Concepts for detection of extraterrestrial life/Chapter 10
The Wolf Trap
When Professor Wolf Vishniac conceived a device to search for life in space it was inevitable that his biologist friends would name it the “Wolf trap.” The original Wolf trap was built to demonstrate on Earth the feasibility of detecting automatically the growth of micro-organisms on Mars. When operated either on the laboratory floor or outdoors, the feasibility model signals bacterial growth within a few hours after activation.
The heart of the Wolf trap is a growth chamber with an acidity (pH) detector and a light sensor; the former senses the changes in acidity which almost inevitably accompany the growth of micro-organisms, while the latter detects changes in the amount of light passing through the growth chamber. Micro-organisms, such as bacteria, turn a clear culture medium turbid (cloudy) when they grow. It is the change in turbidity which the light sensor measures. The pH measurement complements the turbidity measurement by providing an independent check on growth and metabolism. When either or both of these changes occur, the sensors can communicate this information to a telemetering device which in turn relays the results back to Earth.
The reason for first searching for micro-organisms on Mars is that even in the absence of higher plants and animals, the basic ecology (the interactions between the organisms in a biological community) would not be changed; it is possible to maintain a planetary ecology by micro-organisms alone, though not by animals and higher plants in the absence of micro-organisms.
The biological reasoning behind the particular approach of the Wolf trap was presented by Professor Vishniac as follows. All of the organisms of an environment must have a source of raw materials and energy for growth. Some, like the green plants, can use light energy to manufacture energy-rich chemicals (food); this process is named photosynthesis. Others, like humans, must consume either the photosynthetic plants, or animals which subsist 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 continously measure the change in turbidity as a result of microbial growth and telemeter to Earth the magnitude of the change. From this is would be possible to plot a growth curve. Similarly, a change of acidity can be signaled by the pH detector in terms of rate change, rather than just a yes-no signal.
An essential feature of the Wolf trap operation is the sampling system. Originally a vacuum chamber was used to gather a sample of dust. When the Wolf trap was placed on the floor, a fragile glass shield was broken, allowing
Figure 14.—Wolf trap experimental breadboard with cover removed. The Wolf trap measures 5 × 7 × 7 inches with the cover in place. The sample-collector is extended on the right. The black bottle on the left is the pickup-gas reservoir. Immediately to its right is a release valve and a pressure regulator which are connected to the pickup by the Teflon tubing. The electronics are packaged underneath the gas reservoir, gas valve, and pressure regulator and cannot be seen in this photograph. In front of the assembly, just to the right of center, are the media reservoir and media dump mechanisms. The culture chamber and sensor unit is partially obscured beneath the media reservoir.
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.
The next step is greater refinement of the entire system to reduce its size, increase its detector sensitivity, improve the sample collection efficiency, and fully qualify the instrument for space flight so that the Wolf trap will be ready for installation in a Mars-bound spacecraft.
Figure 15.—Wolf trap optics.