Source: New Scientist (via UASR)
Date: Sept 12, 1998

IN THE near vacuum of interstellar space, temperatures hover just above absolute zero, where even the wobbling of atoms grinds to a halt. Dotting this empty, frigid world are huge globs of gas and dust grains, so numerous they block out nearly all light. And outside the shelter of these interstellar clouds, bombarding cosmic and ultraviolet rays slash most molecules to shreds.

Millions of light years away, under the sunshine and palm trees of Mountain View, California, in a lab tucked away in the grounds of NASA's Ames Research Center, Lou Allamandola and his colleagues have been recreating that world of extremes. And in doing so, they have uncovered tantalising hints that life may have emerged not from some warm primordial slime on Earth, but on a dust grain in the icy heart of space.

It's long been suspected that comets made the Earth habitable by delivering water and gases, and that they perhaps even provided some of the simpler chemical building blocks of life. But what the NASA Ames team has found goes far beyond that. When they recreate the harsh conditions of space in their lab, not only do they generate astonishingly complex organic compounds similar to those vital for life on Earth today, but also curious cell-like structures that may have housed our planet's earliest life forms.

The origin of these intriguing findings is coincidentally enough also the birthplace of stars and planets. With 10 000 atoms per cubic centimetre, interstellar clouds of gas and dust are sufficiently dense that individual blobs within them collapse under their own gravity, forming stars and, eventually, swirling systems of planets. (While this density is crushing by space standards, the air on Earth is about 25 000 trillion times thicker.) In space, the density of these clouds also offers another advantage that may have helped generate life on Earth. Like people in a crowded dance club, molecules of gases, such as methane, carbon monoxide, water vapour and ammonia, are continuously banging and bumping into dust "seedlings"--grains of silicate the size of smoke particles that have been ejected from old stars.

While most astronomers have concentrated on the gases, Allamandola has always had his eyes on the grains. Just as water vapour molecules rising from bubbling soup will hit a cold kitchen window on a frosty night, stick and freeze, says Allamandola, the same thing happens on the silicate seedlings. When gas molecules smash into the cold grains they stick, creating an icy skin of frozen gases. Infrared telescopes first detected the gas particles and silicate grains in the 1970s. But no detection device was, or is to this day, powerful enough to see how they interact in space. So Allamandola, who trained as a cryogenics chemist, and physicist Mayo Greenberg of Leiden University in the Netherlands, set out to do what most people thought impossible: build bits of interstellar space here on Earth.

Nowadays, a deafening buzz permeates Allamandola's lab. It's the sound of cryocoolers--high-powered refrigerators--keeping the simulated interstellar clouds at a cool 10 degrees above absolute zero. The "clouds" are actually vacuum-sealed chambers the size of shoe boxes. A gaseous mixture of water, methane, ammonia and carbon monoxide flows through a metal tube into each chamber, freezing onto what in the lab counts as a dust grain--a few square centimetres of aluminium or caesium iodide. As the molecules pile up and freeze, a thin white layer of ice forms.

And since grains in space receive occasional doses of ultraviolet light from stars, the simulated grain and its accumulated chemicals are also bathed in ultraviolet radiation. Each hour of radiation in the lab is reckoned to equal what a grain in space would receive in a thousand years, a mere blip in the Universe's 15-billion-year history. But that's enough to get the action started. As soon as those rays hit the molecules on the grain, they start breaking chemical bonds, producing highly reactive radicals such as ·OH and ·NH2. The icy temperatures provide the ideal mixer. Frozen to the spot, the radicals are forced to rejoin with their neighbours in ways that would never occur if they were still part of a gas free to fly off and join more suitable partners. The result is a profusion of complex, organic compounds.

The work, however, is a little like cooking with a recipe that doesn't tell you what the dish will be. Allamandola and his colleagues know what they start out with--simple, water-soluble gases--but analysing the chemically complex end result is trickier. Their latest chamber is fitted with both a mass spectrometer and an infrared spectrometer which together give a rough idea of the type of molecules that form on the simulated dust grain. So far, the NASA Ames team has found a slew of alcohols, ketones, aldehydes, alkanes, a giant molecule called hexamethylenetetramine or HMT, and other organic molecules, some with as many as 40 carbon bonds. But apart from that broadbrush description, Allamandola and his team have yet to put names on most of the chemicals they have created in this simulation of an interstellar cloud.

