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THEMIS observes possible cave skylights on Mars

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1] Seven possible skylight entrances into Martian caves were observed on and around the flanks of Arsia Mons by the Mars Odyssey Thermal Emission Imaging System (THEMIS). Distinct from impact craters, collapse pits or any other surface feature on Mars, these candidates appear to be deep dark holes at visible wavelengths while infrared observations show their thermal behaviors to be consistent with subsurface materials. Diameters range from 100 m to 225 m, and derived minimum depths range between 68 m and 130 m. Most candidates seem directly related to pit-craters, and may have formed in a similar manner with overhanging ceilings that remain intact.
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THEMIS observes possible cave skylights on Mars
G. E. Cushing,
1,2
T. N. Titus,
1
J. J. Wynne,
3
and P. R. Christensen
4
Received 16 May 2007; revised 23 July 2007; accepted 14 August 2007; published 15 September 2007.
[1] Seven possible skylight entrances into Martian caves
were observed on and around the flanks of Arsia Mons by
the Mars Odyssey Thermal Emission Imaging System
(THEMIS). Distinct from impact craters, collapse pits or
any other surface feature on Mars, these candidates appear
to be deep dark holes at visible wavelengths while infrared
observations show their thermal behaviors to be consistent
with subsurface materials. Diameters range from 100 m to
225 m, and derived minimum depths range between 68 m
and 130 m. Most candidates seem directly related to pit-
craters, and may have formed in a similar manner with
overhanging ceilings that remain intact. Citation: Cushing,
G. E., T. N. Titus, J. J. Wynne, and P. R. Christensen (2007),
THEMIS observes possible cave skylights on Mars, Geophys. Res.
Lett.,34, L17201, doi:10.1029/2007GL030709.
1. Introduction
[2] The existence, physical properties and possible bene-
fits of extraterrestrial caves have been a subject of scientific
discussion for as long as spacecraft have observed planetary
surfaces. Oberbeck et al. [1969] compared lunar and ter-
restrial lava-tubes, and Carr et al. [1977] examined similar
structures on Mars. Horz [1985] suggested using lunar lava
tubes as shelters for human habitation, and Boston et al.
[2004] discussed the same for Mars along with favorable
implications for astrobiology. Even now, engineers are
developing technologies to allow both human and robotic
missions to physically explore extraterrestrial caves [Boston
et al., 2004; Boston and Dubowsky, 2005]. Unfortunately,
cave detection by spacecraft is difficult because their instru-
ments have limited resolution and generally point nadir
(straight downward), requiring detectable cave entrances to
face skyward and be large enough to be resolved from orbit.
[3] The Martian surface experiences a range of signifi-
cant hazards. Micrometeoroids, solar flares, UV radiation,
high-energy particles from space and intense dust storms
regularly bombard the surface [Mazur et al., 1978; Kuhn
and Atreya, 1979; Frederick et al., 2000; Boston et al.,
2004; Schulze-Makuch et al., 2005], where absolute temper-
atures can double over a single diurnal cycle [Cushing et al.,
2005]. Caves may be among the only structures on Mars
that offer long-term protection from such hazards.
[4] We have identified seven candidate cave entrances
(skylights) located on or near the flanks of Arsia Mons
(southernmost of the three massive shield volcanoes of
Tharsis Montes, Figure 1). This region has widespread pit
craters and grabens, suggesting an abundance of subsurface
void spaces [Ferrill et al., 2003; Wyrick et al., 2004]. These
candidates may have formed in a manner comparable to pit
craters (which are usually found nearby) except that an area
of competent surface materials may have remained intact to
form a ceiling as subsurface materials collapsed and drained
into the subterranean voids below (see Figure 2 and auxil-
iary material).
1
These candidate skylights appear to descend
100 meters or more beneath the surface and may either
(1) open laterally into cavernous spaces; (2) plunge deeply
into subsurface faults; or (3) may not be caves at all, instead
being deep cylindrical shafts with sheer vertical walls. With
currently available data, we cannot determine which of
these cases is correct because THEMIS only observes from
nadir and can’t see whether the candidates have vertical or
subvertical walls. In any case, these are extremely unusual
and interesting features worthy of further investigation.
