Ken Tapping, May 9, 2007

Finding life on a world other than our Earth would be one of the most momentous discoveries in history. Although most scientists believe that there is life out there at other places in our universe, it is not the same as actually knowing there is.

From what we know at the moment, other than our Earth, the best candidate world in our Solar System for bearing life is Mars. Billions of years ago Mars was warmer than it is today. It had a thicker atmosphere, with rivers, lakes and oceans. Life could very well have appeared there too. However, Mars is smaller than the Earth and further from the Sun. Its atmosphere leached away into space and the planet became a frozen desert. Despite this, the descendants of ancient Martian life could still be there, below the surface. The big question is how should we look for it? Mars is a very big place.

People are flexible and can better respond to serendipity, so manned expeditions would be best. Unfortunately, we cannot yet send people to Mars; we are limited to sending robots. These are not as good, since they are less adaptable to unexpected situations. Having to observe from the Earth or from space is worse. Unless we are poking around on the surface of a world, we need simple observations that can yield evidence of life, such as searching for methane in a planet's atmosphere.

Methane molecules are made up of four hydrogen atoms attached to one carbon atom. Methane is a common chemical in the gas clouds between the stars. Planets and stars form from these clouds. When a star starts to shine, the high temperatures destroy any methane picked up with the primordial material. Large planets lying a long way from the Sun, such as Jupiter, Saturn, Uranus and Neptune can retain methane indefinitely. However, small planets lying closer to the Sun cannot. In our Earth's atmosphere, methane can diffuse upwards where it is destroyed in the ozone layer, and any getting past that is broken down by solar radiation. On our world methane won't last for more than a few decades. Despite this, there is quite a lot of it in our atmosphere. It is coming from rotting vegetable material in bogs and swamps, and from the digestive systems of cattle, termites, and to some extent, from us. In other words, the presence of methane in our Earth's atmosphere is evidence that there is life here on Earth. We have detected methane on Mars too. Is it proof of Martian life?

On Mars, any methane would vanish in a few centuries. Something has to be topping up the supply. Volcanoes can release methane from the remains of ancient life buried in the rocks. However, Mars is now completely solidified and volcanism stopped on Mars billions of years ago. An additional piece of interesting evidence is that the methane is not uniformly spread throughout the atmosphere. In some areas there is more than in others. Is this where we need to start searching? Of course microscopic creatures living under the ground are not going to be easy to find. Space missions are planned that will dig into the ground to look for tiny Martians. It is ironic that the best evidence for life might come from decomposition and flatulence.

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Ken Tapping is an astronomer at the National Research Council Herzberg Institute of Astrophysics (NRC-HIA), and is based at the Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9 Tel (250) 493-2277, Fax (250) 493-7767,
E-mail: ken.tapping@nrc-cnrc.gc.ca