麻豆传媒

It doesn鈥檛 carry the weight of the world 鈥� or even of Saturn鈥檚 rings. The best images yet of Saturn鈥檚 small moon Atlas show that it is a fluffy oddball with less responsibility than we thought.

On 12 April, NASA鈥檚 Cassini spacecraft dove close to Atlas to bring us the best images of this odd little moon we鈥檒l see. With a resolution of up to 84 metres per pixel, these new photographs improve our view of Atlas by a factor of two. This is the last and closest flyby of the moon before the Cassini mission ends in September.

鈥淚t鈥檚 very interesting to see it close up after decades of knowing it was there,鈥� says at NASA鈥檚 Jet Propulsion Laboratory, who discovered the moon in 1980. 鈥淔inally seeing it as a real piece of real estate is very, very interesting.鈥�

Some of Saturn鈥檚 moons, called shepherd moons, use their gravity to keep the planet鈥檚 famous rings in check. But Atlas appears to be a shepherd moon that isn鈥檛 responsible for shepherding ring particles.

鈥淎t the time [we found it], we thought the satellite was holding out the edge of the A ring,鈥� Terrile says. But instead, the ring鈥檚 shape is held by two other moons, Janus and Epimetheus. 鈥淭his is a really interesting kind of dynamical dance that these moons do with the ring particles.鈥�

We already knew Atlas has a UFO-like ridge around its equator, but surprisingly, the new images show that ridge is smooth.

鈥淚t looks more subdued than I expected,鈥� Terrile says. 鈥淚t looks like it鈥檚 covered in some kind of fluffy material.鈥�

鈥淲hat is especially interesting is the extent to which the soft material seems to bury and mute any crisp-looking topographic features even on the central 鈥榗ore鈥� structure,鈥� says at Cornell University in Ithaca, New York, who helped plan the flyby.

Another moon, Pan, also has an equatorial ridge, but another recent flyby showed its ridge is rough with tension cracks and craters. The two moons are approximately the same size, but Pan is embedded within a ring, and Atlas is along the outer edge.

The dominant theory for how the ridges form is that because the moons鈥� diameters are so much larger than the ring鈥檚 thickness, they gather material along their equators as they plow through stray ring particles. But it might not be that simple. 鈥淚t may also be some kind of gravitational tidal effect from being near all this ring material,鈥� says Terrile.

Understanding Pan and Atlas may be key to understanding gravity鈥檚 role in all of Saturn鈥檚 moons. 鈥淭he same gravity that causes all these weird phenomena that we鈥檙e seeing on these little moons causes energy to be pumped into some of the larger ones,鈥� says Terrile. 鈥淎nd that energy can create under-ice oceans, maybe even habitable zones.鈥