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FARSIDE Lunar Dome Survey (FLDS)Part II a progress
report By Raffaello Lena and Rodrigo Viegas - ALS and GLR group

1-Introduction The International Farside Lunar Dome Survey (FLDS) is a joint effort of the American Lunar Society (ALS) andthe Geologic Lunar Research group (GLR).As the USGS digitises all Lunar Orbiter images[1] athigh resolution it will be possible to do an inventory of farsidedomes.

Since the project is a long term study [2], any report of specific farside domes-swellingsshould not be viewed as a final report. Digitised images and notes will continue to be accepted and included in our catalogue.

The purpose of this study was :

• to classify some structures according to their morphologic characteristics;
• to gather data on associated features in order to provide information on their origin and to understand their geologic setting.

This first articledescribes Mare Orientale and Lacus Veris.

2-General classification

Several volcanic features were identified duringa morphologic survey ofthe Mare Orientale-Lacus Veris region.Table 1 reports theirlocation and characteristics. A classification scheme describes a complex structurein the most general terms. Swellings on the maria, irregular swellings, low plateaux and steep-sided hills may be referred to as domes even though many of these features may not be true volcanic domes.

J. Westfall in his article "A generic classification of lunar domes" [3],gives the following definition for a lunar dome: "A discrete, regular swelling whose ratio of major axis/minor axis, when corrected for foreshortening, does not exceed 2:1, and whose maximum slope, not including secondary features, does not exceed 5°. Domes may exhibit secondary features, such as pits, clefts, ridges, and hills, as long as any single such feature does not occupy more than a quarter of the area of the dome ". It is of interest that some features in Mare Orientale-Lacus Veris region, likely, do not meet the Westfall criteria for the secondary features.

Domes have been also classified according to their morphology and mode of origin by James Head and Ann Gifford [4] who considered the characteristics of over 200 domes and grouped them in seven categories, as follows:

Classes 1-2-3 refer to volcanic features resembling terrestrial shields.

Class 4 refers to dome-like features associated with ridges and mare arches. Because of the complex topography of ridge/arch it is not certain if this class represents a dome type.

Class 5 includes domes that originated from lava mantling of pre-existing highland topography.

Class 6 includes domes with higher albedo than mare material and steeper slopes compared with ordinary domes.

Class 7 are complex maria domes with irregular outline and topography (i.e. MariusHills).

Classes 1, 2, 3 and 7 are in most cases the direct result of volcanic activity. Classes 5 and 6 are the result of the draping of pre-existing topography (secondary volcanic effects such as flooding of highland material and draping of lavas to produce irregular dome like topography). Class 4 structures are mainly associated with mare ridges.

Several volcanic features ( Table 1) were identified duringour survey.

Their characteristics are summarised as follows:

• they have a flat summit and a general irregular outline;
• the flat summit and irregular outline is more abundant in the Orientale-Lacus Veris regionthan elsewhere on the Nearside;

3-Farside list

FIG. 1

Lunar Orbiter frame IV-195-H2, here reported as figure 1, shows a low dome, elliptical in shape with a flat complexsummit,located near the craterHohmann (under the entry #2 in Table 1). Another irregular structure lies near the crater Maunder (entry #1); it may be described as a possible swelling. The presence of dark halo craters near both craters (see figure 1)is also of note. These two swellings are blanketed by ejecta from Maunder.

FIG. 2

Anotherdome-like plateau was found in Lunar Orbiter frame IV-195-H1 here reported as figure 2. It lies near the crater Shulejkin (entry #3). Because of its shape one may question whether this dome-like plateau should be classified as a dome.

Mutch in his book “Geology of the Moon” [5] calls it "an intermediate-level lava plateau around protective basement spines". Its summit is rather complex, and ithas some odd looking edges. The structure itself extends into the southern ridge. A similar but larger structure can be found in Lacus Mortis, around crater Bürg.

FIG.3

In this region there is another irregular feature, here reported in figure 3, that covers a collapse depression.This irregular object, located near the crater Kopff, could be another swelling. It is reported in Table 1 under the entry #4.

FIG.4

Lunar Orbiter frame (IV-181-H2), here reported as figure 5, shows interesting features located in Lacus Veris, to the SW of the craterLallemand. The marked feature A (reported under the entry #5) is a complex structure closely related with two lateral low plateau like structures. Feature A may be considered as a group of separate units (it appears to have 3-4 structures). The central feature, with an irregular shape, is bisected by a rille.

FIG.5

In the same figure 5 there is another dome, marked as B, that is located at longitude -84.76° and latitude -17.67° (x = -0.948 h= -0.308). It shows anirregular shape with a flat summit with an elongated depression or fissure (it may be described in the Westfall system asDW/2d/5g/7j). In addition, we detected two low features marked as C and D in figure 5, which can be interpreted as possible swellings or low domes. Feature D is closely associated with highland units; the presence and shape of the rilles on its summit suggests a laccolithic nature for this feature. Nevertheless, it can also be interpreted as being the result of secondary volcanic effects, so it can also be described as Class 5 in Head’s classification scheme.

It is interesting to note that feature B can also be seen in LO-IV-187-H2, here reported as figure 4. It appears in theA.L.P.O. Lunar Dome list as x -0.947 h -0.313 (entry #6). Head reported this dome at 85.0° W and 18.0°S. Interestingly the central craterlet is estimated as 1.5 km. Using the Westfall classification scheme, it can be described as DW/ 2a/5g/7j.

FIG.6

Finally, Lunar Orbiter frame IV-187-H2, here reported as figure 6, shows dark haloed craters suggestive of cones or explosive processes ofendogenous nature. Nevertheless, they could have formed as a result of impacts that penetrated the surface ejecta layer and removed the underlying dark soil.

4-Conclusion:

This study shows that it is possible to find more unreported domes or dome-like features on the Moon farside. Also, it can be said that most domes on the farside are low features.

It is necessary to find more farside domes all over the lunar disk, in order to discuss if this low slope is a characteristic common to all farside domes.

Despite the limitation in the lighting conditions, the use of high resolution imagery makes a good tool for the search of domes that otherwise could never be seen through the eyepiece.

Acknowledgements:Many thanks to Maria Teresa Bregante for her stimulating discussion.

An international participation will be welcometo foster increased cooperation among lunar observers worldwide.

References:

[1] www.usgs.gov

[2] Raffaello Lena, Selenology,2004, vol.23.2

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

[4] James Head and Ann Gifford, lunar mare domes: classification and modes of origin, The Moon and Planets,22,1980

[5]Mutch in his book “Geology of the Moon”, 1970, p. 151.

TABLE 1 Farside dome Survey (Rukl Chart Libration zone VII)

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