The Universe in the Classroom

Responsible Exploration: Protecting Earth and the worlds we explore from cross contamination

How do you recognize life?

Whether on Earth or in space, looking for living organisms–or evidence of life — can be a difficult task. Consider life on Earth. Large animals can be difficult to 'see' because they have adaptations like camouflage, hibernation, living underground or inhabiting extreme places like ocean depths. Microscopic organisms and some plants and fungi aren't visible with the naked eye, and other organisms — like viruses, parasites or symbiotic algae in corals — live some or all of their life cycles associated with other organisms. Finding life isn't easy.

The search for life in Astrobiology requires that we recognize life when we see it — and that's not a straightforward task. Spacecraft have already been to Mars, the moon and numerous other places in the solar system, but we haven't found life anywhere yet. So far, all the habitats we've visited have been extremely harsh places, with conditions that would be very stressful to living organisms (based on life as we know it). So what do we look for when we go to a place like Mars? And why do even continue to look when it seems to be so 'empty' of life?

Our interest in searching for life in far off places is driven in no small part by what we know about life on Earth. We've begun to recognize that Earth is much more 'alive' than we previously thought possible. In the past few decades, we've discovered microbial life, and in some cases, weird and different macroorganisms as well, in environments that were once thought to be totally incompatible with living organisms. For example, living organisms have been found in the darkness and extreme pressures of the deepest ocean, in thermal hot springs and dry deserts, as well as in Arctic and Antarctic permafrost. Microbial life has also been found inside of rocks kilometers beneath the Earth's surface, and in extremely inhospitable locations such as highly acidic waters or chemically toxic soils. In addition, we know that microbes can survive in dormant states for very long periods of time (consider for example anthrax spores, or dormant, yet viable microbes lodged inside salt crystals for nearly 250 million years!).

Punishing Environments

If life can be very small, and found in very unusual, extreme environments on Earth, why not in space? Take for example, Mars. If life did arise during a warmer, wetter period in Mars' history (as some scientists now believe), perhaps it managed to migrate into warmer, more clement regions of the planet before the surface became uninhabitable. Maybe if we look at martian rocks and soils, we might be able to find evidence indicative of that life–either live or dormant organisms, fossil evidence, 'biomaterials' or bits and pieces of living organisms, or even chemical cues in the form of molecular 'signatures' associated with life.

Thus, when we someday return samples from Mars (like we did from the Moon during the Apollo program), we will have a strategy that covers the waterfront, so to speak. We'll try to include a variety of rock types — and a variety of tests — to study for evidence of life in a variety of forms.

When Mars rocks and soils are returned to Earth (in about a decade from now if all goes as planned), the sample container will be opened inside a special containment and quarantine facility where three types of testing will be done: 1) physical and chemical testing 2) life detection analyses, and 3) biohazard tests. Because we do not know what martian life might look like, if it exists at all, information from all three categories will be essential to determine if there is any evidence of life. In general the tests will scan for the same types of evidence as we would look for to detect Earth microbial life:

The first thing that will be done with samples is to 'look' for visual signs or chemical evidence of life. On Earth, all life has some shape or structure, regardless how small or simple. Highly sophisticated microscopes and instruments will 'look' for obvious signs of structures, probing in cracks and fissures, or other obvious non-uniform parts of the rocks. The samples will also be analyzed chemically to search for elements usually associated with biological systems (the most obvious are carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur). For example, the biological element abundances in martian samples would be compared with those found on average in typical terrestrial microbes.

If no organic carbon is found in the samples, and no structures of any sort are detected (at scales as small or smaller than all known bacteria and microbes), the probability is be very low that there would be life in the samples. Even so, attention will focus on even finer detail.

Following the preliminary physical and chemical testing, a battery of life detection tests will also be conducted to search specific signs of life. For example portions of the samples will be scanned using a variety of instruments and laboratory techniques to look for biochemical signs of 'life as we know it' (for example, biological 'signatures' such as amino acids, DNA, peptides, lipids, enzymes, cell wall materials, etc). They will also attempt to culture extracts from the samples using standard microbiological plating procedures under a variety of laboratory conditions and growth media. Cultures will be monitored over time to look for colony growth or chemical changes that might indicate metabolism by some organism.

Even if all the tests above show no signs of life, or are inconclusive, there will be yet another set of biohazard tests done to determine whether there might be anything in the sample that could potentially harm either Earth life or its environment. These tests will use cell and tissue cultures with a variety of representative species to test for indications of hazards — toxic, pathogenic, life-cycle altering, capable of causing mutations, altering behavior or disrupting ecosystems, etc.)

If we verify that life exists in samples from Mars, it would be a profound and significant discovery in many ways. The obvious follow-up question would return us to yet another comparison with Earth life. All organisms that we have studied to date have the same biochemical and genetic makeup, sharing similar DNA and showing evolutionary relationships on the universal tree of life. Would martian life be related to Earth life, perhaps indicating that life had been carried between Earth and Mars — or vice versa — in meteorites? Or would extraterrestrial life be distinctly different, perhaps using some other 'alphabet' of amino acids for its genetic code, or different molecules altogether for its basic biochemistry. If life has happened at least twice in one solar system, would it mean we live in a universe that is 'biofriendly', and that life may be found in more places as well? All these questions — and all the potential answers and interpretations — depend on how carefully we are able to study samples from places like Mars, and whether we we'll be able to recognize life when we see it.

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