|Penny Boston and Diana Northup taking a pH or ORP measurement in the Ragu passage (Cueva de las Sardinas. Tabasco, Mexico).|
Credit: Kenneth Ingham, 2001
Penny Boston is one of the leaders of the SLIME team – that’s Subsurface Life in Mineral Environments. She studies bizarre microorganisms that live, often under extreme conditions, in subterranean caves. At the recent NASA symposium “Risk and Exploration: Earth, Sea and the Stars,” in Monterey, California, she talked about the relevance of her work below ground on Earth to the search for life on other worlds. Astrobiology Magazine will present her talk in two parts. In this second part she describes some of the cave environments she has explored and the life forms she has encountered and explains what caves can teach us about extraterrestrial life.
Cave environments are radically different from the surface. Exploration of caves in Saudi Arabia by a very well known caving team, John and Susie Pint, has shown that even in these very hot blasted sand deserts, when you get into these very large bell-shaped caves there are diveable pools. The air in these caves is near saturated humidity. It’s a complete change from the overlying environment, even in caves that are not sealed. Just the barrier of above and below provides this radically different environment. This is a big message for astrobiology, that what is dominant on the surface of a planet is not necessarily the key to where you have to go to look for the life.
Cave environments obviously have no sunlight, so any organisms living within them have to make there living some other way, either by detrital organic material washing in, or in the case of a lot of organisms we’re studying, by being rock-eaters. These guys are disaggregating the parent rock with the organic acids that they give off, and then other organisms come along within these little micro-communities and oxidize the metals in the rock. This is how they get the energy to run their entire ecosystem. Caves are very high humidity environments. In contrast to the surface they’re very thermally stable; even a cave with a big gaping open entrance still remains very thermally consistent on the interior. Nutrients are usually very low. They’re very rich mineral environments. And there’s no conventional weather. So it is a very different planet in the near-crustal caves than it is on the surface.
|Snottites/Biovermiculations are slimy, dripping stalactites made of goo, that contain bacteria in abundance and beautiful microscopic gypsum crystal formations.|
Credit: Diana Northrup
As a result of all these tremendously different conditions that you get in caves, the caves are unique mineral factories. There are vast numbers of unique mineral formations that are found in caves. The explanations for the occurrence of these are very much in their infancy. One of the things that we are working on extensively is which of these types of mineral-formation processes are biogenic. It turns out that there are a lot of them. And the organisms are not simply passive observers or users of the environment, they are mineralogically interactive. They are changing the caves. They are actually interacting with the bedrock and they are guiding, and in some cases controlling, the kinds of mineral deposits that are left.
I would venture to say that the bulk of the organisms that we find are novel; they’re not known to science. From one little cave puddle to the next, we have perhaps 80 percent novel organisms. These are truly evolutionarily self-contained environments. Many of them are physically isolated from the surface, little miniature planetary systems within our own crustal environment.
Not only do caves house these amazing arrays of organisms, but also they’re wonderful preservation environments. Not only do the organisms live there, but they often self-lithify. They’re engaged in self-fossilization while they’re alive. There are some formations known as “U loops” in Lechuigilla Cave that look very organic. They’re entirely rock now. But we have been studying their living counterparts in modern caves, and we can see that the U loops are clearly the fossil remains of microbial mats that were inhabiting Lechuigilla 4 to 6 million years ago, when the cave was actively forming. When we examine the fossils in this material, we find fossil microbial filaments and mats and even preserved drapey biofilms. So if you are looking for biosignatures, caves are the place to look.
There are a number of different kinds of exotic environments that we work in in caves. We tend to pick them for their specific chemical properties. We’re looking for caves that have poisonous atmospheres, that are very hot, that are very cold, that are extreme in some sense, so that we can look at the limits to life on this planet, and learn what adaptive strategies may be used by life on other bodies in the solar system.
|Microbial filaments and green slime on a rock.|
Credit: Diana Northrup
Cueva de Villa Luz is one of the most amazing caves that we’re studying. It’s a sulfuric-acid-saturated cave in Tabasco, Mexico. Gases from the nearby volcano, El Chichonal, come into this cave and make it an extremely poisonous environment within which to work. There are tremendous amounts of hydrogen sulfide, carbon monoxide, carbon dioxide, even aldehydes and other noxious things in there. It requires complete protection form that environment. But this is the most biologically rich cave of any that we’ve ever seen. And it’s because of these poisonous gases. These poisonous gases are not poisonous to the organisms that are living there. It’s home sweet home. We’re not looking at extreme environments just to look at extremes where organisms are just barely hanging on. We’re looking at them to look for organisms for which that is the most comfortable environment, because those are representatives of what we may find as the average conditions on other bodies.
So, we’re trying to write the field guide to unknown life. This is a really tough thing to do. But the place where it makes the most sense to do this is in these kinds of protected and evolutionarily sequestered environments. A lot of the material we look at doesn’t even look alive. In one cave we found this white muddy looking stuff on the walls. It was living mud. It was made out of cells and filaments that coat themselves with calcite minerals. These organisms are actively producing this material in caves all over the world. In another location we found these little tiny white dots on the walls. These organisms were busy dissolving basalt in a lava tube and making their living there. So even though something may not look alive – and sometimes we have to work very hard to show that it is alive – all of these environments contain amazing life forms that also leave traces of themselves.
The kind of cave work that we do is also giving us operational experience that is very valuable to future life-detection missions, whether they be robotic or ultimately crewed teams in the future. We are operating in extreme environments that are hazardous, with a sensitive indigenous alien biology. In this case the alien biology is on our own planet. But nevertheless it’s very different from our surface environment. We have to take precautions to avoid contaminating them, while at the same time managing not to kill ourselves off.
So the caves are out there. I know that as time goes on and we explore the planets in the solar system, we’ll find better and better ways of detecting them. We’ll find ways to get into them. We’ll find ways to drill into them, which will be a lot easier than sinking a core down into solid rock. And they will have amazing structures, amazing minerals, and perhaps even amazing life.