LUNAR PITS: SUBLUNAREAN VOIDS AND THE NATURE OF MARE EMPLACEMENT

42nd Lunar and Planetary Science Conference (2011) 2771.pdf

LUNAR PITS: SUBLUNAREAN VOIDS AND THE NATURE OF MARE EMPLACEMENT. J. W. Ashley1,
A. K. Boyd1, H. Hiesinger2, M. S. Robinson1, T. Tran1, C. H. van der Bogert2, R. V. Wagner1, and the LROC Science
Team. 1Lunar Reconnaissance Orbiter Camera, School of Earth and Space Exploration, Arizona State University,
Tempe, AZ 85287-3603 (james.ashley@asu.edu); 2Institut für Planetologie, Westfälische Wilhelms-Universität,
Münster, Germany.

Introduction: The possibility for shallow, subsur- region within Oceanus Procellarum at approximately
face lunar voids has existed in the minds of researchers 14.09°N, 303.31°E. The pit was estimated to be ~ 65 m
at least as far back as the 1960’s, often under the head- in diameter, and 80-88 ±10 m deep, with a ceiling 40-
ing of lunar caves [1]. Such voids were anticipated as 60 m thick [12]. The SELENE team deduced the for-
the straightforward result of lava tube drainage with mation mechanism for the Marius Hills pit to be partial
ceiling failure and subsequent collapse [2]. Any future collapse of a lava tube based on its location within a
long-term human presence on the Moon will require sinuous rille, and by elimination of other less plausible
reliable protection from surface hazards (radiation, mechanisms. Direct imaging of cavernous interiors
micrometeorites, temperature fluctuations, solar flares, beneath their ceilings is arguably necessary to confirm
etc.), which can be accomplished effectively using this interpretation, and requires the combination of
extant lava tube caverns [3-5]. Subsurface void spaces optimal solar incidence and high angle slew (off-nadir)
may also preserve unique geologic environments sig- imaging.
nificant to scientific exploration, and pits and/or sky-
lights would naturally serve as points of access to those !"#$%&'()*%"*+%,
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environments. It is unknown whether these systems are
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present as intricate “plumbing” networks, or occur as
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isolated caverns of limited extent. However, it is possi-
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ble that such caverns could extend for tens or even
hundreds of kilometers.
Features described as pit craters, but having a vari-
ety of suggested origins have also been observed on
Mars [6, 7], Mercury [8], and the Jovian moon system
[e.g., 9, 10], with possibilities for larger-scale pit fea-
tures on Venus [11]. In some sources, however, a dis-
tinction is made in discussion between pit craters and
skylights, the latter term having an implicit mode of
origin as that of lava tube ceiling collapse [12-14].
a
N

Depth Shadow Diameter (m) Slew Incidence
A
Location Image
(m) length m Max Min angle (°) angle (°) B
M106662246R 102 60 97 86 0.00 30.52
Mare M126710873R 106 53 95 85 0.00 26.56
Tranquilitatis M137332905R 92 70 98 81 7.05 37.37
M144395745L NM NM 100 NM -50.46 47.92
Marius Hills M114328462R >32* NM 62 48 6.80 61.38
M122584310L 36 19 57 46 0.00 28.08
M133207316L >8* NM 76 61 29.13 82.84
M137929856R NM NM NM NM 42.86 33.79
Mare Ingenii M115225180L >24 NM 146 107 0.00 74.52
Tuesday, January 4, 2011
M121124338L 68 87 125 97 0.00 52.00
M123485893R 47 38 104 68 -5.45 39.05
M128202846L 76 105 125 68 0.00 54.25
M136465172L >47* NM 110 73 15.51 55.47
M138819477R 52 42 101 73 0.00 38.91
* depth estimate less certain due to high slew angles ~ 100 m
NM not measureable

Table 1. Catalog of images.
Figure 1a: Marius Hills pit imaged from a slew angle of
Background: The first candidate for such a lunar 43° and a solar incidence angle of 34°. Red arrow shows sun
environment was identified with JAXA’s SELenologi- azimuth. N-S pixel scale is 0.55 m; E-W pixel scale is 0.71
cal and Engineering Explorer (SELENE; a.k.a. Ka- m. Note mare bedrock layering in the pit walls. Figure 1b
guya) Terrain Camera and Multi-band Imager at 10 m shows the imaging geometry in cross section. Dotted lined
resolution during an imaging campaign optimized for zones have not been imaged and are speculative.
skylight detection [12]. Nine images (with solar inci-
dence angles ranging from ~ 15° to 73°) were collected Features identified as ‘pit-floor craters’ have been
by SELENE of a pit feature named Marius Hills Hole found on Mercury during the January 14, 2008, flyby
by its discoverers, which is located in the Marius Hills of the MEcury Surface, Space ENvironment, GEo-

