METEORITES AND ORGANIC GEOCHEMISTRY
Meteorites
A significant area of exobiological research is the investigation of a special class of stony meteorites known as "carbonaceous chondrites." It is increasingly apparent that almost all life-detection concepts rely on the eventual analysis of the solid materials that may be available on Mars and other planetary surfaces. Cosmic dust and meteorites are two classes of material bodies that reach the Earth from outer space. The carbonaceous chondrites are the only extraterrestrial materials known to contain organic carbon.
The study of meteorites has generated an astonishing diversity of hypotheses. There is agreement at only one point: that meteorites are preserved chunks of very ancient, perhaps primordial, planetary matter and that when we are able to understand the curious structures and chemical and isotopic variations in the meteorites, we will also know a great deal about early planetary (and perhaps preplanetary) history.
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 4½ 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 10-23 to 10-26 of the parent body.
Carbonaceous Meteorites
Analysis and characterization of the chemical constituents (organic) of carbonaceous chondrites, including the possible mechanism of their formation, may be expected to improve methods of analyzing samples from the Moon and planets and of interpreting remote automated biological analyses on the planets' surfaces.
Carbon has been detected in all meteorites analyzed; 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 analyses on large numbers of meteorites is given in [table I] ([ref.26]).
| Meteorite group | Number analyzed | Mean carbon content, percent by weight |
|---|---|---|
| Pallasites | 10 | 0.08 |
| Ureilites | 2 | .69 |
| Bronzite chondrites | 12 | .05 |
| Hypersthene chondrites | 8 | .04 |
| Enstatite 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 constituents, 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 analyses 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 illustrated 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 space.
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 analysis 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 relationship.