Did Comets Make Life on Earth Possible?

Stefan Lovgren
for National Geographic News
October 2, 2003
An ambitious new NASA research project aims to answer perhaps the most vexing and profound of scientific mysteries: How did life on Earth begin?

The multimillion-dollar undertaking, led by the NASA Goddard Space Flight Center in Greenbelt, Maryland, brings together an interdisciplinary team of scientists from around the world to study how organic molecules are created in interstellar clouds and delivered to planets as they form.

The research will focus on the role of comets. Many scientists believe there is increasing evidence that comets supplied at least part of the raw material for the origin of life on Earth. The theory is changing the way scientists think about life in the universe and raises the possibility of alien worlds.

"Our mission is to gain a greater understanding of the origin and evolution of organic material on Earth," said Michael Mumma, a comet expert and director of the Goddard Center for Astrobiology, NASA Astrobiology Institute, who is leading the research. "The key question is: Were water and organic molecules delivered to Earth by cometary impact and does [that process] extend to planets elsewhere?"

Dirty Snowballs

Astronomers believe that stars, planets, and comets form in a massive chain reaction that begins when a cloud of interstellar material collapses under its own gravity. Some of the material forms the star—like our sun—and some of it gets spread out in a disk around the nascent star.

Some material in this disk later aggregates and forms planets. Close to the sun, where it's warm, leftover debris (rocky material) turns into asteroids. In the outer regions, where it's cold, icy chunks of rock and dust turn into comets.

It is generally believed that organic molecules, which contain carbon atoms and are present in all life forms known to science, are trapped in large amounts in both interstellar clouds and comets.

"We have extremely definite evidence from our radio observations that there's quite an array of organic molecules in interstellar space," said Bill Irvine, a professor of astronomy at the University of Massachusetts in Amherst, who is measuring radio waves from celestial objects as part of the research effort.

There's other evidence that comets contain organic material. When European spacecraft analyzed dust particles from the Halley comet in 1986, it turned out to be some of the most organic-rich material measured in the solar system. Meteorites that have hit Earth contain a whole suite of molecules, including amino acids, which play an important role in terrestrial biology.

"If such material exists in meteorites, which come from a class of asteroids, there's every reason to think it must also exist in comets," Irvine said.

Panspermia Goes Primordial

Most scientists have long believed that life on Earth began as a "primordial soup" in a lake or pond some four billion years ago. According to this theory, chemicals from the atmosphere combined with some form of energy necessary to make amino acids—the building blocks of proteins—to create the first primitive organisms, kicking off the evolution of Earth's species.

But the primordial soup theory is being increasingly disputed. Many geophysicists now say the Earth did not have enough gases, like ammonia and methane, from which organic material like amino acids could be produced.

Instead, a growing cadre of scientists believes the organic material needed to create life may not have been produced on Earth, but was instead brought here by comets. The newly formed Earth was likely subjected to a fierce bombardment of comets four billion years ago. These comets may have brought with them the organic compounds that later evolved into living matter.

According to the most radical theory, known as "panspermia," life in a ready-made form is ubiquitous in the galaxy and is brought by comets to new planets. Few scientists subscribe to this hypothesis, however.

Perhaps the main question is whether organic molecules can survive space travel or if they break up and contribute the atoms that are necessary to ultimately make biological material and water?

"Our museums contain examples of primitive meteorites that likely are very similar to the material delivered by comets," said Mumma. "The key point is that small bodies deliver their organics intact to Earth's surface. This must have been a common event on the early Earth."

Many scientists are now leaning toward a combination of the comet impact theory and the primordial soup thesis. Some chemical building blocks may have come from comets, but the assembly into life took place on Earth.

"The comet impact theory fits in with the primordial soup theory," Mumma said. "They can be complimentary."

Drilling Into Comets

Scientists will measure the molecular make-up of comets to better understand what chemical reactions formed them. This may provide clues to the evolution of carbon-based chemistry on Earth in its early history.

The new research will combine laboratory experiments, observations with telescopes and spacecraft, and missions to sample comet and asteroid material. A European mission not associated with NASA is even going to land on a comet and drill into it as the comet journeys toward the sun.

An important part of the project will focus on water, which is seen as an essential ingredient of life. Scientists want to know if the Earth's water was incorporated into the Earth as the planet formed or if the water arrived on Earth as a result of cometary impacts after the Earth was already formed.

"Earth's new oceans were filthy, and should have been full of organic molecules and dust particles carried to our planet by comets and primitive meteorites," said Mumma. "We want to learn how significant their contribution was to the genesis of life on Earth."

The comet research could have a tremendous impact on the quest to find life on other planets. After all, comets have slammed into many other planets. If they supplied the raw material to form life on Earth, what is to say the same thing hasn't happened on other planets?

One thing is for sure, however. Finding out exactly how life on Earth began, and if it extends elsewhere, will take some time. The first stage of the NASA project will last five years, but Mumma thinks it could take as much as 25 years before scientists have a definite answer about the origin of life on Earth.

National Geographic Resources on Comets and Asteroids

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Pluto: Planet or Comet?

Killer Asteroids: A Real But Remote Risk?
Is a Large Asteroid Headed for Impact With Earth in 2880?
Fires From Asteroid May Have Spared Some Regions
"Dinosaur-Killer" Asteroid Crater Imaged for First Time
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Mass Extinction That Led to Age of Dinosaurs Was Swift, Study Shows
Researchers Rethink Dinosaur Die Off Scenario
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Fossil Leaves Suggest Asteroid Killed Dinosaurs
U.S. Summons Experts to Draft Asteroid Defense Plan
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Fighter Jet Hunts for "Vulcanoid" Asteroids

Universe Reborn Endlessly in New Model of the Cosmos
First Evidence for Early Meteorite Bombardment of Earth
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