Significant Achievements in Space Bioscience 1958-1964 Part 2

Meteorites provide a more representative sample of average planetary matter than the highly differentiated crust of the Earth. Although it is known that the meteorite parent bodies ceased to be geochemically active shortly after their formation, some 4 billion years ago, there is no consensus on the nature of the meteorite parent bodies, not even on such basic properties as size, location, and multiplicity. This is not surprising because the meteorite samples commonly available for study represent only about 10? to 10?6 of the parent body.

Carbonaceous Meteorites

a.n.a.lysis and characterization of the chemical const.i.tuents (organic) of carbonaceous chondrites, including the possible mechanism of their formation, may be expected to improve methods of a.n.a.lyzing samples from the Moon and planets and of interpreting remote automated biological a.n.a.lyses on the planets' surfaces.

Carbon has been detected in all meteorites a.n.a.lyzed; however, both the amount and forms present vary considerably. Among the forms of meteorite carbon are diamond, graphite, cohenite (Fe,Ni,Co)3C, moissanite SiC, calcite CaCO3, dolomite (Ca,Mg)CO3, bruennerite (Mg,Fe)CO3. A summary of the results of carbon a.n.a.lyses on large numbers of meteorites is given in table I ([ref.26]).

Table I.-_Meteorite Carbon_

----------------------------------------------------------------- Meteorite group Number Mean carbon content, a.n.a.lyzed percent by weight ----------------------------------------------------------------- Pallasites 10 0.08 ----------------------------------------------------------------- Ureilites 2 .69 ----------------------------------------------------------------- Bronzite chondrites 12 .05 ----------------------------------------------------------------- Hypersthene chondrites 8 .04 ----------------------------------------------------------------- Enstat.i.te chondrites 8 .29 ----------------------------------------------------------------- Carbonaceous chondrites 16 2.04 -----------------------------------------------------------------

Most meteorites possess only traces of carbon, and studies of this carbon indicate that it is composed largely of graphite, cohenite, and moissanite, with some diamond. However, studies of the carbon in the carbonaceous chondrites have failed to detect any of these forms. Some carbonates are present in a minority of the carbonaceous group, but account for only a small percentage of the total carbon (perhaps about 10 percent of the total C in type I only).

The carbonaceous chondrites contain organic carbon. The word "organic"

is not used in a biological sense, merely as a chemical term to describe compounds of carbon other than carbonates, bicarbonates, and carbides.

No evidence has been found of any form of carbon other than organic, except for traces of carbonates.

Various studies have demonstrated possible methods of estimating the total amount of organic matter present in meteorites. Wiik ([ref.27]) has suggested that organics can be estimated by measuring the loss of weight on ignition. Unfortunately, this method has several disadvantages and gives very low values. Corrections must be made for weight gains due to oxidation of reduced const.i.tuents, such as FeO, Fe, Ni, and Co, and for weight losses due to H2O, S, etc. The water loss is exceedingly difficult to estimate, as part comes from the combustion of organic hydrogen and part comes from the loss of mineral-bound water. The carbon also proves difficult to combust completely, and high temperatures (over 1000 C) are required for efficient conversion to CO2.

In one study the major fraction of organic matter removed proved to have a carbon content of about 47 percent ([ref.28]). Thus, if all the meteorite carbon is present as organic matter of approximately this composition, total organics must be approximately double the carbon content; that is, 2 percent by weight carbon indicates 4 percent by weight organic matter. This estimate may be too low, for Mueller ([ref.29]) has extracted a major organic fraction containing only 24 percent carbon; however, this work has not been confirmed for other meteorites.

Briggs and Mamikunian ([ref.26]) have pointed out that only 25 percent of the organic matter has been extracted, and only about 5 percent of this has been chemically characterized. Most of this 5 percent is a complex mixture of hydroxylated aromatic acids together with hydrocarbons of the aliphatic, napalicyclic, and aromatic series. Small amounts of amino acids, sugars, and fatty acids are also present.

Thus far, these chemical a.n.a.lyses point to an abiogenic origin for the organic matter, and no conclusive evidence exists of biological activity on the meteorite parent body. Microbiological investigations of samples of the carbonaceous chondrites have yielded only inconclusive evidence on the problem of "organized elements."

Several of these microstructures from different carbonaceous chondrites are ill.u.s.trated in a paper by Mamikunian and Briggs ([ref.30]). It has been difficult to identify the organized structures, and most do not have morphologies identical to known terrestrial micro-organisms.

However, they may prove to be a variety of mineral grains, droplets of organic matter and sulfur, as well as a small amount of contaminating terrestrial debris.

A comparison between the photographs of the organized elements observed in the Orgueil and Ivuna meteorites and the synthetic proteinoid microspheres observed by Fox ([ref.25]) point to similarities between the two. One inference from this finding is that the organized elements in carbonaceous chondrites were never alive but, rather, should be considered as natural experiments in molecular evolution. Also, these similarities strengthen the belief that the laboratory experiments are similar to the natural experiments in s.p.a.ce.

