 5.32° over the structure).
Fig. 2 A. Bares
Another observation (figure 3) was made by G. Mengoli on february, 14, 2003 at 19:45 UT using a Mewlon 20cm (C 68.06° , H 6.53° over the structure).
Fig. 3 G. Mengoli
In addition, Figure 4 shows the Lunar Orbiter
frame IV-163H2 taken on may, 23, 1967 at 6:34 UT(C 79.87° , H 16.78° over the
structure). This image reveals several dome-like features, which correspond
also to numbers 1-4 on figure 1.
Fig. 4 LOrbiter (in comparison
to Fig 1 By Fattinnanzi)
Visual observations carried out by Lena and Viegas confirmed the Rumker features
as seen in the images.
Measurements
Our images, converted as a zenithal view, were rotated so that the shadows were oriented orizontally along the pixel rows. On these images the position and the diameter of the positive forms was measured in units of pixels. An image of Mons Rumker extracted from Plate F2-a of the Orthographic Atlas of the Moon was sent to us by H. Jamieson. Plate F2-a is part of plate W231 taken at the Mount Wilson Observatory on july, 31, 1924 at 12:24 UT (C 229.17°, H 5.80° over the Rumker). The F2-a image was also converted in a zenithal view. Comparing this grided photograph taken at sunset with our figure 1 taken at sunrise we estimated the coordinates of the principal features under discussion (Table 1).
Furthermore, the scale of the images was obtained (0.66 and 0.44 km per pixel in Figure 1 and in Plate F2-a respectively) which allowed diameters and shadow lengths of the concerned features 1-4 (in fig. 1 and 4) to be expressed in km.
Our results (including a classification of
the positive forms) are summarized in Table 1 and 2.
Table 1
|
|
|
|
km |
classification |
|
|
|
|
|
2d/6f/7j |
|
|
|
|
|
2a/6f/0 |
|
|
|
|
|
2b/6f/7j |
|
|
|
|
|
2a/6f/7j |
Table 2
|
|
Solar alt. Shadow Length H ° pixel km |
Solar alt. Shadow Length H ° pixel km |
Solar alt. Shadow Length H ° pixel km |
|
|
3.24 9 6.0 | 4.73 penumbra |
|
|
|
3.84 penumbra | 5.34 Not visible | 6.54 Not visible |
|
|
3.69 10 6.6 | 5.23 penumbra | 6.45 penumbra |
|
|
3.94 7 4.6 | 5.48 penumbra | 6.70 penumbra |
Results and discussion
The central region of the complex is lower than the surroundings.
This gives Rumker a ring-like appearance when very close to the terminator. A great concentration of domes located at West and South of Rumker implies a centre of great volcanic activity.
We note that two domes have been previously reported by Head and Gifford [4] at: longitude -57.8° latitude +42.0° and at longitude -58.2° latitude +40.5° (similar to our feature 3) respectively.
Of these elevations, features 1, 3 and 4 show summit craterlets, likely representing the central vent, and are steeper.
Lunar Orbiter imagery (fig.4) reveals other craterlets in this region, but the angle of illumination makes it difficult to determine if these contain rims or are volcanic in nature. On the other hand, feature 2 looks as a low dome material (fig.1). In fact feature 2 , under a low solar altitude does not cast a black shadow, only a penumbra for the partly illuminated flank.
Close inspection of the "raw images" reproduced in Fig.1-3, reveals the shadow profile or penumbra of the elevations. A penumbra (not black shadow) is visible from H = 5.0° (fig.2) to H = 5.8° of the Orthographic Atlas of the Moon image plate F2-a, and certainly up to H = 6.0° of our image reported in Fig. 3.
From this we estimate an average slope < 5° for features 1,3 and 4 (Table 1). On the other hand feature 2 has a lower height resulting in a moderate average flank slope < 3.8°.
Due to the fact that an evident sloping soil is present in Rumker, the estimated heights must be regarded as close approximations rather than strictly accurate (at about 327m, 400m and 310 m for features 1,3 and 4 respectively). On the other hand, we can estimate the average slope of the volcanic forms under discussion (last column of table 1), in agreement (as best we can tell) with the Westfall criteria [5].
Rumker consists of both low domes material and steeper domes on its summit . A possible interpretation is that the plateau was formed first, by fluid lavas; then, the domes formed as the lavas cooled down and became more viscous. But it is also possible that at least part of the plateau formed from outpouring of fluid lavas from the domes, so we cannot state a definite order in the magmatic processes. Moreover, it is difficult to identify the edges of the domes that cover the south and western part of Rumker, because they are superimposed on one another.
Finally we reported the diameter, position
and slopes of 4 single volcanic forms on its summit.
Acknowledgements
We wish to thank Eric Douglass for his stimulating
discussion and
Harry Jamieson for furnishing the orthographic map of Rumker and for many
stimulating discussions. Many thanks also to Bill Dembowsky for the suggestions.
References