42nd Lunar and Planetary Science Conference (2011) 2771.pdf

chemistry, and Ranging (MESSENGER) spacecraft plain, without obvious indications of additional subsur-
[8]. While these features have a different morphology, face voids.
scale, and other interpreted modes of origin [e.g., 8] Mare Tranquilitatis Pit: In contrast to the Marius
than lava tube skylights, their presence is interpreted to Hills and Mare Ingenii pits, the Mare Tranquilitatis pit
be associated with near-surface magmatic activity, (8.34°N, 33.22°E) is nearly circular in planform, with
which may increase the odds of finding lava tube sys- diameters ranging from 85 m to 97 m, and a depth of
tems during higher-resolution surface reconnaissance 100 ± 6 m (Figure 2). It was imaged on four orbital
after MESSENGER achieves orbital insertion in passes with a maximum slew angle of -51°. Figure 2a
March 2011. In addition, pit features interpreted to be shows approximately 39 percent of the floor in direct
lava tube skylights to subsurface caverns have been sunlight. Figure 2b is a highly oblique view of the pit,
found in volcanic regions on Mars [13]. The confirma- showing excellent detail within the subsurface layering
tion of such features within volcanic materials on the and a conspicuous funnel-shaped slope to the rim. An
Moon lends credibility to these interpretations and irregular wall profile is suggestive of differential unit
possibilities, and encourages their continued explora- strength within the layered materials, possibly resulting
tion. from intermittent emplacement of mare deposits with
Lunar Reconnaissance Orbiter Camera (LROC) regolith buildup between these events.
Narrow Angle Camera (NAC) images have pixel scales
up to 0.5 meters at a nominal 50-km altitude over a 2.5
km-wide swath [15], and have been collected for three a b
pits found in lunar mare, including the Marius Hills pit
imaged by [12]. Each of these are discussed below.
Marius Hills Pit: LROC NAC was used to re-
image the Marius Hills pit under a variety of lighting
conditions on four orbits, revealing morphologic de-
tails with pixel scales ~ 0.5 m. An oblique (43° slew ~ 100 m
angle) image reveals portions of the pit floor beyond
the nadir view pit perimeter, and beneath the overhang-
ing ceiling rock, thus confirming the ‘sublunarean’ Figure 2. Mare Tranquilitatis pit imaged nadir (0.00°)
extension of this pit (Figure 1). (2a), and -51° (2b) slew angles; images M126710873R and
NAC images show pit diameters to range from 60 M144395745L, respectively. Note layering complexity, dif-
m to 47 m with a depth of ~ 36 m. The pit outline is ferentially modified pit wall profile, and funnel-shaped rim
elliptical with a faintly trapezoidal modification and in 2b (red scale bars are ~ 100 meters along each length).
walls that exhibit a layered structure.
The sun angle was calculated for both directions Summary: The direct high-resolution imaging of a
using the sub-solar point and the center of the image. sublunarean void floor present beneath a sinuous rille,
The calculation of the height difference between the together with additional pit features of similar appear-
shadow and the object producing the shadow was then ance, provides supporting evidence to their interpreta-
performed in the two orthogonal directions independ- tion as collapsed lava tube skylights by confirming a
ently and compared for error checking. Figure 1b cavernous subsurface, as has been anticipated through-
shows the cross-sectional geometry of the Marius Hills out the literature based on the geomorphology of mare
oblique imaging. The vertical Marius Hills pit reveals a deposits and an understanding of lava flow behavior on
multilayered structure of near-surface mare flood ba- Earth, but never directly imaged before.
salts. Thickness estimates for the coarser units (high- References: [1] Halliday (1966) Bull Nat Speleol
lighted as A and B in Figure 1a have been calculated to Soc, 28,167-170. [2] Kauahikaua et al. (1998) JGR-
be 4 - 8 and 6 - 10 m (± 1 m), respectively. Solid Earth, 103, 27303-27323. [3] Horz et al. (1985)
Mare Ingenii Pit: The Mare Ingenii pit is located Lava Tubes: Potential shelter for habitats, LPSI. [4]
at 35.95°S, 166.06°E, and was imaged by the NAC six De Angelis et al. (2002) JRR, 43, S41-S45. [5] Coombs
times with a range in incidence angle of 39° to 75°, et al. (1992) NASA-CP-3166. [6] Wyrick et al. (2004)
and from both nadir and off-nadir positions, allowing JGR-Planets, 109. [7] Scott et al. (2002) JGR-Planets,
for weak stereoscopic viewing. The pit is oblate with a 107. [8] Gillis-Davis (2009) Earth and Planetary Sci.
moderately trapezoidal outline and a long axis aligned
Let., 285, 243-250. [9] Schenk (1993) JGR-Planets,
approximately NNE-SSW. Its dimensions are roughly
101 by 66 m, with a depth of 60 ± 15 m. The pit walls 98, 7475-7498. [10] Croft (1983) JGR, 88, B71-B89.
have slopes that range from nearly vertical on the [11] Senske et al. (1992) JGR-Planets, 97,13395-
NNW side to approximately 45° on the SSE side. Rub- 13420. [12] Haruyama et al. (2009) JGR, 39. [13]
ble covers the floor in areas that are illuminated, and Cushing et al. (2007) JGR, 34. [14] Witter er al. (2007)
rises close to the pit rim, forming a ramp-like structure. JGR-Solid Earth, 121. [15] Robinson et al. (2010)
The mare terrain surrounding this pit is a level cratered Space Sci. Rev., 150, 81-124

LUNAR PITS: SUBLUNAREAN VOIDS AND THE NATURE OF MARE EMPLACEMENT

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