The question of whether life exists on other planets has, in the past half century, become a topic for science rather than just a fictional scenario or the passion of a few visionaries. Only in the past 20 years has science amassed the tools and knowledge to launch a serious search for life “out there.” Fortunately, we have also learned that our own planet, in some of its most extreme environments, may harbor life forms that could help us understand the forms life could take elsewhere in the universe.

A good example: This year’s evidence of possibly recent liquid water flow on Mars has given us further reason to believe the Red Planet’s subsurface may indeed be the place to look for life. If the martian subsurface is a good place to look for life — and if the subsurface regions of other planets or moons are as well — then the subsurface is a good place to start looking on Earth too. Thus it was in 1992 that several of my space-science colleagues and I started looking at caves in a new light.

We are studying microbial life in some of Earth’s most bizarre caves — caves that are chemically unusual or receive little or no organic input from the surface. In some caves, life has apparently proceeded for millions of years without much interference from the surface. Often we researchers are the aliens in these caves, unable to cope with extreme conditions in which the natives happily flourish.

We are investigating life’s occurrence and activity level, metabolic byproducts, effects on mineralogy, and macroscopically visible indicators. Searching for life in caves offers some of the same research challenges scientists will face when searching for life in other planets.

We try to observe “caves” from a microbes-eye view: A cave can be as small as the space between sand grains. Caves give humans access to study Earth’s subsurface. At the same time, some caves are relatively or completely closed systems that have been artificially opened. They often  are extremely old and their microbial communities have had considerable time to evolve. Many closed caves have no higher life forms, thus enabling us to study a microbial system unaffected by grazers or predators. Caves have no weather in the ordinary sense of the word, so we can study what the microbes can do when they’re not exposed to destructive surface processes. Because caves are dark beyond their entry zones, they provide a perfect opportunity to closely study non-photosynthetic food chains of both high and low productivity.

In a 1992 issue of the journal Icarus, Chris McKay, Misha Ivanov and I suggested that life in the subsurface of Mars may have survived much longer than any of the planet’s surface life. Indeed, it might still be present in subsurface habitats. At the time, we were not focusing on caves, but rather on the subsurface in general. We knew that the martian subsurface might harbor the moisture and inorganic compounds necessary for chemolithoautotrophic metabolism (when organisms change carbon dioxide into sugars and starches, just as photosynthetic organisms do, by using the energy released from reactions with inorganic materials like sulfur and manganese and iron compounds).

Such habitats would provide protection from the extremes of the surface: intense cold, high fluxes of ultraviolet radiation and dryness exceeding that of any place on Earth. We imagined a system relying on reduced gases coming from a still active planetary interior, a system that would have no interaction with the barren, hostile surface. Possible dry lakebeds tentatively identified on Mars may contain carbonate formations in which subsurface cavities could have formed earlier in the planet’s history. And we knew Mars harbored huge volcanoes that may have created lava tubes as caves. Caves seemed like Earth’s most obvious analogy to the Red Planet’s subsurface.

Lechuguilla Cave

Lechuguilla Cave at Carlsbad Caverns National Park in New Mexico came to our attention because it was deep, isolated from the surface until artificially opened in 1986, and contained considerable amounts of sulfur as well as iron and manganese compounds. The Viking missions had detected high concentrations of sulfur, iron and manganese in martian surface materials that microorganisms could use as energy sources. Such materials might also be abundant in the martian subsurface. When we decided that Lechuguilla Cave looked like a promising prospect for a Mars subsurface analog, we contacted Kiym Cunningham (then of the U.S. Geological Survey), who was acting as scientific liaison for Lechuguilla Cave. Cunningham had discovered what he interpreted as microorganisms in scanning electron micrographs that he had taken from some of Lechuguilla’s secondary cave decorations (such as stalactites and stalagmites and other less known but fantastic mineral formations). Lechuguilla was especially rich in interesting secondary mineral formations, partly due to its sulfur-dominated chemistry. Gypsum in all its myriad forms abounds in Lechuguilla.

We decided to go caving there with Cunningham and Dale Pate and Jason Richards, cave specialists at Carlsbad. Chris McKay and Larry Lemke, both of NASA’s Ames Research Center, and I accompanied Cunningham, fellow microbiologist Larry Mallory of Biomes Inc. and a group of other experienced cavers into Lechuguilla Cave to see if it offered promising research fodder.

We were novices and had no idea what we were getting into. It was the most grueling trip of my life (five days 1,000 feet below the surface) and my only redeeming thought was that, should I live, I would never have to go back again. Of course, later, on the surface when my bruises were healing and the memory of my extreme discomfort and fatigue was fading, I realized that I had to go back.  First, I had to learn to cave without killing myself.  Then, I had to study the weird fluffy stuff referred to as “corrosion residue.” A blob of it fell into my eye and gave me an instantaneous, raging eye infection that healed within hours after I reached the surface. I knew that Cunningham was right: There were microorganisms in and on everything in this cave that at first had seemed devoid of life. What were they doing there? How could they live on apparently so little organic material? Were they making their own organic material chemolithoautotrophically, just as we had assumed life would in a martian subsurface biosphere?

