Identification of a dome near the lunar crater Hansteen: morphometric analysis and proposed intrusive origin.
The apparent internal origin of lunar domes was a major factor in endogenic interpretations of the maria, and their low profiles suggest a volcanism characteristic of fluid mafic magmas. (1) In previous works we have introduced a novel classification scheme for effusive lunar domes which is based on their spectral and morphometric properties, and have examined for a variety of lunar mare domes the relationship between the conditions in the magma source regions and the resulting eruption conditions at the surface. (2,3) The typical hemispherical or flat mound with summit pit is widely accepted to be the lunar equivalent of terrestrial small shield volcanoes. (4) Many other lower rises have more gentle slopes and lack the summit pit, and sometimes the roughly circular outlines of classical domes.
The Geologic Lunar Research (GLR) group has an ongoing project to study lunar domes, with the purpose of classifying them based on rheological properties. The Consolidated Lunar Dome Catalogue published online (8) contains all lunar domes which have been studied in detail by the GLR group and for which reasonably accurate morphometric properties could be determined. The catalogue is continuously updated according to ongoing observing and modelling activities.
In previous studies we have examined a set of lunar domes with very low flank slopes which differ in several respects from the more commonly occurring lunar effusive domes. (5,6) Some of these domes are exceptionally large, and most of them are associated with faults or linear rilles of presumably tensional origin. Lunar domes of possibly intrusive nature were formed in different dome fields and are associated with a large variety of lava types.
All described candidate intrusive domes are characterised by very low flank slopes in the range 0.1[degrees]-0.9[degrees]. Accordingly, these domes might be interpreted as surface manifestations of laccolithic intrusions formed by flexure-induced vertical uplift of the lunar crust (or, alternatively, as low effusive edifices due to lava mantling of highland terrain, or kipukas, or structural features). Intrusions are subsurface concentrations of magma that have locally uplifted the mare but do not erupt, a mechanism reported for terrestrial laccoliths and described in detail in Johnson & Pollard. (7) Laccoliths have recently been proposed to explain various geological features such as domes or floor-fractured craters on the surface of the Moon and also Mars and Mercury. (9)
Johnson & Pollard (7) recognise that laccolith formation is characterised by three distinct stages. During the first stage, a thin sill-like unit undergoes lateral growth. The second stage consists of vertical growth caused by flexure of the overlying strata due to pressurised magma. If the flexure-induced vertical uplift exceeds a few hundred metres, piston-like uplift of a fault-bounded block may occur during the third stage of laccolith formation in comparison to the Earth. Michaut (10,11) shows, based on a numerical model of magmatic intrusions, that the smaller gravity and dryer crust of the Moon would lead to an increase of the characteristic elastic length scale for laccolithic intrusions by a factor of about two, which would explain the systematic differences in size between terrestrial and putative lunar laccoliths.
Due to their low profile, Lunar Orbiter and Clementine images do not show these intrusions very well, due to the typically high solar angle under which these images were acquired. Hence, as part of our program of observing and cataloguing lunar domes, we have used high-resolution telescopic CCD images taken under oblique illumination conditions.
In this paper, our goal is to assess the evidence for an intrusive origin of the lunar dome Hansteen 2 (Ha2) located to the north of Mons Hansteen. We examine the morphometric characteristics by making use of a combined photoclinometry and shape from shading approach. (2,12,13) The resulting values are used to derive information about the physical parameters of dome formation under the assumption of an intrusive mode of formation, providing a geological interpretation of our morphometric data.
A telescopic CCD image of the dome Ha2, examined in this study, is shown in Figure 1. The image was taken under strongly oblique illumination conditions using a telescope with aperture of 200mm. For image acquisition an Atik CCD camera was employed. The image was generated by stacking several hundred video frames, using the Registax software package. The scale of the images is 370m per pixel on the lunar surface. Due to atmospheric seeing, however, the effective resolution (corresponding to the width of the point spread function) is not much better than 1km. All the images shown in this paper are oriented with north to the top and west to the left.
The image shown in Figure 1 was taken on 2009 October 30 at 02:11UT. Using the Lunar Terminator Visualization Tool (LTVT) software package, (14) we determined the selenographic positions of the examined dome to 10.57[degrees]S and 48.20[degrees]W. LTVT is a freeware program that displays a wide range of lunar imagery and permits a variety of highly accurate measurements in these images. Selenographic coordinates, sizes, and shadow lengths of features can be estimated based on a calibration procedure. This calibration allows LTVT to make the spatial adjustments necessary to bring the observed positions of lunar features into conformity with those expected from the Unified Lunar Control Network (ULCN). The ULCN is a set of points on the lunar surface whose three-dimensional selenodetic coordinates (latitude, longitude, and radial distance from the lunar centre) have been determined by careful measurement. Typically these points consist of very small craters. According to Davies et al., (15) the three-dimensional positions are expressed in the mean Earth/polar axis system.
