Page:Concepts for detection of extraterrestrial life.djvu/47

Rh because we can be certain that if life exists at all on Mars, there will be at least one photosynthetic species that captures energy from the Sun.

The composition of the medium is one of the most interesting problems connected with the Gulliver experiment. Obviously, the success of the experiment depends on the correct choice of nutrients. There are reasons for believing that if life exists on Mars it will be carbonaceous life, as it is on Earth. One can therefore feel reasonably confident that organic compounds of some kind will be metabolized by Martian organisms. However, as the number of possible organic compounds is virtually limitless, this premise does not narrow the range of choices very much. What we need for the Gulliver medium are organic substances that are readily decomposed into carbon dioxide by living organisms and that are of widespread occurrence in the solar system. This problem can be approached experimentally. In fact, the experiment has already been done and has been referred to in the introduction to this book; that is, the experiment of irradiating a mixture of gases simulating the primitive atmosphere of the Earth and planets. As Dr. Stanley Miller first showed, this experiment yielded a number of organic acids, such as formic, succinic, and lactic acids. These acids have exactly the characteristics referred to above: they are readily metabolized to carbon dioxide by terrestrial life, and there is reason to believe that they were formed in large amounts on primitive Mars. It is contemplated that these and a number of other compounds of a similar nature will be among the radioactive nutrients in the Gulliver medium.

Several working models of Gulliver have been built. The model shown in figure 13 has been tested under a variety of conditions: from the sand dunes of Death Valley to above tree-line at the 12,000-foot elevation on White Mountain, California, and from the salt desert of southern California to the woods of Rock Creek Park in Washington, D.C. In all of these places, Gulliver was able to detect microbial life in a matter of a few hours.