2. Observations
[5] The majority of data were collected by THEMIS,
which observes the surface from nadir at both visible and
thermal-infrared wavelengths. The thermal-infrared camera
(IR) observes nine bands ranging between 6.3 15.3 mm
with a spatial resolution of 100 meters per pixel. The
visible-wavelength camera (VIS) observes at 18 or 36 m/
pixel in 5 bands, though only band-3 images (0.65 mm)
are used for this investigation. THEMIS observes with both
VIS and IR cameras late in the afternoon (1500 1700 hrs,
when the sun has past transit and doesn’t shine as deeply
into the candidates). Only the IR camera is used for
predawn observations (03000500 hrs) which are nec-
essary to provide a diurnal range of thermal coverage
[Christensen et al., 2004].
[6] Cave entrances detectable by THEMIS are probably
rare because they must face skyward and have minimum
diameters of 100 m. While VIS observations can resolve
smaller features than this, they do not show sufficient detail
to verify definite cave-like characteristics. We therefore use
100-meter IR observations to confirm that each candidate
exhibits smaller amplitudes of diurnal temperature varia-
tions than the immediately surrounding terrain. This re-
quirement of minimum 100-m diameters and skyward
facing should seriously limit the number of cave entrances
detectable by THEMIS (the only thermal imaging system
currently orbiting Mars.).
GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L17201, doi:10.1029/2007GL030709, 2007
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Full
A
rticl
e
1
Astrogeology Team, U.S. Geological Survey, Flagstaff, Arizona, USA.
2
Department of Physics and Astronomy, Northern Arizona University,
Flagstaff, Arizona, USA.
3
Department of Biological Sciences, Northern Arizona University,
Flagstaff, Arizona, USA.
4
Department of Geological Sciences, Arizona State University, Tempe,
Arizona, USA.
Copyright 2007 by the American Geophysical Union.
0094-8276/07/2007GL030709$05.00
L17201
1
Auxiliary materials are available in the HTML. doi:10.1029/
2007GL030709.
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Figure 1. MOLA shaded relief map of Arsia Mons. Caldera floor is centered at approximately 239°E, 9°S. Locations of
the seven candidate cave skylights are labeled.
Figure 2. Seven candidate cave skylights: (a) Dena, (b) Chloe¨, (c) Wendy, (d) Annie, (e) Abby (1) and Nikki (2), and
(f) Jeanne. Arrows signify directions of solar illumination and of North. To facilitate our photoclinometry routine, each
candidate is map-projected with sunlight coming from the 9 o’clock direction.
L17201 CUSHING ET AL.: CAVE SKYLIGHTS ON MARS L17201
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[7] VIS images showing dark, circular features in the
midst of pit-crater chains gave our first indication of
potential skylight openings. These candidates are obviously
distinct from typical pit craters because of their lack of
sloped walls or visible floors, and they also lack the visible
characteristics (such as raised rims or ejecta patterns) that
would associate them with impact craters. Additionally,
thermal behaviors confirm these are not misidentified sur-
face features such as dark sand or rock (analysis section).
Illuminated upper-rims can be seen in several THEMIS VIS
images, and off-nadir observations from other orbiting
instruments (possibly HiRISE) will image these candidates
from the side, (and at much higher spatial resolution)
showing us the interior wall structure, and possibly provid-
ing ceiling thicknesses if such ceilings exist. Observations at
earlier times of day will allow us either to see the floors or
to constrain minimum depths to deeper values than we have
at present (analysis section). Observations of illuminated
floors will provide us with definite depth values, but will
not necessarily determine cave-like characteristics.
3. Analysis
[8] For a convenient aid in identification and visualiza-
tion, we informally identify these ‘seven sisters’ on Arsia
Mons as: Dena, Chloe¨, Wendy, Annie, Abby, Nikki and
Jeanne (Figures 1 and 2, Table 1). Diameters of these
candidates are estimated by taking an average of several
measurements using the 18 m/pixel resolution of
THEMIS VIS, and range between 100 252 meters. Only
minimum values of each candidate’s depth can be estimat-
ed because the floors are completely in shadow during
THEMIS observations. These minimum depths (d
min
)are
easily constrained using observed diameters (D) and their
respective solar incidence angles (i) where d
min
= D/tan(i).
Our ‘useful’ incidence angles ranged between 61.5° 69.9°
and returned respective d
min
values between 68101 m
(Table 1), though actual depths could be considerably
greater. We also model topographic profiles across each
candidate using a 1-dimensional photoclinometry (shape
from shading) routine described in the auxiliary material.
[9] The Mars Orbiter Camera (MOC) observed ‘Dena’
with a partially illuminated floor earlier in the afternoon
with an incidence angle of 40.15°(1430 hrs, Figure 3b).
This valuable observation allows us to tightly constrain the
floor’s depth at the edge of the shadow to 130 m while
the THEMIS minimum depth is only 80 m.
[10] THEMIS IR shows that diurnal temperature varia-
tions are much smaller in the candidate skylights than on
any of their surrounding surfaces. In Figure 4, notice how
‘Annie’ is warmer in the afternoon than the shadows of
adjacent collapse pits, while cooler than the sunlit portions.
Nighttime temperatures, meanwhile, are warmer than all
nearby surfaces. This behavior is consistent in all seven
candidates, and would be expected of cave interiors that
Table 1. Physical Parameters of Candidate Skylights
a
Name Longitude Latitude Diameter Incidence Angle Minimum Depth Minimum d/D Ratio Elevation THEMIS VIS ID
Annie 240.03°E 6.52°S 225 m 65.9°101 m 0.44 11055 m V18340001
Dena 239.02°E 6.31°S 162 m 63.6°80 m 0.49 9100 m V18053001
Jeanne 241.38°E 5.57°S 165 m 65.7°75 m 0.45 9970 m V18315002
Wendy 240.32°E 7.84°S 125 m 61.5°68 m 0.54 15500 m V17716001
Chloe 239.21°E 4.29°S 252 m 83.1°N/A N/A 5700 m V13448001
Abby 240.54°E 6.713°S 100 m 84.4°N/A N/A 11150 m V14334002
Nikki 240.55°E 6.708°S 180 m 84.4°N/A N/A 11150 m V14334002
a
Diameter, D; depth, d. Minimum depths are not calculated for observations with very high solar incidence angles (>80°). Minimum d/D ratio is the ratio
of minimum depth to the diameter of each candidate. Small impact craters on Mars typically have d/D ratios of about 0.2.
Figure 3. (a) A THEMIS observation of Dena at 18-m resolution with an incidence angle of 63.6°and a minimum depth
of 80 m. (b) A MOC observation (R0800159) at 6-m resolution with a partially illuminated floor (40.15°incidence
angle). The depth at the edge of the shadow is 130 m.
L17201 CUSHING ET AL.: CAVE SKYLIGHTS ON MARS L17201
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receive little or no daily solar insolation [Howarth, 1980;
Pflitsch and Piasecki, 2003].
[11] The candidates Wendy, Dena, Annie and Jeanne
have both VIS and diurnal IR coverage, and are large
enough to give distinct IR signatures at 100-m resolution.
Chloe¨ , Abby and Nikki have the same visible and thermal
characteristics as the others, and are also strong candidates,
but their minimum depths could not be constrained to useful
values because they were observed late in the afternoon
when sunlight comes from the side and doesn’t shine deeply
into them (observed incidence angles >83°).
4. Formation Mechanisms
[12] Most of the candidates are either adjacent to pit
craters, or are directly in-line with pit-crater chains, suggest-
ing similar formation processes, though pit craters consis-
tently show distinct walls that slope inward at the angle of
repose (30°). Recent investigations and some terrestrial
analogues suggest these pits are likely caused by the
drainage of loosely consolidated surface materials into deep
extensional fractures or faults that could reach down to 5 km
into the crust [Ferrill et al., 2003; Wyrick et al., 2004].
[13] Some terrestrial pit craters found in Hawaii are
visibly similar to our candidates [Okubo and Martel,
1998]. These form in young basalt with walls that collapse
inward over short geologic timescales. If these Hawaiian pit
craters are truly analogous to the candidates discussed here,
then seismic activity could possibly be an ongoing process
on Arsia Mons at the present time. It’s also possible that
these candidates are much older and fairly stable, and might
be a halted intermediate stage in pit-crater formation. In
either case, some of these Hawaiian pits such as ‘Devil’s
Throat’ [Okubo and Martel, 1998] may still be excellent
terrestrial analogues (the Hawaiian pits do not open into
cavernous spaces, but have been observed to crosscut lava
tubes). Some suggested causes for Martian pit-crater chains
include dike intrusion [e.g., Wilson and Head, 2002; Scott et
al., 2002], and collapsed magma chambers [Mege et al.,
2000]. See Wyrick et al. [2004] for a more detailed expla-
nation of possible pit-crater formation mechanisms. Regard-
less of the mechanism, there is general consensus that
subsurface void spaces with sufficient volume are necessary
to accommodate the immense volume of collapsed materi-
als, which may be up to tens of km
3
for a single pit [Wyrick
et al., 2004].