Clementine imagery: Surface composition
For spectral analysis we utilise the Clementine UV-VIS five-band data set as published by Eliason et al. (16) For all spectral data extracted in this study, the size of the sample area on the lunar surface was set to 2x2[km.sup.2]. Variations in soil composition, maturity, particle size, and viewing geometry are indicated by the reflectance [R.sub.750] at 750nm wavelength. Another important spectral parameter is the [R.sub.415]/[R.sub.750] ratio, which is correlated with the variations in Ti[O.sub.2] content of mare soils. A corresponding relation was established by Charette et al., specifically regarding different basaltic units in Mare Tranquillitatis. (17) A comprehensive characterisation of spectral features attributable to titanium in lunar soils is provided by Burns et al. (18)
Relying on Ti[O.sub.2] abundance data obtained with the Lunar Prospector neutron spectrometer, Gillis-Davis et al. (19) demonstrate that other factors such as ilmenite grain size or FeO content may give a significant contribution to the UV/VIS ratio. According to these analyses, Ti[O.sub.2] content is monotonously increasing with the [R.sub.415]/[R.sub.750] ratio, but the correlation is only moderate and the data display a strong scatter. The third spectral parameter, the [R.sub.950]/[R.sub.750] ratio, is related to the strength of the mafic absorption band, representing a measure for the FeO content of the soil and being also sensitive to the optical maturity of mare and highland materials. (20) The spectral data of the dome Ha2 are listed in Table 1.
Clementine UV-VIS imagery indicates a 750nm reflectance of [R.sub.750] = 0.0956, a moderate value for the UV/VIS colour ratio of [R.sub.415]/ [R.sub.750] = 0.6116, indicating a moderate Ti[O.sub.2] content, and a high [R.sub.950]/[R.sub.750] ratio of 1.0312 indicating a high optical maturity and thus a high exposure age of the dome surface. The absence of a spectral contrast between Ha2 and the surrounding surface indicates that the dome is not a piece of pre-existing elevated terrain later embayed by basaltic lava, a so-called kipuka.
Lunar Orbiter and WAC (LRO) imagery
As very low solar illumination angles are required to reveal the gentle slopes of lunar domes, most of these subtle structures do not appear in the available sets of orbital images. Lunar Orbiter image IV-149-H2 of Ha2 dome is shown in Figure 3b. The Lunar Reconnaissance Orbiter (LRO) WAC image (cf. Figure 3 a) shows a flat surface and an elongated shape of the dome, with the presence of some embayed non-volcanic hills on its summit.
Digital elevation map
Generating an elevation map of part of the lunar surface requires its three-dimensional (3D) reconstruction. Recently, a global lunar digital elevation map (DEM) obtained with the Lunar Orbiter Laser Altimeter (LOLA) instrument on the Lunar Reconnaissance Orbiter (LRO) spacecraft has been released. It has a lateral resolution of 1/64 degrees, or about 500m, in the equatorial regions of the Moon (http://pds-geosciences.wustl.edu/missions/lro/ lola.htm).
A section of the LOLA DEM displaying the region around the dome Ha2 is shown in Figure 1b, a rendered image obtained using LTVT and assuming the same illumination conditions as in Figure 1a. In the LOLA DEM, the elevation difference between the dome centre and its western border, corresponding to about 80m, may be regarded as an approximate value of the dome height.
A rendered image displays the shadow length cast by a dome and is useful for simulating particular situations, showing how rapidly the appearance of domes changes with increasing solar elevation. LOLA DEM was thus used for rendered images with different solar illumination angles, which show the dome Ha2 reported in Figure 1a (solar altitude 1.56[degrees]), under 0.65[degrees], 0.90[degrees] and 1.10[degrees] of solar altitude respectively (Figures 2a-2c).
We have also generated an elevation map of the dome based on our telescopic CCD image. A well-known image-based method for 3D surface reconstruction is shape from shading (SfS). The SfS approach aims to derive the orientation of the surface at each image location by using a model of the reflectance properties of the surface and knowledge about the illumination conditions, finally leading to an elevation value for each image pixel. (12) The iterative scheme used for photoclinometry and the SfS approach is described in our two preceding articles published in this journal, and is not repeated here. (21,22)
The dome Ha2 has an elongated base area of 21.0 x 16.7 [km.sup.2] and a corresponding circularity (minor axis divided by major axis) of 0.79. Its height was determined to 85 [+ or -] 10m using the image shown in Figure 1, resulting in an average flank slope of 0.53[degrees] [+ or -] 0.05[degrees]. A cross-sectional profile of the northern part of the dome is shown in Figure 2c. Assuming a parabolic dome shape the edifice volume corresponds to 11.8 [km.sup.3]. The morphometric properties inferred for the dome Ha2 are summarised in Table 2.
