|
Even though, among the many planetary systems known, we have not managed to find a planet that resembles the Earth very closely, there are already some fairly clear ideas about what we should look for in the atmospheres of terrestrial type planets so as to determine whether they host at least primitive life forms, like those we see in the above image.
These are the Desulfotomaculum acetoxidans bacteria, noted for their strange characteristic of using sulphur-rich molecules, such as those from the decomposition of proteins, in their respiration and metabolism. For these bacteria sulphur is as essential to life as oxygen is for us, and as we release carbon dioxide into the atmosphere, these microscopic beings release hydrogen sulphide as a waste product.
If a terrestrial type planet were to host large colonies of bacteria similar to these, its atmosphere may contain unmistakable traces of their presence on the surface. So is it sufficient to identify hydrogen sulphide (poisonous for us) in a planet's spectrum to conclude that the planet hosts alien life?
Well, things aren't quite that simple unfortunately. Apart from the observational difficulties, the presence of water in the atmosphere can mask the hydrogen sulphide, and also, any hydrogen sulphide detected could actually have come directly from intense volcanic activity.
However, the problem of detecting the signature in the atmosphere can be overcome by noting that increased levels of hydrogen sulphide also cause increased levels of elemental sulphur in the atmosphere, and this has a much clearer spectroscopic signature.
Renyu Hu, a doctoral student at MIT (Massachusetts Institute of Technology), simulated terrestrial type planets, orbiting within the habitable zone, with nitrogen-rich atmospheres but with 1000 times more sulphur than that of Earth. The results showed that the presence of sulphur bacteria can increase the levels of sulphur in the atmosphere by a factor of 10, compared to a planet with no life. This might be a good starting point for understanding how much life there is in the Cosmos, but should probably be used in combination with other bio-signatures given the potential confusion with volcanic gasses.
|
