Life ingredients probably didn't ride in on a comet, new study shows

When comets hit Earth billions of years ago, they pelted the planet too fast and too steep for them to have delivered fragile organic molecules, according to a new study. 

Using advances in computer simulations, a University of Cambridge-led team of researchers ran scenarios of comet strikes, comet interactions with an atmosphere, and gravity's influence on comet movements through space to determine whether they could have brought the chemical ingredients for life to Earth. 

Rather than look at a bunch of important molecules, the researchers focused on one in particular: hydrogen cyanide, or HCN. Made of one hydrogen, one carbon, and one nitrogen atom, it's a simple molecule commonly carried on comets. They found that the immense heat would have literally vaporized most of those molecules on impact, challenging a theory on the origins of life that has existed for about a century.

Even in the rare case that HCN survived a crash to Earth, it would not have come in large enough quantities, said Richard Anslow, lead author of the paper, which appears in the journal Scientific Reports.

"It would really have to be a one-in-a-million occurrence," said Anslow, a Cambridge astronomer, in a statement. "We can't rule out for certain that it happened that way. But it seems to be a really rare occurrence — at best, a one-off fluke." 

Comets are chunks of ice, dust, and rock left over from when the solar system began about 4.6 billion years ago. Most range from a few hundred yards to dozens of miles wide, and they travel on long, stretched-out orbits. When a comet swings close to the sun, its heat converts some of the comet's ice into gas, which creates the glowing cloud and long tail. Scientists long suspected they helped start life on Earth.

In 1992, scientists Christopher Chyba and Carl Sagan studied the moon's craters and found that space dust, not comets, brought most of the carbon that reached Earth. But space dust holds its carbon in tough compounds that don't easily react, so it couldn't supply something like HCN. 

The Rosetta space mission took images of comet 67P/Churyumov-Gerasimenko in November 2014. Credit: ESA / Rosetta / NAVCAM

Because comets do carry HCN in a usable form, they have remained a contender as a source of life's building blocks. Lab experiments show that concentrated HCN can produce fats, sugars, and the nucleic acids that make up DNA and RNA.

The researchers selected HCN as the best case study because it's the only starter molecule that comets carry in useful amounts, and its triple bond makes it tougher than more complex molecules. In short, the scientists figured if HCN can't survive a comet crash, others chemicals probably won't either. 

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Recent computer models show that only comets wider than about 300 feet make it through the early Earth's atmosphere without breaking apart or burning up. Those models also show that earlier studies overestimated how much HCN can actually survive a high-speed collision. 

But if not comets, what could have brought life's ingredients? The researchers suggest asteroids, which slammed into Earth far more often. Their impacts may have triggered the critical chemical reactions to make prebiotic molecules. Or, an even simpler source: Maybe Earth made them on its own, without any extra help from space. 

The study showed comet delivery was slightly more probable for Mars. It's smaller than Earth with a thinner atmosphere, so comets had a better chance of reaching its surface intact. But the more likely answer is that it didn't happen on either planet, Anslow said. 

"Historically, scientists thought that comets were important for the origin of life on Earth because they carried carbon and nitrogen-bearing molecules, so these giant space snowballs were almost like a get-out-of-jail-free card that brought all the molecules a planet would need to seed life," Anslow said. "It’s kind of a romantic idea, but our results suggest it's highly unlikely."