Has the Cassini-Huygens space mission found something remarkable?
Reflections on the meaning of a news item…
About a week ago, I heard a report on Australia's ABC radio network about scientific findings on the possibility of life existing on Saturn's moon Titan.The findings were based on data from the Cassini-Huygens space mission, a joint project of American and European space agencies.
Since I heard the radio report, I've done some web searching, and found that quite a few media outlets round the world have reported about the findings: the UK Telegraph 1, Time Magazine 2, New Scientist 3, China's People's Daily 4…
I also found that one of the scientists working in this area, Chris McKay of NASA, has provided internet users with a concise "horse's mouth" account of the latest research about Titan... 5
Why am I writing this article? Not just to repeat what's been said elsewhere, but to reflect on the significance of what has be noticed about Titan's atmosphere.
What has actually been found on Titan? Neither an actual organism, nor substances clearly produced by one; but an absence – a lower-than-expected amount of certain substances in levels of the atmosphere near the surface...
The point is, these substances (the ones which aren't there in the quantities generally expected) are compounds which a few scientists (including Chris McKay) had earlier suggested that organisms at the surface of Titan might be consuming. 6
As a general rule, it is difficult to draw conclusions from something that is absent – it is like an "argument from silence" – drawing conclusions, not from what someone said, but from the fact that they didn't say something. Then again, a footprint is also a sort of absence – a bit of sand or mud not in its usual position.
Has the Cassini-Huygens mission found chemical footprints of something remarkable?
The organisms which may exist on Titan are described as methanogens – producers of methane (CH4), a substance long known to exist in Titan's atmosphere.
There are methanogens here on Earth – all microscopic – some of which make methane by combining hydrogen (H2) with carbon dioxide (CO2). The suggestion is that methanogens on Titan combine hydrogen with hydrocarbons such as acetylene (C2H2), and ethane (C2H6).
Why make a fuss about a microbe or two such a long way away?
The discovery of extraterrestrial acetylene-consuming methanogens would be of great interest to biochemists (people studying the chemistry of life), no matter how small or simple the organisms themselves might be.
Also, if the organisms are changing the composition of Titan's atmosphere, there would have to be big populations of them... We would not be talking simply about a particular sort of organism, but about a particular sort of world.
Several decades ago, James Lovelock and Lynn Margulis began to write about the Gaia hypothesis, raising the question of how a system of interdependent living things (animals, plants, fungi, bacteria...) interact with the world they live on. They used the name of Gaia, the Greek goddess of the earth, to express the idea that a world with life is a self-regulating system like a gigantic organism.
According to Lovelock and Margulis, the atmosphere of a world that has life will be quite different from that of a world without life. 7
For instance, the atmospheres of both Mars and Venus have a high percentage of carbon dioxide (CO2). That is not surprising, because carbon dioxide is a stable molecule that can form naturally out of atoms of carbon and oxygen, both of which are quite plentiful on Mars and Venus... and on Earth as well. The interesting point is that Earth has a much lower proportion of carbon dioxide in its atmosphere. This is because CO2 is constantly being removed from Earth's air by plants, algae and bacteria.
How to find life elsewhere in space?
By landing miniaturized laboratories to dig up the ground in various places and test it for living things?
Or by developing a bigger picture of different worlds, looking for instance at what their atmospheres are made of, and asking whether there is anything about the big picture that suggests something is living there?
The dig-and-test approach sounds logical, but raises questions about where to dig, how to conduct the tests, how to assess the results. The Viking Program landers that reached Mars 30 years ago were an attempt to find life this way. The machinery did what it was supposed to do, but failed to find organic (carbon-chain) molecules in the areas tested, and didn't resolve the question of whether Mars has life. 8
The second approach to finding life – the big-picture approach – is more in line with the Gaia hypothesis. It is what scientists have been doing with the data from the Cassini-Huygens mission to Titan...
