"You wouldn’t know an alien if it came up and bit you on the bum!"


Well, maybe. But what is life anyway? How do we know that biting each other on the bum isn’t the universal intergalactic way of saying, "Greetings O strange alien, we come in peace."

We’re life, of course. But, as Captain Barbosa articulated so well in the first Pirates of the Caribbean movie: "Yes, we know that."

But what about other life? Would we even know it if we saw it?

Life on Earth comes in two flavours - plants and animals. Plants are squishy green machines powered by sunlight, which use the medium of water and the handy electrical properties of carbon to capture and store energy.

It sounds more a bit more complicated than that when you start foolishly digging around in textbooks. But that’s just scientists trying to sound clever by using long words - the last recourse of the socially challenged.

The goal of it all - where the energy goes - is to make copies of themselves.

Animals are machines too - though not usually green, except after a hard night - which also use carbon to store energy and make copies of themselves. The difference between them is that animals are parasitic. They can’t take energy directly from light, or use minerals directly from the ground.

So they let plants do all the hard work, then steal what they need. Viruses are remnants of the ancient chemical soup of the Earth’s first oceans, four billion years ago. They’re just big molecules, really, left behind by evolution when cell membranes evolved and the first simple bacteria came along.

They’ve adapted now to guerrilla tactics, specialising in invading and hijacking the cells of higher creatures.

Once they’ve taken over a cell, viruses set out to wreck the neighbourhood by attacking others nearby. This is how they make copies of themselves.

Yet they’re only barely alive - not alive at all, really, while outside a cell. Viruses lie on the borderline of life as we know it.

But they ruled - if that’s the right word - the Earth for 10,000 times as long as the human race has existed.

Life channels energy. Everything runs down. It’s called entropy. In time, the universe itself will run down.

Life uses this energy flow. Our bodies use the chemistry of iron to take oxygen from the air, hold it in red blood cells, then release it into our muscles as energy.

In our search for life in the universe, we look for mobile environments, where energy and food and raw materials float around where they can be grabbed easily.

The Cassini probe has observed all these things on surface of Saturn’s giant moon Titan.

If aliens were to look at Earth, they would know that life might be here, because of chemicals in our atmosphere that shouldn’t be there, unless something was replacing them. Unstable things, the most noticeable being free oxygen.

Oxygen is a nasty compound, highly reactive, which is why it’s so useful for life. But its presence in our atmosphere is only due to its continual replenishment by plants. Plants came first, before animals, and the planet’s atmosphere was not oxygenated until around two billion years ago.

Prior to that, it comprised carbon dioxide, ammonia, methane and a mixture of other hydrocarbon compounds.

There is no free oxygen in Titan’s hazy atmosphere, but all the rest are there. Not only that, Cassini has now also detected the next step - a very important one - chemical anomalies on the surface.

One is the unexplained absence of the compound acetylene which, like oxygen on Earth, is a molecule in which energy can be stored.

There should be a certain amount of the stuff in the atmosphere, but there isn’t. The inference is that something is reacting with it.

The second odd thing is an excess of hydrogen molecules in the atmosphere near the surface, but none actually on the surface. Hydrogen is very inert, and although energy can be extracted from it, there would need to be a catalyst or helper compound there to trigger it. In any case, it is the lightest of all gases and shouldn't be near the surface anyway. It's supposed to be drifting out into space from the top of the atmosphere.

Finally, the surface is continually being covered with a layer of hydrocarbons, the food of the first life forms on Earth. Methane rain washes it off into rivers and seas, but it falls continually, recoating the mountains and valleys again.

It was once thought that liquid water was necessary, a mobile medium for the chemistry needed to support life. But now it is possible to see methane as playing the same role in Titan’s rivers and oceans, using acetylene and hydrogen as fuel. The chemistry works.

Whether there actually is life on Titan remains an open question. But these new results mean that the possibility is no longer just speculative.

Finding life on Titan would change our outlook on the world completely. To evolve separately on two bodies in the one star system would mean that life evolves easily, and that it must be everywhere throughout the universe.