What's NASA Up To And Other Space Stuff

[Wraith]

If you can move around Ganymede, you really should just be making an orbital ring.

do you really think the ice crust is moving around that much?

No idea! I admit that I can't keep my Jovian moons straight, but I know that on at least one of the moons the ice surface is believed to be shifting around at a fairly frantic pace due to Jupiter's gravity, and I don't know that this isn't the case on Ganymede. It's tidally locked and with a stable orbit, so it probably isn't too bad there, but even slow shifts are going to involve levels of energy that steel can't deal with.

Interesting side note, have you seen the pics of Ceres? There's some talk there could be a liquid layer there too, which sort of upends the tidal heating explanation for the big planet moons.

Tidal heating isn't a requirement for subsurface liquids. I can think of a bunch of possibilities that would allow Ceres a liquid subsurface ocean - it's closer to the sun, so it gets more energy from there, radioactive heating, an ocean of ammonia instead of water, etc. This is honestly the first I've heard of it having a liquid layer, though.

[EyeKhan]

If you can move around Ganymede, you really should just be making an orbital ring.

No idea! I admit that I can't keep my Jovian moons straight, but I know that on at least one of the moons the ice surface is believed to be shifting around at a fairly frantic pace due to Jupiter's gravity, and I don't know that this isn't the case on Ganymede. It's tidally locked and with a stable orbit, so it probably isn't too bad there, but even slow shifts are going to involve levels of energy that steel can't deal with.

The OP pic shows a lot of craters, so thinking there can't be a huge amount of shifting....

This is honestly the first I've heard of it having a liquid layer, though.

It was just mentioned in passing in an article I read about the probe going into orbit, so don't quote me....

[Wraith]

The OP pic shows a lot of craters, so thinking there can't be a huge amount of shifting....

The lighter spots? It might just be the resolution, but those don't look like craters to me. It's likely that they're damage from strikes, but we don't know age (a Jovian moon should be getting far more frequent strikes than the moon), and they look like the ice healed over them. Compare to this image of the moon:



Look at the edges - you can clearly see that they're craters there. I don't see that in the Ganymede pic.

I'm not really sure why I'm talking about this though; you're probably right and there isn't much shifting. I only see a handful of the fracture lines that usually indicate movement like that.

edit: I'm taking it back! This is Ganymede:

[EyeKhan]

The lighter spots? It might just be the resolution, but those don't look like craters to me. It's likely that they're damage from strikes, but we don't know age (a Jovian moon should be getting far more frequent strikes than the moon), and they look like the ice healed over them. Compare to this image of the moon:



Look at the edges - you can clearly see that they're craters there. I don't see that in the Ganymede pic.

I'm not really sure why I'm talking about this though; you're probably right and there isn't much shifting. I only see a handful of the fracture lines that usually indicate movement like that.

edit: I'm taking it back! This is Ganymede:

Even recent craters on an icy world are not well defined since the ice melts and fills the hole. And the marks that remain are relatively few presumably because the surface shifts around. The question is what time scale of shifting are we talking about - do crater marks get erased within decades, millennia, or is it more like millions of years? On Earth craters on the continents can last a long time, depending on how arid the region is, but generally get filled relatively quickly. And erosion and geology here is stable enough to build cities and infrastructure of all kinds. Probably icy moons have something analogous to the plates of Earth's crust with some relatively stable regions in the plate centers and messy fault zones at the edges.

Interesting in the Ganymede pic the obvious fracture/ stress lines are pitted with apparently well defined craters. I wonder if the smaller strikes don't cause the melting that the larger ones do.

On a side note, I went to the big crater out in Arizona. Very commercialized - it's privately owned - but pretty neat. And it's not far from the Petrified Forest National Park, which is also pretty neat. In between is Winslow Arizona, notable (only) for the Eagle's lyric. There's a statue of whoever sang it (Glen Fry?) on the corner and the song plays on outdoor speakers all day.