November 1997

Looking Inside Earth for Life on Mars

Continued from page 1

By Steve Nadis

Lechuguilla, a world unto itself, has no large natural openings, which means that very little air or water flows in from the outside. The cave is also of geologic interest because of its rich sulfur deposits. Various microorganisms have been found on earth that derive energy from sulfur, and similar lifeforms may exist on Mars, which both the Viking and Pathfinder missions have shown has a high sulfur content.

"A highly diverse group of organisms lives in the cave, including numerous bacterial strains we've never seen before," notes Larry Mallory, a microbiologist at the University of Delaware's Center for Marine Biotechnology, who has made repeated visits to Lechuguilla since 1992. "A quarter mile down, you enter a microbial world. There are no multicellular lifeforms beyond that point except for cavers."

So far, hundreds of microbial strains have been found in the cave, the bulk of which are previously unknown, says Boston. Ultimately, she and her colleague, University of New Mexico biologist Diana Northup, plan to examine the DNA of the microbes to compare it with the DNA of known microbes. "We're trying to find the closest relatives of these strains, figure out where they came from-the soil or ancient ocean-and determine how long they've been separated from the world above," Boston says.

The researchers also hope to determine the energy source that drives this sunless ecosystem. "The key is finding organisms that can make food by chemical means," McKay explains. Mallory has identified a strain of bacteria that oxidizes manganese, but he is still trying to determine whether the creatures actually derive energy from that process. On subsequent trips, he plans to search for iron-oxidizing bacteria at the "Rusticles," an iron formation in the northeast portion of the cave that looks like a "runny popsicle." Boston also intends to gather more samples in the hopes of capturing both iron- and sulfur-oxidizing organisms.

Figuring out what these "bugs" eat is a tough problem, she says. The most likely food sources are either in the air or in the rocks. If adequate funding can be secured, she and Northup hope to conduct gas chromatography studies of the air and do "whole-rock analyses" to see what in the rock wall is edible.

Boston has already had close encounters with some weird things growing in the cave, such as an odd fluffy material called "gorilla drek" that clings to the crumbling cavern walls. "It feels like wet, graphite cotton candy," she says. "When you put it between your fingers, it wilts down to nothingness." The scientists eventually determined that bacteria and fungi live in the fluff, but Boston suspected that from the start: "It gave me an eye infection, which made me think that something was growing there."

The "gorilla drek" episode taught Boston an important lesson: "It's difficult to tell when things are alive, even with a whole crew of investigators crawling around, taking samples, and examining them later in the lab. It's possible that when we look on Mars, we won't find organisms that fit our preconceptions." Detection tools that work here on earth, she says, may not work there.

She and McKay hope to draw on their experiences in Lechuguilla while designing pending missions to Mars. In fact, both have proposals pending with NASA for the 2001 Mars Surveyor mission. "By looking at life underground on earth, we hope to determine whether it's reasonable to look for life underground on Mars, figure out how deep to look, and see if there might be fossil remnants we could recognize," McKay says.

"We'd like to know what kind of clues these bugs might leave behind," Boston adds. If life on Mars retreated to the subsurface and then died out, there still might be a variety of "signatures"-actual fossils, geochemical traces, tell-tale isotopic shifts, or characteristic weathering patterns on the surface of rocks. "Studying a community in a cave that died out and gets fossilized may help us know what to look for in a radically different environment."

The studies of Lechuguilla could shed light on other questions pertaining to the smallest closed ecosystem that can survive. "Some argue that it takes a whole planet to sustain an ecosystem; others claim you could keep a viable system going in a bottle," Boston says. "It's an unanswered question that is applicable to earth or any other planet."

Although she is still heavily invested in Mars, and even calls herself a "Martian" at times, Boston acknowledges that the research at Lechuguilla will be of interest for terrestrial biology, independent of extraterrestrial concerns. Mallory agrees, saying that the cave research can contribute to our understanding of where life exists on this planet and how far down it goes.

The Lechuguilla work may also have a number of practical applications. Mallory has founded a corporation called Biomes to test the disease-fighting capabilities of cave organisms and their byproducts. Some organisms appear to produce compounds that are specifically toxic to certain types of breast cancer cells. He is also investigating the antibacterial, antiviral, and antifungal properties of other compounds derived from the cave.

The exploration of Lechuguilla, meanwhile, is proceeding slowly, given the difficulties of underground navigation, plus the fact that admittance to the cave is severely restricted by the National Park Service. The cave is closed to visitation except for research and maintenance, and the park service lets in only about half a dozen research teams a year. Boston is happy to be among the few admitted, offsetting to some extent her concern that humans may not set foot on the Red Planet for quite a long while. For now, she consoles herself by saying, "This cave is my Mars."