When biologist David Deamer of the University of California in Santa Cruz heard what was going on at the astrochemistry lab at Ames, he was soon knocking at Allamandola's door. Deamer had been studying the Murchison meteorite, which landed in Australia in 1969 and has since kept numerous researchers busy identifying its rich array of organic molecules. Deamer had found something intriguing deep within the loose sandstone-like rock: hundreds of microscopic globules. Grinding the stone into powder and flushing it with a solvent to rinse out organic molecules revealed what looked like tiny two-layered vesicles swimming in the liquid. When he published his findings in Nature in 1985 (vol 317, p 792) it caused a considerable fuss, not least because no one had any idea what these vesicles were, nor what sort of chemistry created them, and where.

Deamer suspected that clues to his "fossil" vesicles lay in the residue in Allamandola's space chambers. After all, the Murchison meteorite is believed to be a remnant of a spent comet, and comets are little more than billions of ice grains piled together in mountain-sized chunks. When Deamer warmed the chamber residue in water and peered at it through the microscope he discovered tiny droplets, each between 10 and 40 micrometres across, up to the size of red blood cells. Their similarity with the Murchison droplets was a sure sign that the meteorite vesicles had had their origins far off in space, not here on Earth. Deamer also discovered that the vesicles fluoresced under ultraviolet light, one more indication that they were made of complex organic molecules.

"It was a remarkable transformation of a few simple, water-soluble chemicals," says Allamandola. "It would have been considered science fiction a few years before." It was time to call in a biochemist.

Last year, Jason Dworkin, who had worked with Stanley Miller, famous for creating amino acids in the 1950s with little more than a spark of electricity and a few hot gases, joined the group.

A closer look

After ruling out contamination, Dworkin has been working on creating enough of the dust grain ice to get a closer look at the molecules that make up the vesicles. It's an onerous task. Running the space chamber for weeks on end yields only about a milligram of organic residue. And it contains dozens of different chemicals. Still, so far he has created enough residue to show that the molecules that make up the outer layer of the vesicles behave like lipids, which are the major component of cell membranes.

Just like soap molecules, one end of each vesicle molecule is attracted to water, while the other end avoids it like the plague. This allows the molecules to self-organise into spheres, sticking their water-loving heads towards the outside, and keeping their water-hating tails tucked away inside.

Even mainstream research into the origin of life, which relies on mixing chemicals under conditions thought to have existed at the start of the Earth's history, hasn't had any success in making lipids or lipid-like structures, says Dworkin. Now Dworkin plans to find out whether the lipid-like molecules from the space chamber can form bilayers similar to the membranes of all modern Earthly cells.

But why get so excited about what looks at best like a very rudimentary membrane that just happens to glow under ultraviolet light? Put simply, the finding matters because many experts believe that life, or at least life as we know it today, could not exist without boundaries, especially on a waterlogged planet like our own ("Life is...", New Scientist, 13 June, p 38).

Without barriers, goes one argument, biologically important molecules would be so diluted in the ocean that no chemistry could ever happen. Modern cell membranes, with the help of proteins embedded in them, also act like a home-security and climate-control system all rolled in one, regulating what leaves and enters, maintaining the correct pH, and providing a means of separating charges so that the cell can, for example, create the energy-carrying molecule ATP. Membranes may even be essential for stabilising peptides, the precursors of proteins.

A crazy idea

"The [Dworkin] results are very exciting," says geneticist and astrophysicist Pascale Ehrenfreund of the Leiden Observatory in the Netherlands, who studies the role of interstellar ice grains in the origin of life. "Membrane formation is a crucial step in the first forms of life." Earth's first life forms would probably have been far too simple to make their own membranes. But whether they were forced to make use of ready-made reaction chambers, or whether they were merely strands of nucleic acids wriggling their way unprotected through the primordial sludge (sticking to a clay surface or happening upon a drying puddle could have concentrated the chemicals) is an open question. If they did need to seek shelter, the Deamer-Allamandola vesicles could be just the thing.

"If you can form bubbles," says Tom Wdowiak, an astronomer at the University of Alabama in Birmingham, "then you've got something that can serve as a capsule." But the Allamandola team has done more than recreate what might have been our planet's first mobile homes. Their reenactment of what happens in an interstellar cloud has also shown that space can generate the right sorts of life-giving chemicals to go inside. In one experiment, Allamandola and Bernstein added water to the giant HMT molecules created in the space chamber, yielding formaldehyde, ammonia and even small amounts of amino acids.

