Saturn’s sixth largest moon, Enceladus, was discovered in 1789 by British Astronomer William Herschel. With a low albedo and close proximity to Saturn, Enceladus is difficult to observe. Because of this difficulty little was known about this moon until the Voyager flybys in the 1980’s. Voyager 1 discovered that Enceladus is located in the densest part of Saturn’s E Ring, and Voyager 2 discovered that Enceladus has diverse and relatively complicated surface features.

The Voyager missions generated a number of questions about the small moon: “Is there a connection between Enceladus and Saturn’s E-ring?” “What is causing the tectonic activity which is deforming Enceladus’s surface?” The recent Cassini mission was able to answer these questions, along with generation new discoveries and new questions.

To answer the first question, Cassini discovered that Enceladus is the fourth known body in the solar system with active volcanism. The other three are Earth, Jupiter’s moon Io, and Neptune’s moon Triton. This volcanism causes icy jets, plumes of water vapor, and other materials to be shot into the atmosphere. It is this cryovolcanism which was determined to be the cause of Saturn’s E Ring. Just recently Cassini photographed the volcanic southern pole. These pictures revealed a geological feature scientists are calling “tiger stripes”. These tiger stripes are 300 meter deep fractures and are surrounded by chunks of ice, and are the source of Enceladus’s volcanism.

Cassini also discovered the cause of the tectonic activity. Enceladus, like many other moons is traped in orbital resonances, this causes tidal heating on the moons interior. Like thought to exist on Jupiter’s moon Europa, this could also cause Enceladus to have a subsurface liquid ocean. Because of the volcanic activity a subsurface ocean on Enceladus is though to be only tens of meters beneath the surface, where the oceans on Europa are thought to be 100 kilometers beneath the surface.

Does Enceladus have a subsurface ocean? If it does, is this another place to look for signs of life? With many more Enceladus flybys to come, we may yet find out if there is a subsurface ocean, but we will certainly have to wait for the right mission if we want to determine if life exists.

Saturn’s largest moon (the solar system’s second largest moon), Titan, was discovered in 1655 by Dutch astronomer Christiaan Huygens. In 1944, Gerard Kuiper demonstrated that Titan’s dense atmosphere has the spectral signature of methane.  Up until the arival of the Voyager 1 in 1980 and Cassini-Huygens in 2004, Titan was somewhat of a mystery with its surface features hidden beneath thick layers of clouds and haze.

Although the surface was still hidden, Voyager was able to learn much about the moon’s planet-like atmosphere. Titan’s huge atmosphere creates a surface pressure of 1.5 bars, a temperature of 94K, and a density of 5.3kg/m3.  This surface temperature is close to the triple point of methane, which could mean that Titan has a methane cycle similar to Earth’s hydrological cycle.           

In 2005, ESA’s Huygens Probe was released from Cassini and entered Titan’s atmosphere. It discovered that Titan and Earth’s atmosphere share a similar altitude/temperature relationship.  On Earth, the temperature decreases with altitude in the troposphere, increases in the stratosphere due to the absorption of UV rays in the ozone, decreases in the mesosphere due to decreasing atmospheric density, and finally increases in the thermosphere due to the release of thermal energy caused by the break up of molecules by solar radiation.  On Titan, the temperature decreases with altitude in the troposphere, and increases in the stratosphere. 

With several Cassini flybys, Titan’s mysterious surface is finally being revealed.  Titan’s surface is incredibly Earth-like with rain-cut river beds, hydrocarbon lakes, and giant equatorial sand dunes.  Much is still unknown about the surface, such as the depth of the lakes, and how the sand dunes are formed.  Cassini Radar observations also confirmed that the entire crust of Titan is floating on top of a massive water ocean.              

With a two year extention to the Cassini mission, including 26 more Titan fly-bys, there is definitely more discoveries to come.  Titan is well worth exploring with complex orbiters and landers, not only to explore Titan’s exotic surface features but also to look for signs of extremophiles.  Any such mission is far off, so in the meantime we can enjoy the only sounds ever recorded on a body other than Earth which were recorded by the Huygens Probe as it descended through Titan’s atmosphere: Sounds of Titan