located at x -0.252 and h -0.012

by Raffaello Lena, Rodrigo Viegas,Eric Douglass and Cristian Fattinnanzi

(Geologic Lunar Research Group)

Introduction

The study of domes provides lunar observers with an opportunity for systematic study of the Moon using visual and CCD imaging techniques.

Lunar domes are formed either by outpouring of magma from a central vent or by a subsurface accumulation of magma that causes an up-doming of the bedrock layers, creating a smooth, gently sloping positive relief [1-5].

Domes seem to occur principally in clusters in the dome rich region of Oceanus Procellarum, near the craters Marius, Gruithuisen, Hortensius, and in the area around Gambart B and C.

Many observers have studied the dome field near the crater Gambart. The A.L.P.O. Lunar Dome list reports several low domes in this region (Table 1). Observation of these domes requires low solar altitude for maximum detail.

Recently this region was very well monitored by the GLR group. In addition, a low feature has been observed in this area, near the crater Turner and it is reported here.

This feature, located at x -0.252 and h -0.012 (longitude 14.62° W latitude0.71° S), does not appear in theA.L.P.O. Lunar Dome listand appears to require very specific lighting conditions to be visible.

Our study of this elusive dome includes CCD images, which has made it possible to extract additional information for its classification.

Table 1-Domes in Gambart region reported in A.L.P.O. list

Observations and images

On December 31 1999 at 05:53 UT Viegas observed a low relief near Turner (Co-longitude 270.67°, Solar Altitude over the dome 2.07°).

FIGURE 1

The structure lies about 100 Km from the well-known Gambart domes.This observation was carried out under good seeing conditions using a 114mm f/8 Newtonian telescope (Figure 1). Then, it was reported to the GLR (Geological Lunar Research Group) and further research was carried out.

FIGURE 2

A CCD image by Fattinnanzi was taken on May 20 2002 at 19:42 UT (Co-longitude 90.49°, Solar Altitude over the dome 1.94°). As depicted in the image (Figure 2), there are several features on its surface including 2 craterlets and at least 1 cleft. The cleft can be seen to bisect it diagonally.

FIGURE 3

Also, in Figure 2 the dome appears low from the fact that its shadow was not black at the time image was taken (Solar Altitude over the dome 1.94°). Figure 3 is from Consolidated Lunar AtlasE-14.

FIGURE 4

On the other hand, this dome was imaged by the Apollo 12 cameras (Figure 4, AS 12-50-7438).

Lunar Orbiter imagery (Figure 5) reveals much finer detail in the dome than can be detected by traditional earth-based imaging, including several rilles.

Using all the available images, we were able to measure the diameter and position of the unlisted dome.These were obtained by enlarging the images, counting the number of pixels in the object of interest, and then converting this into kilometers (Table 2).

The dome located at x -0.252 andh-0.012is a flat structure of 19.5 km in diameter and circular in shape.

TABLE 2 Dome near Turner

Geology

Domes are volcanic products. These structures may form as effusive shield-like volcanoes or may remain subsurface as laccoliths. In the former scheme, thin lava flows accumulate around the vent, slowly building up a volcano on the lunar surface. As the lava itself has low viscosity (due to the low silica content) [6], these volcanoes have little positive relief, and have slopes of very low incline. In the latter scheme, magma accumulates within the lunar crust, increasing in pressure slowly, causing the crustal rock above it to bow upward. This creates a structure of low positive relief, having slopes of very low incline, without ever necessarily having external eruptions (though they can rupture the roofs) [7]. For many domes, it is not possible to determine the mode of formation, as the clues of a central pit and/or lava flow boundaries are missing (these would point to an effusive volcano). In the present example, neither of these characteristics are present. However, in the present example one clue does point towards this being a laccolith. In the northern part of this dome there is a rille (figure 3, marked ‘A’). Now, most rilles that form in extrusive volcanism are sinuous (i.e., they meander) and flow out onto a mare surface, with the rille being traceable for some distance on the mare surface (cf. Hadley Rille). These form as a result of thermal erosion/positive construction from running lava [8], and so their course is not dictated by subsurface geologic structures. However, this rille is straight and ends where the dome reaches the mare surface. Such a rille is likely due to tensional stresses, and these do show structural control by subsurface geology (e.g., a fault). Such tensional stresses are most consistent with laccolith formation. Here the magma accumulating beneath the surface produced not only an upbowing of the surface rock layers, but also failure in the rock strata (fracturing). If these fractures remained in the uppermost layers only, then no magma would be released, and the fracture would remain sharply visible (once magma is released, these features become filled and/or smoothed). Note that it is possible for an effusive dome to have such a fracture, due to dike intrusion, but it is unlikely that a dike would terminate at both edges of the dome without extending onto the surrounding mare surface. Thus, our impression is that this structure is a laccolith.

