Signs of life

In the shallow waters of Shark Bay, Australia are strange, mushroom-shaped rocks, called stromatolites. These are produced by living creatures.

Millions of bacteria form slimy mats which trap sand and mud particles. The bacteria then grow up through it to form a new layer, and the rocky structure grows larger and larger.

A cross-section of one of the "mushrooms" shows the layers like tree rings. These structures are really robust and over time become incorporated into rocks as fossils.

Their tree-ring like structures are found in rocks up to 3.7 billion years old, showing that liquid oceans existed back then, when the sun was about 30 per cent fainter and our planet should have been frozen solid.

Conversely, if things were not frozen solid then, our brighter sun means we should be frying now. That this is not the case indicates how living things change their environment to suit themselves, up to a point.
 
This idea was first put forward by James Lovelock, who suggested that evolution in living creatures is not just a process of adapting themselves to their environments, but also one of changing their environments to suit themselves.

In the case of the Earth, living creatures changed the atmosphere. When life first appeared, the atmosphere was rich in methane and carbon dioxide, both greenhouse gases. These made our planet warm when the sun was fainter.

Plants take in carbon dioxide, and use sunlight to convert it and water into carbohydrates, releasing oxygen, which is not a greenhouse gas, as a waste product. Over time the sun brightened, and plants removed more and more carbon dioxide, replacing it with oxygen, and incidentally stabilizing the Earth's temperature.

The idea that living creatures may involve themselves in a little "terraforming" influences how we may search for life on other planets.
 
On Earth, to a large extent the proliferation of plant life is dependent on the carbon dioxide supply. Animals, like us, depend on oxygen to live, breathing out carbon dioxide. The more plants there are, the more oxygen there is, allowing more animals to live, producing more carbon dioxide.

Oxygen is highly reactive, which is why it is so useful to our life processes. It also means it would rapidly disappear from our atmosphere by combining with iron and other elements, unless continuously topped up.

On Mars, the atmosphere is very thin, but what there is of it is 95 per cent carbon dioxide, with about 0.1 per cent oxygen and occasional intriguing traces of methane. There is no real evidence of a  biosphere.

Did Mars have a biosphere in the distant past? There is evidence that there was once liquid water on its surface, forming rivers and lakes, which in turn requires a thicker atmosphere than there is now. In addition, the fact that Mars is often referred to as the Red Planet is another clue.

The red comes from iron oxide, which suggests Mars originally had much more oxygen in its atmosphere. The red rocks in many parts of our world get their colour from iron oxide, formed when plants flooded the atmosphere with oxygen.

This environmental modification idea suggests that for life like ours to develop, we needed our sun to brighten as we replaced greenhouse gases with oxygen.

We often think of red dwarf stars as good places for life-bearing planets, because they are so stable. However, that would mean that life forms taking in the carbon dioxide would cool their worlds, making them less habitable.

They would never share the fecundity of our world in producing plants and animals. We should look for life like ours on planets like ours orbiting Suns like ours.

• After sunset, Venus shines brightly in the west; Jupiter lies in the south and Saturn in the southeast.
• Mars, bright and red, and just past its closest approach to us, rises around midnight.
• The moon will reach last quarter on the 4th.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.