All living tissue contains proteins. As luck would have it, a California firm, Ciphergen Inc., was developing an instrument for biomedical research that could fill the bill. The instrument, which became available just last year, consists of tiny, thin strips of metal coated with different chemically active molecules that can extract and identify other molecules, including proteins, from complex mixtures. Think of it as a hypersensitive nose. She wrote a proposal that was immediately accepted by NASA's National Astrobiology Institute, an interdisciplinary consortium at the Ames Research Center south of San Francisco that is dedicated to the search for extraterrestrial life.
Some molecules are very light, such as amino acid.
A typical protein consists of a chain of amino acids, and is thus much heavier. Amino acids are essential for life to form, but their presence doesn't necessarily mean life is there. The presence of protein, on the other hand, indicates that amino acids have combined to form the basic structure of organic life. So it's those larger molecules, or proteins that bear the clear signature of life. Some heavy molecules can be formed by non-biological processes, and sorting out which molecules resulted from which process is the task that now stretches before Fogel.