by Emily Lakdawalla

May 23, 2007
from ThePlanetarySocietyWeblog Website

Today's set of image releases from the Mars Reconnaissance Orbiter HiRISE team included this one, of a fairly bland-looking lava plain to the northeast of Arsia Mons. Bland, that is, except for a black spot in the center.

Mars Reconnaissance Orbiter


What's that black spot?


It's a window onto an underground world.

Cave entrance on the flank of Arsia Mons


In this HiRISE image captured on May 7, 2007, a black spot mars the flank of Arsia Mons. The spot is most likely a skylight onto a subterranean cavern.

Credit: NASA / JPL / U. Arizona

This black spot is one of seven possible entrances to subterranean caves identified on Mars by Glen Cushing, Tim Titus, J. Judson Wynne and Phil Christensen in a paper they presented at the Lunar and Planetary Science Conference in March (read it far below).


Here's the figure from their paper that shows the seven caves, which they refer to by the names Dena, Chloe, Wendy, Annie, Abbey, Nikki, and Jeanne:

Possible cave entrances on Mars

Seven dark spots seen in Mars Odyssey THEMIS images could be the entrances to underground caves on Mars. The researchers who identified these caves have given them the following names:

  • Dena (-6.084 N, 239.061 E)

  • Chloe (-4.926 N, 239.193 E)

  • Wendy (-8.099 N, 240.242 E)

  • Annie (-6.267 N, 240.005 E)

  • Abbey & Nikki (-8.498 N, 240.349 E)

  • Jeanne (-5.636 N, 241.259 E)
    Credit: NASA / JPL / U. Arizona / G. Cushing et al. 2007

Their identifications were based upon Mars Odyssey THEMIS images, which achieve resolutions of a little better than 20 meters per pixel; having spotted the caves, they requested that the sharper-eyed HiRISE camera on Mars Reconnaissance Orbiter target the spots for more detailed imaging. The image above is the first one of these, and it shows the cave entrance called Jeanne.


So what more can we learn from the HiRISE image?


Let's check it out at full resolution (you'll have to click to enlarge for the full glory of 25 centimeters per pixel, a number I still goggle at every time I think about it).

Cave entrance on the flank of Arsia Mons

At its highest resolution of 25 centimeters per pixel, the HiRISE camera can see the detailed shape of the slightly scalloped edge of a hole on the flank of Mars' Arsia Mons (left), but no amount of image enhancement (right) can bring out any further details inside the hole. That means that the walls of the cave are overhanging - the cave is larger below the ground than the entrance we can see at the surface - and that it is very deep. Mars' dusty atmosphere produces enough scattered light that "skylight" would illuminate the floor of a shallow cavern well enough for HiRISE to detect it.

Credit: NASA / JPL / U. Arizona

The hope for the HiRISE images was that we could see some details from inside the hole. But as you can see by the highly stretched version at right, there is absolutely nothing visible inside that hole. It's black black black black black. HiRISE is a very sensitive instrument, and Mars' dusty atmosphere scatters quite a bit of light around, so there is certainly light entering that cave hole and bouncing around the interior.


But it seems that the cave is so big and so deep that almost none of the light that enters the cave comes out. It's deep, and it's big; the hole that we see really is just a skylight on a big subterranean room.


How big?


We'll never know for sure without visiting it, but I expect that Cushing and his coauthors and the HiRISE team will be crunching the numbers on the illumination conditions and the sensitivity of the camera to put a lower limit on how deep that cave must be for HiRISE to be able to see nothing at all inside it.


Think about that.


All these orbiters at Mars, and most of them are just seeing the surface and atmosphere. To be sure, there are two instruments up there - MARSIS on Mars Express and SHARAD on Mars Reconnaissance Orbiter - that are probing the shape of the subsurface with ground-penetrating radar. But neither of those instruments have the resolution necessary to tell us what the inside of this cave looks like. It might as well be in the greatest depths of space. Here there be dragons.


What's down there? Are there stalactites and stalagmites and crystals, or is it just a vast open room or tunnel?

Maybe these spots will be explored by Martian speleologists someday. But that day is a distant one, I'm sure. Earth speleologists are only now exploring some of the biggest holes in our world.





THEMIS Observes

Possible Cave Skylights On Mars

by G.E. Cushing1,2, T.N. Titus1, J.J. Wynne1,2, P.R. Christensen3,1

Lunar and Planetary Science XXXVIII


U.S.G.S. 2255 N. Gemini Dr. Flagstaff, AZ 86001

2Northern Arizona University, Flagstaff, AZ 86011

3Arizona State University, Tempe, AZ 85287

from LunarAndPlanetaryInstitute Website




Here we report the discovery of seven candidate skylight entrances into subterranean caverns (Figure 1). All seven are located on the flanks of Arsia Mons (southernmost of the massive Tharsis ridge shield volcanoes), a region with widespread collapse pits and grabens which may indicate an abundance of subsurface void spaces [1,2].




Subterranean void spaces may be the only natural structures on Mars capable of protecting life from a range of significant environmental hazards. With an atmospheric density less than 1% of the Earth’s and practically no magnetic field, the Martian surface is essentially unprotected from micrometeoroid bombardment, solar flares, UV radiation and high-energy particles from space [3,4,5,6].


Additionally, intense dust storms occur planet wide, and some regions exhibit temperature ranges that can double over each diurnal cycle [7]. Besides general geological interest, there is a strong motivation to find and explore Martian caves to determine what advantages these structures may provide future explorers. Furthermore, Martian caves are of great interest for their biological possibilities because they may have provided habitat for past (or even current) life [5,6,8].

