Jupiter and Saturn come into view in July
Gary Sparks
Jupiter and Saturn are well placed for viewing during July, especially by mid evening. Jupiter will be higher in the evening sky, although by the 31st the two planets will be at similar altitudes at 10pm, with Jupiter further west than Saturn. The moon passes Jupiter on the morning of July 14. The two are 4 degrees apart, shortly before moonset at about 5am.
One of the moon’s most fascinating features, one hidden from our view on its farside, is a huge impact basin known as South Pole-Aitken (SPA). It’s so named because its 2000km diameter extends from the crater Aitken on its north rim to the lunar south pole. It’s immense, second only to Mars’ Hellas basin as the largest impact in the solar system.
SPA isn’t particularly obvious in spacecraft photos, a somewhat darker centre hints of metal-rich material dredged up from the Moon’s deep crust or uppermost mantle. Topographically however it is an unmistakable pit up to 13km deep. Early this year China’s Chang-e 4 lander dropped onto one particularly deep spot near the basin’s center. The final data collected by NASA’s Cassini spacecraft shows that Saturn’s rings are a complex mix of stripes, speckled textures and other odd features that appear to be sculpted by the planet’s moons. Saturn’s rings are largely made up of particles of ice and when these get far enough away from the planet’s gravity, they can clump together to form icy moons.
There are 62 named moons orbiting Saturn, ranging from the massive Titan, thousands of kilometers across, to much smaller moons closer in. Images from Cassini’s cameras reveal in unprecedented detail how Saturn’s rings are affected by moons like Daphnis, which is just 10 kilometers across and embedded in the A ring, the outermost of the planet’s main rings.
Daphnis creates a gap in the A ring and kicks up waves in the gap’s outer edge, which form as a result of the interaction of gravity from Saturn, Daphnis and the ring particles themselves. The new images clearly show these waves, and also how they settle down in Daphnis’ wake.
The final Cassini images also reveal further secrets about strange disturbances in the rings called “propellers” because their shape is reminiscent of an old-fashioned twobladed aeroplane propeller. The largest examples of these disturbances are up to 100km or so wide, and have been nicknamed Bleriot, Earhart and SantosDumont after famous aviators. Scientists are excited about understanding how Saturn’s ring disc works because it can give clues about how solar systems like our own form. Saturn’s rings are analogous to a “baby solar system” and provide a “natural laboratory”. It’s possible that some of the planets in our solar system could have once looked like Daphnis.