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SHARON SHANKS | Cosmos Comets brought water to Earth



Published: Sun, May 27, 2001 @ 12:00 a.m.



Astronomers have theorized for decades that comets may have brought water and other life-necessary ingredients to Earth.

After all, comets are primarily large, dirty snowballs containing some of the most primitive leftovers from the formation of the solar system, and it makes sense that most of the water ices trapped within their cores could survive an impact with a newly-forming Earth.

Ah, but there is water -- and there is water. Until recently, astronomers were faced with a problem: Much of the water ices found in comets so far were the wrong type of water.

The recipe for water is one of the easiest in nature: two atoms of hydrogen and one atom of oxygen, two of the most abundant elements in the universe. But there are variations to this recipe. The universe is a master chef and mixes ingredients to taste by using isotopes: the same elements, but with an extra neutron or two in their nuclei cores. So-called heavy water is an isotope: It is water, but made with hydrogen that has an extra neutron in its core. (This type of water is called deuterium.)

What studies show: Research done on the chemical reaction of gases under low temperature indicates that comets that formed in the very cold region far away from the sun, approximately near the orbit of Neptune, have greater amounts of deuterium than those that formed in the relatively warmer area near the orbit of Jupiter. Comets that formed closer contain the type of water common here on Earth.

Studies of comets Halley, Hyakutake and Hale-Bopp showed that their water was primarily heavy water, and thus these comets probably formed in the area near Neptune. These types of comets couldn't have supplied Earth with our life-giving water.

But just released studies of Comet LINEAR show that it has the right stuff, ordinary molecules of hydrogen and oxygen that are so important to life.

Comets are notoriously hard to study. Although astronomers would love to have large pieces to study in a lab, they're forced to do their observations at long distances and for short periods of time, limited to a comet's closest approach to the sun.

Using spectrometers and other instruments, astronomers can look at the composition of a comet by studying the gases it releases when the sun warms it.

Distinction: Every element has its own spectral fingerprint when it is heated, and by comparing the pattern of bright lines emitted by the gases, astronomers can determine their composition and amount.

Researchers aren't limited to visible light. They can also use telescopes sensitive to wavelengths in the infrared and other areas of the broad electromagnetic spectrum to study comets at a distance.

Comet LINEAR provided astronomers with a rare glimpse into a comet's hidden secrets when it disintegrated upon its close approach to the sun last year. By breaking up, the comet released its materials into a cloud of particles and gases, allowing astronomers to focus their many instruments on the remains and study their composition and abundance.

The comet was discovered in September 1999 by the Lincoln Near-Earth Asteroid Research program in New Mexico, and was named LINEAR-S4. It was a first-time visitor to the inner solar system, and thus brought with it a full complement of primordial materials that had never been warmed by the sun.

Current theories about the solar system's formation indicate that comets formed from the original cloud of gas and dust that provided the raw material for the sun and planets.

What happened: The comets that formed in the cold regions beyond the orbit of Jupiter were pushed by the gravity of the large planets into the Kuiper Belt or Oort Cloud, two relatively stable reservoir areas of comets and asteroids.

Here they stayed until they were nudged by the slight gravity of a passing star or other body into an orbit that took them closer to the sun.

Many of the long-period comets discovered today are thought to have originated in the Kuiper Belt or Oort Cloud. The cold-formed comets also have a different composition: They have more carbon monoxide, methane, ethane and acetylene. It was cold enough in the outer reaches of the solar system for these volatile elements to freeze and become trapped in the comets.

The comets that formed near Jupiter are different. They have less of these elements because it was too warm for them to freeze into place.

They are also less numerous because Jupiter's immense gravity hurtled them out of the solar system into interstellar space. But when the early solar system was forming, these near-Jupiter comets were numerous, and thus impacted Earth in great numbers during the period of heavy bombardment that marked the early life of the solar system.

They and the water and organic molecules they brought with them may have provided many of the ingredients for the soup of life on Earth.

Comet LINEAR was an exception to the Jupiter-region comets in that it was held in the Oort Cloud instead of being ejected from the solar system.

It had never before been heated by the sun and thus had fresh elements to release, and because it disintegrated when it approached the sun, it showed eager scientists a good picture of all its elements.

Goodbye to comets: Many comets don't survive their first close approaches to the sun, where the increased gravity and temperature can pull more fragile, loosely packed comets apart. We watched the fragments of Comet Shoemaker-Levy 9 collide with Jupiter in July 1994 after the giant planet's gravity pulled the comet apart and then pulled it in. Comet West broke up in 1976, and Comet Ikeya-Seki fell apart in 1965.

Another result from the study of Comet LINEAR looked at the rate at which it released water vapor when it broke up and estimates that the comet, with a nucleus estimated between 2,500 and 3,300 feet in diameter, carried about 3.6 million tons of water.




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