Date Published: September 14, 2004
Publisher: Public Library of Science
Author(s): Chris P McKay
Abstract: If there is life on distant worlds, how would we go about finding it?
Partial Text: I need a “tricorder”—the convenient, hand-held device featured on Star Trek that can detect life forms even from orbit. Unfortunately, we don’t have a clue how a tricorder might work, since life forms don’t seem to have any observable property that distinguishes them from inanimate matter. Furthermore, we lack a definition of life that can guide a search for life outside Earth. How can we find what we can’t define? An answer may lie in the observation that life uses a small, discrete set of organic molecules as basic building blocks. On the surface of Europa and in the subsurface of Mars, we can search for alien but analogous patterns in the organics.
The obvious diversity of life on Earth overlies a fundamental biochemical and genetic similarity. The three main polymers of biology—the nucleic acids, the proteins, and the polysaccarides—are built from 20 amino acids, five nucleotide bases, and a few sugars, respectively. Together with lipids and fatty acids, these are the main constituents of biomass: the hardware of life (Lehninger 1975, p 21). The DNA and RNA software of life is also common, indicating shared descent (Woese 1987). But with only one example of life—life on Earth—it is not all that surprising that we do not have a fundamental understanding of what life is. We don’t know which features of Earth life are essential and which are just accidents of history.
The Viking missions to Mars in the late 1970s were the first (and as yet, the only) search for life outside Earth. Each Viking conducted three incubation experiments to detect the presence of metabolism in the Martian soil. Each lander also carried a sophisticated Gas Chromatograph Mass Spectrometer for characterizing organic molecules. The results were unexpected (Klein 1978, 1999). There was a detectable reaction in two of the incubation experiments. In the “Gas Exchange” experiment, a burst of oxygen was released when the soil was exposed to water. The “Labeled Release” experiment showed that organic material was consumed, and that carbon dioxide was released concomitantly. In the Labeled Release experiment, this reaction ceased if the soil was first heated to sterilizing temperatures, but the reaction of the Gas Exchange Experiment persisted.
Table 1 shows a categorization of life as we have observed it. Using this diagram, we can speculate about how life might be different on Mars or Europa. At the bottom of the table, life is composed of matter—a reasonable assumption for now. Carbon and liquid water are the next level; this makes Mars and Europa likely candidates, because they have carbon and have, or have had, liquid water. Other worlds may have a different chemical baseline for life. The usual speculation in this area is that the presence of ammonia and silicon, rather than water and carbon, might be preconditions for life on other planets. Such speculation has yet to lead to any specific suggestions for experiments, or to new ways to search for such life, but this may just reflect a failure of human imagination rather than a fundamental limitation on the nature of life.
If we were to find organic material in the subsurface of Mars or on the ice of Europa, how could we determine whether it was the product of a system of biology or merely abiotic, organic material from meteorites or photochemistry? If this life were in fact related to Earth life, this should be easy to determine. We now have very sensitive methods, such as PCR and fluorescent antibody markers, for detecting life like us. This case would be the simplest to determine, but it would also be the least interesting. If the life turned out to be truly alien, then the probes specific to our biology would be unlikely to work. What, then, could we do to determine a biological origin?