More recently, the NASA Ames team turned to polycyclic aromatic hydrocarbons, the largest reservoir of carbon in the universe. Carbon is an essential part of all known life, and PAHs spewed out by the sloppy combustion of stars in the process of being born, contain between 10 and 20 per cent of all the carbon in the Universe. And although PAHs are never seen in normal, healthy cells, they are highly biologically active--for example, PAHs in soot from car engines and factories are carcinogenic simply because they can wiggle their way into DNA.

The NASA Ames team decided to use their space chamber to find out what might be happening to the PAHs deep within the interstellar clouds. The result was totally unexpected. When they fired gaseous water and PAHs such as naphthalene and anthracene one at a time on to the simulated "dust grains" in the space chamber, and bombarded the mixture with ultraviolet light, it produced compounds uncannily similar to those needed for life on Earth. "It's the UV light that makes PAHs useful [for life]," says team member Scott Sandford. The rays broke the water molecules apart, and the separated hydrogens and oxygens--locked in place by the cold temperatures--reattached themselves to the PAH, creating a huge array of complex chemicals.

"This change is antithetical to what everyone thought," says Wdowiak. "Everyone thought that PAHs would just break down [when exposed to UV light]. This is great. We didn't think PAHs would turn into anything of use. And suddenly we had this huge reservoir of carbon we never thought about before."

Just this July, space-in-a-lab simulations by NASA Ames team member Max Bernstein produced compounds called quinones and alkaloids from PAHs. (It is far easier to analyse the irradiated PAHs, than the residue made from squirting methane, carbon monoxide, water vapour and ammonia into the space chambers.) Alkaloids are ubiquitous in the plant world. Meanwhile, quinones help all cells move electrons around, are crucial for photosynthesis in plants and are broken down for energy in human muscle and brain cells. The NASA Ames team "is showing that carbon [from space] is coming in, in a form that is rich, that can be utilised by life," says chemist Richard Zare of Stanford University in Palo Alto.

Bernstein speculates that interstellar quinones raining down on Earth provided a tasty treat for early organisms until, at some point, the primordial forerunner of the plant took advantage of the quinones to harness sunlight. But Allamandola envisages a more radical scenario. Sure, the compounds raining down on Earth in cometary dust could have provided a nutritious meal for struggling primordial life, he says. But what of those vesicles? Is it possible--and this, Allamandola knows, "is a crazy idea"--that they and the complex organic molecules jump-started life before reaching Earth?

Consider the inside of a comet, he says. There, under layers of icy material, molecules created by ultraviolet light falling on PAHs and other gases stuck to silicate grains would be shielded from the worse radiation. Perhaps the comet swoops by a star, warming the outside just enough to melt some ice, providing water for the cell-like vesicles to form, just as they did when Deamer thawed the residue from the space chamber.

Whizzing through space in the belly of the comet wouldn't be a smooth ride. Gradually, the amino acids, PAHs and other organics would jiggle their way into the vesicles. And voilą, you'd have reaction chambers chock-a-block with complex organic molecules primed to generate the very first cells. "Maybe they're just sitting there waiting like seeds in a packet to hit the right place," he says. Allamandola's "crazy idea" could be checked out early next century when the European Space Agency's International Rosetta Mission is scheduled to drill into a comet's core and brings whole pieces back to Earth.

"It's a big stretch from making vesicles and encapsulating organic compounds to promoting life," says John Cronin, a prebiotic chemist at Arizona State University in Tempe. "It's hard to imagine how that can take you to anything as complex as a nucleic acid that can store and reproduce information." But, he adds, "it's hard to imagine that process anywhere, so maybe it's not such a big step."

Now, Bernstein and Allamandola plan to mix PAHs in their space chamber with all the gases found in an interstellar cloud--water, plus methane, ammonia and carbon monoxide. "It's going to be like the Wild West. Anything can happen," says Allamandola.

"Anything can happen" could be space's new motto. The space-chamber shows that under extreme conditions simple organic ingredients can produce a rich banquet of potentially life-spawning chemicals far more easily than anyone expected. In short, it could happen just about anywhere.

"The most amazing thing is that we start with something really simple. And then suddenly we're making this enormous range of complex molecules," says Allamandola. "When I see this kind of complexity forming under these exceedingly extreme conditions, I begin to really believe that life is a cosmic imperative."

Gretel Schueller is assistant editor at Audubon magazine in New York

© Copyright New Scientist, RBI Limited 1998

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