A study about Highland
domes: a generic classification of the dome near Piccolomini, located at 28,69º
E and 27,50º S .
By Raffaello Lena, Rodrigo Viegas and Cristian Fattinnanzi
(Geologic Lunar
Research group, GLR)
Introduction
Lunar domes are "positive relief features", which are similar to the low shield volcanoes found on Earth [1-2]. They represent the terminal phase of lunar volcanism and they occur mostly in the maria. There are a few in the highlands, but these are usually difficult to discern because of the brightness and ruggedness that characterises the surrounding highland materials. A firstOne theory argues that 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 [3-5]. This idea is in agreement with the majority of highland domes being steep because low Fe and Ti lavas are less dense viscous and thus can be thought to pile up more easily, producing steeper constructs. The classical example of Highland dome is Gruithuisen gamma which has a higher albedo than the nearby Mare Imbrium and also a steeper slope. This refers to " an early volcanism which was both unrelated to the physical maria and was also of different mineralogy " [5]. A second theory argues that highland domes are mare domes covered by a thin veneer of ejecta from the Imbrium impact [4].
Two papers which recently appeared in Selenology, the Journal of the American Lunar Society, described two highland domes, one near Zollner in [4] and the other near Piccolomini in [5]. The latter dome is the subject of this report. As described by some of us in [5] this dome "was originally drawn by Harold Hill on February 18, 1987 [6], without specific details as to the dome position or presence of summit craters". By using our images we described the position of this highland dome, not listed in the ALPO dome list, at 28,69º E and 27,50º S (Xi 0.426 and Eta -0.462). In this study we report further measurements and include CCD imaging. This has made it possible to extract additional information for its classification and interpretation.
Instruments and Measurements
This report is based on an analysis of 12 visual observations and 6 images taken under different solar altitudes and sent to us. We strongly encouraged observers to participate in organized, simultaneous observations. Table 1 lists the 6 observers, their instruments, and the number of observations they submitted. Utilising a digitised version of the Lunar Orbiter Atlas frame 4-77H1, with a resolution of 300 metres per pixel (figure Figure 1), the diameter values were obtained (Table II). For each observation, we calculated the solar altitude (H) and azimuth (A) as seen from the dome, and the colongitude (C), using the Lunar Observer's Tool Kit software by Harry Jamieson.
Information about the vertical cross-section was obtained using the Ashbrook method, as described in [7]. Using this method, we estimated for different solar altitudes the fraction x of the dome’s east-west diameter that is covered by black shadow.
According to Ashbrook [7] the average slope of the dome flank can be equalledis equal to the solar altitude when x is 0.25 (where x is the fraction of the dome’s east-west diameter that is covered by black shadow).
The height (H) of the dome was then calculated by the formula (1):
H = R (tan s)
Where R is the radius of the dome and (tan s) is the tangent of the average slope angle when the dome is ¼ covered by black shadow (see Table III). Furthermore, we were able to distinguish between the black shadow and the dark grey shading of the dome flank which represent grazing illumination by sunlight.
On the best image, the dome diameter and the length of its shadow were both measured in pixels. The corresponding scale of the image was obtained, allowing diameters and shadow lengths to be expressed in kilometres. From the lengths of the cast black shadows and the local solar altitudes, we were able to calculate the height of the dome.
Estimated x values (visual and imaging) were obtained both under evening (E) and morning (M) illumination; the results are summarised in Table III and IV. Furthermore, Table IV reports the classification according to Westfall [8].
Table I Contributing
observers and instruments, where (M) and (E) refer to morning and evening illumination
respectively.
| Contributing
Observers |
D and F/D |
|
|
| Bares A. |
250 mm f/10 |
|
|
| Fattinnanzi C. |
250 mm f/6 |
|
|
| Konkel G. |
250 mm f/10 |
|
|
| Lena R. |
100 mm f/15 |
|
2 (E) |
| Nardella S. |
150 mm f/6 |
|
1 (E) |
| Viegas R. |
114 mm f/8 |
|
2 (E) |
Table II Dimensions
of the dome localized at 28,69º E and 27,50º S
|
|
|
|
|
|
|
|
Table III: Slope
observations of the dome under evening (E) and morning (M) illumination: X
is the fraction of the dome east-west diameter that is covered by black shadow.