In cooperation with the Smithsonian Astrophysical Observatory, NASA has a network to track meteors in the Midwest (South Dakota, Nebraska, Kansas, Oklahoma, Iowa, Missouri, and Illinois). Photographs of meteor trails are used for scientific study, and attempts are made to track and recover meteorites for examination for traces of organic material of extraterrestrial origin.

Fundamental research in terrestrial organic geochemistry has shown that ancient sediments and drill core samples subjected to organic a.n.a.lysis contain certain stable biochemical components of past life. This preserved record is significant not only in studies of early-life chemical pathways but also in studies of the interaction of organic matter with the geological factors. Since life on any planetary body will interact with the soil, or surface material, it is of interest to understand the relations.h.i.+p.

CONCEPTS FOR DETECTION OF EXTRATERRESTRIAL LIFE

It is not possible to present completely convincing evidence for the existence of extraterrestrial life. The problem often reduces to probabilities and to estimates of observational reliability. In almost all cases the evidence is optimistically considered strongly suggestive of-or, at the worst, not inconsistent with-the existence of extraterrestrial life. Alternatively, there is a pessimistic view that the evidence advanced for extraterrestrial life is unconvincing, irrelevant, or has another, nonbiological explanation.

In studies of the laboratory synthesis of life-related compounds and its significance concerning the origin of life, several results seem to suggest that organochemical synthesis is a general process, occurring perhaps on all planets which retain a reducing atmosphere. The temperature ranges must be such that precursors and reaction products are not thermally dissociated. The reaction rates for the synthesis of more complex organic molecules diminish to a negligible value when the temperature range is below 100 C.

Besides the planetary parameter of temperature, an even more fundamental necessity for a living state exists-a liquid solvent system. For terrestrial life forms, water serves this purpose. Water has this and other properties of biological significance because of hydrogen bonding between adjacent molecules in the liquid state.

Ultraviolet radiation could serve as an extraterrestrial energy source for organic synthesis. Research shows that, while an atmosphere is important, living systems can survive a wide range of ambient pressures and are little affected by a wide range of magnetic field strengths.

Oxygen is not a prerequisite for all living systems. While it is sometimes concluded that free oxygen is needed for all but the simplest organisms, less efficient metabolic processes coupled with higher food collection efficiency-or a more sluggish metabolism-would seem to do just as well. Earth is the only planet in the solar system on which molecular oxygen is known to be present in large amounts. Since plant photosynthesis is the primary source of atmospheric oxygen, it seems safe to infer that no other planet has large-scale plant photosynthesis accompanied by the production of oxygen.

The possibility of the existence of extraterrestrial life raises the important question of man's being able to detect it. Research on extraterrestrial life detection is predicated on the ability to develop ways to detect it even when the living systems are based on principles entirely different from those on Earth.

The subst.i.tution of various molecules for those of known biological significance to living organisms as we know them has been investigated; the subst.i.tution of NH2 for OH in ammonia-rich environments leads to a diverse, and biologically very promising, chemistry. The hypothesis that silicon may replace carbon does not support the construction of extraterrestrial genetics based on silicon compounds. (Silicon compounds partic.i.p.ate in redistribution reactions which tend to maximize the randomness of silicon bonding, and the stable retention of genetic information over long time periods is thus very improbable.)

Evidence relevant to life on Mars has been summarized by Sagan (ch. 1 of [ref.10]):

_The Origin of Life_

In the past decade, considerable advances have been made in our knowledge of the probable processes leading to the origin of life on Earth. A succession of laboratory experiments has shown that essentially all the organic building blocks of contemporary terrestrial organisms can be synthesized by supplying energy to a mixture of the hydrogen-rich gases of the primitive terrestrial atmosphere. It now seems likely that the laboratory synthesis of a self-replicating molecular system is only a short time away from realization. The syntheses of similar systems in the primitive terrestrial oceans must have occurred-collections of molecules which were so constructed that, by the laws of physics and chemistry, they forced the production of identical copies of themselves out of the building blocks in the surrounding medium. Such a system satisfies many of the criteria for Darwinian natural selection, and the long evolutionary path from molecule to advanced organism can then be understood. Since nothing except very general primitive atmospheric conditions and energy sources are required for such syntheses, it is possible that similar events occurred in the early history of Mars and that life may have come into being on that planet several billions of years ago. Its subsequent evolution, in response to the changing Martian environment, would have produced organisms quite different from those which now inhabit Earth.

_Simulation Experiments_

Experiments have been performed in which terrestrial micro-organisms have been introduced into simulated Martian environments, with atmospheres composed of nitrogen and carbon dioxide, no oxygen, very little water, a daily temperature variation from +20 to -60 C, and high ultraviolet fluxes. It was found that in every sample of terrestrial soil used there were a few varieties of micro-organisms which easily survived on "Mars." When the local abundance of water was increased, terrestrial micro-organisms were able to grow. Indigenous Martian organisms may be even more efficient in coping with the apparent rigors of their environment. These findings underscore the necessity for sterilizing Mars entry vehicles so as not to perform accidental biological contamination of that planet and obscure the subsequent search for extraterrestrial life.

