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Enceladus is the sixth-largest moon of planet Saturn. It is approximately 500 kilometers in diameter, about a tenth of that of Saturn's largest moon, Titan.
Because there is water, energy, and organic compounds, Enceladus is yet another place in the solar system to look for life.
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Enceladus is mostly covered by fresh, clean ice, reflecting almost all the sunlight that strikes it, making its surface temperature at noon reach only -198 °C (-324.4 °F). Despite its small size, Enceladus has a wide range of surface features, ranging from old, heavily cratered regions to young, tectonically deformed terrains that formed as recently as 100 million years ago.
Enceladus was discovered on August 28, 1789, by William Herschel, but little was known about it until the two Voyager spacecraft passed nearby in the early 1980s. In 2005, the Cassini spacecraft started multiple close flybys of Enceladus, revealing its surface and environment in greater detail. In particular, Cassini discovered water-rich plumes venting from the south polar region.
Cryovolcanoes near the south pole shoot geyser-like jets of water vapor, other volatiles, and solid material, including sodium chloride crystals and ice particles, into space, totaling approximately 200 kilograms (440 lb) per second. Over 100 geysers have been identified. Some of the water vapor falls back as "snow"; the rest escapes.
These geyser observations, along with the finding of escaping internal heat and very few (if any) impact craters in the south polar region, show that Enceladus is geologically active today. Like many other satellites in the extensive systems of the giant planets, Enceladus is trapped in an orbital resonance. Its resonance with Dione excites its orbital eccentricity, which is damped by tidal forces, tidally heating its interior, and possibly driving the geological activity.
Evidence of liquid water on Enceladus began to accumulate in 2005, when scientists observed plumes containing water vapor spewing from its south polar surface, with jets moving 250 kg of water vapor every second at up to 2,189 km/h (1,360 mph) into space. In 2006 it was determined that Enceladus's plumes are the source of Saturn's E Ring.
The plumes' "salty" composition is one of the evidence of a salty subsurface ocean.
Gravimetric data from Cassini's December 2010 flybys showed that Enceladus likely has a liquid water ocean beneath its frozen surface.
Measurements of Enceladus's "wobble" as it orbits Saturn—called libration—suggests that the entire icy crust is detached from the rocky core and therefore that a global ocean is present beneath the surface. The amount of libration (0.120° ± 0.014°) implies that this global ocean is about 26 to 31 kilometers deep. For comparison, Earth's ocean has an average depth of 3.7 kilometers.
A model suggests that Enceladus's salty ocean (-Na, -Cl, -CO3) has an alkaline pH of 11 to 12. The high pH is interpreted to be a consequence of serpentinization of chondritic rock that leads to the generation of H2, a geochemical source of energy that can support both abiotic and biological synthesis of organic molecules such as those that have been detected in Enceladus's plumes.
Fortuitously, Cassini flew through this gas cloud during the July 14, 2016, encounter, allowing instruments such as the ion and neutral mass spectrometer (INMS) and the cosmic dust analyzer (CDA) to directly sample the plume. INMS measured the composition of the gas cloud, detecting mostly water vapor, as well as traces of molecular nitrogen, methane, and carbon dioxide.
Visual confirmation of venting on Enceladus came in November 2005, when ISS imaged geyser-like jets of icy particles rising from Enceladus's south polar region. The November 2005 images showed the plume's fine structure, revealing numerous jets (perhaps issuing from numerous distinct vents) within a larger, faint component extending out nearly 500 km from the surface. The particles have a bulk velocity of 1.25 ±0.1 km/s. Cassini's UVIS later observed gas jets coinciding with the dust jets seen by ISS during a non-targeted encounter with Enceladus in October 2007.
Observations during a flyby on March 12, 2008, revealed additional chemicals in the plume, including trace amounts of simple hydrocarbons such as methane, propane, acetylene and formaldehyde. The plumes' composition, as measured by the INMS, is similar to that seen at most comets.
Cassini also found traces of simple organic compounds in some dust grains.
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March 15, 2017: Cassini probe sees more heat than expected below the ice surface of Enceladus:
A Study published in the journal Nature Astronomy reports that the south polar region of Enceladus is warmer than expected just a few feet below its icy surface.
This suggests that Enceladus' ocean of liquid water might be only a couple of miles beneath this region, closer to the surface than previously thought.
The excess heat is especially pronounced over three fractures that are not unlike the "tiger stripes" - prominent, actively venting fractures that slice across the pole - except that they do not appear to be active at the moment.
The seemingly dormant fractures lying above the moon's warm underground sea point to the dynamic character of Enceladus' geology, suggesting it might have experienced several episodes of activity, in different places on its surface.
The finding agrees with the results of a 2016 study by a team independent of the Cassini mission that estimated the thickness of Enceladus' icy crust. The studies indicate an average depth for the ice shell of 11 to 14 miles (18 to 22 kilometers), with a thickness of less than 3 miles (5 kilometers) at the south pole.
