Concepts for detection of extraterrestrial life/Chapter 3
The Vidicon Microscopes
The detection of life by looking for it sounds elementary; however, this seemingly simple technique is extremely complex and involves numerous technical problems. The usefulness of a visual method lies in the extensive background of classical terrestrial biological observation from the macrocosm to the microcosm. In addition, the morphological approach does not depend upon assumptions concerning the nature of extraterrestrial biochemistry. Certain structural attributes, with varied degree of elaboration and application, are not expressions of particular specific form, but of life itself.
A television (vidicon) microscope for planetary exploration has been suggested by Dr. Joshua Lederberg of Stanford University. The investigation of this idea is being carried out in his Instrument Research Laboratory and in Dr. Gerald Soffen’s laboratory at the Jet Propulsion Laboratory of the California Institute of Technology. These groups are assessing the problems of using the microscope as an instrument of life detection.
Terrestrial atmosphere and soil contain a multitude of viable or moribund microscopic organisms, Bacteria, algae, fungi, protozoans, and diatoms are commonly found. Fragments of organisms and fossil forms are also frequently among the components. Special parts of organisms, such as seeds, pollen grains and spores, comprise an important fraction because of their capability of surviving rigorous environmental conditions.
Recognition and identification of micro-organisms by microscopy is often difficult and uncertain; however, in many cases, characteristic morphology is highly indicative and sometimes conclusive. Specific form, size, symmetry, optical properties, surface features, pigmentation and intricate internal architecture are among those typical details which have made the microscope useful. In addition to conventional use, the microscope may be extended to carry out microspectrophotometry, microhistochemistry and microfluorometry, which would provide chemical information concerning the object or materials in the field of view.
The simplest model of a microscope for space use is called the “abbreviated microscope.” This is a fixed-focus, impaction, phase-contrast instrument
Figure 4.—The abbreviated microscope. An aerosol for carrying particles is injected into the instrument and is impacted onto the impaction plate through a nozzle implanted in the condenser lens. The objective lens and the lamp are fixed in relation to the plane of focus. The sample is collected through a gas-operated aerosol aspirator. The instrument has no mechanical moving parts.
which offers a unique solution to the “deposition” of a sample for optical examination (fig. 4). An aerosol sample is injected into the plane of focus of the microscope through an orifice in the condenser lens. The image is transmitted by vidicon. The lens system observes a 100-μ field with 0.5-μ resolution. The vidicon picture, when telemetered to Earth, would require a great deal of data transmission; it might take hours to send a single picture. When more power and larger antennas are available, and with special data handling techniques, this time may be reduced significantly.
A more complex idea under development employs spectral and spatial scanning as a criterion for the selection of objects of interest. Specific ultraviolet absorption of particles is carried out by scanning microspectrophotometry. This has been developed to include the fluorometric capability of detecting the primary fluorescence of native compounds and fluorescence due to products formed by specific reactants. The scanning technique is also being investigated for use with biological stains.
Other areas being explored are automatic focusing, changes in magnification, the use of more sensitive imaging devices, and improved sample preparation to remove the inorganic fraction.