© 1975-1979, 2008 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization, First Edition, Xenology Research Institute, Sacramento, CA, 1979; http://www.xenology.info/Xeno.htm

5.4.3  Astrogeology

While the skies and seas of alien worlds are fascinating subjects for discussion, it is mainly upon the surface of a planet (its crust, or lithosphere) that life evolves and flourishes. Scientists who study mountain-building (orogeny), tectonic and seismic activity, and the construction of worlds generally, call themselves “astrogeologists” or “astrogeophysicists.”^598,2144

As Dole has pointed out, our knowledge of the forces responsible for earthquakes, volcanoes, and mountain-building is still incomplete.²¹⁴ One suggestion is that quakes and volcanoes are more likely on planets with higher gravitational compression and more internal heat generation due to radioactive decay. Planets smaller than Earth would tend to have less gravitational contractive force, relatively larger surface areas (compared to total mass) across which to radiate heat off to space,¹²³⁷ and relatively smaller volumes of heat-producing radioactive substances. Small worlds will thus tend to have lower internal temperatures,¹²³⁷ thicker and more solid crusts, and therefore much less volcanism and seismic activity.

Larger planets have relatively great volumes of radioactive material, higher gravitational compressive energy, and comparatively smaller surface-to-volume ratios (so it’s harder to get rid of heat).¹²³⁷ They should have larger molten cores, mantles that rise closer to the surface, and thinner crusts that can buckle and slip around more easily. If these suppositions are true in general for high-mass terrestrial worlds, more frequent and more severe quakes might be predicted, as well as higher levels of volcanic activity.

This theory squares with the reported characteristics of planets in our own solar system. The lightest world that has been intensively investigated is the Moon, within which only the faintest tremors have been detected deep below the surface.²⁰⁵⁶ The lunar lithosphere has solidified down to a depth of roughly 1000 kilometers.^1291,2043 When the core loses heat and contracts, the mantle is so thick and rigid it cannot buckle. Consequently, there is no real geologic surface activity on the Moon.^1291,2043

Mercury, the next most massive world examined by astrogeologists, is believed to have no surface tectonic activity at this time -- although various surface migrations and volcanism a few eons ago are evident.^1565,2040 Mars apparently has seismic activity. The red planet also seems to have some lithospheric collapse due to mantle contraction, but there is no clear and convincing evidence for horizontal plate movements across the surface. It has been suggested that on Mars we may be seeing “incipient plate tectonics...where one plate is beginning to break away...like the Earth, about two hundred million years ago.”⁵⁹⁸ The towering Olympus Mons (formerly “Nix Olympica”¹³²³), at 26 kilometers high the largest mountain in the solar system, bears mute testimony to the presence of extensive and fairly recent volcanism on Mars.²⁰⁷²

Earth has well-developed tectonic activity, plenty of active volcanoes, and a crust only about 30 kilometers thick.³⁶⁷ Radar probes of Venus, our sister world, have found low mountain chains suggestive of at least a moderately active lithospheric environment.^1214,2041

Presumably, the core of a still larger terrestrial planet would be more massive and hotter, pushing the mantle closer to the surface. The thinner crustal sheet would buckle, slip and shake far more readily than does Earth’s rocky skin. Quakes would probably be more violent and more numerous, and breakthroughs in the crust by hot magma (volcanoes) should be widespread and commonplace.

What kinds of mountains are alien worlds likely to possess? The building of mountains is an extremely complex process, depending on planetary mass, gravity, composition, heat flow rate through minerals, air pressure and wind velocity, and a host of other factors. For instance, on larger worlds rivers may flow downhill faster because of the higher gravity, which may cut deeper valleys and canyons.

Perhaps one of the most significant astrogeological advances in this century has been the development and elaboration of the theory of continental drift. Continents are now known to be small plateaus of granite embedded in much larger “tectonic plates.” The entire Earth’s crust is believed to be fragmented into a mosaic of perhaps eight of these plates, rigid shifting masses of solidified lithosphere which have been described as great tabular “icebergs” of rock floating on the surface of a “sea” of denser subjacent mantle material.^2140,2141

Plates are believed to be about 100 kilometers thick,²¹⁴⁰ and may move literally thousands of kilometers across the surface of the planet in only 100 million years or so.²¹⁴² Convection currents in the deep mantle have been proposed as the prime mover of the plates, circulating the viscous magma in localized “cells” much like the currents of water in a flat pan which is heated from below.²¹⁴¹

Because the continents are always on the move (though they change shape very little as they travel piggyback around the world²¹⁴²), each has a trailing edge and a leading edge. The trailing edge is tectonically stable, so mountain-building is minimal. But the leading edge is forced downward with the descending mantle currents; the lighter, more siliceous materials that comprise the continents pile up at the site of subduction.²¹⁴¹ Great mountains are born. (One of the clearest examples of this process occurred during the Cenozoic Period, when the Indian Plate smashed into and dove under the Eurasian Plate, throwing up the mammoth Himalayan ranges.²¹⁴⁰)

From the arguments presented earlier, it is at least plausible to advance the hypothesis that more massive planets will have more internal energy available to drive the thermal convection currents in the mantle, and should therefore produce greater tectonic thrusting and more extensive mountain chains.

