Like giant interstellar sperm, comets might transport the seeds of life from collapsed space clouds to fledgling and otherwise barren planets, depositing their life-giving substances in a colossal impact.
While as of yet unproven, a new computer model shows that at least one building block of DNA could develop in space when giant clouds of molecular matter collapse under their own gravity, squeezing substances and forcing chemical reactions.
If the controversial theory gains support it would be a shot in the arm for an idea more than 20 years old: that life on Earth originated in space. It might also explain how life emerged so quickly after our planet formed, and it certainly proposes that life is not as rare or as tough to kick-start as we might think.
"Life is not at all mysterious," said Sandip Chakrabarti, who along with his wife developed the computer model. "It is a property of a collection of extremely complex molecules. That's it."
Chakrabarti said he and his wife set out to show that the formation of life's most basic elements is a generic process. Working at the S. N. Bose National Center for Basic Sciences in Calcutta, the pair generated chemical reactions on a computer, simulating processes inside a hypothetical interstellar cloud.
Genesis of DNA?
As the cloud collapses, the density of carbon, hydrogen, nitrogen, oxygen and other complex molecules increases, explained Chakrabarti, raising the probability of reactions. He likens the process to driving into a city, where one encounters more and more cars.
Over the course of a million years, the simulated cloud collapses to a fraction of its previous size. The amino acids glycine and alanine form, followed by an abundance of adenine -- one of the four fundamental components of DNA, the complex organic molecule that packs the blueprints for life.
The Chakrabartis' work is discussed in the current issue of New Scientist magazine and will appear in an upcoming issue of the journal Astronomy and Astrophysics.
Doubting the model's assumptions
Tom Millar, a physicist at the University of Manchester Institute of Science and Technology, said a lot of assumptions went into the computer simulation, some of which he does not believe are plausible.
Millar told space.com that estimated reaction speeds could be off by enough to have overestimated the amount of adenine by a factor of 100 million. There simply isn't enough energy in a frigid interstellar cloud to generate the reactions assumed in the model, Millar said.
The simulation relies on an energetic reaction involving hydrogen cyanide, a substance common to both interstellar space and terrestrial chemical labs.
Said Millar: "If you assume, as does Chakrabarti, that HCN is reactive in interstellar clouds where the temperature is lower than on Earth, then one would expect the process to work also on Earth, and bottles of hydrogen cyanide should be turning into adenine -- clearly this does not happen."
Millar noted that a similar process involving ice instead of gas might yield adenine. Interstellar ice particles gather energy from starlight, he explains, and would not be as apt to fly apart during chemical reactions.
"You can keep all the relevant atoms and molecules together for a million years or so, and even though the process of building a complex species like adenine may be inefficient, you have a very long time to do it." If such a process worked, Millar added, "all comets might contain biological molecules and hence provide a widespread platform for life to start. Whether or not comets in our solar system are directly related to interstellar dust grains is a matter of debate at present."
Support for panspermia
The theory that life comes from space -- and hence is practically everywhere -- was proposed in 1977 by British researchers Fred Hoyle and Chandra Wickramasinghe, of the University of Wales at Cardiff. Their work holds that comets spread life's seeds to Earth and other planets. In its broadest sense, the theory maintains that the stuff of life exists throughout the universe -- a concept called panspermia.
(In a recent article, Hoyle and Wickramasinghe propose that
.)The Chakrabartis' computer model does not create life, but rather a building block of DNA. But a sudden introduction of building blocks might make for a neat explanation of why life emerged just a few hundred million years after the Earth formed, 4.5 billion years ago.
"Comet dust could have ensured that the appropriate ingredients for life were available on the Earth's surface and explain why the history of life on Earth began so early," said Matthew Genge of the Department of Mineralogy at the London Natural History Museum.
Genge said meteorites from asteroids are known to contain some, but not all, of the building blocks of DNA.
"Dust particles are a great way of delivering such bio-molecules to Earth because they slow down at high altitudes in the atmosphere (because they are so light) and are less heated, Genge told space.com.
But a computer model is just that -- a model. Even Chakrabarti is quick to say that it's not yet clear whether life comes from space. In an e-mail interview, however, he did offer an explanation for how it might happen.
After the simulated cloud collapses, a star is born, around which comets form and, eventually, planets.
"Even though the temperature is raised during planet formation, it is impossible that all these molecules would be destroyed, as comets smuggle these molecules out to cooler regions," Chakrabarti said.
If such a comet once slammed into the young Earth, some of the molecules inside would have remained intact, he said. "If the temperature and composition of the atmosphere are favorable, a colony will flourish. Otherwise, they would be too damaged."
Computer-generated adenine image (top) provided courtesy of
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