In science, it's often the case that solving one mystery just raises more questions. Take Saturn's moon Enceladus. For almost a decade, scientists have been puzzled by the gossamer plumes that waft up from its surface. Data from NASA's Cassini spacecraft now indicates that these may might be due to present-day hydrothermal activity in the vast ocean beneath the crust of the frozen moon, raising the possibility that Enceladus may harbor life.
In 2005, the unmanned Cassini orbiter first saw signs of the wispy plumes flowing from the unusually warm south polar regions of Enceladus. The plumes, which consist of ice, water vapor, salts and organic chemicals, remained a mystery until 2014 when a gravitational study of Enceladus indicated that it had a 6-mi (10-km) deep ocean hiding beneath an ice crust 19 to 25 mi (30 to 40 km) thick. If it's warm enough for deep oceans to exist on Enceladus, the reasoning goes, then it may be active enough to generate the plumes.
The current view is that the plumes are due to hydrothermal activity. In other words, the core of Enceladus is hot – possibly due to tidal forces caused by Saturn's giant mass. Just as the heat inside the Earth can give rise to volcanoes and geysers, a similar mechanism may be to work on Enceladus.
The idea is that Enceladian seawater seeps down into the crust of the moon where it heats and blows back up as a mineral-laden solution like an extraterrestrial Old Faithful. According to NASA, this model is supported by new insights based on two independent research projects, the first in Germany and Japan, and the second in the United States and France.
The first discovery was made using data from Cassini's Cosmic Dust Analyzer (CDA), which detected microscopic particles of silica in 2004 before the spacecraft went into orbit around Saturn. By modeling, experiments, and a process of elimination, the CDA team determined that the particles were silica grains, such as that found in sand and quartz, up to 6 to 9 nanometers in size.
According to the team, the particles were caused by waters seeping into the porous rocky core of Enceladus, where minerals were dissolved in the now hot, slightly alkaline, salty water of at least 194° F (90° C). As they came into contact with colder water in a subsurface ocean, their properties were altered before they were then blown 30 mi ( 50 km) out into space due to Enceladus' very low gravity. The small size also indicates that the particles left the seabed for space very quickly.
The second study focused on the source of the large amounts of methane detected in the plumes. Based on computer modeling, the high pressures and low temperatures in the ocean on Enceladus should cause any methane present to form clathrates. That is, materials that capture methane molecules inside a water crystal lattice and sink to the bottom as solids.
The models indicate that clathrates should from so fast that they deplete Enceladus' ocean of methane, yet Cassini detected methane in the plumes. This indicates that one of two things is happening. Either hydrothermal vents are pumping methane into the water faster than the clathrates can absorb it, or the geysers generating the plumes are pulling along clathrates, which release their methane as they heat up and the pressure drops. However, the presence of silica has the scientists leaning towards the hydrothermal hypothesis.
The importance of these discoveries is that they paint a picture of Enceladus that looks very much like the deep oceans of the Earth, where hot vents turn otherwise lifeless expanses of sea floor into oases of life. These volcanic vents produce heat and provide minerals that feed microbes that act as the basis for self-contained ecosystems that aren't dependent on the Sun for energy and are home to many extremophile life forms that thrive in environments that other organisms would find deadly, such as extreme heat or acidity. If similar vents exist on Enceladus, then they may host life.
"These findings add to the possibility that Enceladus, which contains a subsurface ocean and displays remarkable geologic activity, could contain environments suitable for living organisms," says John Grunsfeld, astronaut and associate administrator of NASA's Science Mission Directorate. "The locations in our solar system where extreme environments occur in which life might exist may bring us closer to answering the question: are we alone in the Universe."
Launched in 1997 from Cape Canaveral Air Force Station Space Launch Complex 40, Cassini has been orbiting Saturn since it arrived in orbit in 2004 after a complex series of flybys of Venus, Earth, and Jupiter. The nuclear-powered spacecraft has since spent the last 10 years making an extensive study of Saturn, its rings, and its moons, including the deployment of the European Space Agency's Huygens probe to Titan in 2005.