Asteroid impacts, which have been fingered for killing off the dinosaurs and other mass extinctions, may have helped kick-start life in the first place, experiments by Japanese researchers suggest.
Earth's oceans formed about 4.3 billion years ago, a time when lunar craters suggest Earth suffered a bombardment of comet and asteroid chunks, note the researchers led by Yoshihiro Furukawa of Japan's Tohoku University. How life's complex chemical building blocks — "organic" molecules, such as amino acids — accumulated on Earth amid extraterrestrial impacts and the vibrant volcanism of a young planet has intrigued researchers for decades.
Perhaps "the bulk of the organic molecules necessary for life's origins were (created) by oceanic impacts of extraterrestrial objects," Furukawa and colleagues say in the current edition of the journal Nature Geoscience. Most asteroids contain some carbon, the basic element in organic molecules. So the researchers investigated whether an asteroid impact could serve as a chemistry lab for cooking up the ingredients for living tissues, the "pre-biotic soup" that some biologists suggest led to life.
In the study, the team reports the results of blasting simulated asteroid pieces with a propellant gun into a water and ammonia mix meant to simulate the early ocean. The goal was to re-create a 4,500-mph dive into the ocean that blasted apart iron and carbon meteorites at 8,500-degree temperatures.
The experimental blasts synthesized more than a dozen organic molecules otherwise absent from the early Earth, the study concludes, ones that would have lingered after an impact. "Impact-formed organic molecules persist following bombardment conditions because they quickly dissolve in (the) ocean," Furukawa says by e-mail. Such organic molecules are the constituents of living tissues and take part in the chemical reactions used by all living things.
"This report lends support to the exciting hypothesis that terrestrial planets and icy moons are predisposed to origins of life" delivered by carbon-rich meteorite impacts, says astrobiologist Lisa Pratt of Indiana University in Bloomington. The new study has the "unexpected twist" of adding newly formed organic molecules from the impact event to any matter already inside the meteorite, Pratt says. "The long list of ingredients in pre-biotic soup just got longer."
Origin-of-life research has heated up in recent years. A 2006 study in Science magazine suggested that underwater volcanic vents might have synthesized similar chemicals. And in October, researchers led by Adam Johnson of Indiana University revisited the 1953 "primordial soup" experiment that kick-started the investigation of early life's chemistry.
Johnson and colleagues found that one part of the 1953 experiment, which had gone unreported, produced far more amino acids than had been reported. The experiment passed a witches' brew of ammonia, nitrogen and other chemicals found in Earth's early atmosphere past a spark generator to simulate lightning. Lightning through clouds of volcanic steam may have zapped together early organic molecules, the researchers suggest.
Asteroids impacts are a "more natural" source of organic molecules than the spark flask experiments, Furukawa says. Asteroids themselves might have delivered organic molecules to Earth, but Furukawa and colleagues argue that the heat of such an impact would have destroyed any organic molecules, requiring creation in the fallout after the object blasted the early ocean.