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6 | NewScientist | 5 May 2012

METHANE clouds scud over an icy landscape that barely registers a temperature above -180 °C. If life exists on Titan, surely it has to be about as otherworldly as any our solar system could support?

Perhaps not: a simulation of conditions on Saturn’s giant frigid moon shows that some of the key molecular precursors of life as we know it are likely to have formed there. The results raise the odds that Titans, if they do exist, might be less alien than we imagine.

There is good reason to hope that a search for life on Titan will prove fruitful. The moon is swathed in a thick, protective, nitrogen-rich atmosphere. Beneath that is evidence of surface liquid – albeit in the form of hydrocarbon lakes.

So far, though, the hunt for living Titans has drawn a blank. The Cassini space mission, launched in 1997, has been orbiting Saturn since 2004. It provides detailed feedback on day-to-day activity on Titan and life signs have proved elusive.

Not that lack of evidence has stopped speculation on how life might arise on the frozen world. The latest scenario was presented at NASA’s Astrobiology Science Conference in Atlanta, Georgia, last month.

Titan’s atmosphere is thought to resemble Earth’s around the time that life began here, more than 3.5 billion years ago. What Titan lacks is the vital spark that generated life on Earth: energy on Titan is in short supply.

The atmosphere is so thick that

Weird sparks on Saturn’s moon may make the existence of Earth-like life possible

Titan – home to life as we know it?

Bob Holmes it filters out the sun’s ultraviolet light, which is rich enough in energy to damage DNA molecules on Earth. Titan also has surprisingly fine weather: Cassini has yet to record a lightning storm there. This energy gap means that compounds on the moon’s surface that contain oxygen are unlikely to relinquish their grip on this key ingredient of amino acids, themselves a crucial component of terrestrial life.

Now Jack Beauchamp, a chemist at the California Institute of Technology in Pasadena and his student Daniel Thomas have identified another energy source:

wind. When windblown dust particles strike one another, they can transfer electrons and set up static charges which later discharge with a spark. Beauchamp grew up in the Mojave desert in the south-western US and often heard these discharges as static on AM radio broadcasts. Similar discharges cause whirlwinds to glow on the surface of Mars.

Much of the surface of Titan is covered by enormous wind-blown dunes of frozen hydrocarbon. To see whether static discharges could generate amino acids, Beauchamp and Thomas filled a reaction vessel with polystyrene and glass beads coated with a thin layer of simple molecules containing carbon, hydrogen and oxygen to mimic Titan’s hydrocarbon sands. The pair then

blasted the beads with cold nitrogen gas to give them a taste of Titan’s bitter winds.

Sure enough, the agitated beads began to spark, and even at temperatures well below 0 °C the energy was enough to kick-start a reaction that produced the amino acid glycine.

“This is a totally new process for making molecules of

astrobiological importance,” says Beauchamp. The results are preliminary – indeed, the pair found glycine only a few days before the conference – but Beauchamp believes that the process should produce other amino acids as well.

Of course, amino acids are only the first step towards life as we know it. Sceptics question

“Static discharges could generate amino acids within Titan’s windblown hydrocarbon dunes”

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5 May 2012 | NewScientist | 7

–Reap the whirlwind–

If asked to choose the most likely place in our solar system to harbour alien life, most astrobiologists would plump for Mars. The Red Planet is much warmer than Saturn’s moon Titan (see main story) and there is abundant evidence that liquid water once existed on its surface.

Conditions are far from hospitable now, though – Mars is cold and arid, with a thin atmosphere that does little to stop the surface from being blasted with ultraviolet rays. Even so, a few terrestrial organisms may be able to survive unaided on Mars.

Wayne Nicholson of the University of Florida in Gainesville and his colleagues extracted bacteria from a sample of Siberian permafrost and put them in a Mars simulator at the Kennedy Space Center near Orlando. The simulator recreates the conditions of modern Mars almost perfectly. “Except for gravity – we can’t do gravity,” says Nicholson.

Out of the tens of thousands of samples they tried, six not only grew under Martian conditions but actually thrived, Nicholson reported at NASA’s Astrobiology Science Conference in Atlanta. DNA sequencing revealed that the hardy bacteria belonged to two species

common in Arctic environments. Nicholson’s bacteria are not the

first to grow under Martian conditions. A few weeks previously, his colleague Andrew Schuerger, also of the University of Florida, extracted an unrelated bacterium from spacecraft-assembly buildings and grew it in the Mars simulator. The two unpublished studies suggest that terrestrial bacteria could contaminate a mission landing on Mars. They also hint that Mars is not as inhospitable as you might think.

Indeed, if life began on Mars during earlier, more favourable times, it could have evolved to cling on as conditions worsened. Brian Wade of Michigan State University in East Lansing and his colleagues took Escherichia coli bacteria, which do not grow well under harsh conditions, and exposed them to Mars-like levels of desiccation and ultraviolet light.

Within 500 generations, the bacteria evolved to cope with the stress, he reported at the astrobiology conference. That strongly suggests that Martian life, too, could have adapted to today’s austerity. n

Martian austerity no barrier to life

in this section n Solar power’s big problem, page 8n Strange fat keeps skin waterproof, page 10n Bacterial hard drives, page 18

in La Jolla, California. Even if conventional life on

Titan is frozen in a form last seen on Earth more than 3.5 billion years ago, exotic life better suited to the chilly conditions on Saturn’s satellite may have fared better.

Steven Benner, a chemist at the Foundation for Applied Molecular Evolution in Gainesville, Florida, points out that, while familiar chemical reactions may not occur on Titan, the icy moon is unlikely to be free of reactions.

Temperatures in Titan’s liquid methane lakes probably plunge to -180 °C, at which point chemical reactions involving the making and breaking of covalent bonds would grind to a halt. At these temperatures, though, Benner says that other molecular bonds

which play a minor role in Earth’s chemistry move to centre stage.

Weak van der Waals forces between molecules, which result from brief imbalances in the charge distribution of atoms, exist fleetingly at room temperature. At -180 °C, though, such forces would have just the right nimbleness to act as covalent bonds, Benner told the astrobiology conference.

In fact, Titan’s cold could be an advantage, reducing the rate at which molecules accumulate random damage. For life adapted to the slow, stable ways of Titan, Benner notes, conditions on Earth would push chemistry to the brink of chaos. “Why we humans don’t spontaneously ignite is amazing, to a Titan.” n

whether the later steps, such as protein building, would move fast enough at Titan’s frigid temperatures to finish the job.

Two new studies suggest that Titan’s atmosphere is a billion years old at most – a fraction of the age of Earth’s atmosphere. This places a severe constraint on the time available for life to have emerged there (The Astrophysical

Journal, DOIS: 10.1088/0004-637X/749/2/159 and 10.1088/ 0004-637X/749/2/160). So any life on Titan could well be stuck in the primitive, pre-cellular stage.

“Titan is a very interesting prebiotic factory, but to go beyond simple molecules is, I think, very unlikely,” says Jeffrey Bada, a chemist at the Scripps Institution of Oceanography

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