Seeding Life on Earth

Life on Earth began roughly 3.8 billion years ago, shortly after a period of heavy bombardment by space rocks. Comets, in particular, have long been suspected of delivering basic chemicals needed to jump-start life, and possibly also the water that created the oceans.

But most models of solar system formation have comets developing out near the current orbits of Uranus and Neptune. These comets later were booted outward by the giant planets' gravity, sequestered for billions of years in a distant halo of objects called the Oort Cloud. A handful were sent inward toward the Sun and possibly Earth.

But if Comet LINEAR was formed near Jupiter's orbit, as is now thought, then it is likely just one of a bevy of objects that could have been hurled Earthward.

"Jupiter is very massive and has a strong gravitational field, so the comets that come near it can be slung all the way out to interstellar space," Mumma said. Or, he adds, toward the Earth.

And water in Comet LINEAR should have a composition similar to the water found in Earth's oceans.

Water is made of two atoms of hydrogen joined to one oxygen atom. But hydrogen comes in different types, called isotopes. They behave the same way chemically, but one called deuterium is heavier because it has an extra neutron in its nucleus. The water in comets that form far from the Sun are enriched in the heavy form of water, whereas Earth's oceans contain more of the lighter variety.

Recent observations of comets Halley, Hyakutake, and Hale-Bopp showed them to be rich in heavy water, meaning they likely formed out near the orbit of Neptune and were not of the type that contributed to Earth's oceans.

LINEAR broke up before its exact water composition could be determined, but a low amount of volatile organic molecules provides a strong indication that it carried the same kind of water that comprises terrestrial seas, Mumma and his colleagues say.

Mumma said much more debris circulated near Jupiter and Saturn compared with farther out, near Uranus and Neptune. This means collisions would have been more frequent near Jupiter, preventing comets from becoming too large, so their delivery to Earth would have been relatively gentle.

"The smaller comets from Jupiter's region impacted Earth relatively gently, shattering high in the atmosphere and delivering most of their organic molecules intact," he said.

And even comets like LINEAR, somewhat depleted of organic compounds, are thought to have contained more of these building blocks of life than was present on Earth.

"This means life on Earth did not have to start completely from scratch," Mumma said. "Instead, it was delivered in kit form from space."

If Mumma and his colleagues are right, then the Oort Cloud is populated with more comets that formed out near Uranus and Neptune, because these planets didn't have the gravitational oomph to move the comets further. Comets like LINEAR are rare simply because most of them got ejected from the solar system, tossed into the Sun, or possibly got tied up in the creation of life.

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