Concepts for detection of extraterrestrial life/Chapter 9
The Radioisotope Biochemical Probe: Gulliver
This instrument, named after Swift’s famous fictional traveller to strange places, is designed to search for microbial life on Mars. The project scientists for NASA are Dr. Gilbert V. Levin of Hazleton Laboratories, Inc., and Dr. Norman H. Horowitz of the California Institute of Technology.
Gulliver consists of a culture chamber that inoculates itself with a sample of soil. The chamber contains a broth whose organic nutrients are labeled with radioactive carbon. When micro-organisms are put into the broth they metabolize the organic compounds, releasing radioactive carbon dioxide. The radioactive carbon dioxide is trapped on a chemically coated film at the window of a Geiger counter. The counter detects and measures the radioactivity; this information will be conveyed to a radio transmitter which will signal it to Earth. Gulliver can detect growth, as well as metabolism, by virtue of the fact that the rate of carbon dioxide production increases exponentially (geometrically) in growing cultures. Exponential production of carbon dioxide would provide strong evidence for life on Mars and would make it possible to estimate the generation time; that is, the time required for doubling the number of organisms in the culture.
In addition to a culture chamber and counter (actually a group of counters in anticoincidence circuitry), Gulliver contains a built-in sample collector. This mechanism consists of two 25-foot lengths of kite line and chenille wound on small projectiles. The windings are made in the manner of harpoon lines to prevent snagging, and the strings are coated with silicone grease to make them sticky. When the space package arrives on Mars, a miniaturized programmer will take charge of Gulliver (actually, at least two Gullivers will be used, one as a test instrument and the other as a control). The projectiles will be fired, deploying the lines over the surface of the planet. A tiny motor inside the chamber will then reel in the lines, together with adhering soil particles. After line retrieval, the chamber of Gulliver will be sealed, and an ampule inside will be broken, releasing the previously sterilized radioactive medium onto the lines.
Both the test and control Gullivers will be inoculated with soil simultaneously, as described above, but the control instrument will be injected with a metabolic poison soon after inoculation. The purpose of this step is to make sure that any carbon dioxide evolution that is detected is of biological origin. If space is available for more than two Gullivers, the nature of the antimetabolite can be varied so as to provide information on the chemical sensitivity—and therefore on the chemical nature—of Martian life.
In principle, Gulliver is capable of performing many different kinds of metabolic and biochemical experiments. For example, a modified version of the current model would be able to detect photosynthesis by measuring the effects of light and darkness on the evolution of carbon dioxide. This application of Gulliver has been demonstrated in the laboratory. It is important 
Figure 13.—A working model of Gulliver, tested under a variety of conditions. 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.