Page:Concepts for detection of extraterrestrial life.djvu/50

40 on the plants, for energy requirements. In photosynthesis plants consume carbon dioxide; animals eat the plants and produce carbon dioxide. Such an interdependent cycling of raw materials is common within a biological environment. A consideration of a known environment allows one to predict with reasonable accuracy the type of micro-organism that will flourish in it. Such predictions have nothing to do with the size and shape of the micro-organism, nor with its microscopic appearance or its molecular structure. They only deal with its physiology: activities, such as photosynthesis, which would enable an organism to flourish in such an environment.

These predictions are the basis for the several culture media now being considered for inclusion in a Mars-bound Wolf trap. The most important consideration in preparing these media is the knowledge that Mars lacks oxygen in its atmosphere. Hence, a number of media are being devised to support the life of probable anaerobic micro-organisms. A variety of media allows the biologist to test fundamental assumptions about the nature of life and its chemistry, and increases the likelihood of detecting at least one possible life form.

The detection principle of the Wolf trap is susceptible to a variety of modifications. The first device can be a simple unit to meet the weight and power requirements of early spacecraft, or it can be an elaborate multichambered experiment with varied media as mentioned above. The latter, of course, would have the greater scientific value.

The feasibility model has been completely redesigned, and a new model—the “breadboard” model—has been built incorporating changes to make it suitable for space flight (fig. 14). One improvement is a more sensitive method of detecting turbidity. The intensity of a beam of light passing through a turbid bacterial suspension will be reduced since some of the light is scattered to the sides. Instruments which can measure this reduction in direct light intensity “see” turbidity when 100 million organisms per milliliter are present. The unaided eye is a better detector since it can tell if a suspension is cloudy at a concentration of roughly 5 million organisms per milliliter. At least a thousand-fold greater sensitivity is possible by measuring the light scattered to the sides by the suspended organisms, rather than measuring the reduced intensity directly. The particular optical geometry which has been selected for the Wolf trap measures light scattered at an angle of approximately 20° off the forward beam. This system is shown in figure 15.

The response of the first model was a simple yes-no answer. It is more informative to continouslycontinuously [sic] measure the change in turbidity as a result of microbial growth and telemeter to Earth the magnitude of the change. From