Looking for Life in All the Wrong Places
Weird space critters could be right beneath our planetary probes.
- By Christen Brownlee
- Air & Space magazine, May 2007
Illustration by Richard Thompson
In 1976, scientists anxiously waited for the first data streaming back from the Viking 1 and 2 landers, sent to search for signs of life on Mars. The results were frustratingly inconclusive; for decades researchers have been debating whether the Vikings detected life. Then last January, two scientists presented a paper arguing that Mars may indeed harbor life, but that the landers’ life-detecting equipment may have killed it. They theorized that Martian microorganisms might contain a mixture of water and hydrogen peroxide; if so, a Viking experiment that doused Martian soil samples with water would have drowned such life-forms.
The idea that Mars may harbor microbes containing hydrogen peroxide is based in part on the presence of what appears to be that chemical on Mars’ surface. The theory that microbes may be the origin of that hydrogen peroxide is not well accepted—not yet, anyway. Most researchers digging for extraterrestrial life are focused on forms containing water and carbon-based molecules—the only forms found on Earth. But a growing number of scientists are speculating that the solar system may harbor what they call “weird life”—forms that contain chemicals not traditionally associated with living organisms.
Thanks to the discovery of unusual creatures on Earth, such as “extremophile” bacteria adapted to the extreme heat of underwater thermal vents, most astrobiologists accept the possibility that life-forms on other planets could have unfamiliar appearances or adaptations. However, most still envision microbes filled with water and carbon-based, or organic, molecules. It’s not unreasonable, says David Grinspoon, astrobiology curator of the Denver Museum of Nature and Science and formerly NASA’s principal investigator for exobiology research. He points out that such compounds have been detected in practically every corner of the universe that has been examined.
However, he and other researchers now suggest that an element other than carbon may serve as the backbone for molecules essential to life-forms on other planets. One proposed substitute is silicon, which occupies a place on the periodic table directly under carbon. Vertical rows on the table represent an element’s most basic behavior, so carbon and silicon’s close positions suggest that one can be swapped for another to form molecules with similar characteristics, says Grinspoon.
Likewise, water isn’t the only solvent that life-forms could use to enable necessary chemical reactions, says Dirk Schulze-Makuch of Washington State University in Pullman, one of the scientists who suggested that Viking may have killed Martian microbes. “Life and environmental conditions on a planet are intrinsically related,” he explains; he champions the idea of Martian organisms containing hydrogen peroxide because it fits with the very cold and dry conditions on that planet. Depending on its concentration in a solution, hydrogen peroxide does not freeze until at –70 degrees Fahrenheit, and when it does freeze, it does not form crystals, which would destroy cells . And the compound absorbs even minute amounts of water vapor from the atmosphere, which would benefit a water-dependent organism in an extremely dry environment like Mars’.
Chemist Steven Benner of the University of Florida in Gainesville suggests that the molecules that might make up weird life and enable it to reproduce may differ from terrestrial proteins and the nucleic acids DNA and RNA. By making some simple chemical tweaks to these molecules, Benner and his colleagues have crafted new variations that still work. “You can pick any one of these [molecules] and easily walk away from its natural structure” while still preserving functionality, he says. Benner and other researchers have come up with a variety of new amino acids, the molecules that string together to form proteins, that don’t exist in nature—at least not on Earth. His group has also constructed new types of DNA with bases different from the adenine, thymine, guanine, and cytosine that form the rungs in the double helix on Earth.
The probes that search for life on other planets use technology that can detect a range of chemicals beyond water and organic molecules. The trick is to devise experimental protocols that do not destroy or miss signs of possible life—a protocol, for example, that does not douse samples with water if hydrogen peroxide is thought to be a possible constituent. Recently, Rafael Navarro-Gonzalez of the University of Mexico in Mexico City and others decided to check the instrument that Viking used to test Martian soil for organic molecules, a gas chromatograph–mass spectrometer (GCMS), which identifies the atomic constituents of a substance. The scientists used the instrument to test soils from areas on Earth that are similar to Mars and known to have organic molecules, but it nonetheless gave negative readings, again casting doubts on Viking’s results. Navarro-Gonzalez says that the Mars Science Laboratory, presently planned to launch in two years, will also use a GCMS, but it will follow a different sample-treatment protocol, one that uses solvents, and is more likely to reveal organic molecules, if any are present.
Another way to increase the chances for finding new life-forms is to send probes to areas where they are more likely to be found—that is, to search creatively. The Mars Science Laboratory will cover a much greater area than Viking did. And NASA’s Phoenix probe, currently scheduled to take off this August, will land in a subpolar area of Mars that is especially cold and higher in atmospheric water vapor—more favorable than the Viking sites for detecting life, especially the hydrogen peroxide-containing organisms Schulze-Makuch envisions. Phoenix will also carry non-chemical tests: two microscopes to study samples for signs of life.