[14] Because Annie and Dena lie directly along the
centerline of pit-crater chains (and Abby, Nikki and Wendy
are immediately adjacent to similar chains), we propose
these candidates may have formed through a similar process
of collapse that has either halted or is still in progress. Local
topography indicates a possibility that these candidates have
intact ceilings above laterally extending cavernous spaces,
or may extend deeply into subsurface voids that have not
been completely filled by the collapsed materials. Chloe¨ and
Jeanne are unique from the other candidates because they
have no nearby collapse features to indicate a local presence
of subsurface void spaces. This could indicate a different
formation mechanism, but otherwise, their sizes, appearan-
ces and thermal behaviors are identical to the other candi-
dates. These two skylights may still open into subsurface
voids although local topography does not indicate such
spaces. Many cave skylights on Earth expose subsurface
voids such as lava tubes or sink-holes that are not otherwise
discernible by local terrain features [Calvari and Pinkerton,
1999; Miyamoto et al., 2005].
5. Conclusion
[15] Appearances and thermal behaviors of all seven
candidates are consistent with those we should expect from
skylight openings into subsurface cavernous spaces, and the
terrain surrounding most of the candidates indicates a likely
presence of subsurface voids. There is much more to be
learned about these features and future observations will
look into these candidates at different angles, different
wavelengths, higher resolutions and at different times of
day which may show internal structures and possibly
provide more clues about how they formed and why they
exist.
[16] As exploration targets, these candidates are too small
and too high in elevation (>5.7 km) to reach with our
current landing technologies. Astrobiological possibilities
may also be poor at these locations because if microbial life
ever did flourish on Mars, it may not have migrated to these
elevations. Furthermore, if these candidates happen to be
Figure 4. THEMIS VIS and IR images show diurnal thermal behavior of the candidate ‘Annie’. (left) The visible image,
(middle) an afternoon IR image observed concurrently with the VIS (1500 hrs), and (right) an early-morning observation
at 0400 hrs. This example shows the thermal behavior typical of all seven candidates.
L17201 CUSHING ET AL.: CAVE SKYLIGHTS ON MARS L17201
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cylindrical shafts without protective overhanging ceilings,
then microbial life would probably not survive due to direct
exposure to sunlight during those seasons and times of day
when the sun is directly overhead.
[17] Evidence of past fluvial activity has been docu-
mented in the Tharsis Montes region [Mouginis-Mark and
Christensen, 2005; Basilevsky et al., 2006], and water-ice
clouds are observed nearly every day of the year around
Arsia Mons [Benson et al., 2003]. Accordingly, further
research and modeling could give valuable insights about
whether these clouds affect conditions at the surface (or in
the candidate skylights). Additionally, possible evidence of
liquid water reaching the surface was recently identified in
Centauri Montes by Malin et al. [2006]. If this is true, then
caves found at lower elevations (probably below 0 km)
could hold some of this moisture, providing a potential
resource for future explorers and improving the possibility
of finding past or present microbial life.
[18] The discovery of potential skylight openings into
Martian caves is an exciting step towards exploration and
discovery. Future observations will provide more detailed
information and inspire deeper insights about the character-
istics and history of these features. A detailed search
covering other volcanic regions across Mars is currently
underway for similar targets—especially for those at lower
elevations which are easier to reach and have a greater
potential for holding some form of water and/or life. This
discovery challenges us with fresh insights and new possi-
bilities for the future of Mars exploration.
[19]Acknowledgments. This article was significantly improved by
insights given by Peter Mouginis-Mark, and from Windy Jaeger and Lazlo
Kesthelyi. Valuable ideas were also gained through conversations with
Danielle Wyrick, Jenny Blue and Kyle Winfree.
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P. R. Christensen, Department of Geological Sciences, Arizona State
University, P.O. Box 876305, Tempe, AZ 85287-1404, USA.
