[Widdekind]

Please allow me to link my to previous efforts:

http://www.astronomyforum.net/plane...netary-cooling-times.html

[Geosciences-Forum.com]

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[Widdekind]

According to the National Geographic documentary Naked Science -- Super Diamonds (TV), natural diamonds are up to 50% softer than artificial "Industrial Diamonds", which are manufactured at higher pressures.

WILD SPECULATION: Smaller and less massive planets, by producing less Surface Gravity, will produce lower Lithospheric [sic?] pressures, and, hence, softer & weaker diamonds. Conversely, bigger planets will tend to produce harder diamonds.

[Geosciences-Forum.com]

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[Widdekind]

Melting Glaciers can cause previously dormant Volcanoes to erupt, by reducing the Pressure pushing down on their Magma Chambers*. Indeed, sudden Pressure drops trigger explosive eruptions**.

* National Geographic Channel Naked Science -- Ice Age Meltdown (TV)
** History Channel How the Earth was Made -- Krakatoa (TV) ; How the Earth was Made -- Yellowstone Super-Volcanoe (TV)

WILD SPECULATION: On large, massive, High-Gravity worlds, where Pressure increases with Depth more rapidly, Volcanism may be more suppressed. Conversely, on smaller, Low-Gravity worlds, Volcanism may be more easily triggered.

[Widdekind]

The Moon and Mars are both comparatively small, Low-Gravity worlds. As such, they are less dense than the Earth.


CONCLUSION (?):

Cave Systems on Low-Gravity worlds will tend to be bigger than those on Earth. Thus, early Space Colonies could likely be built in such huge Caves.

Indeed, Mankind lived in Caves for hundreds of thousands of years, specifically b/c Caves make excellent shelters.


ANALYSIS:

Cave Systems can be created by Water [liquid water] & Lava [liquid rock]*. Now, all Rocky Worlds have Volcanic Activity, at least early in their Planetary Lifecycles (previous posts). Thus, Volcanic Cave Systems are quite common, and tend to increase in size, w/ decreasing Surface Gravity (and increasing Volcanic Activity, see footnote):

On the Moon, several "Sinuous Rilles" are probably Lava Tubes or channels that fed the vast Basaltic [Magma-rocks] Lava Lakes that once pooled in the Maria (dark Lunar "seas"). The Apollo 15 Astronaughts landed next to a huge Rille, Hadley. They stood at the canyon's rim, and photographed the Basalt layers exposed on the opposite wall, but the slopes were too dangerously steep to enable them to descend and sample them. Almost a mile wide, and a thousand feet deep today, Hadley's lava torrent probably melted its way through several older Lava Beds when it formed 3.3 billion years ago. Some portions of the Lava Channel are still roofed over, forming a Lava Cave, or Tube. The greater volume of Lunar Basalt Eruptions, combined w/ the Moon's lower gravity, created spectacular lava-cut valleys that dwarf those on earth...

[A large] Lava Flow cut the 75-mile [long] Hadley Rille, target of the Apollo 15 Moon Landing. Far longer than any found on Earth, several of the Moon's Rilles are visible from Earth through telescopes**.

Thus, there may be dozens of miles of apparently stable (having survived for over 3 billion years of Impacts & Moon Quakes) "pre-fabricated" Cave Systems, of possibly immense scale (mile-wide & 1000' tall (!!)), on the Moon.

* http://en.wikipedia.org/wiki/Caves
** Tom Jones & Ellen Stofan. (National Geographic) Planetology, pp. 120-121. Even larger Volcanic formations exist on Venus, including a 4200 mile Lava Flow Channel, the longest such structure in the Solar System. In the absence of full-fledged caves, Canyons, Cliffs, & Rock Shelters would still afford some protection, from Cosmic Rays, Solar Radiation, and (possibly) Planetary Weather.

Presumably, Mars also has similar such Volcanic Cave Systems, perhaps near Olympus Mons. And, Mars also shows ample evidence of Liquid Water, and even Oceans, in its past*.

* (Cool Science) Exploring Mars ; (DK Eyewitness) Mars. cf. http://sciencenow.sciencemag.org/cgi/content/full/2009/410/1?etoc

[Widdekind]

On Earth, all movements of Mantle Magma happen at about the same rate*.

* http://www.geosciences-forum.com/ftopic654.html

CONCLUSION (?): To lowest order of approximation, all Exoplanets may be characterized by a single "Mantle Magma Flow Rate".