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AN UNLISTED DOME NEAR CAUCHY, located at 36.75° Eand 11.06° N

By Raffaello Lena, Christian Wöhler, Jim Phillips, Maria Teresa Bregante and Paolo Lazzarotti

1) Introduction

The study of domes provides lunar observers with an opportunity for systematic observations of the Moon.

Mare Tranquillitatis occupies a pre-Nectarian impact basin and the lavas are extensive but thin [1].In Mare Tranquillitatis there are numerous wrinkle ridges and obviously domes. Moreover, in the Cauchy region lie the well-known features Rima Cauchy (a graben) and Rupes Cauchy (a fault). These were probably caused by the shock wave from the Imbrium impact [2]. Several domes are developed along the extension of Rupes and Rima Cauchy, which are oriented radially to the Imbrium basin. Domes associated with a graben along a strike are interpreted to be formed by dikes that are very shallow [3].

The region near Cauchy has been very well monitored by the GLR group. In this study we report measurements and include CCD images of the lunar dome located at 36.75° E and 11.06° N (Xi 0.587 Eta 0.192). It is, to our knowledge, previously unreported by any lunar dome survey.

FIG.1

For each of the observations, the local lunar altitude of the Sun (H) and the Sun's selenographic colongitude (C) were calculated using the Lunar Observer's Tool kit by H. D. Jamieson (ALPO). Figure 1 displays the dome under a lower solar altitude. This image was taken by J. Phillips on October 3, 2004, at 07:45 UT (H = 0.60°, C = 142.78°). Figure 2 displays the Cauchy dome field. This image with a scale of 365 m per pixel was taken on April 13, 2005, at 19:24 UT by P. Lazzarotti (H = 2.55°, C = 325.84°). The dome’s eastern flank does not show a black shadow on the raw image, but a dark grey shading (penumbra) of the dome’s flank. Figure 3 shows an image taken by C. Wöhler on January 15, 2005, at 16:56 UT (H = 9.52°, C = 333.26°) with a scale of 954 m per pixel.

FIG. 2

Information about the vertical cross-section was obtained using the Ashbrook method [4]. Using this method, we estimated on the raw image of Figure 1 the fraction x of the dome’s east-west diameter that is covered by black shadow. The corresponding scale of the image amounts to 0.460 km per pixel, allowing diameters and shadow lengths to be expressed in kilometres. According to Ashbrook [4], the average slope of the dome flank is equal to the solar altitude when x = 0.25, assuming a hemispherical shape of the dome. The height H of the dome was then calculated by equation (1):

H = r (tan s)

where r is the radius of the dome of (6.1±0.460) km and (tan s) is the tangent of the average slope angle when the dome is covered by one-fourth with black shadow (x = 0.25). Moreover, we were able to distinguish between the black shadow and the dark grey shading of the dome flank which represents grazing illumination by sunlight. It turns out that the summit of the dome is (61±15) metres higher than the surrounding plain. The average slope was thus measured as 0.60°±0.15° in Figure 1. The dome appears to be associated with a ridge segment. Due to the extraordinarily gentle slope of its flank and the lack of a summit crater we believe that it is of intrusive origin.

The height values reported in Table 1 were derived by photoclinometric analysis (cf. [8] and references therein). From the image shown in Figure 2 the height of the newly discovered dome was measured as (75±15) m, in agreement with the result of the Ashbrook method.

Fig.3

Table 1: Morphometric properties of features in Figures 2 and 3 (see also Figure 4).

Due to the higher solar altitude, this feature appears only faintly on Lunar Orbiter frame IV-052-H2 (see Figure5).

Fig.4

Table 1 also reports the well-known domes Cauchy Tau and Omega. For the dome Cauchy Tau our estimation indicates a height of (190±20) and a gentle slope of (1.28°±0.14°). For Cauchy Omega we obtained a height of (125±15) m and a slope of (1.17±0.15°). As a note of interest, in Table 2 we state for Cauchy Omega the data published in a file report of the US Geological Survey [5]. In this work the authors present data about the morphometric properties (diameter, height, depth of the summit crater) of 18 lunar volcanoes. These were computed from Lunar Topographic Orthophotomosaics, Lunar Orbiter imagery, and Apollo images. A recent LPOD item focused on the dimension of these Lunar Volcanoes [6].

Table 2: Morphometric data for Cauchy Omega according to [5]

Fig 5.

Clearly these preliminary data about the Cauchy dome field and the unlisted dome at 36.75° E and 11.06° N described in this paper can be improved by new specific observations. Any observations that readers can make about these unlisted domes will be gratefully received for our GLR survey (lena@glrgroup.org).

The activities of the GLR group are described athttp://www.glrgroup.org and at http://it.groups.yahoo.com/group/domilunari

References

[1] D. Rajmon, A.M. Reid, P. Spudis, Geology and Stratigraphy of Mare Tranquillitatis, Meteoritics & Planetary Science, vol. 34, Supplement, p. A96, 1999.

[2] D. Wilhelms, The geologic history of the Moon, USGS Prof. Paper 1348, 1987.

[3] J. A. Petrycki, L. Wilson,Volcanic Features and Age Relationships Associated with Lunar Graben, 30th Annual Lunar and Planetary Science Conference, March 15-29, 1999, Houston, TX, abstract no. 1335.

[4] J. Ashbrook, JALPO, vol. 15, 1-2 (1961) .

[5] Pike and Clow, Revised Classification of Terrestrial Volcanoes and Catalogue of Topographic Dimensions, With new Results of Edifice Volume, US Geological Survey Open-File Report 81-1038, 1981.

[6] C. A. Wood, Dimension of Lunar Volcanoes, 2004.

http://cwm.lpod.org/DataStuff/volcano-sizes.html

[7] J. Westfall, A Generic Classification of Lunar Domes, JALPO, vol. 18, no. 1-2, pp. 15-20, 1964.

[8] B. K. P. Horn, Height and Gradient from Shading, MIT technical report, AI memo no. 1105A, 1989.

http://www.people.csail.mit.edu/people/bkph/AIM/AIM-1105A-TEX.pdf