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Looking Inside Earth for Life on Mars

November 1, 1997

It’s your typical spelunking saga: You squirm through tunnels no wider than a watermelon in diameter, then rappel over the edge of a 200-foot cliff into a void so black you can’t even see your feet, let alone the bottom-assuming there is a bottom. If you survive that ordeal, you’ll still have to negotiate the slippery slope of the “Freakout Traverse” or the dreaded crevasse known as the “Great White Rift.” A subterranean tour through Lechuguilla Cave in New Mexico might also include other highlights and lowlights: “Misery Hole,” “Death Pit,” and the “Parallel Universe.”

Some explorers enjoy the thrill of probing an uncharted, underground wilderness. Others find solace in the quiet and darkness of America’s deepest cave, which extends more than 1,500 feet below the surface. Then there are those who come searching for life on Mars.

Life on Mars? Indeed, as scientists explore the Red Planet via the Mars Pathfinder and a series of subsequent NASA missions planned over the next decade, other researchers are studying the microbial lifeforms that inhabit this vast cave in Carlsbad Caverns National Park, evaluating the possibility of analogous creatures lurking beneath the Martian surface. This work has been supported by NASA’s exobiology program from a budget designated for the study of “terrestrial analogs” to other planets. “We’re reconsidering the subsurface environment on Mars partly because the subsurface on earth is turning out to be very rich,” says Penny Boston, a biologist and NASA consultant at Complex Systems Research in Boulder, Colo. In fact, scientists are finding microorganisms almost everywhere they look-near vents on the floor of the deep ocean, inside Antarctic rocks, and at oil- and gas-drilling sites, thousands of feet underground.

The basic premise of the analogy to life on Mars goes like this: Some three to four billion years ago, Mars was a warmer and wetter place owing, in part, to a thick atmosphere of carbon dioxide that heated the planet through the greenhouse effect. Eventually this carbon dioxide combined with water to form carbonic acid which, in turn, reacted with rocks on the surface to create carbonates such as limestone and dolomite. This process depleted the carbon dioxide in the atmosphere, leaving the surface frozen and inhospitable with no liquid water to be found anywhere. “Now we’re saying that if life couldn’t have survived on the surface, maybe it moved underground,” explains Christopher McKay, a planetary scientist at NASA’s Ames Laboratory who has accompanied Boston and others on several Lechuguilla expeditions.

The entrance pit to Lechuguilla cave was discovered in 1914. But it wasn’t until the 1950s that investigators began to suspect there might be a cavern somewhere beneath the pile of rubble on the bottom of this pit. Once the rubble wall was eventually broken through in 1986, intrepid spelunkers tunneled in and began to chart more than 90 miles of passages. The full extent of Lechuguilla is not yet known, but Dale Pate of the National Park Service in Carlsbad, N. Mex., is convinced that “there’s a lot more cave out there than we know about, possibly 1,000 or 2,000 miles worth.”

Some call it the most beautiful cave in the world, with brilliant colors, pristine pools, and spectacular mineral formations that resemble flowers, crystals, chandeliers, and coral. Although scientists can appreciate this splendor, they are drawn to the cave mainly because it is the perfect place to study a virtually uncontaminated underground environment. “We do our best to limit the impact because we don’t want to contaminate and destroy the thing we hope to study,” Boston explains. “We’re looking for exotic organisms that are easy to distinguish from contaminants brought in by humans. But if we’re not careful, these contaminants might outcompete indigenous organisms.”

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.”

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