Ken Tapping, February 24, 2010

One of the things that make our planet nice to live on is water. At the temperatures we have on our world, we get water as a vapour, as a liquid and as a solid. In a few places, such as in volcanic areas, we even get it as steam.

When water evaporates from the surface of the sea and lakes, it absorbs solar energy. If the sea is warm, as it is in the tropics, it can produce a lot of water vapour. When the temperature in the atmosphere falls low enough for the water vapour to condense, it liberates the heat. It is this heat that drives most of our weather. It is what makes clouds boil upwards and it is what drives storms. It also moves energy from one part of the world to another. When ice forms, it releases heat. When it melts again it absorbs it, and is an important part of the temperature control mechanism at high latitudes.

There is only one other place in the Solar System where there is a substance doing the job that water does for us; it is Titan, the largest moon of the planet Saturn. The Cassini spacecraft has observed from space a surface covered with lakes, just like much of Canada. When the Huygens spacecraft landed on Titan, it ended up on the bed of a dried up river. On Titan it rains, feeding temporary rivers; there is wind, and there is weather.

However, Titan lies about 9.5 times as far from the Sun as we are, so it gets only a little more than 1% of the sunlight and heat that we receive here. The temperature on Titan's surface is about – 178 degrees Celsius, so the liquid in those lakes and in Titan's rain cannot be water. We believe it to be a mixture of methane and ethane, both hydrocarbons. Methane is a very effective greenhouse gas, which makes it an undesirable thing to have in our atmosphere. However, for Titan it is just what the Doctor ordered, saving it from being even colder.

At those low temperatures Titan's atmosphere cannot be much like ours. It is a thick smog of hydrocarbons. Sunlight is acting on the methane and ethane in the upper atmosphere and producing a witches' brew of organic chemicals. Since organic chemicals are usually soluble in hydrocarbons, those substances would wash down in the methane and ethane rain, ending up in the lakes.

Billions of years ago the Earth's atmosphere was much like Titan's, loaded with methane and other hydrocarbons, but of course a lot warmer. This chemical brew in the oceans probably helped along the beginnings of life on our world. Those life forms gulped down the hydrocarbons and the early plants released oxygen, making our world as it is today.

The lakes on Titan are at least tens of metres deep, so they should be permanent and stable enough for life to form. It really would be great to land another spacecraft on Titan, to make more detailed and longer term studies of the planet, its lakes and its atmosphere, and to search for life. Some more robot explorers would be a great idea, but knowing what we are like when we lay our hands on large reservoirs of hydrocarbons, it might be politic to hold off on large-scale visits until we no longer have a hydrocarbon-based economy.