Blasting the atmosphere of Saturn's moon Titan with X-rays can produce a base component of DNA, a new laboratory study suggests. While the effect may only occur periodically, when meteoroid impacts deliver water to the moon's surface, the finding adds to evidence that Titan may be ripe for life.
In some ways, Titan is more like Earth than any other body in the solar system. It has continents, lakes, clouds, and perhaps even rain – but where Earth has rock and water, Titan has ice and methane. It may also harbour an ocean of liquid water beneath its icy surface that could host life.
With its nitrogen-rich atmosphere and abundance of organic material, Titan seems like a model of the early Earth.
But how did life on Earth get started, and might a similar process have a chance on Titan? For decades, researchers have been trying to recreate life's appearance on the early Earth by zapping materials thought to be present there with electricity or high-energy photons. The first such trial, called the Miller-Urey experiment, was performed in the early 1950s and produced amino acids, the building blocks of proteins.
In the 50 years since, dozens of teams have expanded on Stanley Miller and Harold Urey's original setup, using a wide variety of energy sources and gases that modelled conditions not only on Earth, but on interstellar dust grains and on Titan.
In 1984, a team that included Carl Sagan created adenine, one of five base components of DNA and RNA, in Titan-like conditions, using a spark of electricity meant to simulate lightning.
But so far no evidence of lightning has been found on Titan. And until now studies hitting Titan-like atmospheres with photons, like those it receives from the sun, have produced only organic compounds like benzene – and none of the components of DNA.
Now, researchers led by Sergio Pilling of the Catholic University of Rio de Janeiro in Brazil have produced the base adenine using photons for the first time.
Instead of using ultraviolet (UV) radiation as in previous studies, however, they used low-energy, or "soft", X-rays. "Soft X-rays can penetrate deeper in Titan's atmosphere and reach denser regions [than UV]," Pilling told New Scientist, adding that X-rays set off different chemical reactions in Titan's atmosphere.
They modelled Titan's current atmosphere using a mixture of nitrogen and methane gas, and added water to it to simulate the conditions when the moon is bombarded with water-bearing comets or asteroids – a situation that occurred much more frequently in the early solar system.
A frozen sheet of salty water ice lay below this 'atmosphere' and caused the gas to condense into liquid droplets, like dew settling onto Titan's icy surface.
Then the researchers bombarded the setup with X-rays for up to three days, representing the radiation that Titan would get from the sun over a period of about 7 million years. Afterwards, the still-frozen surface contained some organic compounds, but nothing that could be called the building blocks of life.
But when they heated the samples to room temperature, adenine appeared.
That means Titan's saucepan of proto-life would need a source of extra heat to activate. If there was a warm period in Titan's history, perhaps prompted by volcanic activity or meteoroid impacts, "a primitive life could have had a chance to flourish there", the researchers write.
And Titan is due to be heated up in the next few billion years, when the sun bloats into a red giant star, expanding to the present orbit of Earth, they say.
Chris McKay, an astrobiologist at NASA, says the work is interesting, but adds that it may be difficult for life to get started on the moon's surface most of the time. "Adenine synthesis is important, but because Titan lacks water and essentially lacks any molecule that includes oxygen, prebiotic synthesis cannot get very far," he told New Scientist.
But if impacts sometimes allow water to exist on the moon's surface, "then things might happen", he says. "It is interesting to see how far the chemistry can go."
Jonathan Lunine of the University of Arizona agrees. "This is interesting but not seminal," he told New Scientist.
He points out that adenine is just one of many molecules used by life on Earth, so its creation in the experiment does not mean Titan has all the elements needed to create life as we know it.
Some researchers have speculated that microbes on Titan might breathe hydrogen, eat organic molecules drifting down from the upper atmosphere and excrete methane. But so far there is no evidence of life on the moon, and if any does exist, it may use entirely different building blocks from that on Earth (see Life - but not as we know it).
Journal reference: Journal of Physical Chemistry A (DOI: 10.1021/jp902824v)
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It's Life Jim. . . but Not As We Know It
Fri Jun 26 01:15:50 BST 2009 by billzfantazy
I wonder @ the hubris of people who believe life can only exist in Earth like conditions....it's all we know.... but not all we can imagine.
It's Life Jim. . . But Not As We Know It
Fri Jun 26 07:04:42 BST 2009 by Julian
I've wondered that same thing for a long time. Look at how every living thing on this planet evolves to live in any environment. I think it's all just time, and in time anything can happen.
It's Life Jim. . . But Not As We Know It
Fri Jun 26 09:28:12 BST 2009 by C Sagan
I agree, we havent ventured past our solar system but many act like they understand the entirety of the universe. The question we should be asking is how we will identify life when we do blunder into it. It may be very hard for us to identify due to our carbon-bias.
Leave Us Alone
Fri Jun 26 01:26:06 BST 2009 by derekcolman
As a Titan, I beseach you to halt this research. We have no wish to associate with you primitive apemen on Earth.
Leave Us Alone
Mon Jun 29 22:54:12 BST 2009 by anonymous
I'm a Titan too; unfortunately some of my kind are grievously shortsighted and make silly comments about stopping research. Be it known that many of us wish to share knowlege with the hot fast creatures that have lately popped up on a planet of the inner Solar System (we thought no planets within Jupiter for several millions of years...it's hard to conduct good astronomical observations from our world, you know).
Do not take advantage of us simply because it takes us six of your hours to write a sentence. We've already developed our super-conducting computers and robots to take care of our metabolic shortcomings AND any hostilities. We've had them for about 100 million years now, and they work faster than you can yet imagine.
They even preserved YOUR world about 65 million years ago...
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