Detection of Buried Empty Lunar Lava Tubes Using GRAIL Gravity Data

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DETECTION OF BURIED EMPTY LUNAR LAVA TUBES USING GRAIL GRAVITY DATA. R. Sood
1
,
L. Chappaz
1
, H. J. Melosh
1,2
, K. C. Howell
1
, and C. Milbury
2
.
1
School of Aeronautics and Astronautics, Purdue Uni-
versity, West Lafayette, Indiana 47907, rsood@purdue.edu,
2
Earth, Atmospheric and Planetary Science, Purdue
University, West Lafayette, Indiana 47907.
Introduction: As a part of NASA's Discovery
Program, the Gravity Recovery and Interior Laboratory
(GRAIL) spacecraft were launched in September 2011.
The sister spacecraft, Ebb and Flow, mapped lunar
gravity to an unprecedented precision [1]. High resolu-
tion data is currently being utilized to gain a greater
understanding of the Moon's interior. Through gravita-
tional analysis of the Moon, subsurface features, such
as potential buried empty lava tubes, have also been
detected [2]. Lava tubes are of interest as possible hu-
man habitation sites safe from cosmic radiation, mi-
crometeorite impacts and temperature extremes. The
existence of such natural caverns is now supported by
Kaguya's discoveries [3] of deep pits that may poten-
tially be openings to empty lava tubes. In the current
investigation, GRAIL gravity data collected at different
altitudes is utilized to detect the presence and extent of
candidate empty lava tubes beneath the surface of the
lunar maria.
Detection Strategy: Previous work done by Chap-
paz et al., (2014) makes use of two detection strategies
based on gradiometry and cross-correlation to detect
subsurface features. Gradiometry technique encompass
the calculation of the gravitational potential from a
spherical harmonics data set. Specific truncation and
tapering are applied to amplify the signal correspond-
ing to the wavelength of the structures. By calculating
the second partial derivatives of the potential function,
the Hessian of the gravitational field is formulated. The
largest eigenvalue and corresponding eigenvector asso-
ciated with the Hessian determine the direction of max-
imum gradient. A secondary detection strategy, cross-
correlation, utilizes the individual track data based on
the relative acceleration between the two spacecraft as
they move along their respective orbits. The gradiome-
try and cross-correlation detection techniques are ap-
plied to localized regions. Gravity models up to degree
and order 1080 with predetermined truncation and ta-
pers are utilized.
Detecting Underground Structures: The objec-
tive of our analysis is to determine the existence of
underground empty structures, specifically lava tubes.
Within this context, several regions in the maria with
known sinuous rilles are considered, in particular a
region around the known skylight of Marius Hills
(301–307°E, 11–16°N). Cross-correlation analysis of
this region is shown in Figure 1, with the red dot mark-
ing the location of a known skylight along the rille.
The bottom-left map in Figure 1 corresponds to the co-
Figure 1: Free-air and Bouguer cross-correlation maps
and free-air/Bouguer correlation along with regional
topography in the vicinity of Marius Hills skylight.
rrelation between free-air and Bouguer maps where a
strong correlation (red) is indicative of potential under-
ground features. However, the structures that are the
object of this analysis are a similar or smaller scale
than the resolution of the gravity data. It is therefore
challenging to determine whether a signal observed on
an eigenvalue or cross-correlation map is, in fact, the
signature of a physical structure or is a numerical arti-
fact. To assess the robustness of an observed signal,
rather than considering a single simulation, several
different spherical harmonic solutions truncated be-
tween various lower and upper degrees are considered
to produce a collection of maps. The cross-correlation
maps in the top row and the bottom-left of Figure 1
yield an averaged map over a few hundred simulations.
The bottom-right map provides a visual reference for
the regional topography along with elevation in the
vicinity of Marius Hills skylight.
The capability of both strategies to identify subsur-
face anomalies has led to the detection of additional
candidate structures within the lunar maria. Figure 2
corresponds to a region around a newly found lunar pit
in Sinus Iridum. The top row of Figure 2 illustrates the
corresponding local averaged maximum eigenvalues
for the free-air, Bouguer potentials, and the correlation
between the two. The red dot marks the location of a
newly found pit/skylight (331.2°E, 45.6°N) within Si-

nus Iridum. The pit itself is approximately 20 m deep
with central hole of 70 m x 33Dde m and an outer fun-
nel of 110 x 125 m. The maps overlay local topogra-
phy, and the color represents the signed magnitude
corresponding to the largest eigenvalue of the Hessian
derived from the gravitational potential. Both free-air
and
Figure 2: Local gradiometry (top), cross-correlation
(bottom) maps for free-air (left), Bouguer (center), and
free-air/Bouguer correlation (right) for Sinus Iridum
pit.
Bouguer eigenvalue maps show gravity low in the vi-
cinity of the lunar pit. The correlation map distinctively
marks the region near the pit as a region of mass deficit
with a potential access to an underground buried empty
lava tube. The cross-correlation technique applied is
shown in the second row of Figure 2. The schematic
shows that for both free-air and Bouguer cross-
correlation maps, the anomaly is detected in the same
region as via the gradiometry technique. Both tech-
niques provide evidence for a subsurface anomaly in
the vicinity of the newly found lunar pit.
Free-air and Bouguer Gravity Anomaly: Contin-
uing the validation of the subsurface anomaly, regional
free-air and Bouguer gravity maps are generated. Fig-
ure 3 illustrates local maps for the free-air gravity on
the left and Bouguer gravity on the right. On closer in-
Figure 3: Local free-air (left) and Bouguer (right) grav-
ity map for Marius Hills skylight with overlay of to-
pography.
spection, the two gravity maps demonstrate a gravity
low surrounding the rille along which the Marius Hills
skylight lies. The Bouguer low adds to the evidence
suggesting a potential buried empty lava tube along the
rille with an access through the Marius Hills skylight.
Similar free-air and Bouguer gravity analysis is car-
ried out for the newly found pit in Sinus Iridum as
shown in Figure 4. The color bar is adjusted to visually
Figure 4: Local free-air (left) and Bouguer (right) grav-
ity map for the newly found lunar pit in Sinus Iridum
with overlay of topography.
distinguish the region in proximity to the lunar pit in
Sinus Iridum. The gravity low shown in both the free-
air and Bouguer gravity suggest an underground mass
deficit in the vicinity of the pit. Although the pit itself
is relatively small, it can potentially be an access to a
larger underground structure as evident from the gravi-
ty maps and the two detection strategies. Additional
maps have also been studied to identify a possible con-
nection of this anomaly to a buried empty lava tube
structure.
Conclusions: Two strategies are employed to de-
tect small scale lunar features: one based on gradiome-
try and a second one that relies on cross-correlation of
individual tracks. The two methods have previously
been validated with a known surface rille, Schröter’s
Valley. Then, a signal suggesting an unknown buried
structure is observed in the vicinity of Marius Hills
skylight that is robust enough to persist on a map creat-
ed from an average of several hundred simulations. A
similar signal is also observed in the vicinity of the
Sinus Iridum pit suggesting a possible subsurface mass
deficit.
The technique has been extended to cover the vast
mare regions. Multiple new candidates for buried emp-
ty lava tube structures have been discovered as a part
of this study. Some of the candidates bear no surface
expression but similar signals are observed from both
the detection strategies as observed for candidates with
surface expressions, i.e., skylights/pits.
References:
[1] Zuber et al. (2013) SSR 178, 1.
[2] Chappaz et al. (2014) AIAA 2014-4371.
[3] Haruyama et al. (2009), GRL 36, L21206.