A combined approach was then used to construct a DEM of the dome using a WAC image superimposed onto the corresponding LOLA 1/512[degrees] DEM (see Figure 4).
Dome classification and laccolith modelling
For candidate intrusive lunar domes, we have introduced three morphometric classes in previous works. (5,6) The first class, In1, comprises large domes with diameters above 25km and flank slopes of 0.2[degrees]-0.6[degrees]; class In2 is made up of smaller and slightly steeper domes with diameters of 10-15km and flank slopes between 0.4[degrees] and 0.9[degrees]; Lena & Phillips: Identification of a dome near the lunar crater Hansteen and domes of class In3 have diameters of13-20km and flank slopes below 0.3[degrees]. While the morphometric properties of several candidate intrusive domes overlap with those of some classes of effusive domes, we show that possible distinction criteria are the characteristic elongated outlines of the candidate intrusive domes, as is the case for the examined dome with an elongated base area of 21.0x 16.7 [km.sup.2] and circularity of 0.79. Due to its large diameter and edifice volume, the dome Ha2 belongs to group In2, representing the known upper limit of this class (see Figure 5). The shape of this dome resembles another flat structure near the Valentine dome (termed V2 in Figure 6), previously described in Lena et al. (21)
Kerr & Pollard23 introduce a laccolith model in which they treat the overburden of the pressurised magma as an elastic plate. The force due to deflection of the plate is compensated by overburden weight and the magma pressure, which decreases from the laccolith centre towards its border. A detailed description of the implementation of the model is given in Wohler & Lena (6) where an iterative scheme is fully described. Given the diameter D, the height h, the volume V, and the curvature radius r inferred from the DEM of an intrusive dome, the model yields values of the intrusion depth, corresponding to the overburden thickness d, the minimum thickness of the uppermost mare basalt layer [h.sub.1] and the maximum magma pressure [p.sub.0]. (6)
For the dome Ha2, we obtained a minimum thickness of the uppermost mare basalt layer of [h.sub.1] = 0.2km, an intrusion depth of d = 1.2km and a maximum magma pressure of [p.sub.0] = 9.5MPa.
Results and discussion
In contrast to effusive lunar domes which are characterised by relatively sharp and circular boundaries, Ha2 is of strongly elongated shape, its surface merges smoothly into the surrounding mare surface, and a clear boundary is absent. Some non-volcanic hills located on its summit are embayed by lavas. Presumably, these hills are part of the underlying rugged basin floor below the mare lavas. Furthermore, Ha2 has no effusive vent (but this may also be the case for effusive domes if they were plugged by lava), and it is characterised by a much larger diameter and a much lower flank slope than all lunar effusive domes examined so far. (2,3)
One might also consider Ha2 as a kipuka, i.e. an elevated 'island' surrounded by the flooding mare lavas. Kipukas usually consist of a different material than the surrounding mare, such that a spectral contrast would have to be observed. A typical example of a lunar kipuka is the formation Darney % located in western Mare Cognitum, an elevated section of highland terrain embayed by mare lavas. (24) The absence of a spectral contrast between the dome Ha2 and the surrounding surface, indicating that both consist of the same material, and the absence of a sharp boundary suggest that Ha2 is most likely not a kipuka. Hence, sheet-like magmatic intrusion of laccoliths appears as a possible and plausible mode of formation for the dome Ha2. Especially, the comparative numerical modelling of laccolith properties, especially their characteristic sizes and thicknesses, under terrestrial and lunar conditions recently performed by Michaut (10,11) is in favour of an intrusive interpretation of large, low, and smooth lunar structures like Ha2 in terms of the elastic plate model. However, alternative modes of formation cannot be definitely ruled out based on the available data.
When assuming an intrusive origin of the dome Ha2, this would indicate that laccolith formation proceeded until the second stage, characterised by flexure of the overburden. (7)
All domes of class In1 show fractures on their surfaces, probably formed by tensional stress. Linear rilles traversing the summit are also detectable on three further large and low domes located in Grimaldi, near Aristillus, and Kies 2, termed Gr1, Ari1, and K2, which have recently been described. (25,26) These linear rilles were probably formed by the stress fields associated with dikes that ascended to shallow depths below the surface.
Table 3 reports the flank slope, diameter, height, and edifice volume of other intrusive domes described in previous studies. (5,6,25,26) The diameter vs. flank slope and volume vs. flank slope relations of these domes are illustrated in Figure 5.
