Viking: the
Search for Life

In 1976 the Viking 1 and 2 landers undertook searches on the Martian surface for the chemical evidence of present or past life on Mars. The images shown below give a picture of one of the backup landers, and two different views of the Martian surface as photographed from Viking 1.

Viking lander (Ref) and two views of the Martian surface from Viking 1 (Ref)

In addition to photgraphing the surface, the Viking landers undertook a series of experiments at two points on the surface to find evidence for life.

The Experiments

The 4 basic experiments that the Vikings carried out to search for evidence of life were:
  1. Gas Metabolism: look for changes in the atmosphere consistent with metabolism in the Martian soil.
  2. Labeled Release: Look for release of radioactive carbon dioxide consistent with metabolism from organic material labeled by radioactive carbon.
  3. Pyrolytic Release: Search for radioactive compounds in soil by heating soil exposed to radioactive carbon dioxide.
  4. Mass Spectrometer: Search directly in Martian soil for organic compounds known to be essential to Earth life.
These experiments were built around the hypothesis that if there were life on Mars it would have a similar metabolism to life on Earth, and that it would have a similar biochemistry based on the same organic compounds important to life on Earth.

The Results

The results of these experiments were complex. The first three gave positive results, but the complete absence of any organic compounds in the Martian soil according to the mass spectrometer experiment suggests that the positive results for the first three were not evidence for life, but rather evidence for a complex inorganic chemistry in the Martian soil. Thus, the Viking verdict was that there was no evidence for present or past life on Mars.

Renewed Interest in Martian Life

This issue has been given renewed impetus by the recent claim (see also this and this) that a meteorite found on the Earth was once part of Mars (because of detailed chemical composition), and that there may be evidence in this rock for past organic activity. However, this is a very open topic at the moment, since there potentially are other explanations of the meteorite's content. We will have to wait on further evidence to clarify this issue.

The Newest Martian Missions