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2005

In the Search for Life on Titan, the Challenge Will Be Recognizing It
The New York Times, January 11, 2005
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Dr. Steven A. Benner is bracing for what could be a spectacular year. When the Huygens mission parachutes into the hazy skies of Titan, the largest moon of Saturn, he will be among the researchers anxiously awaiting a report.

After several days, Dr. Benner and his colleagues will receive the Huygens measurements of Titan's chemical composition. They will comb through the data to see whether the moon harbors life.

But Dr. Benner, a professor of chemistry at the University of Florida, will not limit his search to familiar life forms of the sort that exist on Earth. He is casting a wider net, in hopes of discovering creatures that defy conventional biology -- what Dr. Benner calls ''weird life.''

For the last year, Dr. Benner has been on a National Academy of Sciences committee that is considering just how weird extraterrestrial life might be. The official title of the committee's project is ''The Limits of Organic Life of Planetary Systems.''

Weird life could conceivably be lurking on several planets and moons in the solar system, including Earth, and the Titan mission is providing Dr. Benner and his colleagues with the first good search opportunity. The challenge is knowing what to look for.

''My job,'' he said, ''is figuring out what kind of signals would be detectable if we don't encounter life as we know it on Earth -- or a green man with a ray gun who shoots at us.''

Dr. Benner's coming work is the latest chapter in a career dedicated to examining the boundaries of life. Life on Earth, despite its diversity, obeys certain rules. All known organisms need liquid water, for example, and they all store genetic information in DNA molecules. Are those rules merely flukes of the history of life on Earth, or is life possible only with this particular chemistry?

Dr. Benner suspects that the rules are flukes. He finds some of his evidence in human cells. To turn a stretch of double-stranded DNA into a protein, cells first have to build a copy of it using a single-stranded molecule called RNA.

When RNA was discovered, it was considered little more than a cellular messenger. Later research showed that it could carry out sophisticated chemical reactions that biologists once thought only proteins could perform, like grabbing certain molecules or cutting them into smaller pieces.

The versatility of RNA led researchers in the 1970's to propose that life today, which relies on DNA, RNA and proteins, evolved from simpler organisms that used only RNA. In the 80's, Dr. Benner drew on experiments in his lab and elsewhere to create a biochemical portrait of these RNA-based creatures.

He continues to explore the ''RNA World'' hypothesis, trying to create these organisms from scratch. He is collaborating with Dr. Jack W. Szostak of Harvard Medical School and Dr. Gerald Joyce at the Scripps Research Institute to insert engineered RNA molecules into oily bubbles to see whether they can feed and reproduce.

If life did start out based on RNA, Dr. Benner suspects, some of the primitive organisms may be hiding somewhere on Earth. He speculated that they might have found refuge where DNA-based life cannot survive. RNA-based organisms would be small --far smaller than even the smallest bacterium -- because they would not be burdened with proteins and protein-making machinery.

''If I get rid of my need to make proteins,'' Dr. Benner said, ''I can get down to very small sizes and I can go live in a very small hole.''

Dr. Benner is planning a search for RNA-based life in rocks known as zeolites. They are rich in water but also have extremely small pores.

''I have a shelf full of them,'' he said.

But Dr. Benner also says life could theoretically take even more exotic forms. His suspicion emerges from experiments he and other scientists have carried out on DNA, seeing how far they can alter the molecule without destroying its ability to store genetic information.

''When you start with DNA,'' he said, ''you just look at that structure and ask how can you change it.''

In ordinary DNA, genetic information is encoded in an alphabet of four ''letters'' known as bases. In manipulating DNA molecules, Dr. Benner and other researchers have increased the genetic alphabet to 12 letters.

Another important part of DNA is its backbone. Each molecule has two backbones, like the sides of a ladder, that are made of repeating clusters of phosphorus and oxygen atoms.

Experiments in Dr. Benner's lab suggest that the advantage of DNA's backbone is that each cluster has a negative charge. The repeating charges repel one another, preventing the molecule from folding into a useless clump. Dr. Benner suspects that backbones with repeating charges can also be fashioned from other atoms.

Dr. Benner's National Academy of Sciences panel is using such experimental results to formulate a strategy to search for weird life. For example, if phosphorus is not essential for a repeating charge on a DNA-like molecule, the search should not be limited to planets rich in phosphorus.

One big question is what is needed to support life. All known life needs water to survive, but as Dr. Benner pointed out in the December issue of Current Opinion in Chemical Biology, life could conceivably exist without it.

Water, for all its virtues, has drawbacks. ''Water attacks DNA and causes it to fall apart,'' he said. ''Your body has got all sorts of enzymes that are constantly running around fixing the damage that water has done.''

Other liquids do not pose such risks, and Dr. Benner theorizes that they might be able to support life as well. Particularly promising, in his opinion, are liquid hydrocarbons like methane and propane that do a good job of dissolving organic compounds.

The best place to look for such hydrocarbon-based life is Titan, scientists say, because Earth-based telescopes have found a blanketing haze of methane and other hydrocarbons. Some researchers have proposed that the atmospheric hydrocarbons are produced by liquid hydrocarbon oceans.

Earthlike life would not be able to survive in the strange oceans, because DNA does not dissolve in liquid hydrocarbons.

''You are not going to have that structure on Titan,'' Dr. Benner said, referring to DNA. ''And that's period.''

But it is conceivable that other forms of life might thrive.

''If you had the ability to put together life without worrying about the toxicity of water,'' he added, ''you'd have all sorts of opportunities on Titan that you wouldn't have on Earth.''

The Huygens mission is to provide the first hard information to test his speculation. It will paint a broad portrait of the physical conditions on Titan, not searching specifically for signs of life.

Dr. Benner said he thought he and his colleagues might be able to wrest clues about the presence of life from the data.

''It will require a little cooperation from the life on Titan,'' he said.

In other words, life on Titan would have to be able to alter the moon's chemistry in a recognizable way.

Cells carry out chemical reactions that differ from those that occur in the absence of life. If you eat a piece of bread, its starch molecules go through a long series of reactions, each step making a minor change in the arrangement of atoms. In the absence of life, organic molecules quickly break down in one or a few reactions.

If life exists on Titan, Dr. Benner cannot say what it eats or how it metabolizes food. But he predicts that the organisms would produce a distinctive series of molecules through a long series of reactions.

Identifying such a signature of life would take a long time, he predicted.

''It's going to go on for months,'' he said. ''You'll see us squirreled up in laboratories and libraries being our typically nerdy selves, trying to figure out what this data means.''

Discovering life on Titan -- or even disproving its existence -- would carry scientists a major step forward in understanding the nature of life.

''It's as fundamental a question as you can ask in biology,'' Dr. Benner said. ''If life is an intrinsic property of chemical reactivity, then it should exist on Titan. But if there's no life on Titan, no life on Mars, and we go somewhere else and find no life, no life, no life, you're going to get this notion that life emerged on Earth through very special circumstances.''

Copyright 2005 The New York Times Company

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