The dome Ha2 shows similar elongated shape and dimension comparable to other domes of class In2, like C11, Pa1, L6, V2, but with large diameter and inferred edifice volume.
Ha2 is also characterised by higher deep intrusion (d= 1.2km) and magma pressure of 9.5MPa when compared to Pa1 and V2. Furthermore, the flat appearance of these domes suggests that the rising lavas did not build up a dome through a series of flows, but that it was more likely formed by rising magma collecting in a subsurface pocket, leading to a subsurface intrusion. The resulting tensional stress does not produce the formation of crossing rilles, characteristic for the class In1 dome, for which are inferred higher magma pressure of 18-100MPa.
The recent numerical modelling and scaling analysis by Michaut (10,11) of the magmatic intrusion processes leading to the formation of laccoliths strengthens the hypothesis of an intrusive origin of large and low lunar domes similar to those described in this study and our preceding works. For lunar laccoliths, an adjustment of the gravitational acceleration and the crustal elasticity (for the dry lunar crust the Young modulus E is assumed to be 2.5 times higher than for the terrestrial crust) leads to characteristic diameters of lunar laccoliths of 12-32km and similar thicknesses for terrestrial and lunar laccoliths, which is also in accordance with the observational data, including the dome examined here.
Summary and conclusion
In this study we have analysed the morphologic, morphometric, and spectral properties of the lunar dome Hansteen 2 [Ha2] located north of the well known highland dome Mons Hansteen. Its height amounts to 85 [+ or -] 10m, resulting in an average flank slope of 0.53[degrees]. The edifice volume corresponds to approximately 11.8 [km.sup.3]. The flattened appearance of Ha2 suggests that the rising lavas did not build up a dome through a series of flows. The dome Ha2 is a large and voluminous structure and belongs to class In2 in the classification scheme of possible lunar intrusive domes. It is characterised by a lower intrusion depth of 1.2km and magma pressure of 9.5MPa, when compared to other candidate intrusive domes of class In1. The smooth cross-sectional dome shape indicates that laccolith formation proceeded until the stage characterised by flexure of the overburden.
Address: Geologic Lunar Research (GLR) group, Raffaelo Lena, via Cartesio 144, 00137 Roma, Italy. [firstname.lastname@example.org]
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Received 2011 July 28; accepted 2011 October 26
Table 1. Spectral properties of the dome Hansteen 2, derived from Clementine UV-VIS data Dome [R.sub.415] [R.sub.750] [R.sub.900] [R.sub.950] Hansteen 2 0.0585 0.0956 0.0981 0.0986 Dome [R.sub.1000] [R.sub.415]/ [R.sub.950]/ [R.sub.750] [R.sub.750] Hansteen 2 0.1010 0.6116 1.0312 Table 2. Morphometric properties of the dome Hansteen 2 and the similarly flat domes V2 and Pal Modelling results for the minimum basaltic layer thickness h u intrusion depth d, and maximum magma pressure Dome h [m] slope size [km] V [[km.sup.3]] [[degrees]] Ha2 85 0.52 21x16.7 11.8 V2 80 0.82 11.0 1.9 Pa1 60 0.50 13.5 4.3 Dome [h.sub.1] d [km] [p.sub.0] [km] [MPa] Ha2 0.20 1.20 9.5 V2 0.08 0.38 2.9 Pa1 0.12 0.91 7.2 Table 3. Morphometric properties of other domes of probably intrusive origin of classes In1-In3 according to earlier studies (5,6,25,26) Dome long. lat. slope D [km] [[degrees]] [[degrees]] [[degrees]] K2 -23.82 -28.30 0.15 51x34 Ga1 -14.84 -0.75 0.57 30 V1 10.20 30.70 0.55 30 M13 -31.53 11.68 0.41 27.8 Ar1 0.71 55.71 0.25 33.0 Gr1 -68.62 -04.45 0.62 36x24 Ari1 05.67 33.28 0.22 54x35 C11 36.75 11.06 0.70 12.2 Pa1 -47.88 -26.63 0.50 13.5 L6 -29.16 47.08 0.70 10 V2 10.26 31.89 0.82 11 C9 34.66 7.06 0.13 13.3 C10 35.19 10.00 0.30 19.2 Dome h [m] V class [[km.sup.3]] K2 55 37 In1 Ga1 140 50 In1 V1 130 42 In1 M13 100 15 In1 Ar1 70 22 In1 Gr1 160 75 In1 Ari1 85 63 In1 C11 75 6.4 In2 Pa1 60 4.3 In2 L6 95 1.5 In2 V2 80 1.9 In2 C9 15 0.5 In3 C10 50 10 In3
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|Author:||Lena, Raffaello; Phillips, Jim|
|Publication:||Journal of the British Astronomical Association|
|Date:||Feb 1, 2013|
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