G. E. Cushing andT. N. Titus, Astrogeology Team, U.S. Geological Survey,
2255 N. Gemini Drive, Flagstaff, AZ 86001, USA. (gcushing@usgs.gov)
J. J. Wynne, Department of Biological Sciences, Northern Arizona
University, P.O. Box 5640, Flagstaff, AZ 86011, USA.
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... 100-225 m diameters, 68-130 m minimum depths. These prospective skylights may (1) open laterally into cavernous spaces, (2) descend deeply into underground faults, or (3) not cave at all but instead be deep cylindrical shafts with steep vertical walls (Cushing et al. 2007; Viúdez-Moreiras 2021). These minimal depths (dmin) are readily determined using measured diameters (D) and sunlight incidence angles (i), where dmin = D/tan (i). ...
... These minimal depths (dmin) are readily determined using measured diameters (D) and sunlight incidence angles (i), where dmin = D/tan (i). however, actual depths may be significantly larger (Cushing et al. 2007). Shielding relies on void geometry D and depth d. ...
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The uncovering of the actual cave of the Seven Sleepers tale through several scientific domains and a new method of Quran interpretation will result in a paradigm shift. We suggest that the Dog Cave near Naples, Italy is the actual cave for this tale. Moreover, the tale occurred in the period of the emperor Vespasian in 71 A.D., and the Seven Sleepers awoke in 380 A.D., the years of the confession of faith of the emperor' Theodosius I. We argue that the new approach will allow us to decode numerous mysteries and contribute to uncovering evidence in many scientific fields, including history, physics, cosmology, and geology. This study examines a heuristic approach to unraveling the tale of the Seven Sleepers. Associating what is stated in the legend with the historical record is critical, and uncovering the evidence that might give the idea of time travel and how this evidence could be formed via a new method of Qur'an interpretation is crucial. Also, attempt to figure out who the Seven Sleepers are. Because the restrictions of specific sources limited previous research of the Seven Sleepers narrative, the outcome was toward a definite place, emperor identity, and date of the tale's occurrence, despite the interest in uncovering the Quran's secret. A few scholars attempt to think outside the box to link Quranic results with scientific fields that consider the preliminary meaning of the word Quran. Furthermore, there are restrictions on the Arabic language rules. Introduction Revealing the actual cave of the Seven Sleepers will be an underground revolution for much of the evidence that is uncovered in the cave in many domains of physics, cosmology, and geology. In addition, it helps explain many concerns that have been lingering about hazy parts of history that have been attributed to myth and legend. For example, the Seven Sleepers have traditionally been seen by researchers in the scientific community as a myth concocted by the ancient people's creative minds. In the period from the 9th to the 13th-century, over 200 manuscripts study the legend of the seven sleepers. Previous work has only focused on the study of the legend's origin. The legend address historical records by identifying the period of two Roman emperors, Decius 251 AD to Theodosius II 448 AD(Archer 2016; Grysa 2015). Our knowledge of the tale of the seven sleepers is mainly based on some information. The research aimed to present a new approach, revealing the cave of the seven sleepers based on literature and linking it with information extracted from the Quran using a new interpretation process. Giving new light on the foggy history of that time and uncovering the evidence that compelled them to travel ahead in time and hypothesize how it formed. In this research, we shall refer to the Seven Sleepers as "tales" rather than "legends." This investigation will be divided into five main sections. The first section summarizes the study's purpose, contribution, and literature review. In the second section, we propose a new method for Quranic interpretation of the cave chapter relating to the tale of the seven sleepers. The third section will outline the advantages of our interpretation procedure and provide the results. The fourth segment emphasizes noteworthy findings. Finally, the last section contains our conclusion.
... Terrestrial planets elsewhere in the Solar System are known to contain similar pit craters features associated with basaltic lava flows. These pit craters have been located on the lunar surface (Greeley, 1971a;Haruyama et al., 2009;Kaku et al., 2017;Sauro et al., 2020), as well as on the surface of Mars Crown et al., 2019;Cushing et al., 2007;Sauro et al., 2020;Zhao et al., 2017). In some cases on the Moon these pits are indicative of more extensive subsurface void spaces that are interpreted as lava tubes based upon corroborating geophysical gravity and radar sounding observations Kaku et al., 2017). ...
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Chapter
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