Comets have inspired both awe and alarm since antiquity, "hairy stars" resembling fiery swords that to many were omens of doom. Nowadays, scientists have found evidence that comets not only may have taken life away through cataclysmic impacts, they may have helped provide life by supplying Earth with vital molecules such as water — possibilities they hope to learn more about from the encounter with Comet Hartley 2 tomorrow (Nov. 4).

Comets as life-givers

It is widely believed Earth was molten when it formed some 4.6 billion years ago and remained that way for its first 50 million to 100 million years. This heat would suggest the young planet also was dry.

"As such, for a long time, people thought water was delivered sometime after the Earth formed and cooled down a bit," said astronomer David Jewitt at the University of California, Los Angeles. "So people looked around at what kinds of things loaded with water might hit Earth, and comets were the obvious answer." The giant chunks of ice called comets are, along with rocky asteroids, the leftovers from the formation of the solar system.

In addition, astronomers discovered that comet surfaces were apparently coated with organic compounds, suggesting comets also may have supplied other key ingredients for life. [How Did Life Arise on Earth?]

"However, this view began to change about 15 years ago," Jewitt explained.

Scientists began observing the levels of standard hydrogen atoms and of atoms of deuterium, which, like hydrogen, has one proton in its nucleus, but also has one neutron.

"The deuterium-to-hydrogen ratios have been observed in four comets now, and these are higher than that seen in Earth's oceans by a factor of two or three," Jewitt said. "The argument was that if the oceans were created by comets, these ratios should be the same, and they weren't."

Water, water everywhere

Some researchers began looking for other plausible sources of Earth's water and other life-giving molecules. Simulations of orbits of objects in the solar system suggested the asteroid belt would be a better source than the more-remote Kuiper belt, from where most short-period comets come — comets that need no more than 200 years to complete an orbit of the sun, which would place them close enough for a chance collision with Earth. [Video - Hunting Asteroids and Comets]

The asteroid belt is simply closer, just beyond the orbit of Mars, while the Kuiper belt is beyond the orbit of Neptune, some 30 to 40 times the distance that Earth is from the sun. Moreover, organic materials such as amino acids have been detected in the outer parts of the asteroid belt.

Analyses of deuterium-to-hydrogen ratios in the asteroid belt also showed a wide range of values, with some matching those found in Earth's oceans. In addition, comets were discovered in the asteroid belt in 2006.

"Now, these arguments are much more complicated than one might initially think," Jewitt cautioned. "First, is it really obvious that the water in the oceans should have retained the same deuterium-to-hydrogen ratios over time?" A number of geological processes might have altered these ratios, such as deep-sea hydrothermal vents.

Also, while short-period comets come from the Kuiper belt, long-period comets (ones that take more than 200 years to complete an orbit) come from the even more distant Oort cloud, and the deuterium-to-hydrogen ratios of those have not been measured yet. "Maybe those are more similar to the ones that formed the oceans," Jewitt said.

Another possibility is that Earth was not so dry when it formed. "It's hard for most people to see how hot rock can trap much water, but the argument there is that, overall, Earth is not all that wet," Jewitt said. "The mass of the oceans is only a few hundredths of 1 percent of Earth's total mass, which is pretty dry.

"My guess is that Earth's oceans were formed as a contribution of all three — comets, the asteroid belt, and the primordial material that went up to make the Earth," Jewitt said. "It's just a question of finding out which was the biggest source."

Comets as death-dealers

Comets may have also lived up to their ancient reputations as omens of doom by inflicting mass destruction on the face of the planet.

"Statistically, comets must hit planets," Jewitt said.

Comets might typically explode in Earth's atmosphere instead of striking the ground, because they possess large amounts of structurally weak ice. "They become balls of dust that are stopped by the atmosphere," Jewitt said.

These airbursts can still prove deadly. The Tunguska explosion in 1908, which flattened some 500,000 acres (2,000 square kilometers) of Siberian forest, is often thought to have been caused by an airburst from an asteroid or comet some 65 feet (20 meters) wide and 185,000 metric tons in mass — more than seven times that of the Titanic.

Close encounter

A close encounter with a cyanide-spewing comet might help determine whether ancient comets once helped to deliver water and the ingredients of life to Earth.

After months of hunting down Comet Harley 2, NASA's Deep Impact spacecraft will come within 434 miles (698 km) of its quarry.

Unlike the four other comets that spacecraft have gotten a close look at, the nucleus or core of Hartley 2 is a factor of five smaller — it's an elongated chunk of ice and dust only about three-quarters to 1 mile (1.2 to 1.6 km) across. It should have been more easily affected than the other four by any events that helped shape comets after the formation of the solar system.

Comparing Hartley 2 with the four larger comets thus should help "give us a better picture on what the properties of the primordial comets that seeded the Earth with water and organics might have been like," said Michael A'Hearn, principal investigator for the mission. "We're trying to understand how they might have changed over time to figure out what they might have started like."

The mission could raise as many mysteries as it helps solve. For instance, during the approach, the probe detected a roughly fivefold surge in the amount of cyanide-laced gas that the comet pumped out. Unusually, "we saw this increase with no increase in the amount of dust," A'Hearn said. "We're trying to figure out how a comet can put out gaseous molecules without the dust."