Preserved evidence of past or present life on Mars might only be found in caves [5,6,8], and such a discovery would be of unparalleled biological significance [3]. Cave deep zones on Earth generally maintain constant climate conditions [9,10] which are ideal for the preservation of organic material.


Accordingly, Martian caves are among the most desirable targets for astrobiological exploration [11,12,13,14].




The Mars Odyssey Thermal Emission Imaging System (THEMIS) collected the majority of data for this study [15]. From a nadir perspective, THEMIS observes both visible and thermal-infrared wavelengths during the afternoon (~ 15001700 hrs), and IR wavelengths only for early-morning observations (~ 0300-0500 hrs.) [15].

The inspection of dark, circular pit-like features at visible wavelengths (VIS band 3, ~.654 μm) gave our first indication of potential skylight openings (nadir-pointing observations prevent us from determining whether these are caverns or deep vertical shafts).


To aid in visualization, we have informally named these ‘seven sisters’ on Arsia Mons as: Dena, Chloë, Wendy, Annie, Abbey, Nikki and Jeanne (Figure 1 below).

Most of the candidates are adjacent to collapse pits or are directly in-line with collapse-pit chains, and appear to have formed by similar processes.


They are visibly distinct from collapse pits, however, by a lack of visible (sunlit) walls or floors. These proposed skylights also lack the visible characteristics (such as raised rims or ejecta patterns) that would associate them with impact craters.


Thermal behaviors furthermore confirm they are not misidentified surface features such as dark sand or rock.

Diameters generally range between 100-252 m (estimated from THEMIS VIS at 18 m/pixel for most images). Only minimum depths can be calculated (because the floors are not illuminated by the sun in THEMIS observations) and range between 73-96 m (diameter ÷ tan(incidence angle)).


However, a fortunate MOC observation of Dena at ~2 p.m. (R0800159) actually does show an illuminated floor, allowing us to tightly constrain the depth using a 1-D photoclinometry routine. This routine returns a depth of ~130 m for the illuminated floor, while the minimum depth estimated from the THEMIS observation is only ~80 m.

Because THEMIS IR observes at 100-m resolution, cavern skylights with diameters much smaller than that are probably not thermally distinguishable from regular temperature variations on the surface.




Analyses of the candidates suggest they are not of impact origin, not patches of dark surface material, and are likely skylight openings into subsurface cavernous spaces. Visible observations show dark holes with sufficient depth that no illuminated floors (incidence angles ≥ 61.5°) can be seen from a nadir perspective (Thermal-infrared data suggest temperatures inside these features remain nearly constant throughout each diurnal cycle.)


Figure 2 shows afternoon temperatures for Annie that are warmer than the shadows of adjacent collapse pits, and cooler than sunlit portions. Meanwhile, nighttime temperatures for this candidate are warmer than all nearby surfaces.


Such is the behavior we would expect of a cavern floor that receives little or no daily solar insolation [9,10].

Wendy, Dena, Annie and Jeanne are the strongest candidates because they have the most complete data sets; i.e., they have both VIS and diurnal IR coverage, and they are large enough to be clearly identified at 100-m resolution. Chloë, Abbey and Nikki are also strong candidates because they have the same visible and thermal characteristics as the other candidates.


Their minimum depths could not be constrained, however, because of late-afternoon observations when the sun is too low to shine deeply into the pits.





Additional observations are necessary - particularly those at different times of day and from an off-nadir perspective. These candidates cannot be physically explored with our current state of technology because the targets are too small and specific, and the atmosphere at these elevations is too thin for a lander to slow down or maneuver sufficiently to reach them.


The astrobiological significance may also be reduced at these elevations because microbial life, if it ever existed on Mars, may not have occurred at these elevations. However, possible evidence of liquid water at the Martian surface was recently identified by Malin, et al. (2006) [16]. If liquid water does exist at or near the surface, then caves at lower elevations could hold natural reservoirs, greatly improving the possibilities for past or present microbial life.

The discovery of potential skylight openings into Martian caves is an exciting step towards future exploration and discovery. New spacecraft orbiting Mars, with greater observational capabilities, can observe these candidates at higher resolutions, at different wavelengths. Future observations will provide more substantial information about the characteristics and history of these features. A planet-wide search for similar targets is currently underway - particularly for those existing at lower elevations.


This discovery presents us with new insights and new challenges for the future of Mars exploration.





[1] Ferrill, et al. (2003) LPSC XXXIV;

[2] Wyrick, et al. (2004) JGR, 109(E6);

[3] Mazur et al. (1978) Space Sci. Rev. v.22, 3-34;

[4] Kuhn and Atreya (1979) J. Mol. Evol. v.14, 57-64;

[5] Boston, et al. (2004) STAIF v.699 1007-1018;

[6] Schulze-Makuch et al. (2005) JGR, 110(E12);

[7] Cushing and Titus (2005) GRL, v. 32;

[8] Frederick (2000) Concepts and App. for Mars Exp. 114;

[9] Tuttle and Stevenson (1978) Nat. Cave Mgmt. Symp. Proc.;
[10] Howarth (1980) Evolution v.34;

[11] Grin et al. (1998) LPSC XXIX;

[12] Boston (2000) Geotimes 45(8) 14-17;

[13] Boston et al. (2001) LPSC XXXII;

[14] Parnell et al. (2002) Astrobio. v.2(1), 43-57;

[15] Christensen (2004) Space Sci. Rev. v.110(1);

[16] Malin (2006) Science v.314 1574-1577.



New View of Dark Pit on Arsia Mons


August 29, 2007

from HiRISE-HighResolutionImagingScienceExperiment Website


Click above image