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CHAPTER 18 BIOLOGICAL OBJECTIVES AND STRATEGY FOR THE DESIGN OF A SPACE VEHICLE TO BE LANDED ON MARS D. A. GLASER INTRODUCTION The age-old question of the existence of extraterrestrial life has been given new and more specific meaning by the remarkable development of molecular biology during the last two decades. The question also has a new immediacy because of the rapid development of space vehicles capable of sending instrumentated packages to the neighborhood of Mars and other planets and of landing them on their surfaces. Our new knowledge of the detailed molecular mechanisms of genetics and of biochemical physiology has led us to delineate our interest in extraterrestrial life in the form of two specific questions: 1. Is it highly probable that living systems will arise in any planetary environment capable of supporting a complicated biochemistry? 2. Is the terrestrial form of life, based on nucleic acids and protein chemistry, unique in the sense that all living systems that can arise in an environment similar to that on Earth share the same bio- chemistry? These questions have particular interest because biologists want to know if their science is characterized by the same universal causal necessity that has been found in chemistry and physics as they apply to the solar system 325

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326 MARTIAN LANDINGS: UNMANNED and the remote regions of the Universe as we know them from astronomical observations. It would be an extremely important new scientific fact if it were found that living systems arise with high probability in favorable environments and that they are always based on approximately the same biochemistry that characterizes terrestrial life. It is probable that the majority of biologists today believe that life will arise and evolve to some stage with high probability in a favorable environ- ment and that it will be based on a biochemistry not very different from that found on Earth. In designing instruments to be sent to Mars on a spacecraft, therefore, they would like to include a variety of instruments that would be capable of establishing the presence of biochemical sub- stances, such as amino acids, proteins, nucleic acids and a variety of other substances found in terrestrial organisms. It is generally agreed among most biologists, however, that one should not overlook the use of detectors that could identify the presence of life even if its chemistry were quite unfamiliar to us and if we had no method for making a detailed chemical study of the strange organisms. Detectors for such unusual forms of life would have to depend on more general properties of living systems such as morphological, dynamical, thermodynamical, or ecological properties. Certain kinds of morphological complexity and symmetry are extremely improbable in non-living systems. Arguments that such unusual shapes are the products of living systems would be much strengthened if many similar copies of such shapes are discovered. Another class of properties possessed by living systems that could be the basis for a detection system is that they exhibit motion of a type that would not be expected for non-living systems. One might imagine a system to detect the presence of bacteria among a large number of non-living particles of about the same size and shape if they were motile and exhibited rapid swimming motions sufficiently vigor- ous to be distinguishable from the Brownian motion characterizing all particles of small size. The motion of larger systems might exhibit a re- sponse to a stimulating signal not expected of non-living systems. One can imagine the Martian spacecraft to be equipped with means of making a noise or flashing a light and observing whether any of its environment begins to move in response to these stimuli. A third characteristic of living systems that might be used to establish their presence on Mars are thermo- dynamic properties having to do with reaction rates and entropy produc- tion. In particular, there have been very speculative arguments that the reaction rates in biological systems must be significantly greater than the reaction rates in their non-living environment in order that the organisms be able to grow, reproduce, and maintain their integrity at a greater rate than the erosive or degradative processes that act on the environment in the weathering of rocks and other such processes. A general characteristic

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Biological Objectives and Strategy for Space Vehicle Design 327 of terrestrial organisms is that they go through a stage of exponential increase in numbers under favorable conditions. In the case of terrestrial microorganisms, such kinetics have been detected calorimetrically. On Mars one can imagine that the spacecraft can be instructed to scoop up a handful of frozen soil and install it in a sensitive calorimeter. When the soil has been warmed up to growth conditions, an exponentially increasing evolution of heat could indicate the multiplication of microorganisms, if other autocatalytic processes can be discounted. Another possible basis for detection of unfamiliar life-forms would be generation by them of various signals not expected to be emitted by objects in their non-living environment, such as bursts of light or sound. If it can be established that optical activity is produced only rarely by inorganic processes, its detection is another fairly non-specific signature of life. In general, the detection of living systems by observations that do not depend on detection of a biochemistry similar to that of terrestrial organ- isms depends on observing some phenomenon that cannot be explained by the laws of physics and chemistry as they are ordinarily applied to phe- nomena not resulting from a long history of sequential events. In this view a b'ving organism is, to be sure, a product of physical and chemical phe- nomena, but has achieved its "living" status through a long evolutionary history of interaction with its environment. To the extent that a phenome- non can be established as being an exceedingly improbable result of short term physical and chemical events, it can be called a living system. An obvious and necessary requirement for the evolution of living systems is the invention of a memory or genetic mechanism that allows the system to retain and use selectively the information corresponding to its successful historical variations. EXPERIMENTAL STRATEGY AND INSTRUMENTATION Many specific instruments and measurements have been suggested for the use of known biochemical and biological techniques for detecting life on Mars. The decision concerning which instrument to incorporate in the actual package involves a balancing among the biological importance of the result to be obtained, the weight of the apparatus, its power require- ments, its volume, its likelihood of successful operation or failure, its data rate requirements, and perhaps still other factors that would be part of an engineering feasibility study. Present policy also requires that the entire spacecraft and all of its equipment be sterilizable in order to minimize the probability of contaminating Mars and thereby destroying possible Martian organisms or modifying the biochemical environment so as to reduce the

