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Comets and planets: a noble link appears.

More than 30 years ago, a young chemist fascinated with the origin of life made a startling discovery. Mixing together water, ammonia, and hydrogen cyanidethree compounds believed to be carried by comets - John Oro of the University of Houston found that this hellbrew formed adenine, a fundamental component of DNA and a relative of adenosine triphosphate, the molecule that provides a key fuel for most living things. Citing his laboratory findings, Oro proposed in a 1961 NATURE article that as comets struck the planets early in the history of the solar system, these icy bodies brought with them the chemical precursors of life.

Although Oro's article became a landmark, over the years many researchers ignored the role of comets in planetary evolution because they didn't believe these objects contributed significantly to the chemical composition of the terrestrial planets. After all, they reasoned, rocky planets like Venus, Earth, and Mars probably originated as agglomerations of rocky, meteoritic bodies. And it seemed that meteorites striking the young planets might account for the concentrations of noble gases - chemically inert substances- in the atmospheres of these planets. Comets simply weren't needed.

But during the past decade, researchers uncovered a major problem with this explanation for the evolution of planetary atmospheres. They discovered that the meteorites that have fallen to Earth contain a far greater abundance of xenon, a heavy noble gas, than that found in the atmosphere of Earth and Mars. At first, scientists suggested that the missing xenon might lie hidden in underground rock or ice deposits, but no extra xenon was ever found.

Now, a team of space scientists proposes a way out of this dilemma. New laboratory experiments suggest that noble gases trapped within the icy nuclei of comets may account for the abundance of xenon in the atmospheres of Earth and Mars, as well as the unusually large amounts of argon found on Venus. Tobias Owen of the University of Hawaii in Honolulu presented the findings this week at the World Space Congress in Washington, D.C.

Two of Owen's colleagues, Akiva BarNun and Idit Kleinfeld of Tel Aviv University in Israel, conducted experiments crucial to understanding the influence of comets. They found that at a certain temperature, amorphous water-ice -- a laboratory stand-in for comets - trapped the noble gas argon but relatively little xenon. Such a pattern approximates, but doesn't precisely match, the relative abundance of these gases in the atmospheres of Mars and Earth, Owen notes. In the experiments, the abundances of gases trapped in water-ice chilled to 50 kelvins most closely approximated terrestrial conditions. That temperature has special significance, Owen says, because it corresponds to the temperature found in the region between Uranus and Neptune, where scientists believe comets first formed.

On the basis of this work, Owen suggests that the present noble gas composition on Earth, Mars, and Venus may represent the mixture of two reservoirs: trapped gas released when comets first rained down on the planets, and gas released in an entirely different pattern from rocky material inside the planets. Different planets - and different regions within those planets -- draw from these two reservoirs in different proportions. For example, says Owen, material from deep within Earth's mantle, shielded from cometary bombardment, exhibits a noble gas composition more akin to that of rocky compounds such as meteorites. He adds that the higher abundance of argon on Venus indicates that the most recent series of comets striking that planet formed at temperatures below 50 kelvins, since the Tel Aviv experiments indicate that chillier water-ice traps more of the noble gases.

Owen emphasizes that his conclusions remain speculative because researchers have yet to measure the actual abundances of noble gases inside "new" comets -- icy bodies that are visiting the inner solar system for the first time and haven't had a chance to warm up and expel most of the gases trapped within them. If future observations confirm the findings, he says, the recent work may provide one of the first direct links between comets and planetary evolution.

The new findings also strengthen the argument that comets played a crucial role in transporting biochemical compounds to the planets, says Oro, who spoke at this week's conference. Adds Owen: "The fact that you have to have comets coming in [to account for the noble gas abundances] means you're also bringing in carbon and nitrogen."

-R. Cowen
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Title Annotation:experiments suggest that trapped gases in comets may account for abundance of xenon in atmosphere
Author:Cowen, Ron
Publication:Science News
Date:Sep 5, 1992
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