Page:Concepts for detection of extraterrestrial life.djvu/52

42 the internal vacuum to suck dust into the culture chamber. The sampling mechanism of the breadboard model, like the early model, is based upon the sucking up of dust. However, instead of a “packaged” vacuum, compressed gas forced through a constricted throat produces a partial vacuum which sucks particles into the collection nozzle and carries them from there to the culture chamber.

When the soil inoculum is initially introduced into the culture chamber of the breadboard there is a relatively high signal which drops rapidly as the heavy sand-sized particles settle out of the suspension. The very small particles settle out of the suspension more slowly. Superimposed on this soil settling curve is the growth curve of the organisms. Starting from some low population level, the microbes begin to multiply. When the number of organisms is large enough (around 100,000 per milliliter in the present device) they begin to form a significant amount of the signal.

Naturally, the system cannot discriminate between soil and micro-organisms. The Wolf trap could send a signal change even if there were nothing living on Mars, as, indeed, could any of the other life-detection devices. Suppose for instance, the Wolf trap lands on Mars and almost immediately signals a marked change in the culture medium; the signals show dense turbidity and the acidity increases greatly. About all such data would mean would be that the Martian surface is extremely dusty and the dust extremely acidic. However, if only a few of the chambers indicate change, and the the changes are signaled over the course of several hours or a day, then it can be reasonably concluded that changes have taken place as a result of microbial activity, especially if the turbidity signal increases exponentially (doubled every hour or so), instead of climbing at a constant rate.

Sample acquisition poses one of the most difficult engineering problems in the Wolf trap, as in other life-detection devices. Light scattered by an abundance of small colloidal-sized soil particles might saturate the detectors, allowing the growth of organisms to go undetected. It would be equally unfortunate if an insufficient sample were collected. The concern over the sampling problem is reflected in figure 14, where fully half the volume of the experimental breadboard is taken up by sampling system components. Although it is more complex, the Wolf trap breadboard is less than one-third the size and one-sixth the weight of the original feasibility model. Yet the device is still not as compact as possible. The design engineers of the Wolf trap point out that the bulky solenoid-operated valves in the breadboard can be replaced by one-shot rupture diaphragms in the flight model. This would represent a considerable saving in weight and space.