_Direct Searches for Life on Mars_

The early evidence for life on Mars-namely, reports of vivid green coloration and the so-called "ca.n.a.ls"-are now known to be largely illusory. There are three major areas of contemporary investigation: visual, polarimetric, and spectrographic.

As the Martian polar ice cap recedes each spring, a wave of darkening propagates through the Martian dark areas, sharpening their outlines and increasing their contrast with the surrounding deserts. These changes occur during periods of relatively high humidity and relatively high daytime temperatures. A related dark collar, not due to simple dampening of the soil, follows the edge of the polar cap in its regression. Occasional nonseasonal changes in the form of the Martian dark regions have been observed and sometimes cover vast areas of surface.

Observations of the polarization of sunlight reflected from the Martian dark areas indicate that the small particles covering the dark areas change their size distribution in the spring, while the particles covering the bright areas _do_ not show any a.n.a.logous changes.

Finally, infrared spectroscopic observations of the Martian dark areas show three spectral features which, to date, seem to be interpretable only in terms of organic matter, the particular molecules giving rise to the absorptions being hydrocarbons and aldehydes. [However, see p. 7 and Rea et al. ([ref.9]).]

Taken together, these observations suggest, but do not conclusively prove, that the Martian dark areas are covered with small organisms composed of familiar types of organic matter, which change their size and darkness in response to the moisture and heat of the Martian spring. We have no evidence either for or against the existence of more advanced life forms. There is much more information which _can_ be garnered from the ground, balloons, Earth satellites, Mars flybys, and Mars...o...b..ters, but the critical tests for life on Mars can only be made from landing vehicles equipped with experimental packages....

Results of Kaplan et al. ([ref.31]) indicate that Mars has no detectable oxygen, but does contain small amounts of water vapor, more abundant carbon dioxide, possibly a large surface flux of solar ultraviolet radiation, and estimated daily temperature variations of 100 C at many lat.i.tudes. Studies have shown that terrestrial micro-organisms can survive these extremely harsh environments. Furthermore, a variety of physiological and ecological adaptations might enable the biota to survive the low nighttime temperatures and intracellular ice crystallization.

Less evidence is available to support the possibility of extraterrestrial life on other planets. The Moon has no atmosphere, and extremes of temperature characterize its surface. However, the Moon could have a layer of subsurface permafrost beneath which liquid water might be trapped. The temperatures of these strata might be biologically moderate.

Studies by Davis and Libby ([ref.32]) on the atmosphere of Jupiter support the possibility of the production of organic matter in its atmosphere in a manner a.n.a.logous to the processes which may have led to the synthesis of organic molecules in the Earth's early history. It is difficult to a.s.sess the possibility that life has evolved on Jupiter during the 4- or 5-billion-year period in which the planet has retained a reducing atmosphere.

The question of extraterrestrial life and of the origin of life is interwoven. Discovery of the first and a.n.a.lysis of its nature may very well elucidate the second.

The oldest form of fossil known today is that of a microscopic plant similar in form to common algae found in ponds and lakes. Scientists know that similar organisms flourished in the ancient seas over 2 billion years ago. However, since algae are a relatively complex form of life, life in some simpler form could have originated much earlier.

Organic material similar to that found in modern organisms can be detected in these ancient deposits as well as in much older Precambrian rocks.

Although the planets now have differing atmospheres, in their early stages the atmospheres of all the planets may have been essentially the same. The most widely held theory of the origin of the solar system states that the planets were formed from vast clouds of material containing the elements in their cosmic distribution.

It is believed that the synthesis of organic compounds preceding the origin of life on Earth occurred before its atmosphere was transformed from hydrogen and hydrides to oxygen and nitrogen. This theory is supported by laboratory experiments of Calvin ([ref.16]), Miller ([ref.33]), and Oro ([ref.34]).

The Earth's present atmosphere consists of nitrogen and oxygen in addition to relatively small amounts of other gases; most of the oxygen is of biological origin. Some of the atmospheric gases, in spite of their low amounts, are crucial for life. The ultraviolet-absorbing ozone in the upper atmosphere and carbon dioxide are examples of such gases.

Significant in the search for extraterrestrial life are the data (e.g., planet's temperature) transmitted by Mariner II, which was launched from Cape Canaveral on August 27, 1962, and flew past Venus on December 14, 1962. Mariner II's measurements showed temperatures on the surface of Venus of the order of 800 F, too hot for life as known on Earth.

The question "Is life limited to this planet?" can be considered on a statistical basis. Although the size of the sample (one planet) is small, the statistical argument for life elsewhere is believed by many to be very strong. While Mars is generally considered the only other likely habitat of life in our solar system, Shapley ([ref.35]) has calculated that more than 100 million stars have planets sufficiently similar in composition and environment to Earth to support life. Of course, yet unknown factors may significantly reduce or even eliminate this probability.