These differences have led some people to think that nothing could live on Titan. E.g. In a book published in 1977, the British astronomer and media identity Patrick Moore says: "There is no suggestion that an atmosphere of this sort could support life even if conditions on Titan were suitable in other ways." 9
David Grinspoon is a NASA scientist who for several years has been arguing that the possibility of life on Titan has to be taken seriously. He uses the term "living worlds hypothesis" to describe a view which he says is "closely related to the Gaia hypothesis". According to the living worlds hypothesis, Grinspoon says, "planets that are geologically and meteorologically alive are much more likely to be biologically alive as well". 10
Observations from Cassini-Huygens and from observatories on Earth have established that Titan is indeed meteorologically alive. It is a world with enormous lakes in its polar regions, 11a world where it rains. 12
The rain and the lakes are not made of water but of liquid methane and ethane.
Titan's surface is too cold for liquid water, although water may possibly exist in liquid form underground.
T.S.Eliot's poem "The Waste Land" (1922) speaks of the disappointment of a place where there is "no water but only rock". 13
When I was growing up in the sixties and seventies the solar system beyond Earth was looking more and more like such a waste land. With the help of better technology, including space probes, the human race was learning fast about our neighbor planets, Venus and Mars. And what we were learning destroyed a lot of fantasies.
Before that, astronomers had observed that Venus has clouds, and this had led to conjectures about oceans or swamps. It turned out that the planet's surface is far too hot for that... In the case of Mars, too, astronomers had seen polar ice caps and seasonal changes of surface colour, which led people to speculate about plants, animals, even artificial canals... However, the Martian surface that the space probes photographed was depressingly like the surface of the Moon... There were, indeed, signs that some liquid had flowed across the surface in the past, but nothing now...
Earth was beginning to look like the one and only oasis in a solar system otherwise dry and lifeless. And no-one knew, then, whether other solar systems existed at all.
Todays, thanks to the Cassini-Huygens mission, we know there is at least one other world where liquid (albeit not water as such) is flowing among the rock.
Methane (CH4) is a simple compound of carbon and hydrogen, found in the Titan's atmosphere. In the upper atmosphere, ultraviolet radiation causes methane molecules to let go of some of their hydrogen atoms, and enables more complex carbon compounds to build up; some containing nitrogen as well as hydrogen and carbon.
Similar chemical process have often been performed in laboratories on Earth since Miller and Urey conducted their classic experiment in 1952. Miller and Urey were not studying Titan; they were looking for a model for the way life here on Earth got started.
Returning to the most recent findings about Titan...
As I've mentioned, the atmosphere just isn't behaving the way most specialists expected.
Something seems to be happening, in the region at or near the ground, that takes out molecules of acetylene (C2H2) and hydrogen (H2). A remarkable finding, but in keeping with what has been noticed about the amount of ethane (C2H6) at the surface...
These very molecules, acetylene and hydrogen, were named, several years ago, along with ethane, as possible energy sources for life on Titan. 14
The big picture seems to be that Titan is a world which not only builds up organic compounds, with the help of solar energy; but also breaks them down again, replenishing raw materials (like methane) and releasing energy.
A process of building up and breaking down happens on Earth as well. We call it the carbon cycle...It's a cycle in which consuming is as necessary as creating. A dance of Kālī.
On Earth today all of the building up, and most of the breaking down, is done by living organisms. For instance sugars such as glucose (C6H12O6) are built up by plants through photosynthesis, and consumed as an energy source by many living things. On the other hand it is well established (by tests such as the Miller-Urey experiment) that an atmosphere like Titan's can build a range of carbon compounds without the help of organisms. The question now is whether Titan can break compounds down again, as efficiently as it seems to be doing, without living things playing a part?
In spite of the obvious differences between Titan and Earth, Saturn's biggest moon is starting to look very like another Gaia.
Colin Robinson, 14 June 2010
7 "Controversy is rife on Mars" (Interviews with Carl Sagan and Lynn Margulis conducted by Stewart Brand); in Brand, Steward; Space Colonies; Penguin 1977; p 123 to 124.
8 "Controversy is rife on Mars" (Interviews with Carl Sagan and Lynn Margulis conducted by Stewart Brand); in Brand, Steward; Space Colonies; Penguin 1977; pp 120 to127.
9 Moore, Patrick and Hardy, David; The New Challenge of the Stars; Hutchinson, Australia, 1977; p 30.
13 Eliot, T.S.; "The Waste Land" Section V "What the Thunder said"; in Eliot, T.S.; The Complete Poems and Plays; Faber and Faber, London, 1969.
© Colin Robinson 2010
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