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  1. Is it possible that subterranean caves do/have existed on Mars?
  2. We know of the past existence of water on Mars. Which region (if any) of Mars would we likely find/expect to find a cave system below the surface?
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Apparently, this was investigated by NASA in the early 2000s in what became known as The Caves of Mars Project. Its goal was to find possible places for humans to live over extended periods of time, safe from the weather and radiation.

Boston et al. (2004) wrote a summary of the results:

  • There is strong evidence for subterranean caves formed from lava tubes on Mars - even under part of Olympus Mons!
  • The caves could be created by a number of different minerals, including "limestone" (likely just calcium carbonate, not like terrestrial limestone), basalt, and crustal rock. Ice could also have played a part in the formation.
  • Lave tubes are easily accessible once they are found. Tunnels and natural overhangs are likely to be found near mountains, and can be dangerous. There are polar structures (subice structures) that would provide shelter, but they're all the way at the poles.
  • The caves should be relatively uniformly distributed over the surface, although they should be situated more densely near areas of past lava flows and mountain ranges.

Cushing (2010) analyzed and published images of possible cave entrances based on the Odyssey and Mars Reconnaissance Orbiter missions.

Here are some possible entrances by a lava flow near Arsia Mons:

enter image description here

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  • $\begingroup$ Lava tubes in limestone? Really? $\endgroup$ – WhatRoughBeast Dec 7 '16 at 23:43
  • $\begingroup$ @WhatRoughBeast Those caves are not lava tubes, no. The bullets refer to caves as a whole. $\endgroup$ – HDE 226868 Dec 7 '16 at 23:48
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Undiscovered Caves

Just recently, a huge amount of frozen water was discovered below Mars' surface in the Utopia Planitia region--enough to fill Lake Superior, meaning 2,903 cubic miles. While this does not directly indicate any caves, it does point out how much of Mars' geology we have yet to explore. So, I'd bet the Caves of Mars project, as described by HDE 226868, could reveal more caves over time, if they exist.

Caves from Ice Mining

One likelihood, though, is that this ice will eventually be mined. The ice deposit in Utopia Planitia ranges in thickness from about 260 feet (80 meters) to about 560 feet (170 meters), but is only covered by 3 to 33 feet of surface soil.

If this ice were mined, the surface might be deemed necessary to leave intact, necessitating reinforcing the roof of the huge hole (approximately 2,900 square miles!) where the ice once was. In addition, in places where the ice is over 500 feet thick, sequential tiers or floors might be built as each ~20 foot deep layer was mined away. This could would give rise to enormous complexes of multiple-story man-made caves. Reinforcing of the roof of the mining area would be absolutely necessary if other deposits significantly deeper below the surface were mined.

So, these areas are likely future locations of man-made caves.

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Caves are likely rare at best on Mars

HDE226868 pointed to lava tubes as potential sources of caves on Mars, and to evidence for them. I do not dispute this at all, but when you picture a cave on earth, the chances are you are picturing a limestone cave, not a lava tube. Limestone caves are common because limestone is very slightly soluble in water, and the action of water will slowly dissolve limestone veins out of larger rock formations over hundreds of thousands of years.

While liquid water is not common on Mars, limestone is not present at all on Mars. It is not formed rarely formed by geological processes and usually by biological ones. Calcium carbonate can be formed by evaporation of a salty ocean, and it appears that it was formed this way on Mars after the oceans of the distant past dried up. However, concentrations of calcium carbonate large enough to make limestone formations are made from the crushed shells of marine invertebrates; if organic processes are not involved, other chemicals will be mixed in.

Lava tubes are also rare on Mars, because of its relative lack of volcanic activity. I am not sure what the latest scientific consensus is, but my impression is that volcanic activity on Mars as basically ceased over the last 500 million years. That is a long time, even on a geological scale, and plenty of time for erosion to collapse caves, even with the rare water and earthquakes of Mars.

In conclusion, there is evidence that there are caves on Mars, but I do not think that there are very many at all, because the processes by which form caves on Earth are uncommon on Mars. However, the lava tubes that may exist throughout the the Tharsis-Olympus region might be what you are looking for.

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  • $\begingroup$ So water is irrelevant when you don't want it to dig out caves but a big deal when you want it to collapse caves by erosion? ...? $\endgroup$ – Zxyrra Dec 8 '16 at 0:59
  • $\begingroup$ @Zxyrra Limestone is primarily from marine organisms. If it comes from inorganic deposition, it is generally referred to as travertine. If not from millions of years of crushed shells, what mechanism do you propose that could deposit cubic kilometers of nearly pure calcium carbonate? There are far too many other chemicals mixed into seawater for evaporating oceans to leave limestone deposits. The answer to your second question is yes; freeze-thaw cycles do lots of damage with little flowing water (and there is little on Mars) $\endgroup$ – kingledion Dec 8 '16 at 1:18
  • $\begingroup$ Edited your answer to the calcium carbonite part into the answer to clarify for other readers. As for "freeze-thaw cycles" - Why should this damage affect large underground caves if these cycles are in small streams on the surface? Collapse of existing caves caused by trickling streams on hills and the weight of an incredibly thin atmosphere is not a plausible mechanism to destroy hundreds of lava tubes, even given millions of years. $\endgroup$ – Zxyrra Dec 8 '16 at 1:39
  • $\begingroup$ @Zxyrra Cave collapses in Turkey have been determined to be caused by freeze-thaw cycles of groundwater in the rocks of the cave (ironically, a limestone cave in this case). It only takes a small amount of water to cause crack propagation that can collapse cave walls. I removed atmosphere and replaced with earthquakes as a more plausible mechanism for cave erosion. $\endgroup$ – kingledion Dec 8 '16 at 1:56
  • $\begingroup$ Not only did you just say tectonic activity stopped - so you can't suddenly resurrect it for earthquakes - but you also just said rock erosive enough to make caves isn't present. Turkey is completely irrelevant here; you cannot erode caves in these ways on Mars. $\endgroup$ – Zxyrra Dec 8 '16 at 2:01

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