The "Nebular Hypothosis" was a theory proposed for the formation of the solar system. It was first proposed in a very simple form by Kant in 1755, but the more mature development came with Laplace in 1796.
What this did was to provide a model for the formation of the solar system that explained how
(1) the planets contain most of the angular momentum in the Solar System
(2) all the planets are in roughly the same plane, which is the same plane
as the Sun's equator
(3) most planets rotate in the same direction
With this model, the solar system forms from a hot rotating nebular of dust and gas. As the rotating gas cools, the nebular decreases in size, but as it contracts, there is a corresponding increase in rotational speed. This would throw out a rotating ring of gas and dust, which would be rather like the ring around Saturn, but composed of dust and gases. Gravitational forces would lead this ring to would contract along its orbit into a planet, and this would repeat, as the central nebula's speed led to further expelling of matter, causing the formation of the planets. So the gravitational collapse of the nebular gives rise to the sun and planets, and in particular, the further out a planet, the older it is, the closer to the sun, the younger.
But for the purposes of science fiction, we need to remember that Laplace's hypothesis dominated the 19th century and left a lasting legacy in the 20th century.
If we take the case of Venus, Venus is seen as a cloud covered planet, and watery and humid, often with jungle where there is a modicum land, but with a great deal of water, and very little land.
In Robert Heinlein's "Between Planets", Venus is portrayed as a world covered in hot, steamy swamps, which are used to explain the constant,
unyielding cloud cover. Edgar Rice Burroughs has a Venus which is a tropical world shielded from the heat of the sun by a perpetual cloud cover. C.S. Lewis story about Venus - Perelandra (in the Old Solar language) has the whole surface of Venus covered by an ocean upon which are free-floating islands - natural rafts of vegetation large enough to support animal life, and there is the exception of a single mountainous land-mass. Isaac Asimov's "Oceans of Venus" has a surface totally of water, colonised by human beings using giant underwater domes, where the native species are all aquatic.
Perhaps one of the best examples of this is John Wyndham's "The Man from Earth", which is set in the far flung future of Venus, where the inhabitants have evolved into intelligent bipeds. But at the time the narrator arrives in Venus, it is past the age of the dinosaurs, and starting the evolution of primitive mammals.
The man from earth arrived as part of a mining expedition, where rivally between two greedy corporations - Metallic Industries Incorporated and International Chemicals leads to a ship to sent to Venus by one of them to exploit the resources of that planet. This is how Venus appears:
It was not a landscape, for in every direction stretched the sea - a grey, miserable waste. Even our relief could not make the scene anything but dreary. Heavy rain drove across the view in thick rods, slashing at the windows and pitting the troubled water. Lead-grey clouds, heavy with unshed moisture, seemed to press down like great, gorged sponges which would wipe everything clean. Nowhere was there a darkling line to suggestland. The featureless horizon which we saw dimly through the rain was a watery circle
They land on an Island, where there is a hot tropical climate, which is the same worldwide.
Our island was permanently blanketed beneath thick clouds. One never saw the sun at all, but for all that the heat was intense and the rain, which seldom ceased, was warm.
The narrator is a saboteur for one of the companies, and kills off the crew one by one. Unfortunately, the space ship is destroyed as well, and he waits for a rescue that will never come; his company has also betrayed him. But as he explores, he gathers two four legged mammals as pets, and comes across a valley in which there are dinosaur like creatures:
Farther on an enormous head reared above the trees, looking directly at me. It was unlike anything I had ever seen before but thoughts of giant reptiles jumped to my mind. Tyrannosaurus must have had a head not unlike that. I was puzzled as well as scared. Venus could not be still in the age of the giant reptiles. I could not have lived here all this time without seeing something of them before
He discovers that this Valley - called by the later intelligent Venusians "The Valley of Dur" has unique properties -
"at some remote date in the planet's history certain internal gases combined in a way yet imperfectly understood and issued forth through cracks in the crust at this place, and this place only. The mixture had two properties. It not only anaesthetized but it also preserved indefinitely. The result, was to produce a form of suspended animation. Everything that was in the Valley of Dur has remained as it was when the gas first broke out. Everything which has entered the Valley since has remained there imperishably. There is no apparent limit to the length of time that this preservation may continue."
He succumbs to the gas, only waking after millions of years have passed, and the descendants of his four legged pets are the intelligent race on Venus. When he finally manages to communicate, they show him the Earth through a telescope:
Gratz looked back at the scarred pitted surface of the planet. For a long time he gazed in silence. It was like the moon and yet - despite the craters, despite the desolation, there was a familiar suggestion of the linked Americas, stretching from pole to pole - a bulge which might have been the West African coast. Gratz gazed in silence for a great while. At last he turned away. "How Long?" he asked. "Some millions of years."
There's a strong ecological message here - the great mining companies have exploited the earth, and all that is left is a barren planet, now devoid of life. Venus on the other hand, has evolved into a planet of intelligent life, and its climate has also changed to a climate not unlike that of Earth's temperate regions - "occasional clouds, occasional rain, warmth that is not too oppressive."
It's a perfect fit with Laplace's Nebular hypothesis, and the evolution of the planets also is mirrored by evolution on the planets. And it reflects
ideas about the cloud cover of Venus, and the idea that Venus is a planet at a more primitive state than earth.
It isn't until the middle of 1960s that we start to see a more accurate picture of the planet Venus as we know it today with the Mariner probes. And Venus is revealed, not as a watery swamp land, but a runaway greenhouse planet, a hell hole of sulphate clouds, with no water:
Radio observations published in 1958 showed an amazingly hot temperature, upwards of 600°K, around the melting point of lead. "It was very disappointing to many people," one of the discoverers recalled, "who were reluctant to give up the idea of a sister planet and perhaps even the possibility of life." Some astronomers worked up arguments that the radio measurements were misleading, representing something in the upper atmosphere, so that life might still exist on Venus. The matter was settled in 1962 when the spacecraft Mariner II flew past and showed unequivocally that the heat radiation came from the hot surface. (1)
And from Venus, the next exploration is an older world, within Laplace's Nebular hypothesis, that of Mars. My next posting on Astronomy and Science Fiction will look at that.
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