I promised an #Enceladus thread yesterday; it starts here! Let's take a look at this tiny, but all the more interesting icy moon of Saturn! https://twitter.com/People_Of_Space/status/1391834523294437384

Though discovered already in the late 18th century by William Herschel, we had long known extremely little about it.

That changed with the Voyager flybys!

This image was taken by Voyager 2 on August 25, 1981, from about 110,000 km away. It shows cratering, but also bright ice, grooves, some crater-poor areas. That suggested parts of the surface were very young - the tiny moon was active!

Here's a mosaic from Voyager 2 images showing the marked difference between the cratered northern hemisphere and the crevassed young terrain of the southern one. Something was going on!

I keep saying "tiny" moon. How tiny exactly?

It's about 500 km across (our Moon is nearly 3,500 km across) - about the length of the Czech Republic, where I live, from west to east!

In the comparison, Enceladus is in near the lower left corner.

That was a puzzle! Anything *that* tiny shouldn't be very geologically active.

(Sure, comets are despite their much smaller size, but that's caused by great differences in the amount of sunlight in their elongated orbit - which is not the case for Enceladus!)

A little moon like Enceladus couldn't have kept its initial accretion heat to drive active geology, nor could it have many radiogenic heat sources.

'External' heating was required: tides!

(Back after a short break :)!)