| Date and times (UT) |
tion |
solar altitude |
|
|
|
| 6
February 2003
20:00 UT |
|
|
|
|
|
| 6
April 2003
23:44 UT |
|
|
|
|
|
| 23
March 2003
05:30 UT |
|
|
|
|
|
| 23
March 2003
04:10 UT |
|
|
|
|
|
| 23
March 2003
03:40 UT |
|
|
|
|
|
| 16 November 2000 09:00 UT |
|
|
|
|
|
| 23
March 2003
02:13 UT |
|
|
|
|
|
| 30
June 2002
03:00 UT |
|
|
|
|
|
| 23
March 2003
02:13 UT |
|
|
|
|
|
| 23
March 2003
02:13 UT |
|
|
|
|
|
| 7
February 2003
00:40 UT |
|
|
|
|
|
| 17
April 2002
19:30 UT |
|
|
|
|
|
| 17
April 2002
19:40 UT |
|
|
|
|
|
| 13
April 2001
06:30 UT |
|
|
|
|
|
| 15
June 2002
23:00 UT |
|
|
|
|
|
| 8
March 2003
19:52 UT |
|
|
|
|
|
| 10
November 2002
18:00 UT |
|
|
|
|
|
| 15
July 2002
23:50 UT |
|
|
|
|
Table IV: Slope
determination and Westfall Classification [8].
|
|
(pixel, km) |
(pixel, km) |
(m) |
|
|
|
|
20 11.4± 0.57 | 9 5.2± 0.57 |
|
|
|
Results
Observations
We received 7 observations from 5 observers for evening illumination and 11 observations from 4 observers for morning illumination (Table I).
Evening Illumination
Figures 2-5 show the aspect of this region as imaged and sketched by several observers. This dome requires a specific solar altitude for it to be observed clearly and requires a narrow solar angle for maximum detail.
The dome appears to be hemispherical with the presence of a small feature on the top that suggests a craterlet, likely representing the central vent. In fact, the Lunar Orbiter imagery (fig.Figure 1) shows a small summit crater and reveals other craterlets in this region, but the angle of illumination makes it difficult to determine if these have rims or are volcanic in nature.
The image proposed
in Figure 5 reveals very fine detail on the dome including the central craterlet.
figure 1
figure 2
figure 3
Morning illumination
Inspection of fig.
6 and 7 reveals that the cast shadow entered the terminator.
figure 6
figure 7
In fig. 8 and 9, made at H values of 2.5° and 4.0° respectively, the West foot was dark but no black shadow (umbra) was cast. Figure 8 was made under very good seeing conditions, making it possible to see the summit craterlet. Undoubtedly, this craterlet is a hard difficult target, which requires low solar altitudes.
On the other hand, under a higher solar altitude, the dome does not project a black shadow but a penumbra on the partly illuminated flank.
figure 8
figure 9
Slope estimation
The visual estimations agree very closely with CCD images. The x values along with the corresponding solar altitude and illumination (Morning or Evening) are summarized in Table III.
A penumbra (not
black shadow) is visible from H = 2.1° to H = 4.0° and certainly up to H = 7.0°
(Table III).
From Table III, it follows that the average slope angle of the dome is about 1.8°. Furthermore, even the steepest parts of the eastern and western flanks have not slopes greater than about 5°. The height of the dome was then estimated using the formula 1 assuming a radius of 5.7 kms, from the Lunar Orbiter images. It turns out that the summit of the Piccolomini dome is 179 meters higher than the surrounding plain.
This estimated
slope and height are comparable with the value measured in the frame shown in
Fig. 5, where we calculated, from length of its shadow, a height of 155± 17
meters and an average slope 1.7°± 0.17.
Geological considerations
The exceptionally low slope of this dome compared with the steepness of ordinary highland domes is an aspect of note.
A highland dome was recently reported near crater Zollner in [4] that also shows a low slope, which appears to be under 2 º.
A highland dome already reported at 24.72 º E and 29.21 º S [4], near crater Rothmann-D, lies about 110 Km km away from the dome we are dealing with in this paper. Its slope is classified as 5 in the Westfall classification scheme, that is, under 2 º.
These three low highland domes have something in common: they are all positioned within the Nectaris basin, in the surroundings of the Altai Scarp. On the moon, there are only 2 more highland domes with slopes under 2 º [4]. There are a total of 8 reported highland domes within the Nectaris basin [4].
The Nectaris basin was partially filled by lavas that formed the Mare Nectaris. Nevertheless, the outpouring of lavas was not enough to cover appreciably all the basin. This "lack of outpouring" can be related with the formation of low instead of steep highland domes. This could imply that the sources of highland dome volcanism are closely related with the sources of mare lavas. This, together with the observation that these three highland domes are low, could favour the second theory mentioned in the Introduction.
