Cassini views equinox on Saturn

via Astronomy Now.

The ringed planet Saturn lies more than nine times the distance from the Sun that the Earth does. The time taken for it to pass round our host star is therefore a lot longer; one Saturn year is close to thirty Earth years. Saturn, like the Earth, has its rotational axis tilted relative to the plane of its orbit, meaning that it experiences seasons. This also means that it experiences solstices (longest day of the year in one hemisphere, shortest day of the year in the other) and equinoxes (day and night of equal length), two of each during every three decades.

From Earth, what we see of Saturn’s equinox is the apparent disappearance of the rings – they lie edge on to the Sun and to us. When Galileo observed an equinox, he believed the disappearance of the rings was due to them being two giant moons in orbit of Saturn. The development of better optics disproved this, but there is another big satellite around Saturn at the moment, the probe Cassini.

The recent equinox fell on August 11th. This provided a perfect opportunity to explore the rings. As with the Moon, it is easier to make out features and make sense of their heights when they are casting shadows, caught during a partial phase than when the Sun is shining straight at them, such as the flat light of a full Moon. In the case of Saturn’s rings, they reflect flat light pretty much all the time except at equinox, when features start to become visible. So what was caught?

The ring plane itself is believed to be about the thickness of the height of a two storey building, though thousands of kilometres in diameter. What the shadows showed, however, was that some of the blocks of ice are three kilometres in size.

Bright streaks were found in rings A and C, relating to impacts of debris on the rings. It is estimated that impactors one metre in diameter were responsible for the 5,000 kilometre structures. A large event such as a massive impact is believed to be the cause of a much larger set of streaks in the rings, known as corrugations. These were seen earlier in the mission on the D ring, but were seen at equinox to extend through the C and up to the B ring.

It isn’t just impacts disrupting the rings though, the various satellites of Saturn all have their own gravitational impacts. Larger moons send waves through the outer rings, whilst small moons can be seen creating ‘propeller’ shapes in the distributions of particles. A propeller seen during equinox was 130km across, casting a 350km shadow, suggesting it is 200 metres high and due to a 400 metre moonlet. The eight kilometre moon Daphnis was seen to be producing four kilometre high wave disturbances, compared to 1.5 kilometre high waves seen earlier.

The Cassini Equinox mission has been celebrated with a slideshow of the best of Cassini and all equinox images can be seen here. A news conference on the mission, taken from NASA’s Youtube Channel, can be seen below:

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