Since that 1994 excursion we have been collaborating with other researchers to study Lechuguilla’s microbiology (Larry Mallory, for example, investigates the pharmaceutical uses of compounds contained within the unusual microbes of caves). We have discovered numerous strains of microorganisms previously unknown.

My colleague Diana Northup has discovered at one site many whose nearest relatives are the Lactobacilli found in milk and cheese.  Her dissertation work has led to the discovery of an entirely new group within the low-temperature Archaea.  This low-temperature branch of the Archaea was entirely unknown to science until 1992 when some were discovered in marine waters.

We have discovered a passage where filaments of manganese and organisms intertwine into a fantastic woven fabric that rains in flakes from the ceiling at an observable rate. We have seen organisms whose morphologies we cannot yet identify and whose metabolisms and chemical talents are not yet understood. Clearly, we can spend a lifetime studying this amazing and enormous cave — more than 100 miles of mapped passage so far and more to go — and still only understand some of its secrets.

What is extreme life?

We know that understanding extreme environments and their inhabitants is important in its own right and critical in developing our search for life on other planets. But what exactly is an extreme environment? On Earth, we have environments ranging from the superheated waters of submarine volcanic vents to the ultra-dry, bitter cold of the Antarctic dry valleys. We find cave organisms living in areas dripping with sulfuric acid and others thriving in intensely alkaline solutions. We find creatures happily existing in saturated salt solutions, enduring megadoses of ionizing radiation or deriving their energy sources from unappetizing inorganic materials like manganese, iron and sulfur compounds. Imagining how we would react to these surroundings, we conclude that all these places are “extreme.”

The idea of extreme life is a slippery one. Undeniably, the concept of extreme conditions centers on human notions of comfort and discomfort. The range of conditions that we can endure is very limited. Does the term “extreme environment” have any real meaning then? For our flavor of life, carbon-based biomolecules dissolved in water, it does. The freezing and boiling points of water, the effects of acid and alkaline extremes, and the presence or absence of oxygen all provide the setting within which such life must operate. But even on Earth, life is very resourceful and has had a great deal of time to find ingenious ways to bend the rules and ooze, squirm or wriggle its way around the apparently rigid physical and chemical boundaries.

Extreme environments are not only interesting, unusual and hard to define, but they also have much to teach us about life’s limits in the broader sense.  They give us practice thinking outside the usual biological norms that we observe in more clement environments. When we go to other worlds looking for the elusive boojum, it may be that we will find a whole planet where all the inhabitants arose and evolved under conditions similar to an extreme Earth environment. They will hardly be “extremophiles,” but rather garden-variety specimens on their planet and not extreme at all!

Field guide to finding life

As we visit the alien worlds in caves, we must perform the same duty that human explorers on other planets will: planetary protection. Protocol for planetary protection entails two problems and the steps that must be taken to prevent them. The first is forward contamination, or accidentally infecting other planets with Earth organisms. Forward contamination could irreversibly compromise any attempt to determine the actual origin of life forms detected in extraterrestrial samples and even threaten an alien biosphere. The second is backward contamination, the admittedly unlikely possibility of contaminating Earth with organisms from another planet (the feared Andromeda Strain effect that looms large in the public imagination). We must guard against either form of contamination both for the integrity of the science and the safety of Earth’s biosphere. This need for planetary protection places stringent constraints on the spacecraft and instrumentation sent on missions to other planets.

In caves, we don’t want to contaminate a site with the myriad microorganisms that are symbionts and casual hitchhikers on us and our equipment, and with the organic particles we shed copiously and continuously. In turn, we would like to avoid being infected by any pathogens that just might be hiding out in the subterranean realm, waiting for a chance to pounce on new, yummy food (us). Thus we are already implementing a planetary protection protocol as we try to do scientific work in an environment about as far from laboratory conditions as you can get.

We must ask many questions as we investigate an environment for life: Is anybody home? If so, who are they? Do they have relatives that are already known to us? How do they get energy and nutrients? How active are they metabolically? How did they get where they are? What, if anything, are they doing to the rocks, air, water and other features of their environment? Can we find any fossilized examples of them or their predecessors?

We will ask the same suite of questions about extraterrestrial environments too, but the answers will be harder to find without the background information and experience we have for Earth environments and organisms. We have two ways to cope with this challenge. One is to try to extract the fundamental clues of the presence of organisms, things that seem to be inescapable properties of life exclusive of specific chemical and evolutionary particulars. For example, thermodynamic disequilibria in an environment’s chemistry can indicate active life processes. The other approach is encyclopedic. We are amassing as complete a database as possible about Earth’s living systems and environments for future comparison to extraterrestrial materials. Our work in caves is contributing to both of these approaches.

Cave work is a great dress rehearsal for studying potential biological sites on other planets. When we are on expeditions in these radically different places, underground for many days, we feel like we are on a different planet. Memories of the surface fade and we have a glimpse of what it will be like for future human explorers as they live, explore and do science in new and undoubtedly hostile environments.

Science is hard work, a touch of monomania and occasional “happy accidents.” Whether some discoveries are really accidents can be debated, but the fortunate juxtaposition of powerful new analytical tools with the broadening of the known limits to life on our own planet gives our imaginations good stretching exercises for finding life “out there.”

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