Thursday, May 4, 2017


I thought we’d talk about Dione today. That was before I found out there were 4 ‘Dione’s’ in Greek mythology and one in the Phoenician mythology of Sanchuniathon. Rather than try to sort through all those, I changed my mind and decided to discuss Dione, a moon of Saturn. (What or who is Sanchuniathon? I may have to come back to that one sometime.)
This moon was discovered by Giovanni Domenico Cassini in 1684. It is also sometimes called Saturn IV.
Dione’s orbit around Saturn is an ellipse, and at its closest approach, it is slightly closer to Saturn’s center than our own moon is to the Earth’s center. Because Saturn is a lot bigger than Earth, Dione races around it, taking 2.74 days to complete an orbit, as well as a Dione ‘day’. It never turns its face away from Saturn. It’s interesting that every time Dione completes one orbit, Enceladus (another Saturn moon) completes two. Each time they pass each other, the gravimetric tugging generates internal heat in both moons.
Also interesting is that Dione is one of a set of triplets. Two other moons of Saturn, Helene and Polydeuces, share the same orbit as Dione. They run around Saturn in single file, one 60° ahead of Dione, and the other 60° behind.
Who knew this kind of stuff could actually happen ‘naturally’? May I should have stuck with mythology after all.
It is believed that Dione is about 2/3 water in various forms, and the remainder is a dense core of silicate rock. The top of the ‘water’ is an ice crust, probably as thick as 99 kilometers ( 62 miles). The temperature at Dione’s surface is about -121°F, which would make the ice so hard, it would act like rock. Between the rock core and the ice crust is about 65 km ( 41 miles) of liquid ocean. The crust does have various features, such as chasms, ridges, long narrow depressions, craters and crater chains.
Dione is pretty well covered in craters, as large as 100 km (62 miles) across. However, most of the craters are on the opposite side as scientists expect them to be. The theory is that on something the size and mass of Dione, anything big enough to make a 35 km (22 mile) crater would be able to spin the moon about. There are enough large craters to indicate Dione did a lot of spinning in the past. So maybe she keeps her back to Saturn, trying to see the next spin-inducing attacker before it hits?
Oh, and let’s not forget the ice cliffs (formerly known as ‘wispy terrain’ when it was discovered by the Voyager space probe). At the time, they were called ‘wispy’ because whatever they were, they didn’t hide the countryside in their vicinity. But more recent photos by Cassini show that these ‘wispy’ lines were, in fact, ice cliffs, fractures created by chasms. We now know that some of them are several hundreds of meters tall.
In 2010, the Cassini probe detected oxygen ions around Dione, but there were so few of them, scientists prefer to call it an exosphere rather than a tenuous atmosphere.

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