Historically, chemists have referred to all compounds of carbon except the oxides, carbonates (e.g., limestone and marble), metallic carbides, and elemental forms (e.g., diamond and graphite) as “organic.” But, despite the term, organic molecules, even elaborate ones, are not necessarily produced biotically. Discussed below are several important characteristics of organic molecules that, as we shall see, are pertinent to the problem of trying to differentiate between biotic and abiotic origin:

1. Structural stability over wide ranges of temperature and pressure;

2. Aromaticity;

3. Aliphatic homologs;

4. Stereoisomerism; and

5. Structural complexity.

These characteristics are discussed in detail in subsequent sections.

Carbon atoms can bond strongly to each other. Chains of atoms can extend indefinitely and, because each atom can form four bonds, with multiple branches. Chains can bend around to form rings, and rings can be fused to form sheets of atoms for which the bonding diagrams look like chicken wire. In such cases, the four bonds at each carbon atom are often arranged to provide double or even triple linkages (see the next section, “Aromaticity”). Such materials—and many smaller, simpler molecules—are capable of outliving our planet. If they are tucked away in some bit of space rock or ice, they are literally waiting to be discovered and interrogated. The chemical structure of a carbon-containing molecule (the bonding pattern and the other chemical elements that are present) provides information about the last time that molecule was warm enough to rearrange spontaneously or to react with another molecule. Depending on the molecule in question, “warm” might mean some temperature more than a hundred degrees below water’s freezing point or some temperature above the melting point of metallic zinc. The conditions monitored would be as follows: