It's been believed for a long time that Mars is likely to be a planet with a lot of caves on it -- perhaps almost as many as on Earth. There are several reasons for this. To begin with, Mars started out with a large supply of surface water, like Earth -- but that water met a different fate.

During Mars' early "Noachian" Era, it had a dense enough carbon dioxide atmosphere that there may have been enough of a greenhouse effect to keep a good deal of that water unfrozen.

But after Mars lost almost all its atmosphere at the start of its "Hesperian" roughly 3.5 billion years ago, its surface chilled down dramatically, and almost all of its large surface water supply froze into a "cryosphere": a zone extending down up to several kilometers below Mars' surface that is riddled with ice, some of it mixed evenly into the ground as permafrost, some of it concentrated in large buried bodies of ice.

But there are still occasional waves of geothermal heat from Mars' diminishing supply of volcanic activity that seem to melt some of that buried ice back into underground liquid water.

The Hesperian Era, which ended somewhere around 2 to 3 billion years ago, was characterized by frequent utterly gigantic "outbursts" of liquid water from underground -- sudden floods which were very brief, but involved amounts of water up to hundreds of times greater than any floods Earth has ever undergone.

The cause of these is still something of a puzzle; but there's a good chance that at least some of them were caused when remaining huge underground pockets of still-unfrozen liquid water were surrounded by the steadily growing solid buried ice of the cryosphere, and the expansion of that water as it newly froze into ice compressed the remaining pocket of trapped liquid water to tremendous pressures until it finally burst through the surrounding wall of ice at some point and spurted violently out across the Martian surface.

There are also much smaller "valley networks" that have been carved on Mars by much smaller, long-lived flows of water-- and it appears that a lot of them may have been formed not as surface runoff channels of liquid rainwater or snowmelt but by "groundwater sapping": the much slower, gentler trickling of water through the subsurface, eroding a slowly growing pipeline through the ground until the hollowed-out roof above the channel finally collapsed into it.

Most of these channels were formed during the warmer Noachian, but there are some later ones that tend to be concentrated around volcanoes and have quite obviously been formed by geo thermally melted subsurface ice.

Indeed, a lot of the smaller "gullies" scattered along some Martian slopes may well have been carved by brief outbursts of subsurface liquid water -- heated either geo thermally or by surface climate changes -- from the walls of the slopes.

There's also good reason to think that Mars has some chemically carved caves. On Earth, caves in limestone -- which is the layered fossil remains of enormous numbers of plankton shelled with calcium carbonate -- are carved because groundwater, made mildly acid by atmospheric CO2 dissolved in it, slowly dissolves the carbonate.

Mars has no limestone beds, and indeed seems to have no surface carbonates at all -- but it does appear to have some inorganically formed carbonate minerals buried underground.

And, in fact, the reason that it has no surface carbonates, as we now know, is precisely because after its initial thick atmosphere disappeared, the planet's lack of an ozone layer meant that solar ultraviolet light could all the way down to the surface and set off reactions in the sulfurous gases vented from its volcanoes to form large amounts of sulfuric acid.

That acid, in turn -- which acts as a very efficient antifreeze -- could mix with the planet's surface water ice to form a potent acid solution, much stronger and more plentiful than any acidic groundwater on Earth, which not only destroyed the planet's surface carbonates but could eat its way quite efficiently through most of Mars' other ground minerals, turning them into sulfate salts and a remaining digested sludge of fine silicate particles, as has happened at the Meridiani plains where the Mars Rover "Opportunity" landed.

(There are actually a few long chains of small, shallow depressions photographed by the rover which some observers think might possibly be tiny sinkholes over the tops of such subsurface tunnels.) And so ancient acid-carved caves may actually be a lot more plentiful on Mars than on Earth, simply because they were carved there by a more potent form of acid.

However, the caves that we have now discovered fall into still another category: lava-flow caves. "Lava-tube" caves and channels are quite common on Earth.

They're made when the upper layer of a flow of lava running along the surface cools and solidifies into a surface crust which acts as a roof over the current of still-liquid lava running along underneath.

(This is usually basalt lava, which is much "runnier" than the gooey granite-based lava found in most continental volcanoes.)

And since basalt is the type of lava usually found on the Solar System's other worlds, surface "rills" (channels left behind after the remaining liquid lava finally drained out of the tube, and its roof later fell in) are common there too: they've been identified on the Moon, Venus, Jupiter's volcanic moon Io -- and Mars.

In particular, they are common on the slopes of the four gigantic "shield volcanoes" located along the top of the continent-like Tharsis Bulge on one side of Mars. Mars, despite being a smaller planet than Earth, tends to go in for Cecil B. DeMille-type surface features dwarfing their similar counterparts on Earth.

This is true of its flood outburst channels, and of its 4000-km long and 10-km deep "Valles Marineris" that splits one whole side of the planet and could easily fit the Grand Canyon into any of its great tributary canyons.

It's also true of Mars' shield volcanoes, which were formed in the same way as the volcanic Hawaiian Islands, but are several times wider and taller. In both cases, a "plume" (an upwards-flowing current of basalt lava from deep in the planet's interior) punched through its surface crust and laid down a wide, shallowly sloping heap of basalt lava from the plume's surface mouth.

But since Earth's crustal plates are slowly but constantly sliding along sideways, the Hawaiian Islands were all laid down as the flow of lava from a single plume pulsed on and off while the plate of ocean-floor crust above slid sideways, so that the plume formed a series of smaller individual volcanic shields running in a horizontal chain like the puffs of smoke from the stack of a moving locomotive.

Mars' crust, however, doesn't slide sideways -- so the four plumes at the center of the volcanic Tharsis region each simply piled up more and more and more lava at a single location before their volcanism finally died away as recently as a few hundred million years ago.

As one might expect, the lava flows on the sides of these gigantic volcanoes possess a huge number of surface grooves that are clearly former lava tubes, now exposed by the collapse of their roofs.

(Indeed, last November the very sharp "HiRISE" camera on MRO (the Mars Reconnaissance Orbiter) located a lava-tube rille in the Tartarus Colles lava-flow region elsewhere on Mars which still has a 23-meter-long section of its original roof still standing, forming a natural arch over the 40-meter-wide channel.)

The Tharsis volcanoes also have a large number of "collapse pits" on their slopes that are clearly not regular impact craters, and many of these are arranged in rings curving sideways around the volcanoes.

However, one study by the Southwest Research Institue's Danielle Wyrick finds that many such pit chains follow faults running down the slopes -- suggesting that they're due not to the cave-ins not of lava-tube roofs, but by "extensional faults" where the underlying rock has been stretched by strain to produce a deep underground fissure.

On many of these, the central segment of overlying rock falls completely down into the crevasse to produce a flat-floored "graben"; in others, the less-stretched top layer of rock stays largely intact above the fissure, except for individual spots where it falls a short distance down into the fissure to produce the pits. In any case, however, these pits can't be called recognizable cave openings -- they're simply caved-in funnels of surface rock.

But we have now definitely found cave openings on the slopes of Arsia Mons ("Mt. Arsia", the southermost of the four great Tharsis shield volcanoes) -- and I'll describe that dramatic discovery, and its implications, in the conclusion of this article.

Email This Article

Related Links
Mars Reconnaissance Orbiter
Space Blogs at SpaceBlogger.com