#36 June 1990
Section 18.104.22.168.036.of the Artemis Data Book
by Peter Kokh
We’ve all heard the phrase “Earth-like Worlds”, but just what does it mean? Different things to different people, of course. And that’s because it is intrinsically imprecise, since there are so many varying conceptions of just what the “essence of Earth” is. Some people use the term in contra-distinction to the gas giant planets like Jupiter and Saturn, Uranus and Neptune. In this sense, they use “Earth-like” to refer to solid-surface rocky silicate planets such as Mercury, Venus, and Mars. (Even the Moon would fall in that generous clumping!)
But for those who yearn for the promised era when humanity will venture beyond the Solar System and reach for the stars, “Earth-like” conjures up images of worlds with continent-studded oceans, climate ranges that include something between subarctic and tropical, sweet oxygen-rich breathable atmos-pheres, and teeming plant and animal eco-systems.
In contrast to the first broader application, this use of the term goes much too far in the other direction. For Earth itself would only have met this test of “Earth-like” in the last current fraction of its long history! It is not a better definition of “Earth-like” that we need to agree upon. Rather we must get to the heart of the question to find an altogether different, less ambiguous phrase.
“Essence of Earth”
Until the second half of this century, it was the common belief that the continents formed in their present position. There was no attempt to explain why this continent had this shape, that one another. These were taken as factitious givens. True, many of the more inquisitive had wondered about the teasing near match of coastlines on both sides of the Atlantic. In 1905. the German meteor-ologist Alfred Wegner proposed a comprehensive picture of how the continents may have all been one and slowly fractured into large pieces which gradually drifted somehow to their present positions. But, for lack of evidence, his theory was widely dismissed.
In one of the most dramatic and exciting of scientific paradigm shifts since Darwin, a sudden flood of evidence from matching mineral deposits, fossils, and traces of paleomagnetism came pouring in during the sixties. These findings confronted unprepared scientists with the incontrovertible proof that Wegner was right in general, although he had many of the particulars wrong. It was not whole continents that were adrift, but rather great plates of oceanic crust on which the continents, or more accurately, pieces of continents rested. Wegner’s suggestion of continental drift was the precursor of modern “plate tectonics.”
Evidently, unimaginably slow currents of molten rock deep below the surface nudge these plates this way and that, helped by repeated volcanic outpourings which rift continents asunder and push apart the pieces and the plates they ride upon. Once continent chunks have split, the actual site of spreading continues as an actively volcanic mid-ocean ridge.
Where something is pushed apart on a surface of set integral size, obviously somewhere else, something has to be squeezed together or give way in some other manner. And so most of the great mountain ranges on Earth have been raised by continental and sub-continental collisions taking millions of years. Hard to grasp at first, but India, once clearly part of the African landmass, still crashing into the underside of Asia, has raised the Himalayas over the past 50 million years. The Atlas and Appalachian mountains were thrust up in a prior collision and rebound drama between North America and North Africa. Unthinkable? A few decades ago, but no more!
Continental collisions are not the only way pressure from spreading continents and plates are relieved. Often in a confrontation, one plate surrenders, so to speak, dives below the other whose edge rides up on top of it. The eastern Pacific ocean bottom is being thrust beneath the advancing west coasts of the two Americas. The coast-long shore-hugging deep ocean trench and the rampart rows of the volcano studded Rockies, Sierras, and Andes are the result.
It is understandable then that geologists still aburst with the enthusiasm afforded by this new onrush of insights into the shape of present-day Earth, are alert for traces of plate tectonics on other planets. Mars’ great 2,500 mile long, 200 mile wide Valles Marineris canyon complex seems to be an example of large scale crustal rifting. It has too often been unfairly and inappropriately compared with Arizona’s Grand Canyon instead of to Earth’s much vaster ocean trenches, or with Africa’s great rift valley system (including the Red Sea bottom). We see signs of incipient, quickly stalled rifting elsewhere, even on the Moon.
As to continent-resembling features, there are the lighter density slag-composed highlands on the Moon, the great Tharsis Uplift on Mars caused instead by eons-long continuous volcanism (the volcano chain of Ascraeus Mons, Pavonis Mons, and Arsia Mons, usually clumped with nearby Olympus Mons). And there are the two suspiciously continental elevations on Venus: Ishtar Terra near the north pole, and Aphrodite Terra along the equator. We’ve photographed volcanoes caught in the act of erupting on Jupiter’s great moon Io, and on Neptune’s great moon Triton, and are teased by a growing number of indirect indicators of current volcanic activity on Venus, such as lightning and the steadily declining amount of sulfuric acid in the atmosphere, indicating a sudden peak about 20 years ago that may result from a great volcanic eruption at that time.