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328 MARTIAN LANDINGS: UNMANNED usefulness of Mars for biological study or utility at some future time. In addition to the need for choosing among the possible experimental pack- ages, it is important to outline some sort of experimental strategy by which the logical relationships of the instruments are defined. Let us remark about the practical requirements simply that we want to do whatever is possible to maximize the rate of data acquisition, power resources, experimental versatility, reliability, sterility, etc. One important way to attain several of these objectives simultaneously is to use a compact general purpose com- puter aboard the spacecraft to correlate, organize, and program the se- quence of experimental operations and the transmission of data to Earth. This computer could replace a large number of relays or other switching elements that would normally be used in each separate experiment to turn the equipment on and off and to operate motors, light sources and other active and passive elements in the proper sequence to carry out the desired measurements. The use of a computer to serve these various programming functions would probably eliminate much duplication and have the over- riding advantage that changes could be made in the sequence of experi- ments, and sequence of operations within a given experiment, so that newly gained knowledge using improved laboratory techniques can be incorpo- rated into the experimental strategy of the space laboratory until rather late in its engineering development and construction. In fact, the com- plexity of the total possible answers that the space laboratory might dis- cover in response to various questions it would ask of the Martian environ- ment is so great that it is unlikely that it will be possible to foresee all exigencies and to program the built-in computer for them in advance. It will, therefore, be necessary to envision a constant dialogue between the computer on the spacecraft and much larger computer facilities on earth. These terrestrial computers can be used to increase the effective data rate from Mars to Earth by a system of logical interrogation in which the com- puter on Earth asks "multiple choice" questions and the Martian computer has only to tell which alternative is correct rather than transmitting an explicit answer. A great advantage of having a fairly flexible computer aboard the spacecraft to control its operations is that new experiments and new ideas can be inserted into its experimental program by biologists who can have programs written on short notice to send for the instruction of the spacecraft. Since it seems unlikely that biologists will be able to develop a system of experimental designs radically different from the customary, we will probably want to design the strategy of operation of the Mars spacecraft along the same general logical lines as those used for a well-designed experiment in a terrestrial laboratory. Perhaps it would go something as follows: On first landing or perhaps while the descent is being made, if that

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Biological Objectives and Strategy for Space Vehicle Design 329 is a practical engineering feat, the spacecraft can be recording the tempera- ture, pressure, perhaps chemical composition of the atmosphere, noise level and spectral light intensity. On the basis of these physical observations it would be possible to make a decision concerning which of the various instruments aboard the spacecraft are capable of carrying out their special measurements at the ambient conditions on the planet. It may well be that some of the unexpected forms of life on Mars will live under conditions that are inappropriate for use of the instruments in the package. In par- ticular, if some of the living systems operate at temperatures below the freezing point of water, those parts of the Martian spacecraft devoted to water-based chemistry will not be useful at first. Probably, we should first attempt to apply our life detection techniques to materials as we find them in the normal Martian environment, to the extent that our physical measure- ments have told us that our instruments will work under those conditions. The second stage would be to collect a sample of the Martian surface or of objects or materials that are close to the spacecraft and gradually begin to change the conditions of the collected samples in the direction of the range of usefulness of our life detection systems. If one knows enough by that time of the cycle of the Martian environment between day and night and among the seasons, perhaps the environmental changes during the experi- ment should be chosen to follow the normal Martian ones as closely as possible, particularly if that route leads in the direction of making a larger fraction of the spacecraft's instruments useful. Thus if we land on a frozen Martian tundra at a temperature of —30 °C we would first try to detect optical activity, to measure infrared spectra, to look at the neutron aIbedo with a neutron source in seeking hydrogeneous materials, make mass spec- troscopic and electromagnetic spectroscope and fluorescence measurements, etc., in the materials under their normal Martian conditions. After that we would try to take some of the material into the spacecraft laboratory and gradually raise its temperature or humidity or perhaps add a small quantity of oxygen or other gases in order to make studies of enzyme activity, or ultraviolet absorption in aqueous solution, and others of the biochemical techniques normally used in assaying terrestrial biochemicals and their reactions. It is very likely that the most efficient use of the facilities of the spacecraft will be made by terrestrial control of the experimental sequence in which the decisions at each time are based on previous findings. The intellectual task of making all these decisions in advance for every possible constellation of results is virtually impossible. One can imagine a team of perhaps 20 biologists sitting at a number of small individual consoles which feed a large central computer in constant contact with the computer on the spacecraft. Each of them will be responsible for a certain problem or a set of instruments. Since the round trip for information to flow from Earth

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330 MARTIAN LANDINGS: UNMANNED to Mars is from 5 to 25 minutes, there will be a certain amount of time between the making of a decision and the receipt of the result of the measurement performed by the spacecraft in response to that decision. Thus, carrying out the experiments by the spacecraft will be rather like the actions of a chess master who plays 20 opponents simultaneously, walking from board to board, making his move, and coming back only after half an hour to see what results his move has produced. The individual players, in this case the individual biologists, will have opportunity to consult their colleagues at other universities to ask for expert advice, to go to the library, to make calculations, and in other ways to carry on their usual scientific intellectual activities as they would do if they were carrying out an individual experiment. One can imagine a large Scoreboard, showing the participating biologist the state of each of the sensor and motor func- tions of the spacecraft at any moment, so that he can see what moves are possible for him at every moment and can have in front of him a summary of the kinds of results his colleagues are getting, for these will certainly be factors on which he will base his decisions and desires as the investigation proceeds. It should be emphasized that this procedure is not necessarily committed to delivering all-or-nothing answers concerning the existence of life on Mars but instead is a program aimed at studying the physical, chemical, biochemical and biological condition of that planet. It may well be that Mars is at an early stage or an arrested stage of biological development and will contain on its surface a rich mixture of organic and biochemicals without displaying any of the features of systems that we would call "living". On the other hand, there may be living systems, but this first attempt may not be able to prove that they are living, but only that the biochemical environment allows the possibility of life. Proper design of the instruments themselves and of the logical procedure for their operation and interpretation is bound to yield much knowledge of importance to geologists, physicists, planetary scientists, and biologists.