Dinosaurs and asteroids

In March of 1992, three astronomers, Carolyn and Eugene Shoemaker and David Levy discovered their ninth comet, hence its name, “Shoemaker-Levy 9.” The team determined that S-L9 was orbiting Jupiter now rather than the sun. Because Jupiter is a heavy-weight planet, exerting an enormous gravitational pull, astronomers call it the solar system’s vacuum sweeper.

In July of 1992, astronomers noticed that Jupiter’s force had broken S-L9 into 23 fragments that they labelled A through W and that each were strung out in single file, like pearls on a string. Most fragments, they estimated, were little more than two kilometers wide.

Astronomers detected that the comet was losing its power, unable to overcome Jupiter’s massive pull, and they calculated the fragments would strike the massive planet the second week of July in 1994, twenty years ago. That week astronomers trained their telescopes, plus the Hubble space telescope, at Jupiter, in anticipation of the spectacular fireworks, and they were not disappointed.

The first fragment struck on July 16, and the last on July 21. Brown smudges and stains dotted Jupiter’s surface, some thousands of kilometers wide, as big as Earth itself, or bigger.

Astronomers estimated that fragment G was the heaviest and largest, three or four kilometers wide, and its impact created a brown smudge double the diameter of Earth’s. “Although only about the size of a small mountain, it created wounds in Jupiter’s surface the size of Earth,” one author wrote.

As the weeks passed, the stains dissipated, but astronomers were convinced. A collision with a single comet would cause immense catastrophes upon Earth, where life exists.

In 1980, Luis Alvarez, a physicist, and his son, Walter, a geologist, published a paper, and in it they proposed that an extra-terrestrial source, such as a comet or an asteroid, had slammed into the Earth millions of years ago, and that it had caused fires, volcanic eruptions, and tsunamis.

With debris and smoke littering the atmosphere for years, a continual winter settled upon planet Earth. Without adequate sunlight, photosynthesis was disrupted, and once the plants died out, so did the herbivores. Then, those atop the food chain, such as the carnivores like Tyrannosaurus Rex, were wiped out. A single comet pushed the dinosaurs into extinction in a few thousand years.

Walter and Luis had noticed a thin clay layer in sedimentary deposits all over the planet and that this layer is unusual because it contained soot, glass, quartz, and diamonds, minerals formed by high temperatures and pressure. They also noted that the clay layer is rich in iridium, a mineral not abundant on Earth, but one that is found in comets and asteroids.

They also observed that the dinosaur fossils reside below the clay layer, but not above it.

The Alvarez’s proposal encountered fierce opposition and debate, but two years after Luis Alvarez died of cancer in 1988, researchers pointed to the crater Chicxulub, 120 miles wide and under the water off the coast of Mexico. The comet that caused that crater, scientists believe, triggered the dinosaurs’ mass extinctions, and as much as 75% of the pre-existing plant and animal life.

In his book, A Short History of Nearly Everything, Bill Bryson said that if a similar comet would strike the Earth, temperatures “would rise to some 60,000 Kelvin, or ten times the Sun’s surface. Everything in its path—people, houses, cars—would crinkle and vanish like cellophane in a flame.”

As if not bad enough, “the blast would blow out a thousand cubic kilometers of rock, earth, and superheated gases. Every living thing within 150 miles would be killed by the blast. Everything standing would be flattened or on fire, and nearly every living thing would be dead.”

Afterwords, volcanoes would erupt, tsunamis would roil the oceans, and “a cloud of blackness” would cover the planet, ushering in an apocalyptic nightmare.

What life forms would survive another collision with a comet? Possibly those that survived before: carrion-eaters, sharks, crocodiles, bacteria, certain insects, some plants, a few small mammals. The primates, if not destroyed, would be changed forever,and this includes human beings, men and women.

All of this is sobering. Human beings reside atop the food chain now, as T-Rex once did, and yet a single rock floating in outer space has the potential to destroy all that humanity has accumulated over several millennium: science, the historical record, the legal code, literary achievements, technology, buildings, military armaments, and systems of government.

We would have little warning, just a few months or weeks if someone saw it approaching, or as little as a single second after it entered the Earth’s atmosphere and heated up. If we did see it weeks in advance, we could do nothing to prevent it from striking Earth, because we have no technology to redirect a comet’s trajectory, nor to launch a nuclear warhead to pulverize it.

The odds are against an imminent collision though. Brian Witzke, a geologist in Iowa, said that a collision with a comet or an asteroid or a massive meteorite occurs only “about once every million years on average.”