Either they are holes in the roofs of "lava tubes" or hollow chambers produced when a solid crust forms on top of a flow or pool of liquid lava, and the remaining molten rock then drains out from underneath -- or else they are "collapse pits" produced where very deep vertical crevasses formed in the solidified underground lava of the volcanic slopes due to strain faults, leaving a still-unbroken layer at the surface which has caved in here and there to produce skylight openings into the deep crevasse underneath.

Seven of them were tentatively located by the THEMIS camera on the 2001 Mars Surveyor orbiter (which is still working well). They were holes in the surface 100 to 250 meters wide where the floor of the hole was not clearly visible, as it is on any meteor crater or collapse pit -- instead, they looked downright black inside. And the infrared camera on THEMIS found that they were colder than the surrounding surface in the daytime, and warmer at night -- exactly as one would expect if we were looking at the mouths of deep caves whose internal temperature would tend to stay stable, instead of seesawing dramatically up and down over the day-night cycle as happens on the surface of Mars with its thin atmosphere.

Finally, one photo from the sharper main camera on Mars Global Surveyor -- which only recently failed after nine extremely spectacular years orbiting Mars -- did manage to catch a glimpse of the underlying floor beneath one hole, fully 130 meters below the hole's sharp rim.

The really dramatic visual proof of what we were looking at, however, came only late last month with an extremely detailed closeup of the 120-meter-wide hole nicknamed "Jeanne", taken by the HiRISE camera on MRO. While THEMIS has a resolution of 18 meters, HiRISE can see details on the Martian surface as tiny as 1/4 meter. What it showed was an amazing sight -- a round hole with a sharp, cookie-cutter-neat rim surrounding an utterly black abyss, confirming that the circular hole must open into a deep underlying chamber (although as yet the HiRISE team hasn't finished analyzing the blackness of the hole's interior at that particular time of day to give us a minimum estimate of its depth).

And the illuminated bevelled edge on the hole's rim is clearly very narrow -- indicating that the solid-rock roof over the chamber, through which the hole punches, must itself be very thin. That thinness suggests to this writer that in this case we're looking not at a hole over a fissure produced by a stretched extensional fault, but at a caved-in hole in the thin roof of a genuine and big underground chamber or tunnel produced by the drainage of liquid lava.

The obvious next step is to have MRO examine Jeanne and the other holes when the Sun is highest in the sky, to try and see the illuminated floor of the underlying chambers as MGS has already seen it for the "Dena" hole. And the next step after that is to examine all the photos taken by the recent four American and European Mars orbiters to look for other cave mouths all over Mars, not only on lava flows but in other places where the caves might have been carved by the other water- or acid-related processes we've mentioned.

But -- aside from their novelty value -- why is it worthwhile to look for Martian caves? Because they are one of the more promising places to look for evidence of Martian life. Their interiors are shielded from the savage solar ultraviolet light that blasts Mars' unprotected surface, and chagne much less in temperature over the brutal Martian day/night cycle (as we've seen). And if they were carved by either water or volcanic processes, there's a genuine chance that at least a few of them might still come in contact with subsurface ice or with geothermally warmed subsurface liquid water (as well as with methane and other volcanic gases that could serve as a food source for bacteria).

Such isolated pockets of still-liquid subsurface water are the most promising place on Mars to find still-living Martian microbes -- for, even if the water is usually frozen solid, germs could survive in suspended animation in it for extremely long periods, waiting to return to life and reproduce during those rare, brief periods when a resurgence of local geothermal heat (or even surface Martian climate changes) melt the ice.

There's another good reason to investigate Martian caves. Their interiors, being below the Martian surface are likely to always be below freezing, even during those rare periods in Mars' post-Noachian history when climate changes (resulting from major volcanic eruptions, or from the fact that the planet's axial tilt keeps slowly increasing and decreasing) may briefly give it enough additional air pressure and warmth for liquid water to exist again on some parts of its surface during the daytime.

And so any liquid water that trickles down into the caves from the surface during such periods will get trapped and permanently frozen inside the cave (as often happens in cold places on Earth, sometimes entirely filling the cave) -- providing a permanently stored record of the chemical composition of that liquid water and the substances dissolved in it. This could allow us to obtain layered samples of ice indicating the climate conditions and atmospheric composition of Mars during such times. Alternatively, if such caves contain ice that was last liquid water all the way back during the Noachian (as a lot of the current underground ice on Mars probably is), this ice, stored inside the cave for eons, could still contain perfectly preserved frozen samples of microbial life that existed on Mars' surface during those (relatively) balmy days.

There's another problem, though: Martian caves today have the same very low air pressure inside as on the rest of post-Noachian Mars' surface -- so low that it is only rarely possible for liquid water to exist on Mars without very quickly evaporating into vapor, even when it is warm enough for ice to thaw. Thus the interiors of caves might very well never contain any liquid water that could allow organisms to return to life there (unlike sealed pockets of deeply buried liquid water elsewhere on Mars, which can only be sampled by deep and difficult drilling operations). And the caves found on the slopes of Mount Arsia are in one of the highest-altitude places anywhere on Mars -- which also means that even during Mars' most halcyon ancient days, little or no liquid water may ever have trickled down from the local surface into them to freeze into preserved samples of the Noachian.

Martian caves at lower altitude, however, could be a different matter -- especially if the caves themselves run deep beneath the surface of Mars, so that the air pressure in their rooms is higher than that on the surface. (The air pressure on most of the surface of Mars is already teetering on the very brink of the level that could allow liquid water to exist for lengthy periods of time.) R.D. Frederick of the Oregon L-5 Society has pointed out that Olympus Mons -- the biggest of the four great Tharsis volcanoes -- has its lowest slopes bordering right on what some scientists think are features left behind by ancient Noachian flood plains. So Martian caves at lower altitudes than this first bunch to be discovered would still be very much worth exploring.

But how do we do something as difficult as exploring a Martian cave with robots, even if you initially set down the lander near the cave mouth? Penelope Boston of New Mexico Tech, who has put a great deal of thought into the scientific potential of Mars caves, actually proposed -- for the round of competitive proposals made for the first Mars Scout mission this year -- a mission called "Mother Goose", in which such a lander would dispatch a six-legged walker robot to stroll up to the cave's mouth, make detailed analyses of the materials there, and release a flock of ten tiny spider-like "gosling" robots weighing on a few kg each that would automatically walk further into the cave, radioing back pictures and simpler chemical analyses of what they found there.

That proposal was rejected as being unlikely to succeed within the Mars Scout program's low cost cap, but Boston is still developing the overall concept -- including one variant in which the goslings would be tiny balls that would actually bounce themselves around inside the cave, allowing a lot more of them to be carried.

At any rate, now that we know beyond doubt that Martian caves do exist, at some point -- even if it's in the rather distant future -- they will certainly be worthy of exploration. And, after seeing that remarkable MRO photo of Jeanne Cave's mouth, one can only wonder how many other intriguing features Mars is still concealing from us.

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