On 13 Marsh, 2006, NASA’s Cassini spacecraft took this edge-on view of Saturn’s rings, capturing the moon Mimas above the ring plane, illuminated both by the sun and by “saturnshine,” the tiny moon Janus just above the rings and larger Tethys below. Cassini took the photo from a distance of about 2.7 million kilometres (1.7 million miles), combining red, green and blue spectral filters to produce a natural colour view. Cassini’s mission ended 15 September, 2017, when the spacecraft, virtually out of propellant after its long, successful mission, was directed to plunge into Saturn’s atmosphere.
The existence of a fifth giant gas planet at the time of the solar system’s formation — in addition to Jupiter, Saturn, Uranus and Neptune that we know of today — was first proposed in 2011. Now astrophysicists at the University of Toronto have found that a close encounter with Jupiter about four billion years ago may have resulted in the fifth giant planet’s ejection from the solar system altogether.
Janus and Tethys demonstrate the main difference between small moons and large ones; it’s all about their shape. Moons like Tethys are large enough that their own gravity is sufficient to overcome the material strength of the substances they are made of and mould them into spherical shapes, but small moons like Janus are not massive enough for their gravity to form them into a sphere.