Broca's Brain - Carl Sagan [84]
PART III
OUR
NEIGHBORHOOD
IN SPACE
CHAPTER 10
THE SUN’S FAMILY
Like a shower of stars the worlds whirl, borne along by the winds of heaven, and are carried down through immensity; suns, earths, satellites, comets, shooting stars, humanities, cradles, graves, atoms of the infinite, seconds of eternity, perpetually transform beings and things.
CAMILLE FLAMMARION,
Popular Astronomy, translated by J. E. Gore
(New York, D. Appleton & Company, 1894)
IMAGINE THE EARTH scrutinized by some very careful and extremely patient extraterrestrial observer: 4.6 billion years ago the planet is observed to complete its condensation out of interstellar gas and dust, the final planetesimals falling in to make the Earth produce enormous impact craters; the planet heats internally from the gravitational potential energy of accretion and from radioactive decay, differentiating the liquid iron core from the silicate mantle and crust; hydrogen-rich gases and condensible water are released from the interior of the planet to the surface; a rather humdrum cosmic organic chemistry yields complex molecules, which lead to extremely simple self-replicating molecular systems—the first terrestrial organisms; as the supply of impacting interplanetary boulders dwindles, running water, mountain building and other geological processes wipe out the scars attendant to the Earth’s origin; a vast planetary convection engine is established which carries mantle material up at the ocean floors and subducts it down at the continental margins, the collision of the moving plates producing the great folded mountain chains and the general configuration of land and ocean, glaciated and tropical terrain varies continuously. Meanwhile, natural selection extracts out from a wide range of alternatives those varieties of self-replicating molecular systems best suited to the changing environments; plants evolve that use visible light to break down water into hydrogen and oxygen, and the hydrogen escapes to space, changing the chemical composition of the atmosphere from reducing to oxidizing; organisms of fair complexity and middling intelligence eventually arise.
Yet in all the 4.6 billion years our hypothetical observer is struck by the isolation of the Earth. It receives sunlight and cosmic rays—both important for biology—and occasional impact of interplanetary debris. But nothing in all those eons of time leaves the planet. And then the planet suddenly begins to fire tiny dispersules throughout the inner solar system, first in orbit around the Earth, then to the planet’s blasted and lifeless natural satellite, the Moon. Six capsules—small, but larger than the rest—set down on the Moon, and from each, two tiny bipeds can be discerned, briefly exploring their surroundings and then hotfooting it back to the Earth, having extended tentatively a toe into the cosmic ocean. Eleven little spacecraft enter the atmosphere of Venus, a searing hellhole of a world, and six of them survive some tens of minutes on the surface before being fried. Eight spacecraft are sent to Mars. Three successfully orbit the planet for years; another flies past Venus to encounter Mercury, on a trajectory obviously chosen intentionally to pass by the innermost planet many times. Four others successfully traverse the asteroid belt, fly close to Jupiter and are there ejected by the gravity of the largest planet into interstellar space. It is clear that something interesting is happening lately on the planet Earth.
If the 4.6 billion years of the Earth history were compressed into a single year, this flurry of space exploration would have occupied the last tenth