Revising Earth’s Biography
The Year of Astronomy is winding down. It’s 150 years since Darwin put us in our place with On the Origin of Species, 400 years after Galileo turned his telescope skyward and put Earth in its place, and 40 years after men retrieved samples of the Moon. What have we learned in all this time?
To gain some perspective, I decided to consult George Gamow, one of the 20th century’s greatest physicists. Some years ago, rummaging in a used book store in Manhattan, I found an old paperback copy of Gamow’s 1941 Biography of the Earth for a snapshot of scientific thought after “modern physics” but before the space age.
Gamow knew our planet was billions of years old. Atomic physics and radioactivity (which Gamow’s work helped to explain) had already made sense of the Sun’s power and longevity, and had turned Earth’s rocks into clocks. But he states that the Earth is two billion years old – less than half its true age.
His ideas about the requirements for life on Earth and other planets seem surprisingly modern. He states that life needs a temperature range permitting liquid water, some energy source, carbon compounds and a stable environment. (Basically what we believe today. Does that mean that he was prescient or we haven’t made much progress?) Yet his views of the other planets were laced with the wishful, geocentric thinking that typically has informed our views of other worlds in the absence of data. Thus he muses about aquatic life on Venus and plant life on Mars.
Gamow describes how the Earth, along with the other planets of the solar system, was born when a chance encounter with a passing star yanked a stream of gas out of the sun. This explains why the interior of the Earth is so hot. Because such stellar encounters are very rare, it means that stars with planets are incredibly anomalous and that our Solar System may be unique.
He explains how the Moon split off from the Earth, leaving the Pacific “ring of fire”. He knew the structure of the Earth’s deep interior from seismic data. Yet, though he notes the attractiveness of continental drift for solving many geophysical and biogeographic problems, he explains that the Earth’s rigid outer shell is just too stiff and unbreakable to permit such motions. It was, he suggests, the thermal shrinking of the Earth’s crust that caused the cracking and shifting that built up mountains.
Less than 70 years later, we know that these ideas about the origin of the Earth, the Moon and mountain ranges, are all completely wrong. The changes in our world view came not from fundamental breakthroughs in physics but insights born of exploration. Earth’s ocean floors, with their mid-ocean ridges and magnetic stripes, were mapped by ships, revealing a global pattern of tectonic motions that build up mountains and constantly re-make Earth’s surface. Plate tectonics became the unifying theory of Earth science. Remote sensing satellites illuminated Earth’s intertwined cycles of air, water, life and rock. Moon rocks and meteorites, combined with more sophisticated isotopic analyses of Earth rocks, taught us that everything in the Solar System is 4.5 billion years old. The chemistry of the Moon showed it was born when a protoplanet collided with the Earth. The planets, by and large, are more alien than scientists imagined before the space age. Neither Mars nor Venus is, on the surface, a good home for life. But exploration of Earth has also taught us that life thrives in extreme environments. And it turns out that planets form as a normal byproduct of star formation. No rare stellar encounters are required, and we have already discovered hundreds of extrasolar planets. These discoveries have renewed our hopes for finding extraterrestrial life despite the forbidding conditions on nearby worlds.
When I gain a window into the minds of some earlier generation of scientists they often seem both unbelievably sophisticated and surprisingly naive. This should give us some perspective on our current knowledge. As we enter this new age of extrasolar planet discovery, climate predictions, gene sequencing and in-depth planetary exploration, I wonder how our confident statements about what we know will seem if anybody reads them in another 50 years.