Cassini Project Scientist Linda Spilker at NASA's Jet Propulsion Laboratory, Pasadena, California, said:
"Finding temperatures near these three inactive fractures that are unexpectedly higher than those outside them adds to the intrigue of Enceladus. What is the warm underground ocean really like and could life have evolved there? These questions remain to be answered by future missions to this ocean world."
Carbon-rich organic molecules, which are building blocks of life, have been discovered in the watery plumes spewing out from fissures in the surface of Enceladus.
Enceladus' plumes are emanating from numerous fractures called "tiger stripes" located in the moon's southern hemisphere, found by NASA's Cassini spacecraft. The source of the plumes is an alkaline and mildly salty subsurface ocean in which life could potentially be found. The plumes conveniently provide a way of sampling the ocean from orbit.
Cassini flew through the plumes on several occasions, analyzing it with its Cosmic Dust Analyzer and Ion and Neutral Mass Spectrometer instrument. Scientists have published in the June 28, 2018, issue of the journal Nature that Cassini detected in the plumes large and complex organic molecules, whereas previously, only very simple organic molecules such as methanol had been identified. Astrobiology Magazine also announced the discovery.
Cassini's instruments were not designed for sampling the plumes or distinguishing between complex organic molecules, because the Encelade plumes were not known before this mission.
The presence of organic molecules does not necessarily mean there is life on Enceladus, as they can form via geochemical processes involving interactions between water and rock. It is of course highly interesting that these compounds come from an habitable zone of Enceladus, with liquid water. When Cassini flew through one of the plumes in October 2015 (See below), it detected molecular hydrogen thought to have formed in geochemical reactions in hydrothermal vents on the sea floor; which means there is also a source of energy there, another condition for life.
A new study published on April 13, 2017, in the journal Science suggests molecular hydrogen (H2) is likely being produced continuously by reactions between hot water and rock deep down in Enceladus' sea.
In 2005, NASA's Saturn-orbiting Cassini spacecraft first spotted geysers of water ice erupting from "tiger stripe" fissures near Enceladus' south pole. Scientists think these geysers are blasting material from a sizeable ocean buried beneath the satellite's ice shell. This means Enceladus has liquid water, one of the key ingredients required for life as we know it. The ocean stays liquid because Saturn's immense gravitational pull twists and stretches the moon, generating internal tidal heat. This makes Enceladus an "habitable world".
A team of researchers led by Hunter Waite, of the Southwest Research Institute (SwRI) in San Antonio, analyzed observations made by Cassini during an October 2015 dive through Enceladus' geyser plume. Waite and his team were able to calculate that H2 makes up between 0.4 percent and 1.4 percent of the volume of Enceladus' geyser plume. Further calculations revealed that carbon dioxide (CO2) makes up an additional 0.3 percent to 0.8 percent of the plume's volume.
Waite and his colleagues concluded the molecular hydrogen is most likely being produced continuously by reactions between hot water and rock in and around Enceladus' core. They considered other possible explanations and rejected them.
A 2016 study by another research group had already which concluded that tiny silica grains detected by Cassini could have been produced only in hot water at significant depths.
The inferred presence of H2 and CO2 in Enceladus' ocean therefore suggests that similar reactions could well be occurring deep beneath the moon's icy shell. Indeed, the observed H2 levels indicate that a lot of chemical energy is potentially available in the ocean, Glein said.
"Now, Enceladus is high on the list in the solar system for showing habitable conditions," said Hunter Waite, one of the study's leading researchers.
The hydrogen in the sub-surface ocean could combine with carbon dioxide molecules in a process known as "methanogenesis," which creates a byproduct of methane. If there are indeed microbes living in the moon's ocean, they could tap that energy source as sustenance. Scientists said the moon appeared to have ample energy supplies to support life "roughly the equivalent of 300 pizzas per hour", Christopher Glein, a geochemist at the Southwest Research Institute in Texas, said.
"This is the first time we've been able to make a calorie count of an alien ocean," he said.
Jeffrey Seewald - not involved in the new Enceladus study - of the Marine Chemistry and Geochemistry Department at the Woods Hole Oceanographic Institution in Massachusetts, wrote in an accompanying piece called "Perspectives" in the same issue of Science:
"The abundance of H2, along with previously observed carbonate species, suggests a state of chemical disequilibria in the Enceladus ocean that represents a chemical energy source capable of supporting life."
On Earth, deep-sea hydrothermal vents support rich communities of life powered by chemical energy rather than sunlight.
Seewald wrote: "Some of the most primitive metabolic pathways utilized by microbes in these environments involve the reduction of carbon dioxide (CO2) with H2 to form methane (CH4) by a process known as methanogenesis."
"Chemical disequilibrium that is known to support microbial life in Earth's deep oceans is also available to support life in the Enceladus ocean."
Seewald is caution on a biological interpretations. noting that molecular hydrogen is rare in Earth's seawater, because hungry microbes quickly use it:
"Is the presence of H2 in the Enceladus ocean an indicator for the absence of life, or is it a reflection of the very different geochemical environment and associated ecosystems on Enceladus?"