Conclusion
This dome at 28,69º E and 27,50º S is a clear example of the elusive nature of highland domes. Using our data, this highland dome may be classified according to the Westfall classification scheme as DU/2a/5f/7j.
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.
In our view, a careful study of the domes inside the Nectaris basin can provide useful clues as to the nature of the highland dome volcanism. For this reason, data on the Zollner dome, described in [4], are already in progress.
Finally, international participation in our programs continues in an apparent favourable response to our efforts to foster increased cooperation among lunar observers worldwide.
Acknowledgements
We wish to thank all the observers for their contribution to this paper.
Legend
Figure 1: Lunar Orbiter frame 4-77 H1. It is visible the central craterlet.
Figure 2: Evening illumination over the dome located at 28.69° E and 27.50° S. Observation carried out by R. Lena on 23 March 2003 at 02:13 UT
(H = 1.8°, A = 269.5°, C = 149.8°) with a refractor 100 mm f/15 at 250 x. Seeing II Antoniadi scale and X =0.25.
Figure 3: Evening illumination over the dome located at 28.69° E and 27.50° S. Observation carried out by R. Lena on 30 June 2002 at 03:00 UT
(H = 1.9°, A = 271.7°, C = 148.8°) with a refractor 100 mm f/15 at 250 x. Seeing II Antoniadi scale.
Figure 4: Evening illumination over the dome located at 28.69° E and 27.50° S. Observation carried out by R. Viegas on 23 March 2003 at 05:30 UT
(H = 0.8°, A = 268.9° , C = 151.1°) with a Newtonian 114 mm f/8 at 225 x. Seeing I-II Antoniadi scale and X =0.50.
Figure 5: Evening illumination over the dome located at 28.69° E and 27.50° S. Image taken by G. Konkel on 16 november 2000 at 09:00
UT (H = 1.7°, A = 272.3°, C = 148.6°) with a digital camera fitted to a
Schmidt-Cassegrain 250 mm f/10. Seeing II Antoniadi scale and estimated X =0.45.
Figure 6: Morning illumination over the dome located at 28.69° E and 27.50° S.
Observation carried out by R. Viegas on 6 April 2003 at 23:44 UT
(H = 0.5°, A = 91.0°, C = 331.2°) with a Newtonian 114 mm f/8 at 225 x. Seeing III Antoniadi scale.
Figure 7: Morning illumination over the dome located at 28.69° E and 27.50° S. Observation carried out by R. Lena on 6 February 2003 at 20:30 UT
(H = 0.6 °, A = 91.2°, C = 331.5°) with a refractor 100 mm f/15 at 250 x. Seeing II-III Antoniadi scale.
Figure 8: Morning illumination over the dome located at 28.69° E and 27.50° S.
Observation carried out by R. Viegas on 7 February 2003 at 00:40 UT
(H = 2.5°, A = 90.4°, C = 333.3°) with a Newtonian 114 mm f/8 at 225 x. Seeing II Antoniadi scale.
Figure 9: Morning illumination over the dome located at 28.69° E and 27.50° S.
Observation carried out by R. Viegas on 15 June 2002 at 23:00 UT
(H = 4.0°, A =
87.7°, C = 335.9°) with a Newtonian 114 mm f/8 at 225 x. Seeing II Antoniadi
scale.
References
[1] Pg Salimbeni, R. Lena, G. Mengoli, E. Douglass, G. Santacana, Strolling Astronomer, The three domes in Rima Birt region, 2000, n. 2 Vol 42, pag 83-87.
[2] Cattermole, P. Planetary Volcanism. Chichester: John Wiley and Sons Ltd, 1996; p. 193.
[3] B. Spudis, P. Volcanism on the Moon; in Encyclopedia of Volcanoes, ed. H. Sigurdsson. San Diego: Academic Press, 2000; p. 705.
[4] E. Douglass, G. Santacana, Highland domes and a Highland dome near Zollner, Selenology, vol 21.2, 2002 , pag 8-12.
[5] R. Viegas, R. Lena, E. Douglass, Piccolomini Highland dome, Selenology, vol 21.2, 2002, pag 15-18.
[6] H. Hill "A Portfolio of Lunar Drawings" .Cambridge University Press, (1991).
[8] J. Westfall A Generic classification of lunar domes, JALPO vol 18, 1-2 (july 1964), pp 15-20.