Over 80% of Venus lies within 1 km of the mean radius of 6051.84 km. If Venus had been further from the Sun and avoided the runaway greenhouse outcome, then it would have a hydrosphere like the Earth:- a mean global ocean of some 3 km. If we were to take the modern topography of Venus and flood it to an average depth of 3km, we would innundate nearly 90% of the surface.

Perhaps this is what Earth would have been like if the Moon had not been formed? Even this is an overestimate of the amount of land. Venus' surface is almost uniformly 500 Ma in age and has not experienced significant rifting or mountain building in that time. Erosion on Venus is very slow in the hot dry atmosphere but on a waterworld erosion would be at least as rapid as on Earth. Planet girdling storms would smash on the exposed shores of any island, and rainfall would stream from the heavens.

Australia as a continent is a similar age to Venus' surface. We no longer have any topography more than 3km above the surface of the continent, and almost all of it less than 2 km, so the continents of Venus would likely have foundered below the waves long ago, if it were further from the Sun. Now perhaps every 500 to 1000 million years, the planet would undergo an upheaval and build new continents and new mountains, but anything that evolved to crawl onto land would be racing against time before the land was swallowed up again

Mars - How to escape a waterworld?

Mars has, in fact, escaped this fate because it is too far from the Sun and all its water is perpetually frozen. It hasn't rained on Mars for at nearly 4 billion Years. The planet is so cold that its surface is dominated by processes driven by CO2. We find dry ice at the polecaps, CO2 vapour in the atmosphere, and in the subsurface there will be both CO2 permafrost and pockets of liquid CO2 (Hoffman 2001a, b, c). In the past, this liquid CO2 occasionally burst out from underground and exploded into giant rockstorms that rushed down valleys, supported on a boiling cushion of CO2 Gas (Hoffman 1999).

Fascinating as this is, we are interested not in how Mars really is, but how it might have been if it were closer to the Sun. Then, the mythical ocean of the "Blue Mars" theorists would have become a reality. How would Mars have evolved?

We can flood modern Mars, of course, and see what happens. With a smaller planet like Mars, there is less water to flood the surface - an equivalent depth of half that on Earth seems likely, so we only have to add 1,500 metres of water to the planet. Finally we get a planet that has significant areas of land. With the lesser gravity of Mars, much taller mountains are possible. Surely they will resist erosion?

However, things aren't as simple - they never are! Mars has no plate tectonics, and does not even have the episodic mountain building processes that Venus does. Those attractively large looking land areas will be swallowed fast. Most of the mountains in the southern hemisphere of Mars are over 3.5 billion years old. They would all be gone by now. Even the dramatic-looking Tharsis volcanic province is deceptive. The flow rate from Mons Olympus is no more than from Hawaii on Earth. Erosion can keep close match to that leading to a broad shallow sea of eroded lava sand, with a small volcanic island in the middle.

And when we look at Mars' heat flow and the activity of its volcanoes, we find that the planet is cooling off and contracting (Hoffman 2001d). For the last billion years, very little has happened on Mars to build mountains. If there were an ocean on Mars, it would have eaten away all the land by now and filled in the basins to make a giant flat ball covered in water - another waterworld.

Gigantism as an escape route?

We have seen that smaller planets than Earth fail because their tectonic engines cool off, and mountain building stops even though they have less ocean to flood the land. What about a larger, more active planet?

The trouble here is that with a larger planet, gravity is larger so mountains are unable to grow so high. (the strength of the crust is a genuine limit to the height that mountains can grow - only where immense tectonic forces squeeze entire continents together can we build mountain chains like the Himalayas. So on a larger world, mountains are lower and streams more erosive because of the stronger gravity.

At the same time, there is proportionally more water to flood the land. And proportionally more "continental" crust to float to the top of the planet and clog up plate tectonics. So it seems that gigantism leads to worse problems, not better.

Conclusion

There really seems to be no way to build a truly Earth-like planet that still retains its original inventory of light "continental" crust. Big worlds flood too deep and wear away the land. Small worlds cool off and lose their tectonic vigour, and then the land gets worn away even though the seas are shallower. Perhaps a very small world might have so little ocean that some land would survive, but worlds so small are prone to lose their atmospheres.

The process that formed the Moon out of Earth's primordial crust has enabled a very odd planet to exist - one that has Continents and Oceans, Land and Sea. It is only on such a planet that life can emerge onto land, and evolve to tool-using fire-burning , mechanical civilisation.

Around countless stars in our galaxy, and innumerable galaxies through space there will surely be Terrestrial planets, yet they will not be Earth-like. They will not have glistening Silver Moons orbiting silently through space around them, but only small dull rocks whizzing in orbit. The worlds will be, almost without exception, waterworlds.

Yes, they may have intelligent life in their seas, such as fish, squid or crustaceans. There may even be air-breathing creatures akin to dolphins and whales. There may be birds in the skies, but these creatures will never discover electricity, or computers, or rockets, or radio telescopes.

The heavens are likely full of glorious ecosystems on delightful planets, but not one of them has the potential to launch an enquiry into space and ask "Where are They?", still less an actual mission to another star.

If we humans can solve our problems on Earth and do develop into a starfaring civilisation, the Universe may be a lonely place for the likes of us, and perhaps we may find it our role to bring fire from the gods, as Prometheus was said to have done in our own prehistory.

The philosophy of living in a Universe such as this is an interesting topic to contemplate - and all because of our Moon!

Back To Part One:  The Moon And Plate Tectonics

Nick Hoffman is a Senior Research Scientist at La Trobe University, Melbourne Australia. He can be contacted via email at [email protected]

References and Further reading

Hoffman, N. and Lark, A. 2001d "The structural state of Mars from MOLA implies a cooling planet" Lunar and Planetery Science Conference XXXII, Houston Tx. Abstract 1494. Available Online

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