Conference PaperPDF Available

Detection of Buried Empty Lunar Lava Tubes Using GRAIL Gravity Data

L. Chappaz
, H. J. Melosh
, K. C. Howell
, and C. Milbury
School of Aeronautics and Astronautics, Purdue Uni-
versity, West Lafayette, Indiana 47907,,
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
Figure 2: Local gradiometry (top), cross-correlation
(bottom) maps for free-air (left), Bouguer (center), and
free-air/Bouguer correlation (right) for Sinus Iridum
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-
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
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
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.
[1] Zuber et al. (2013) SSR 178, 1.
[2] Chappaz et al. (2014) AIAA 2014-4371.
[3] Haruyama et al. (2009), GRL 36, L21206.
... Frequently, however, the more relevant parameter is the existence and the depth of an interfacing boundary, such as the bottom of Europa's ice shell (e.g., Nimmo et al., 2003Nimmo et al., , 2007, the thickness of buried ice sheets on Mars (e.g., Bramson et al., 2015;Stuurman et al., 2016), the location of ice-rich materials on the Moon and/or Mercury (e.g., Rubanenko et al., 2019), or the ceiling and floor of lunar and Martian caves/lava tubes (e.g., Kaku et al., 2017;Chappaz et al., 2017;Sauro et al., 2020;Sood et al., 2016aSood et al., , 2016b. The case in Figure 1 has clearly demonstrated that, although noise dominates the measurement below 3.5 km, a boundary at 4.7 km can still be detected. ...
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We modified and justified the radar equation for ground penetration by including the backscattering effect. We propose a strawman system on an orbiter 50 km above the surface based on lunar conditions. The Tightly coupled dipole arrays antenna substantially reduces the size and mass and consists of an array with 8 × 8 cells on a light weight plate that is 1.6²‐m² wide with cells 0.25‐m high providing increased directionality and beam‐steering capability. The system includes 8 transmitters and 16 receivers that are able to measure wave polarization so that it is sensitive to subsurface water/ice. The system is controlled by central computers with transceivers all programmable providing more functionalities and flexibilities enabling systems of an even larger number of transceivers. It uses the synthetic aperture radar method to enhance the spatial resolution along the orbit track and has phase steering capability to increase the resolution in the cross‐track direction. The total transmitted power is 160 W. The radar is able to operate in three modes: a fine‐resolution mode that provides the full Multiple‐Input Multiple‐Output ultrawideband (MIMO UWB) capability with vertical range resolution up to 0.2 m, a deep penetration mode that is able to penetrate 100 m for high attenuation targets or over few kilometers for low attenuation targets, and a survey mode that provides overall characterization of the planet economically. This system can provide internal properties and structures of a target and detailed information about most useful resources such as ice, water, minerals, and shelter caves, for human space exploration on the Moon, Mars, and asteroids.
... The technique consists of identifying feasible set of parameters that yield the best fit model relative to the observed signal of a lava tube from the detection techniques [69]. In-depth forward modeling analysis is documented to support the existence of mass deficit consistent with buried empty lava tubes [70]. ...
... While a collection of smaller lava tubes could also produce a gravity signature that would match GRAIL observations, the general pattern of volcanic flows on the Moon is one characterized by a relatively small number of high-volume flows. The interpretation favored here and in Chappaz et al. (2014aChappaz et al. ( , b , 2016 and Sood et al. (2016a ), therefore, is that these gravity anomalies are each caused by a single, large vacant lava tube buried at some non-zero distance under the surface. ...
Mounting evidence from the SELENE, LRO, and GRAIL spacecraft suggests the presence of vacant lava tubes under the surface of the Moon. GRAIL evidence, in particular, suggests that some may be more than a kilometer in width. Such large sublunarean structures would be of great benefit to future human exploration of the Moon, providing shelter from the harsh environment at the surface—but could empty lava tubes of this size be stable under lunar conditions? And what is the largest size at which they could remain structurally sound? We address these questions by creating elasto-plastic finite element models of lava tubes using the Abaqus modeling software and examining where there is local material failure in the tube's roof. We assess the strength of the rock body using the Geological Strength Index method with values appropriate to the Moon, assign it a basaltic density derived from a modern re-analysis of lunar samples, and assume a 3:1 width-to-height ratio for the lava tube. Our results show that the stability of a lava tube depends on its width, its roof thickness, and whether the rock comprising the structure begins in a lithostatic or Poisson stress state. With a roof 2 m thick, lava tubes a kilometer or more in width can remain stable, supporting inferences from GRAIL observations. The theoretical maximum size of a lunar lava tube depends on a variety of factors, but given sufficient burial depth (500 m) and an initial lithostatic stress state, our results show that lava tubes up to 5 km wide may be able to remain structurally stable.
... Using gravimetric techniques to search for subsurface anomalies associated with candidate cave entrances may assist in solving this problem. Using Gravity Recovery and Interior Laboratory (GRAIL) mission data, researchers from Purdue University recently identified subsurface anomalies (presumed void spaces) associated with lunar pits [23]. Their results suggest some pits on the moon actually connect to large cave systems! ...
