Concepts for detection of extraterrestrial life/Chapter 11

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CHAPTER XI

The Multivator Life-Detection System

The multivator is a miniature laboratory for conducting a variety of biochemical or biological experiments on Mars. The nature of the experiments is limited only by those biological properties which can be measured by a photomultiplier as an output transducer. The device was conceived by Dr. Joshua Lederberg at Stanford University. The experiment which has received Dr. Lederberg’s particular attention is the detection of phosphatase activity. This is because:

1. phosphatase is widespread among terrestrial organisms;

2. it catalyzes the hydrolysis of phosphate esters with moderate specificity;

3. it is involved with the role of phosphorus in metabolism and energy transfer which may be a universal characteristic of carbon-based aqueous living systems; and

4. it is capable of being detected with relatively high sensitivity.

A functional test for the presence of hydrolytic enzymes, such as phosphatase, detects the catalysis of ${\ce {AB{}+ H2O {\overset {enzyme}{->}}AH{}+ BOH}}$. The basis of the phosphatase test is the release of AH which differs from AB in being fluorescent. In this case, A is a fluorescent residue and B is a phosphate that permits the fluorometric assay of phosphatase. The multivator is designed to carry out such assays as well as many others. It does this by mimicking in miniature a great many of the kinds of instruments used in a typical biochemical laboratory. The basic elements of the instruments are a light source followed by a filter; the sample under investigation; another filter centered at either the same wavelength as the excitation filter for colorimetry or light scattering, or at a different wavelength if fluorometric observations are to be made; and finally, a light detector, usually a photomultiplier. Figure 16 shows several cut-away views of the multivator.

The most recent version of the multivator consists of 15 modules arranged in a circle around an impeller (figs. 17 and 18). Each of the modules basically consists of a reaction chamber, solvent storage chamber, tapered valve pin, explosive-charge bellows motor, and a filtered light source. The entire solvent chamber is sealed prior to operation by a thin diaphragm which is placed in front of the pointed valve tip.

In operation, dust-bearing air is drawn through the impeller and in front of the reaction chambers. The impeller imparts sufficient velocity to particles above 10μ in diameter to fling them into the reaction chambers where they tend to settle. Upon completion of the particle-collecting operation, the explosive-charged bellows motors are electrically ignited. Expansion of the bellows results in the sealing of the reaction chambers and the injection of the solvent. The substrate materials, which have been stored dry in the reaction chambers during flight, are dissolved and the reaction begins.
Figure 16.—Layout of the multivator assembly. After a preset reaction time, the excitation lamps are turned on sequentially and the light signal, or fluorescent level in the case of the phosphatase assay, is detected by the photomultiplier tube. This information is then reduced to digital form and transmitted. One reading per chamber every 15 minutes would be satisfactory, requiring a fraction of a bit per second for telemetry. Certain chambers of the instrument are designed so that they will not collect soil. This permits a comparison of the behavior of the solvent-substrate mixtures subjected to the same conditions of voyage and Martian environment with the results from those reaction chambers receiving dust samples. This helps to ensure that the information concerning a sign of life is not due to a faulty test.

Modular design of the multivator offers several advantages. First, the entire multivator becomes potentially more reliable with 15 independently operated modules. Secondly, each module may be filled with different types of solvents, thereby increasing the range of experiments that can be performed with a single multivator package. Thirdly, the modular design allows more flexibility in making the final choice of the actual experiment to be performed. It also permits postponing this choice to a relatively short time before the launch date of the mission. More than a full complement of modules could be under design and development; postponement of final choice would not interfere with orderly spacecraft development and construction as long as the experiments met the very simple interface parameters characteristic of the multivator experimental modules.

Figure 17.—Multivator, with housing partially removed.	Figure 18.—Assembled multivator.