Jupiter's moon Europa is thought to be one of the most likely abodes for microscopic life in our solar system. The ice-covered world may have liquid water, energy, and organic compounds - all three of the ingredients necessary for life to survive.
|Jupiter's moon Europa is thought to be one of the most likely abodes for microscopic life in our solar system.
Streaks of reddish-brown color highlight cracks in Europa's outer layer of ice. Some scientists have speculated that microorganisms suspended in Europa's ice may be the cause of these colorations. To test this theory, planetary geologist Brad Dalton of the NASA Ames Research Center compared the infrared (IR) signature of Europa's ice with the IR signature of microorganisms living at hot water vents in Yellowstone National Park. Dalton discovered that the IR signatures are very similar.
"Just on a lark, I asked a colleague of mine who did a lot of work at Yellowstone if he had any IR spectra of extremophile bacteria," Dalton says. He was shocked by how well they matched Europa's spectrum.
However, the Yellowstone microorganisms were cyanidium, a photosynthetic algae that is not a likely colonizer of Europa. Also, the measurements were made at room temperature.
A more realistic comparison requires spectra obtained at temperatures similar to those found on the icy moon. So Dalton decided to subject the extremophile bacteria Deinococcus radiodurans and Sulfolobus shibatae to Europa-like conditions and then analyze their IR signatures. In order to determine whether a spectral match was due to any intrinsic quality of extremophiles, or just related to bacteria in general, he also looked at the ordinary gut bacteria Escherichia coli (E. coli).
E. coli love moderate temperatures around 37 C (98.6 F), and a neutral pH of 7. The sulfur-oxidizing hyperthermophile Sulfolobus shibatae grows best at an extremely acidic pH of 2 and at temperatures around 80 C (176 F). D. radiodurans can survive the harsh ionizing and ultra-violet radiation of space, as well as extreme cold, vacuum conditions, and oxidative damage.
|Electron photomicrograph of D. radiodurans. Deinococcus radiodurans is the most radiation-resistant organism known which would make it the most likely analog for life on Europa.
Of the three bacteria, the extremophile D. radiodurans would seem to be the most likely analog for life on Europa. However, Sulfolobus shibatae could conceivably colonize the ocean that may lie under Europa's ice. In the lab, biologists grow Sulfolobus in a medium containing high concentrations of magnesium sulfates and sulfuric acid - both of which have been predicted to be plentiful on Europa.
Europa's average surface temperature is minus 162 C (minus 260 F), and it has an almost non-existent atmospheric pressure of 10-7 of a bar. (In comparison, the average atmospheric pressure at the surface of the Earth is approximately 1 bar.) Dalton measured the infrared spectra of the bacteria at ten-degree intervals ranging from freezing (0 C, 32 F) down to 100 Kelvin (minus 173 C, minus 280 F). He then evacuated the chamber down to an atmospheric pressure of 0.01 millibar (10-6 bar).
The initial results of these experiments show that even under the most extreme conditions, the IR signatures of the bacteria correlate with certain aspects of Europa's IR signature.
According to Dalton, "The primary water absorption bands at 1.0, 1.25, 1.5, and 2.0 microns in the samples shifted to the positions observed in the Europa spectra. The distorted and asymmetric features of the 1.5 and 2.0 micron bands were reproduced in detail."
Europa's distorted IR signature was detected by the Galileo spacecraft's Near Infrared Mapping Spectrometer. When taking measurements of the undisturbed portions of the moon, the spectrometer obtained spectra consistent with water ice. But when examining chaotic features, such as the cracks and dark lines that crisscross the moon's surface, it discovered several distorted spectra.
The distorted infrared readings indicate that the colored patches are composed of water bound to some other material. Many scientists believe that a mixture of salt minerals or sulfuric acid contained in the ice best explains the spectra. The salts could be further evidence of a salty ocean lying beneath the ice, as is indicated by magnetometer data from the Galileo spacecraft.
If salts are the cause of the distorted IR signature, they are most likely water-impregnated salty minerals like natron and Epsom salts (magnesium sulfate), which only form in the presence of liquid water. But while the general spectra of salt minerals give a good match to the Europa spectrum, no single mix of salt mineral spectra has been found that exactly matches the Europa spectrum.
"All we know for certain is that the surface of Europa is partly composed of some material which contains water in a bound form: either as a hydrated mineral, hydrated salt, or other unfamiliar hydrate," says Dalton. "Living cells contain water in a number of bound forms, including hydrates. That is why I wanted to investigate them."
While the extremophile bacteria are good candidates for explaining Europa's IR spectrum, like the salts they are not a perfect match.
"We certainly wouldn't expect E. coli on Europa," says Dalton. "Something would have to be pretty specialized to thrive there. Because the IR signature of the common E. coli bacteria is so close to that of the extremophiles, it could be that any number of possible microorganisms that may have evolved on Europa also produce the same sort of IR signature."
|Artists conception of a possible liquid ocean beneath Europa's surface. Terrestrial extremophile bacteria possibly could live in this ocean.
None of the known terrestrial extremophile bacteria could survive the harsh conditions of Europa's surface. They possibly could live in the supposed liquid ocean under Europa's ice crust, however. Of the three bacteria he sampled, Dalton says that the two extremophilic bacteria are the better candidates for life on the icy moon. These species also happen to be pink and brown, which would help explain the colored patches on the moon's outer layer. (Salts, on the other hand, are white).
"The microorganisms could be blasted out to the surface in some kind of eruption and flash frozen," says Dalton. "Or they could be emplaced by more gradual means and uncovered by surface processes like micro-meteorite impacts."
In addition to the IR spectra that matched Europa's signature, Dalton found two additional spectral bands in the bacteria's signature that don't match up with Europa. He says these extra bands are due to amide bonds in the cells' protein coatings. Dalton suggests that if the bacteria were subjected to radiation similar to what Europa receives, the protein bonds would disintegrate and the bacteria's spectral signature would be an even closer match to Europa's.
"Protein amide bonds would disintegrate over time due to particle bombardment at the Europan surface," says Dalton. "Experiments involving radiation processing of frozen materials must be conducted before it will be possible to determine whether the results of this study could indeed be evidence for life on Europa."
Ronald Greeley, a planetary geologist with Arizona State University, says that the only way for us to really know what is causing the IR signature is to take samples of the surface of Europa.
"This is an interesting finding, but hardly definitive," says Greeley. "We really need 'ground truth' missions on Europa to gather data that we can then calibrate with the remote sensing data."
Dalton plans to present his preliminary results at the Lunar and Planetary Science conference in March.
"This is just one piece of a very large puzzle," says Dalton. "What it really demonstrates is that there are many materials in addition to salts that could explain the IR signature of Europa."
"I'm as surprised as anyone, and I'm trying very hard to be skeptical," he adds. "I am not claiming to have found life on Europa. More work needs to be done."