Janus and Tethys demonstrate the main difference between small moons and large ones. It’s all about the moon’s shape.
Images & Text: NASA/JPL-Caltech/Space Science Institute
Although Janus should be the least lonely of all moons – sharing its orbit with Epimetheus – it still spends most of its orbit far from other moons, alone in the vastness of space.
Moons like Tethys are large enough that their own gravity is sufficient to overcome the material strength of the substances they are made of (mostly ice in the case of Tethys) and mold them into spherical shapes. But small moons like Janus are not massive enough for their gravity to form them into a sphere. Janus and its like are left as irregularly shaped bodies.
The two large craters on Tethys, near the line where day fades to night, almost resemble two giant eyes observing Saturn. In this image Tethys is significantly closer to the camera, while the planet is in the background. Yet the moon is still utterly dwarfed by the giant Saturn.
Here, the giant impact basin Odysseus on Saturn’s moon Tethys stands out brightly from the rest of the illuminated icy crescent. This distinct coloration may result from differences in either the composition or structure of the terrain exposed by the giant impact.
Like a cosmic bull’s-eye, Enceladus and Tethys line up almost perfectly for Cassini’s cameras. Since the two moons are not only aligned, but also at relatively similar distances from Cassini, the apparent sizes in this image are a good approximation of the relative sizes of Enceladus (504 kilometers across) and Tethys (1,062 kilometers across).
In reality, Janus and the rings both orbit Saturn and are only weakly connected to each other through their mutual gravitational tugs. At specific locations in the rings, these gravitational tugs result in orbital resonances, which lead to some beautiful waves being created in the rings.
Like most moons in the solar system, Saturn’s moon Tethys is covered by impact craters. Some craters bear witness to incredibly violent events, such as the crater Odysseus (seen here at the right of the image). While Tethys is 1,062 kilometers across, the crater Odysseus is 450 kilometers across, covering about 4.5 percent of the moon’s surface area. A comparably sized crater on Earth would be as large as Russia!
Tethys, dwarfed by the scale of Saturn and its rings, appears as an elegant crescent in this image taken by NASA’s Cassini Spacecraft. Views like this are impossible from Earth, where we only see Saturn’s moons as (more or less) fully illuminated disks.