NASA scientists are exploring the possibility that microbial life exist inside Enceladus, where no sunlight reaches, photosynthesis is impossible and no oxygen is available.
Until the two Voyager spacecraft passed near Enceladus, the sixth-largest moon of Saturn, in the early 1980s, very little was known about this small moon except for the identification of water ice on its surface. The Voyager missions showed that Enceladus is only 500 km in diameter and reflects almost 100% of the sunlight that strikes it. Voyager 1 found that Enceladus orbited in the densest part of Saturn's diffuse E ring, indicating a possible link between the two, while Voyager 2 revealed that despite the moon's small size, it had a wide range of terrains ranging from ancient, heavily cratered surfaces to young, tectonically deformed terrain, with some regions with surface ages as young as 100 million years old.
The Cassini spacecraft performed several close flybys of Enceladus in 2005, revealing the moon's surface and environment in greater detail. In particular, the probe discovered a water-rich plume venting from the moon's south polar region. This discovery, along with the presence of escaping internal heat and very few (if any) impact craters in the south polar region, shows that Enceladus is geologically active today.
Given the level of tectonic resurfacing found on Enceladus, a critical factor in the evolution of life on Earth, has been an important driver of geology on this small moon. Enceladus the fourth body in the solar system to have confirmed volcanic activity, along with Earth, Neptune's Triton, and Jupiter's Io.
There are three ecosystems discovered on Earth that could mirror possible lifeforms on Enceladus. Two are based on methanogens, which belong to an ancient group related to bacteria, called the archaea -- the hardy survivalists of bacteria that thrive in harsh environments without oxygen. Deep volcanic rocks along the Columbia River and in Idaho Falls host two of these ecosystems, which pull their energy from the chemical interaction of different rocks. The third ecosystem is powered by the energy produced in the radioactive decay in rocks, and was found deep below the surface in a mine in South Africa.
NASA's Cassini spacecraft discovered a surprising organic brew erupting in geyser-like fashion from Saturn's moon Enceladus during a close flyby on March 12, 2008. Scientists were stunned that this tiny moon is so active, "hot" and teeming with water vapor and organic chemicals.
"Enceladus has got warmth, water and organic chemicals, some of the essential building blocks needed for life," said Dennis Matson, Cassini project scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "We have quite a recipe for life on our hands, but we have yet to find the final ingredient, liquid water, but Enceladus is only whetting our appetites for more."
"A completely unexpected surprise is that the chemistry of Enceladus, what's coming out from inside, resembles that of a comet," said Hunter Waite, principal investigator at the Southwest Research Institute in San Antonio. "To have primordial material coming out from inside a Saturn moon raises many questions on the formation of the Saturn system."
"Enceladus is by no means a comet. Comets have tails and orbit the sun, and Enceladus' activity is powered by internal heat while comet activity is powered by sunlight. Enceladus' brew is like carbonated water with an essence of natural gas," said Waite.
The Casssini Ion and Neutral Mass Spectrometer saw a much higher density of volatile gases, water vapor, carbon dioxide and carbon monoxide, as well as organic materials, some 20 times denser than expected. This dramatic increase in density was evident as the spacecraft flew over the area of the plumes.
New high-resolution heat maps of the south pole by Cassini's Composite Infrared Spectrometer show that the so-called tiger stripes, giant fissures that are the source of the geysers, are warm along almost their entire lengths, and reveal other warm fissures nearby. The warmest regions along the tiger stripes correspond to two of the jet locations seen in Cassini images.
"These spectacular new data will really help us understand what powers the geysers. The surprisingly high temperatures make it more likely that there's liquid water not far below the surface," said John Spencer, Cassini scientist on the Composite Infrared Spectrometer team at the Southwest Research Institute in Boulder, Colo.
Previous ultraviolet observations showed four jet sources, matching the locations of the plumes seen in previous images. This indicates that gas in the plume blasts off the surface into space, blending to form the larger plume.
At closest approach, Cassini was only 30 miles from Enceladus. When it flew through the plumes it was 120 miles from the moon's surface. Cassini's next flyby of Enceladus is in August.
The first step toward answering the question of whether life exists inside the subsurface aquifer of Enceladus is to analyze the organic compounds in the plume. Cassini's March 12 passage through the plume provided some measurements that help us move toward an answer, and preliminary plans call for Cassini to fly through the plume again for more measurements in the future. Ultimately, another mission in the future could conceivably land near the plume or even return plume material to Earth for laboratory analysis.
Organic chemicals were part of the raw material from which Enceladus and Saturn's other moons formed. The origin of Enceladus' heat is less clear, but there are several possibilities that could have given Enceladus a layer of liquid water that persists today. Early on, it could have been heated by decay of short-lived radioactivity in rocks, with the heating prolonged by tidal influences.
Or perhaps an earlier oblong orbit could have brought more tidal heating than exists there today. A past tidal relationship with another moon could have caused the heat. Another theory says the heat could have been produced from a process called serpentization, where chemical binding of water and silicate rock could occur at the upper layer of the moon's core. This increases the volume of the rock and creates energy in the form of heat.
Any of these heating mechanisms might have created a liquid subsurface aquifer solution rich in organics, allowing Enceladus to serve up a suitable prebiotic soup.
The deep sea vent theory for the origin of life on Earth might apply to Enceladus as well. In this scenario, life on Earth began at the interface where chemically rich fluids, heated by tidal or other mechanisms, emerge from below the sea floor. Chemical energy is derived from the reduced gases, such as hydrogen-sulfide and hydrogen coming out from the vent in contact with a suitable oxidant, such as carbon dioxide. Hot spots on an Enceladus sea floor could be locales for this type of process.
We don't know how long it takes for life to start when the ingredients are there and the environment is suitable, but it appears to have happened quickly on Earth. So maybe it was possible that on Enceladus, life started in a "warm little pond" below the icy surface occurring over the last few tens of millions of years.
For life to persist once it has been established requires an environment of liquid water, the essential elements and nutrients, and an energy source. On Enceladus, there is evidence for liquid water, but we don't know its origin. The March 12 close flyby indicates there are some complex organic chemicals, as well. An energy source of some sort is producing geysers. As Cassini's exploration continues, NASA is seeking to bring together more pieces of this intriguing puzzle.
Posted by Casey Kazan. Adapted from materials provided by NASA
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