Like all material bodies, mountains are subject to the Square-Cube Law. This principle is, quite simply, that volume increases faster than area as size increases. For a mountain to remain standing and not collapse, it must be strong enough to support its own weight. This weight is distributed over an area. The weight that must be supported, however, increases with the volume. (For example, mountains with eight times more mass have only about four times more base area to support that mass.) Consequently, a mountain should be less capable of sustaining its own bulk as it increases in size.

The maximum height of rocky ranges is therefore proportional to their weight, the product of the mass and the force of gravity (Figure 5.10). Higher gravity planets will have smaller, squatter mountains, because the limits of compressive strength of rock are reached much sooner. At least down to about 0.1 M_earth or so, smaller worlds should tend to have taller formations. As has been discovered with craters on the bodies in our solar system,¹²⁷⁷ the height of mountains should statistically vary inversely as the force of surface gravity.*

Figure 5.10 Maximum Size for a Planet's Mountains¹²⁷⁹

The graph above gives the “maximum statically loaded topography” supportable by a range of different materials. The curves are based on the assumption that if the interior pressure created by building the mountain exceeds the compressive strength of the materials, then the mountain will “fall down.” Planetary radius R is the horizontal axis, and h, the maximum height of mountains (or depth of depressions), is the vertical axis, both in kilometers. For weaker materials -- such as water-ice -- the topographic relief must be far less than if rock is used. No materials are expected to have much greater strength than taenite, so all planets should be found below this line. (Note the extreme position of Jinx, a hypothetical egg-shaped planet devised by science fiction writer Larry Niven.⁴⁵¹) Note the relative weakness of the ices -- if Titan has only ammonia-ice mountains, they cannot be larger than two or three kilometers.

Maximum mountain heights in our solar system are roughly as follows: Mercury -- 3 km,¹⁵⁶³ Venus -- from 1-2 km,²⁰⁴¹ Earth -- from 8-11 km, Luna -- highest peak is 6.8 km high (Theophilus). Mars -- highest peak is 26 km high (the volcano Olympus Mons).²⁰⁷²

Mountain size will also be related to the compressive and shear strength of the building materials used.^1233,1279 The maximum height of ranges will vary approximately linearly with the compression strength (Table 5.12). For Earth mountains, rock is the usual orogen** with a maximum sustainable load of about 10⁷ kilograms/meter². However, were we to find mountains of carbon dioxide on another planet, the greatest height would be far lower. This is because the compressive strength of “dry ice” is less than 10-30% that of rock.¹⁵⁶⁹

Volcanism could be a peculiar affair on other worlds. On a planet as cold as Titan, for instance, water could be an orogen instead of a thalassogen. If sufficient crustal radioactivity exists, and if the planet is roughly terrestrial-sized, we might observe cold volcanoes spewing forth molten water instead of lava.¹⁹⁴⁷ Dr. Donald M. Hunten, a physicist at the Kitt Peak National Observatory, believes that Titan may possess just such a subsurface magma of liquid water.²⁰⁴⁶ The magma would lie atop a rocky mantle and would contain large amounts of dissolved ammonia. The relatively thin crust should then be a mixture of methane and water-ice, frozen solid.

A curious phenomenon is the flowing of glaciers (mountains of water-ice). There is some evidence that this may be virtually a unique property of H₂O “mountains,” One of the more unusual characteristics of water is its ability to drop its melting point when subjected to pressure. Underneath a glacier pressures rise to hundreds of atmospheres. A lubricating layer of melted ice can form at the base, and the object proceeds to slide downhill on this thin, slippery film of water.

While ice exhibits the freezing point depression effect up to pressures of more than 2500 atm, solid carbon dioxide and other ices cannot duplicate this behavior. Only water-ice will flow rapidly down valleys like rivers. One Alpine formation, the Quarayaq Glacier, is known to flow between 20 and 24 meters per day.¹⁸⁵⁰ (Of course, CO₂ glaciers are still subject to slow creep,¹⁵⁶⁹ but this is far less dramatic.)

If mountains are subject to the Square-Cube Law, are not worlds as well? Small, mountain-sized hunks of matter may be very irregular in shape, because the internal stresses are relatively low. But as mass increases, pressures build: Inside any terrestrial planet rock begins to flow and seek a spherical shape -- energetically the most stable configuration.

Stephen Dole has estimated that the largest mass of a body that can maintain a highly irregular shape is on the order of 10^-5 to 10^-4 M_earth.²¹⁴ To get some idea of the degree to which an object may deviate from sphericity, Table 5.13 gives the largest size of a body whose mountains are as tall as the planetary radius itself (e long axis is twice the short). These worlds must be very small to retain their egg-shape.

Finally, returning once again to peculiar surface effects, the astrogeologists may have some real surprises in store for us on other worlds. For example, we know that Venus’ air is deficient in oxygen, and one explanation is that the surface rocks have all been well-oxidized. But at temperatures beyond 620 K and pressures above 50 atm, superheated steam dissolves alumino-silicate rocks. If the oxygen depletion theory is correct, Venus might once have been molten to considerable depths and served as a factory for huge, exquisite gemstones.¹²⁹³ The surface of the Morning Star may well be studded with garnets, sapphires, rubies and topaz!

* Astrogeologists will recognize that I have made a gross oversimplification here. The mountains of large differentiated planets are actually supported by isostatic forces. Only small bodies can accurately be considered to have statically loaded topography.¹²⁷⁹

** Derived from the Greek roots, meaning, literally, “something that produces mountains.” I use the word to signify “any substance capable of forming planetary mountains.”

Last updated on 6 December 2008