Extraterrestrial caves

[nschorgh]

Hundreds of potential cave entrances have been cataloged on Mars and on Earth's Moon, and a data interchange format suitable for extraterrestrial and terrestrial caves simultaneously is an attractive option. Here I discuss various issues that I have encountered during implementation:

• Whereas one could have a dedicated field for 'planetary-body', it is simpler to use the 'country-name' field for the name of the planetary body. At this point of time, an international treaty guarantees that no planetary body can be owned by a nation, hence a country name is never appropriate for an extraterrestrial cave.

• A side-effect of using the 'country-name' field to indicate the planetary body, is that one would also like to have ISO2 or ISO3 country name abbreviations. ISO two-letter country codes have reserved codes starting with X for other uses. CaveXML assigns XO to the Moon, XA to Mars, and XT to Titan, the three bodies which have by far the most cataloged cave entrance candidates. However, there is quite a number of solar system bodies where someone has claimed detection of a potential cave entrance, so ultimately 26 two-letter codes will not be enough.

• Geographic longitudes on Mars and the Moon conventionally go from 0 to 360 degrees, not from -180 to 180. It's unclear what to do about it. Checking that longitudes are within range is a useful feature. Within XML 1.0 it is not possible to validate entries depending on other entries, and changing to XML 1.1 would be a major structural change. For planetary bodies there can also be a confusion about planetocentric and planetographic coordinates, and positive-east versus positive-west. (Currently, CaveXML requires all longitudes to be within -180 and +180, positive east.)

• Negative altitudes must be allowed. Cave candidates can be below the datum (zero elevation) level defined for a planetary body. Negative altitudes are already useful for underwater caves on Earth (usually at shallow depths), but many terrestrial cave databases do not allow them. They are definitely needed for planetary bodies, such as Mars. One also has to ponder whether expressions like ~-100 (approximately minus one hundred) should be allowed, which would readily lead to a syntactic confusion between ~100 and -100.

• The types of data available for extraterrestrial caves are rather different than for terrestrial caves. With very few exceptions, the available data are all based on orbital imagery. Among the very few exceptions are radar and gravity data, and, so far, all data are based on remote sensing methods rather than in-situ exploration, although in-situ exploration of lunar and martian caves is increasingly realistic over the next few decades. A core data element for catalogs of extraterrestrial caves are the type and size of entrances. Entrances are of interest on Earth too, for those who want to enter a cave, so these data have often also been collected for terrestrial caves. I envision that a classification of cave entrance types could be developed that can be simultaneously used for terrestrial and extraterrestrial caves.