Designing a Search Strategy

Given these “lessons from the Earth,” is it possible to design a strategy that will allow us to search for life with some degree of confidence? The success of any search will depend upon the ability to define the general features of life, to develop methods for measuring such general features, and to employ these methods for remote sensing, for in situ studies, and for the analysis of returned samples.

Studies of Earthly life suggest that metabolic evolution, one of the keys to life becoming a global phenomenon, was already in full swing more than 500 million years ago. Most of the Earth’s geology, and many of its atmospheric properties, which we still see today, were in place by that time. If we want to search for life elsewhere, we must keep in mind that there is no guarantee that a particular planet will have evolved to the same advanced stages we have on Earth - a historical perspective is thus key to developing a strategy for life detection. To put this another way, we must know the early history of a planet in order to frame the search for life properly.

Since Earth is the only place where we are certain that life exists, it will serve as our laboratory for the development of the search strategy. The overall strategy is still in its early stage of definition, but a general idea of it involves three phases: a) the development of non-Earth-centric biosignatures for life detection; b) the testing of these biosignatures on Earthly samples to see just how good they are; and, c) the eventual use of these biosignatures and tests for the analyses of extraterrestrial samples.

Figure 9. Searching for life in the universe. Searching for life when we do not know what it looks like may represent one of the truly great challenges facing human scientists. One must move to fundamental definitions of life, and develop biosignatures to aid in the search - biosignatures that are not dependent upon known earthly molecules, but which would never miss earthly life if it was encountered.

For most biologists this entire process is a new endeavor, asking new questions. It is rare that a biologist is handed a rock and asked: “Is it alive? Or, from this sample, can you prove whether there was ever life on Earth?” Yet that is what we will be faced with in a few years when samples are returned from Mars. In fact, if another planet was teeming with life, as is Earth, this would not be a difficult task, even with very old rocks. It would be relatively easy to tell that Earth was (and is) alive from almost any distance, and especially so if samples were available for detailed physical and chemical analysis. However, if the signs of life are subtle or unfamiliar, then the task becomes much more difficult.

This difficulty is demonstrated by the present controversy surrounding the now famous Mars meteorite, ALH 84001. Four years ago, this 4.5 billion-year-old rock was reported to contain evidence for life on Mars. But even now, after extensive research, the jury is still out as to whether the evidence is convincing. The problems stem from many fronts, including the age of the sample, the difficulties in separating indigenous signals from those due to Earth contamination, and the very definition of life and how one proves that it is (or was) present. What this meteorite really has taught us is that we have a lot to learn about how to distinguish life from non-life.

Biologists may not even be the right group of scientists to answer the question, “Is there life in this rock?” As a biology student, I was never asked such a question. Rather I was given a frog and asked, “How does it work?” or “What is it made of?” These days, the questions have changed to “What genes are there, and how do they function?” but the general problem remains - biologists study life they already know how to detect, they do not seek to detect life they do not know about. This is a question inherently interdisciplinary in nature, and perhaps best suited to those who are willing to define life in general terms that would include all life on Earth, but not exclude life made of different types of molecules.

As a group, we biologists should be extremely well suited to detect life as we know it primarily because we understand its chemistry so well. There are molecules that can be detected at very high sensitivity, allowing us to find a single bacterium in a liter of water. However, if such key indicator molecules are not there, the search becomes much more difficult, and the likelihood that life elsewhere would contain the same key molecules certainly cannot be depended upon. The problem then takes on a different aspect: because we rely on Earth-centric indicators of life, we biologists may unwittingly be the least well suited to detect life that might differ in its chemical make-up.

To this end, our astrobiology group is focusing on what we feel are two fundamental properties of all life, structure and chemical composition, both of which can be detected and measured. Historically, structures are the paleontologists’ keys to recognition of past life on Earth. It is structures that characterize life as we know it, and which should be expected to characterize any new forms of life we encounter. We do not know in advance the nature of the structures or the size scales over which to search, but we do expect there to be structural elements associated with any life forms. When one is hunting in a new spot, dependency on known structures has a number of potential traps, including the fact that one might discard structures simply because they are unfamiliar. It will be important to remain open-minded about the types and sizes of structures found in samples from new sites.

While we believe that life will be linked to some structural elements, structure alone will not prove the existence of life. However, coupling structural analysis with the determination of chemical content may well provide a tool for strongly inferring the presence of life. On Earth, life is carbon-based with a peculiar and remarkably constant elemental composition (hydrogen, nitrogen, phosphorous, oxygen, carbon, etc.), which is remarkably out of equilibrium with the crustal elemental abundance of our planet. In other words, there are more or less of some elements than would be present if there were no life on Earth. Life is, almost by definition, a source of negative entropy: a structure composed of groups of chemical monomers and polymers that are not predicted to be present on thermodynamic grounds, given the abundance of chemicals in the atmosphere and crust of the Earth. The exact nature of these chemicals is not so important as the fact that they are grossly out of equilibrium with their surrounding geological environment. So, if methods were available for analysis of the chemistry of structures at the proper size scale(s), then the possibility of detecting extant (or even extinct) life would be greatly increased. While there are other properties of life that may be measurable (such as replication, evolution, and energetic exchange with the environment), and which may leave traces in the geological record, we believe that if life does or did exist, then it will be detectable by the existence of structures and their distinctive chemistries.

Ultimately, we would like to have samples from many places in our solar system and beyond, but realistically, we will probably need to make measurements remotely and in situ, and be satisfied with these as our indicators of life. As our ability to measure structures and chemistry improves, the possibility of answering the question of whether life does or does not exist beyond Earth will improve as well. A strategy for exploration, sample collection and return, and finally, sample analysis will be needed. Given the number of other solar systems already known to exist, and the emerging numbers of planets around far away stars, it seems unlikely that life will not be found elsewhere. Development of the proper strategy, and definition of those conditions that do and do not support life, will be key to the ultimate discovery of extraterrestrial life. With the proper strategy and approach, the question seems to be not one of whether, but when.

Contributed by: Dr. Kenneth Nealson