Magellan, the powerfully equipped synthetic aperture radar probe that has recently [‘95 rewrite] spent over a year surveying the Venerean topography, has showed great basinland trenches but however, has failed to reveal “coastal” volcano chains, signs of ancient “coastlines” or “beaches”, mountain chains created by the upward pressure of subducting plates, or other traces of Earth-normal plate tectonics. If Venus ever had oceans, they did not survive long enough to play a role in shaping our “twin” planet as we see it today.
“Play a role”? Yes, for it seems that water has acted as a essential lubricant in the incredibly slow movement of crustal plates and continents. The ocean bottom crust is saturated with water, and when an ocean bottom plate is thrust below the advancing edge of a continental plate, drags that complement of water with it, down perhaps as far as 400 miles below the surface to erupt later as volcanic steam. Indeed we are likely to find that on any world where there has not been a significant hydrosphere (ocean), incipient tectonic activity has been an abortive self-snuffing episode. — Earth is a “HYDRO-tecto nic” world!
Testing this new definition
Is this the “new and improved” definition we have been looking for? It could well be. It is the oceans with the internal heat of the planet that have shaped and continue to shape our world. Is this Earth-style water-assisted plate-tectonics necessary to create an “Earth-like” planet? Well, without it, we would not have the rich mineral deposits that have fueled our technological crescendo. Nor might we be in the first place. For it is not only the slate-clearing by periodic asteroidal impacts that has allowed rut-stalled evolution to explore new manifestations of life, it is also the continual separation and recombination of continental chunks. And without on-going renewal via mountain-building plate collisions, any original continents would long since have eroded away, washing into the sea - we would have only ocean with relic shallow water rises where continents used to be. Only “hydro-tectonic” worlds can be truly “Earth-like” in both the geological and biological senses of the term.
So when you next day-dream of joining some interstellar exploratory expedition, searching for “M-class planets” to use the Star Trek term, you’ll know exactly what you are looking for. You will recognize young planets still devoid of life as being squarely on the track, and you’ll be able to tell which equally young planets will never make the grade. We need to look for planets with ocean, not so much ocean that there are only submarine continents, not so little that there isn’t a globally continuous hydrosphere. And we need active plate tectonics. Life will be vastly more probable on such worlds and should develop in a catastrophe-punctuated way similar to that which “Gaia” (Earth-life as a living global ecosystem) has experienced. The state in which we find such worlds will largely depend upon their age which can be estimated from Earth via an in-depth study of its sun’s spectrum and circum-galactic orbital characteristics.
Some HTWs [hydro-tectonic worlds) will be impetuous raw planets still awaiting the quickening of life. Others will still be dominated by early microbial life. And, more interestingly, there will be those dominated by multi-cellular metazoan life, as has been Earth for only the last 13% or 1/8th (600 million years) of its existence. And there will be some on which the hydro-tectonic engine has slowly ground to a halt, bringing any such planet to the final eons-long chapter of its history. Eventually its sun would shine on a world gone the way of Atlantis as the last continental remnants, no longer being renewed, erode away and wash into the thickening sea-brine.
Can hydro-tectonic planets be detected from Earth? Yes and no. The next generation of space and lunar surface telescopes after Hubble might be able to detect planets in the same size/mass range as ours, and as close to their parent suns as ours. Next we can look for the characteristic signatures of oxygen and methane. These gases can only appear on a hydro-tectonic world on which life has already arisen and evolved to the point where it can transform an original volcano-derived carbon dioxide atmosphere.
It would be interesting to equip a deep space probe with instruments capable of detecting such a characteristic signature, and see how far out from the Sun we can clearly detect Earth and tell what kind of world it is. The proposed TAU mission would go out one Thousand Astronomical Units, a thousand times further out than Earth from the Sun, or not quite 6 light days. (Neptune is only 30 AU, 4 light hours away.) It is the intention to use the advantage of such a distance to greatly refine our 2 AU baseline parallax calculations of star distances. A HT-Signature device (HTS) would make a great bus-mate for TAU.
While it is unlikely that life can arise in a fresh start except on a hydro-tectonic world, it should be transplantable to other less friendly locations where it might endure for long enough to suit human (sentient) purposes. And good transplant locations may be much more numerous than hydro-tectonic womb-worlds. But that’s another story!
[For an excellent, very readable, and well-illustrated book on the new plate tectonics, we recommend “The Restless Earth” by Nigel Calder, Viking Press 1972, SBN 670-59530-6.] - PK
Contents of this issue of Moon Miners' Manifesto