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A STUDY ABOUT AN UNLISTED DOME NEAR PROMONTORIUM LAPLACE
located at 28.66° W and 47.17° N

by Raffaello Lena, Christian Wöhler, Alexander Wöhler, Jim Phillips,KC Pau, Paolo Lazzarotti and Maria Teresa Bregante

GLR group

Introduction

Sinus Iridum is a plain of basaltic lava that forms a northwestern extension to Mare Imbrium. It is surrounded from the northeast to the southwest by the Montes Jura. The protruding part at the southwestern end is named Promontorium Heraclides, while at the northeastern rim Promontorium Laplace is located. Different lithological units, included in USGS lunar geologic map I-602, are apparent in the Sinus Iridum region.

Sinus Iridum does not contain any important impact craters, except the crater Heraclides E in the south and the crater Laplace A at the eastern edge [1,2]. No lunar domes are reported near Promontorium Laplace in the ALPO catalogue. USGS lunar geologic map I-602 reports a dome as an Id unit at 27.83° W and 40.66° N [3].

The region near Promontorium Laplace, mapped as Im2 unit in USGS lunar geologic map I-602, has been very well monitored by the GLR group. In this study we report measurements and include CCD images of a previously unlisted lunar dome located at 28.66° W and 47.17° N (Xi -0.326 Eta 0.733) and a shallow, possibly intrusive, dome which lies to its west. Table 1 lists the seven observers and their instruments. The images are oriented approximately with north at the top and west (IAU) to the left. For each of the observations in Table 1, the local lunar altitude of the Sun H and the Sun's selenographic colongitude C were calculated using the Lunar Observer's Toolkit by H. D. Jamieson.

Observations

Using our available images, the diameter of dome 1, corrected for foreshortening, amounts to (9.0 ± 0.5) km. This structure, located at 28.66° W and 47.17° N, requires a specific solar altitude to be observed clearly, “disappearing” under higher illumination angles.


FIG.1

Figure 1 displays dome 1 under a low solar altitude. This image was taken by K. C. Pau on July 16, 2005, at 13:27 UT (H = 2.70°, C = 30.98°).


FIG.2

Figure 2 was taken on October 27, 2005, at 04:58 UT by Paolo Lazzarotti (H = 3.11°, C = 203.45°). The eastern flank of the dome does not show a black shadow in the raw image, but a dark grey shading.

FIG.3



Figure 3 shows an image taken by Jim Phillips on November 12, 2005, at 00:40 UT (H = 4.51°, C = 36.32°).



FIG.4

Another image (Figure 4) was taken by Alexander and Christian Wöhler on October 23, 2004, at 19:37 UT. In this image, a shallow, possibly intrusive structure (dome 2) to the west of dome 1 was detected.


FIG.5

Figure 5 was taken on January 10, 2006, at 02:24 UT by Phillips (H = 2.99°, C = 35.47°), while Figure 6 shows an image by Wöhler taken on December 11, 2005, at 17:35 UT (H = 5.02°, C = 37.67°).

FIG.6

The height values for dome 1 (Table 2) were derived by a combined photoclinometry and shape from shading analysis ([4], cf. also [5] and references therein) and shadow length measurements performed based on Figures 1, 2, 4, and 5. The 3D reconstruction of dome 1, obtained based on Figure 2, is shown in Figure 7.


FIG.7

FIG.8


FIG.9

Table 1: Observations

Table 2: Morphometric measurements of dome 1, located at 28.66° W and 47.17° N

Results and discussion

The height values reported in Table 2 for dome 1 were obtained by determining elevation differences between the summit of the dome and its surrounding on the corresponding 3D profiles derived by photoclinometry and shape from shading analysis. The dome height on the images shown in Figures 1 and 2 was measured as (130 ± 20) m and (125 ± 15) m, respectively. These height values are consistent with the measurements carried out on the image shown in Figure 4, yielding (120 ± 20) m, and Figure 5, resulting in (140 ± 20) m. From Table 2 it follows that the average slope angle of the dome is smaller than 2°, corresponding to a dome having a gentle slope. The height of dome 1 was also computed from Figure 5 using the shadow length method according to the relation h = l tan H, where l is the shadow length, corrected for foreshortening and measured in km, and tan H the tangent of the solar altitude. A height of (115 ± 15) m was obtained, resulting in an average slope of 1.46° ± 0.10°. This value is slightly lower than the height values obtained with the photoclinometry and shape from shading method since the shadow only partially covers the western flank of the dome and therefore does not account for the full dome height.

A crater pit may be present on the dome summit. Several small prominences (see Figures 1-6) represent hummocky material units likely embayed by the dome. In the Clementine 750 nm image (Figure 8) these three bright spots are located near the centre of dome 1, but most of the dome surface is as dark as the surrounding surface (mapped as an Im2 unit in the USGS map I-602 and assigned as Imbrian in age).

Dome 2 is a shallow, possibly intrusive structure situated to the west of dome 1. From Figures 4 and 5 the dome radius (summit to bottom at its western rim) of the shallow structure amounts to (7.7 ± 0.6) km. The height was determined to (110 ± 20) m in Figure 4 and to (80 ± 15) m in Figure 5, corresponding to a flank slope of 0.8° ± 0.15° and 0.6° ± 0.1°, respectively. It may represent a lava bulge or also a low domical structure of intrusive nature, e. g. a subsurface intrusion similar to a terrestrial laccolith, where magma has flowed under a surface of solidified lava and lifted it up [6]. Clementine UVVIS imagery reveals that domes 1 and 2 spectrally appear extremely red and have thus formed in basalts of very low TiO2 content (Figure 9).

Any observations that readers can make about the domes described here will be gratefully received for our GLR survey (lena@glrgroup.org).

References

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

[2] C. Wood, “The Modern Moon, A Personal View”, Sky Publishing, 2003

[3] USGS, Lunar geologic map I-602

[4] R. Lena, C. Wöhler, M. T. Bregante, C. Fattinnanzi, “A combined morphometric and spectrophotometric study of the complex lunar volcanic region in the south of Petavius”, Journal of the Royal Astronomical Society of Canada, article in press.

[5] B. K. P. Horn, “Height and Gradient from Shading”, MIT technical report, AI memo no. 1105A, 1989. http://people.csail.mit.edu/people/bkph/AIM/AIM-1105A-TEX.pdf

[6] J. W. Head, A. Gifford, “Lunar Domes: Classification and Modes of Origin”, Moon and Planets,vol. 22, pp. 235-258, 1980.

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