By Sabine Stanley, Ph.D., Johns Hopkins University
Saturn has many small moons, but three of them are explored a bit more closely, and orbit the planet at a closer distance: Dione, Enceladus, and Mimas. One of them is the smallest spherical moon, one is the Death Star, and one is a potential place for life to form. Read on to find out which small Saturnian moon is which!
Dione and Tethys
Dione has a diameter of 700 miles and is made of ice and rock. This heavily-cratered Saturnian moon has wisps on its surface, and is accompanied by two smaller moons in its orbit. Helene orbits 60° ahead of Dione, whereas Polydeuces orbits 60° behind. These two tiny moons are not spherical and are not disturbed by Dione’s gravity. Polydeuces has a diameter of only three kilometers, and Helene of 43 kilometers with evidence of gullies and dust flows.
(Image: ManuMata/Shutterstock)
A little closer to Saturn there is Tethys, with the same size as Dione, but the least dense of all the round moons in the solar system: only 984 kilograms per cubic meter, which is less dense than water.
Tethys is heavily cratered and has many pores, making it look like the ‘Death Star’. It has two Trojan moons, Telesto and Calypso, similar in size to Dione’s Trojan moon, Helene.
Learn more about water on Mars and prospects for life.
Enceladus, the Sixth-largest Moon
Being the sixth-largest moon among the small Saturnian moons is not difficult. Enceladus has a diameter of about 313 miles, the most reflective surface in the solar system, and a private ring. It is embedded in Saturn’s E-ring. This moon has very young areas on the surface and giant fractures all across. Conclusively, there is a tectonic activity or a heating mechanism there.
At the southern pole, there are huge jets, ejecting icy water particles, other gasses, and even organic molecules into space. They result from the ‘tiger stripes’, which are warmer faults on the surface, expelling around 250 kilograms of water vapor every second. These particles make up the E-ring.
Below the frozen surface, Enceladus has an ocean about 25–30 kilometers deep, feeding the jets. The surface is 20-30 kilometers thick on average, but perhaps about one to five kilometers near the tiger stripes.
Below the Ocean in Enceladus
The jets are probably a result of cryovolcanism, where melted pressurized water erupts at the surface through the cracks. The cracks have been there since the first images of the moon were taken in 1980.
Enceladus has a rocky core under the ocean, making up 50% of the moon. The tiny amounts of radioactive elements in the rocks eventually decay and create enough heat for water ice to melt. Thus, there is liquid water under the surface that erupts from thinner surface ice. This small Saturnian moon is a place to look for life, due to water and heat at the core.
This is a transcript from the video series A Field Guide to the Planets. Watch it now, on Wondrium.
Mimas, the ‘Death Star’ Moon
The reason Mimas is likened to Death Star is that it has a large, complex structure crater, with about the same relative size and location to the super laser on the Death Star. The Cassini division, the largest gap in Saturn’s rings, is caused by Mimas, the smallest possible round moon.
This moon, with a diameter of 240 miles, seems to have no tidal heating, which is odd considering its close distance to Saturn. Mimas has a much smaller rock fraction than Enceladus, which means Mimas is likely to have less radiogenic heating.
The Tiny Minor Saturnian Moons
Hyperion is the first tiny moon, which is too small to be round. This sponge-like moon spins around itself chaotically as it orbits Saturn, due to its eccentric orbit and oblong shape, plus gravitational interactions with the nearby giant moon, Titan.
The next tiny moon is Phoebe, orbiting about four times farther from Saturn than Iapetus, the outermost round moon. It was the first object imaged by Cassini in 2004, as an irregular moon with a retrograde orbit. The moon is 200 kilometers across, spewing enough debris into orbit from micrometeorite impacts to make its own ring around Saturn.
Phoebe is also ice-rich but looks much darker due to the debris covering its heavily-cratered surface. Most probably, it was a captured Kuiper belt object.
There is so much to explore in the Saturn system, and scientists are eager to get back there with various mission concepts. Titan and Enceladus are both excellent candidates for future missions to search for signs of life.
Learn more about comets, the Kuiper Belt, and the Oort Cloud.
Common Questions about Small Saturnian Moons
Saturn has at least 62 moons, seven of which are the major ones. Thus, small Saturnian moons are around 55, orbiting the planet at different distances.
The seven major moons orbit Saturn in almost circular orbits, but the smaller Saturnian moons have diverse orbits, sometimes with very high inclinations and sometimes orbiting in a retrograde manner.
Mimas is the smallest of spherical Saturnian moons with a diameter of just under 400 kilometers.
