Life in a Lava Tube
|Peering Into A Cerberus Fossae Trough MGS MOC Release No. MOC2-576, 16 December 2003
Image Credit: Mars Global Surveyor, Malin Space Systems
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, Boston 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 first part she tells why she focuses her research on cave environments and explains that we have a lot to learn from lava tubes.
I’ve been trying to get to Mars ever since I was a little kid. My solution to this over the last decade or so is to go down instead of go out. That may be as close to going to another planet as I ever get. After my colleagues and I published a paper in 1992 [suggesting that if there were life on Mars it was probably underground], we began looking for ways to study the subsurface. And, of course, the most immediate obvious thing was the new information we were getting from drill holes. People were beginning to drill and to try to look at deep subsurface microbiology. But we discovered that caves were there, and thought that maybe they would be cheaper to get into. So after the early part of my career, working in extreme environments on the surface, I decided to try caving.
The first cave I ever caved was Lechuigillia Cave in New Mexico, which is a notoriously difficult cave. All I thought at the time was, “Am I going to live to get out of this? I just have to live to get out of this cave.” But after the pain faded, I realized that it was an amazing environment, that I’d never been in any place that was so potentially fascinating for the kinds of exotic microbiology that I was interested in as an astrobiologist. I realized that I could learn how to cave safely, go to these places, and refocus my work to tap into an entire area of biology and mineralogy that had not been studied before. This is a new field, really, in terms of what we’ve been doing on Earth. It is immediately applicable to consideration of life beyond the planet. So most of my research, now, is focused on one sort of cave or another.
If you have only been in caves as an occasional tourist in a show cave, you may think that caves are a rare phenomenon, but really there are a tremendous number of subsurface voids on Earth, of all kinds. They aren’t just in calcium carbonate types of environments, which are the ones we often come in contact with, but they really occur in every major rock type. And this is an important lesson for trying to apply our knowledge of Earth caves to other bodies in the solar system. There are many, many ways to make caves. One of the areas of active research that we’re engaged in is a set of thought experiments, looking at the basic physics and chemistry of environments and trying to imagine ways that subsurface voids on other planets could be formed.
|Venus up-close, as photographed by the Soviet Venera 13 lander, which parachuted to the Venusian surface on March 1, 1982. The surface is hot enough to melt lead
The type of caves that we absolutely know exist elsewhere in the solar system are what are known as lava tubes. These are natural outgrowths of flood-basalt type, quiet, flowing lava eruptions. They are essentially rivulets that freeze on the outside. The rock on the outside freezes and forms a very good insulator that then allows the interior to remain molten and to continue to flow through. Eventually, when the eruption stops, they empty out and you have these very beautiful tubes. That’s a very different class of cave from the kinds of dissolution-dominated caves that we often think of.
It was known and recognized by Ron Greeley and other colleagues, even in the Apollo era, that a lot of structures that they were seeing on the moon were lava tubes, or unroofed sinuous rills (lava tubes without their tops). As we have gotten ever-better imaging of the planet Mars, we have seen that there are lava tubes scattered widely over the planet. They are quite easy to pick out.
The gravity on Mars is much lower so lava tubes there scale accordingly. Not only does Mars have enormous examples of volcanism, but it has big whompin’ lava tubes. The biggest lava tube on Earth is about 90 kilometers (56 miles) long, in Hawaii. That’s the record-holder on Earth, but typically when you look at these features on Mars they’re hundreds of kilometers long. And the diameters are equally great. On the average they’re 3 to 10 times the size of the average diameter on Earth. They are truly enormous.
|Jupiter’s volcanic moon, Io.
And when we look at the radar imaging data from Venus missions, we can see that there are tube-like structures associated with even those weird-looking types of volcanic features that we find on Venus. Even Io, which is such a cooking little moon out there, with its tremendous sulfur component, seems to have clear evidence of lava tubes. My dream is that some day we’ll get a really good image of one that’s made entirely out of molten sulfur.
But these are not only fabulous features, they’re also places, at least on the moon and Mars – I wouldn’t recommend astronauts going to Io or Venus – that can actually be exploited as human habitat. We just finished a study for NIAC (NASA Institute for Advanced Concepts), looking at enabling technologies that we would need to make these usable as structures for astronauts, for future bases on the moon and Mars.