By: Rodrigo Viegas, Raffaello Lena, Eric Douglass

Geologic Lunar Research Group (GLR)

Abstract

This study describes a previously reported dome that is not on the ALPO list or classified. Positioned at 28,69º E and 27,50º S (Xi 0.426 and Eta -0.462), this highland dome lies about 85 km NW of the 87-Km-wide majestic crater Piccolomini and stands inside the Nectaris basin, about 30 Km nearer its centre than the Altai Scarp.

Introduction:

Lunar domes are gentle swells between 3 and 60 Km across and, at most, a few
hundred metres high. Most have very low angles of inclination, not more than a few degrees. Many have a central 'pit crater', which occurs upon magma withdrawal with collapse around the vent (1). Domes also occur in the highlands, but are usually difficult to see because of the brightness and ruggedness of the region. In this report, we will examine an unclassified highland dome occurring near crater Piccolomini.

General geology of the Highland Domes:

The term ‘highland domes’ refers to domes that are of a higher albedo than the mare regions. The classic example of a highland dome is Gruithuisen gamma, which has a considerably higher albedo than the nearby Mare Imbrium. The higher reflectance of these domes suggests that they have a different mineralogy, and this is most consistent with lava having a lower iron and titanium content (2) This, in turn, suggests that the domes have a different source region than the usual lunar mare. Given the minerals involved, it is suggested that its source is from the lower crust (3) which differs from the mare source region (upper mantle). If this assessment is correct, then highland domes are related to the Apennine Bench formation (a high albedo, volcanic region inside the Imbrium ring), and likely formed before any of the now visible mare. Further, this would suggest that all highland domes are relatively ancient structures, and so would have experienced comparatively more impact erosion than mare domes.

Finally, it is of note that it is just because highland domes have a higher albedo, that they are relatively difficult to identify with respect to surrounding highland structures.

Observations:

The dome was observed by Rodrigo Viegas on April 13^th 2001. This observation was carried out under good seeing conditions (Antoniadi I to II) using a 114 mm Newtonian at 225 magnifications. Figure 1 shows the appearance of this region as observed and sketched by Viegas. Then, it was reported to the GLR (Geological Lunar Research Group) and further research was carried out.

April , 13 2001 from 05:40 to 07:15 UT Drawing by Rodrigo Viegas using 114 mm newtonian. At 06:30 : Solar altitude: 3,335 º Solar azimuth: 270,050 º
Colongitude: 148,328 º
 

This dome was originally drawn (as best we can tell) by Harold Hill on February 18, 1987. In his book "A Portfolio of Lunar Drawings" (4), Hill states: "A dome-like feature is indicated one diameter of Piccolomini to the West". Interestingly, this feature is identified without specific details as to the dome position or presence of summit craters.
The dome was then photographed by Alessandro Bares and Giorgio Di Iorio, of the GLR group (figures 2 and 3, respectively).

Fig.2 November 5 2001 at 03:07 UT by Alessandro Bares using 250 mm newtonian.
Solar altitude: 8,204 º Solar azimuth: 275,617 º Colongitude: 141,427 º
 

Fig.3 April 28 2001 at 20:44 UT by Giorgio Di Iorio using 102 mm refractor with digital camera Camedia 3030. Solar altitude: 7,311 º Solar azimuth: 87,703 º
Colongitude: 338,836 º

It can be observed at co-longitudes between 332º and 320 º (waxing moon) and between 140º and 152º (waning moon). In the image made by Bares, a craterlet is clearly visible on the top of the dome. Figure 4 shows also the image reproduced from the Consolidated Lunar Atlas.

Fig 4 Consolitated Lunar Atlas image SF9. September, 4, 1966 10:32 UT Solar altitude: 6,869 º Solar azimuth: 275,277 º Colongitude: 142,838 º
 

It should be noted that the ALPO dome list does contain a dome near Piccolomini, but at different coordinates (28,63º E 19.39 º S).

Dimensions and classification:

From our available images and using maps from "The Times Atlas of the Moon", the position of the dome was determined: 28,69º E and 27,50º S (Xi 0.426 and Eta -0.462). Utilising a digitised version of the Lunar Orbiter Atlas, frame 4-77h1, with a resolution of 300 metres per pixel, (figures 5 ), the following diameter values were obtained (reported in Table I).

Fig 5 Lunar Orbiter crop from frame 4-77H1.

Table I
 

E - W diameter (Km)	N - S diameter (Km)	Major - Minor Axes ratio
11.4	9.8	1.163

The height of this dome is difficult to estimate as we lack high-resolution satellite imagery of this region. However, the outer border of the dome seems steep and the images reproduced in fig 3- 4 reveal the presence of a shadow cast by the dome at moderate height of the sun (7.3° and 6.9° respectively).

Description and interpretations:

This dome at 28,69º E and 27,50º S is another clear example of the elusive nature of highland domes. At the same time, this study shows that a combination of careful visual and CCD observations made by different observers provides powerful tools for the study and interpretation of lunar domes.