FIGURE 5

Next it is of note that this dome has other rilles either on this dome or in its vicinity. The rille marked ‘C’ (fig. 3, 4) is clearly on the surface of the dome itself, but the exact nature of this rille is difficult to ascertain, given that it is near the resolution limit of the imagery. Thus, this rille remains undefined in this paper. The rille marked ‘D’ (fig. 3, 4) is difficult to place with respect to the dome, as the western edge of the dome is of small incline with respect to the surrounding surface (i.e., it doesn’t cast a western shadow at the sun angles observed), and it appears to intersect with a mare ridge (fig. 2 and fig. 5, marked ‘E’). Our impression is that this rille is on the surface of the ridge, but not on the dome itself. The rille marked ‘B’ (fig. 3, 4 and 5) is clearly not on the dome, but on the ridge. This latter rille (‘B’) appears to have scalloped edges, as if composed of multiple craterlets instead of being continuous (esp. see fig. 3). Examples of ‘multiple craterlets’ with a linear arrangement do appear on the moon (figure 6), and here the interpretation is that they are segments of a collapsed lava tube [9]. Given the limits of the resolution of our imagery, and the advanced state of degradation of the structures, it is impossible to be sure of this interpretation here. Other possibilities include pits from eruptive degassing (possible), a fissure eruption (possible; for another possible example, see [10]), and volcanic drainage pits (unlikely due to their elevated location).

FIGURE 6

Next, there is another interesting feature in this section. Running through this region is light section (marked in figure 7), which is the bright ejecta from a distant impact. This light material covers the darker lava beneath. However, at a later time, a small impact strict within this lighter material, piercing its thin layer, and excavating the darker material beneath. This material was spread out in the small impact's ejecta, appearing as a dark halo crater (marked in figure 7 as 'A'). Note that this is a contrast-enhanced image, because erosion has decreased the contrast that was originally present, making it difficult to visually identify. 

FIGURE 7

As a final comment, it is of note that the western edge of this structure is difficult to identify, given the low inclination of the structure with respect to the lunar surface. It is possible that the structure itself extends into the western/southern ridge. This would then be the manifestation of a sub-surface volcanic dike with sill formation (the extension of a non-viscous flow laterally between bedding planes) [7].

Conclusion

Using the Westfall Classification Scheme [11], we categorize the dome asDW/2a/5g/7n9n.

The lunar dome we have described here (x -0.252 h -0.012) is another clear example of the elusive nature of these volcanic structures on the moon and the need for more work in this challenging area.

Finally, international participation in our GLR dome project continues in a favorable response to our efforts to foster increased cooperation among lunar observers worldwide.

Acknowledgement

Many thanks to Giorgio Di Iorio for his stimulating discussion.

References

[1] Salimbeni, Pg. , Lena, R., Mengoli, G., Douglass, E., Santacana, G., J.A.L.P.O.,2000, Vol 42,2.

[2] Lena, R., Salimbeni, Pg. , Douglass, E., Santacana, G., Higashida, M., J.A.L.P.O, 2003, Vol 45, 1.

[3] Viegas, R., Lena, R., Douglass, E., Selenology, 2002, Vol 21, 2.

[4] Lena, R., Pau, KC, Fattinnanzi, C., Selenology, 2003, Vol 22,4.

[5] Lena, R., Viegas, R., Fattinnanzi, C., J.A.L.P.O, 2003, Vol 45, 3.

[6] Mursky, G. “Introduction to Planetary Volcanism.” New Jersey: Prentice Hall, 1996.

[7] MacDonald, G. “Volcanoes.” New Jersey: Prentice-Hall, 1972.

[8]Frankel, C. “Volcanoes of the Solar System.” Cambridge: CambridgeUniversity Press, 1996.

[9] Wilhelms, D. “The Geologic History of the Moon.” USGS Prof. Paper 1348. Washington: GPO, 1987.

[10] Masursky, H., Colton, G., and Farouk, E. “Apollo over the Moon.” Washington: GPO, 1978.

[11] Westfall, John; J.A.L.P.O. 1964, Vol 18, no 1-2.

FIGURES

Figure 1: R. Viegas, observation carried out On December 31 1999 at 05:53 UT (Co-longitude 270.67°, Solar Altitude over the dome 2.07°). South at the top and West (IAU) at the right.

Figure 2: C.Fattinnanzi observation carried out on May 20 2002 at 19:42 UT (Co-longitude 90.49°, Solar Altitude over the dome 1.94°).South at the top and West (IAU) at the right.

Figure 3: Consolidated Lunar Atlas, E 14. In this image North is at the top and West (IAU) at the left.

Figure 4: AS12-50-7438, (from: Apollo 12, Preliminary Science Report) In this image North is at the top and West (IAU) at the left.

Figure 5: Lunar Orbiter 4, H-113; from [9]. In this image North is at the top and West (IAU) at the left.

Figure 6 : Lunar Orbiter 5, M-182; from [9] .

Figure 7: Lunar Orbiter 4, 114-H1

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