... Using gravimetric techniques to search for subsurface anomalies associated with candidate cave entrances may assist in solving this problem. Using Gravity Recovery and Interior Laboratory (GRAIL) mission data, researchers from Purdue University recently identified subsurface anomalies (presumed void spaces) associated with lunar pits [23]. Their results suggest some pits on the moon actually connect to large cave systems! ...
Lava tubes are buried channels that transport thermally insulated lava. Nowadays, lava tubes on the Moon are believed to be empty and thus indicated as potential habitats for humankind. In recent years, several studies investigated possible lava tube locations, considering the gravity anomaly distribution and surficial volcanic features. This article proposes a novel and unsupervised method to map candidate buried empty lava tubes in radar sounder data (radargrams) and extract their physical properties. The approach relies on a model that describes the geometrical and electromagnetic (EM) properties of lava tubes in radargrams. According to this model, reflections in radargrams are automatically detected and analyzed with a fuzzy system to identify those associated with lava tube boundaries and reject the others. The fuzzy rules consider the EM and geometrical properties of lava tubes, and thus, their appearance in radargrams. The proposed method can address the complex task of identifying candidate lava tubes on a large number of radargrams in an automatic, fast, and objective way. The final decision on candidate lava tubes should be taken in postprocessing by expert planetologists. The proposed method is tested on both a real and a simulated data set of radargrams acquired on the Moon by the Lunar Radar Sounder (LRS). Identified candidate lava tubes are processed to extract geometrical parameters, such as the depth and the thickness of the crust (roof).
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Solar particle radiation and galactic cosmic radiation pose a direct threat to crew safety on extended lunar and interplanetary missions. A few meters of rock are sufficient to reduce background radiation to a safe level. Rather than bringing heavy shielding material from Earth, or excavating material from the Moon’s surface to cover habitats, the quickest way to shield habitats from radiation would be to place the habitat in an existing cave or other naturally protected structure. Such natural structures exist on the Moon and Mars. Lunar and Martian Lava Tubes were formed as a result of volcanism during Late Heavy Bombardment (LHB) evolution period of our solar system some 300 million years ago. Lunar lava tubes can be several hundred meters wide and tall, and many kilometers long. One of the difficulties posed for lava tube exploration is ease of access. Until recently it was thought that the only way to access such structures was to drill into them. Recent high-resolution images of both the Moon and Mars show breaches in lava tubes called “skylights” that are large enough for entry. Data obtained by the GRAIL and LRO missions show that lava tubes exist on the Moon in relative abundance. There are likely many more that could not be detected or surveyed by the coarse sensors on these missions. In order to map and catalogue the size and locations of these lava tubes, this proposal calls for a two- phase dedicated discovery class mission to search for and survey lava tubes and subsurface caverns, while also prospecting for water ice and volatile deposits. Similar technology can be used to search for both lava tubes and volatiles. The first phase would be to use a satellite with Ground Penetrating Radar (GPR), allowing for high resolution data penetrating several kilometers below the surface. After gathering data by satellite, the second phase will be surface operations; using rovers to perform additional ground based GPR measurements on promising locations found from phase one, and follow this up by robotic exploration of the lava tubes themselves. Once optimal candidates in terms of accessibility, stability, and radiation protection are identified, long term crewed missions to the lunar surface can commence, knowing that astronauts will be protected from adverse radiation exposure while in their long duration tours of duty in lunar habitat modules. Upon refining this method on the Moon, it can also be used to search for and identify habitat locations on Mars, a crucial first step to establishing a permanent human settlement on the planet.
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Future permanent habitats on the Moon may facilitate space exploration by serving as an outpost for manned missions to other planets. Safety and resilience of those habitats are the main concerns, especially given the existing dangerous conditions and hazards such as temperature fluctuation, radiation, seismic activity, and meteorite impacts. Underground habitats in the form of “lava tubes” are good candidates for permanent human shelters because they provide immediate protection from such hazards. Evidence for their existence under the surface of the Moon is provided by GRAIL, SELENE spacecraft, and the LRO. Data from GRAIL suggests that the width of the lava tubes can be as large as 1–2 km. However, the size and geometry configurations under which the lava tubes are stable are not well-defined and there are no well-established criteria for their stability. The paper provides analytical and numerical …
Speculation on near-term scientific reasons for the exploration of lunar pits is offered alongside comments on possible longer-term human exploitation. It is proposed that in order to determine whether or not one or more of the pits offer access the large subsurface voids e.g. a non-collapsed lava tube, a preliminary reconnaissance mission solely focused on obtaining lateral images (and/or LiDAR maps) is needed. Possible concept options for such a preliminary reconnaissance mission are discussed. It is suggested that one of the best possible strategies is to employ a micro-sized probe (~0.3 m) that would hop from a nearby main landing spacecraft to the selected pit. After the surface position of the main lander is determined accurately, the probe would perform a ballistic hop, or hover-traverse, a distance of ~3 km over the lunar surface using existing propulsive and guidance technology capability. Once hovering above the pit, the probe or a separate tethered imaging unit would then be lowered into the pit to acquire the necessary subsurface void topology data. This data would then be transmitted back to Earth, directly, via the lander, or via a store-and-forward orbiting relay. Preliminary estimates indicate that a probe of ~14 kg (dry mass) is viable using a conventional hydrazine monopropellant system with a propellant mass fraction of less than ~0.2 (20%) including margins, suggesting a piggyback architecture would be feasible.
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