Cassini-Huygens, the two-part spacecraft named after astronomers Giovanni Cassini and Christiaan Huygens launched in 1997 by NASA was the first spacecraft to orbit Saturn, and has been circling the planet since 2004. It was only on October 28, 2015 that it came back into focus as it captured stunning images from 38,000 miles above the surface of the planet.
Currently, the unmanned ship is flying close to one of Saturn’s 62 moons, Enceladus, with a jet of icy plume of water erupting from a fissure on its frozen crust.
So what do we know about Enceladus?
Enceladus is the sixth largest moon of Saturn. At about 250km across, it is the size of an asteroid.Its surface is an icy crust that is about 45km thick and covers a sub-surface ocean of liquid salt water. This liquid water was discovered by the Cassini orbiter when the craft imaged Enceladus in 2005.
Plumes at the active southern hemisphere on Enceladus. (Image courtesy: NASA/Cassini)
Cassini also captured pictures of the moon throwing up liquid water and salts in the form of high speed eruption jets. These jets are called cryovolcanoes: 'cryo' for ice. Cryovolcanoes eject ice, water, and water vapor instead of lava and magma that are released by volcanoes on Earth. These jets are called 'plumes.' The plumes on Enceladus spew out up to 200kg of water, ice, and salts per second. Most of this material feeds the second outermost, hazy ring of Saturn, the E-ring. The rest fall back on the moon as snow.
Volcanic activity of any sort on a body requires geological activity. This occurs on Enceladus due to tidal forces exerted on the moon by Saturn. As the planet tugs and pulls at the moon, it stretches and flexes the interior of the moon, causing heat on the inside. However, paradoxically, it's only the south pole of Enceladus that is active. The northern polar regions have a large number of impact craters, indicating the surface is old. The southern pole shows smooth surface as a result of constant volcanic activity that covers the surface with new material repeatedly. In fact, there is a region on the regularly active South Polar Terrain (SPT) with four parallel, extremely active depressions on the south pole that is hotter than the rest of the moon, called Tiger Stripes.
Thermal map of the Tiger Stripes region. Image courtesy: NASA/Cassini-Saturn/JPL
The Cassini orbiter flew through one of Enceladus’s plumes on October 28th, giving us some spectacular images of the moon. Cassini had flown through plumes before, in 2008, but could not collect data due to a software malfunction. This time around, though, we expect to get back data that is rich in detail over the next few weeks. Cassini has contributed majorly to deducing the presence of a salty global ocean as well as discovering cryovolcanism.
Geothermal activity is particularly important when we look for habitability because it enables water to remain in liquid form, that is so essential to life on earth. Enceladus also ejects hydrocarbons and ammonia in its plumes; these are important components of life as we know it. Yet another biomarker on Enceladus is the presence of silica formed when salt water rubs against rock, just like on Earth. All of these factors -- water, nutrients, energy source -- make Enceladus theoretically friendly to life as we know it. Therefore, the moon is one of the most important bodies we can study in our solar system to understand habitability and to hunt for potential microbial life.