Saturn’s Moon Helene


Helene is a small and faint moon of Saturn likely made mostly of rock-hard water ice, with a diameter of 36 x 32 x 30 kilometers (22 x 20 x 18.6 miles). Helene orbits Saturn at the considerable distance of 377,400 km (234,505 miles). The moon is also designated S/1980 S 6, Saturn XII (12) and Dione B, for it shares its orbit with the larger moon Dione.

Helene is called a Dione Trojan satellite; it orbits at about 60 degrees ahead of its larger companion. The term comes from the behavioural resemblance to the Trojan asteroids which orbit the Sun within Jupiter’s path – again 60º in front and behind. These orbital positions are known as Lagrangian points (L4 and L5, respectively.)

Although conventional craters and hills appear, the above image also shows terrain that appears unusually smooth and streaked. Planetary astronomers are inspecting these detailed images of Helene to glean clues about the origin and evolution of this floating iceberg.

Helene rotates synchronously, meaning it completes one day (one rotation) for each time it orbits Saturn. As a result, one side always faces Saturn. The opposite side always faces away.

The Saturn-facing hemisphere of the moon is covered with strange gully-like features that probably represent slides of dry material into local topographic lows. There are two things that are very strange about these gullies.

One is to see them at all. Features like this, if seen on Earth or even Mars, would be assumed to have something to do with water, but there is no possibility of liquid water on Helene. These gullies must form by a dry process in which material – likely very powdery dusty stuff – cascades toward local topographic lows.

The other thing that is very strange is the strong difference in color between the higher-standing stuff and the smooth gully slide areas in between them. The color differences are most obvious on the right side of the image, where the Sun hits Helene directly and there aren’t many cast shadows; color differences fade as you get toward the lower and lower light near the terminator at the left side of the image.

There are a couple of dozen little tiny bowl-shaped impact craters scattered across the image, and there are eroded features that are almost certainly older, larger impact craters, but really there are not very many craters considering Helene’s location in the shooting gallery of the Saturn system, so whatever process makes these gullies has also very likely been active recently and has wiped away past smaller craters. This inference becomes even more interesting when you look at the opposite face of Helene, the one that faces out from Saturn, which is heavily cratered as you might expect.