APOD: Mimas: Small Moon with a Big Crater (2014 Oct 21)

[dlw]

There have been studies of impact on spherical bodies containing liquid, e.g., (US) football players' heads. The bottom line is that a significant impact on one side may cause damage on the far side, directly opposite to the impact. If Mimas does have a liquid interior, it might show damage on the other side.

Are there any high res pictures of the other side?

[Chris Peterson]

There have been studies of impact on spherical bodies containing liquid, e.g., (US) football players' heads. The bottom line is that a significant impact on one side may cause damage on the far side, directly opposite to the impact. If Mimas does have a liquid interior, it might show damage on the other side.

Not liquid so much as fluid.

[Nitpicker]

There have been studies of impact on spherical bodies containing liquid, e.g., (US) football players' heads. The bottom line is that a significant impact on one side may cause damage on the far side, directly opposite to the impact. If Mimas does have a liquid interior, it might show damage on the other side.

Not liquid so much as fluid.

http://en.wikipedia.org/wiki/Mimas_(moo ... cteristics

Mimas's most distinctive feature is a giant impact crater 130 kilometres (81 mi) across, named Herschel after the discoverer of Mimas. ...<snip>... The impact that made this crater must have nearly shattered Mimas: fractures can be seen on the opposite side of Mimas that may have been created by shock waves from the impact travelling through Mimas's body.

My reading of the recent study, is that the possibilities are either a somewhat oblong solid core, or a liquid water ocean beneath the surface (but above a more spherical solid core). The (rocky?) core may have a certain fluidity, but I imagine -- without much evidence to back me up -- that would require more energy than would be necessary for a liquid ocean above it. (To me, "fluid" is a term which includes liquids and gases, where molten rock would be a more viscous liquid than water in liquid phase.)

[Chris Peterson]

My reading of the recent study, is that the possibilities are either a somewhat oblong solid core, or a liquid water ocean beneath the surface (but above a more spherical solid core). The (rocky?) core may have a certain fluidity, but I imagine -- without much evidence to back me up -- that would require more energy than would be necessary for a liquid ocean above it. (To me, "fluid" is a term which includes liquids and gases, where molten rock would be a more viscous liquid than water in liquid phase.)

I wasn't commenting on the interior of Mimas, only pointing out that the work looking at the propagation of impact energy through bodies considers fluid interiors, not simply liquid. In fact, the interior of nearly any large body is going to be some combination of solid and fluid. At the pressures found far below the surface, few materials can exist in liquid form (including water).

[Nitpicker]

My reading of the recent study, is that the possibilities are either a somewhat oblong solid core, or a liquid water ocean beneath the surface (but above a more spherical solid core). The (rocky?) core may have a certain fluidity, but I imagine -- without much evidence to back me up -- that would require more energy than would be necessary for a liquid ocean above it. (To me, "fluid" is a term which includes liquids and gases, where molten rock would be a more viscous liquid than water in liquid phase.)

I wasn't commenting on the interior of Mimas, only pointing out that the work looking at the propagation of impact energy through bodies considers fluid interiors, not simply liquid. In fact, the interior of nearly any large body is going to be some combination of solid and fluid. At the pressures found far below the surface, few materials can exist in liquid form (including water).

I can't tell if you are saying water can or cannot exist in liquid form at high pressure (is water one of the few materials or not)? I assume you must be saying that water (and other materials) can only exist in supercritical form (behaving like both a liquid and a gas) beyond a certain temperature and pressure. But are you still talking about a large body as small as Mimas? I'm fairly confident that at some point in between the surface and the centre of Mimas, the conditions might be right for liquid water to exist.

Edit: ultimately, what I think you are saying is that the work looking at the propagation of impact energy through bodies, will probably not be able to help determine which possibility from the recent Mimas study, is more likely. I'll agree with you there (not that I was disagreeing with you before, just not quite understanding).

[Chris Peterson]

I can't tell if you are saying water can or cannot exist in liquid form at high pressure (is water one of the few materials or not)?

At high pressures water exists only as exotic forms of ice.

I assume you must be saying that water (and other materials) can only exist in supercritical form (behaving like both a liquid and a gas) beyond a certain temperature and pressure.

No.

But are you still talking about a large body as small as Mimas? I'm fairly confident that at some point in between the surface and the centre of Mimas, the conditions might be right for liquid water to exist.

Sure, there could be a region of liquid water a few tens of kilometers below the icy surface. But the pressures very deep would result in solid forms, which would very likely be fluid (like most of the Earth's interior, although that consists of fluidic silicates in solid phase, not ice).

[Nitpicker]

My simple estimation of the pressure at the core of Mimas is ~14.5 MPa. (Based on <mean density>*<surface gravity>*<mean radius>, which works quite well for Earth's core pressure of ~350 GPa.)

At that pressure in Mimas, the phase diagram for water shows ice/solid phase only below 273 K, much like in my kitchen freezer. (Though the situation changes rapidly above pressures of a few hundred MPa, a situation I wasn't previously aware of, so thanks for making me read up on it.)

[Chris Peterson]

My simple estimation of the pressure at the core of Mimas is ~14.5 MPa. (Based on <mean density>*<surface gravity>*<mean radius>, which works quite well for Earth's core pressure of ~350 GPa.)

At that pressure in Mimas, the phase diagram for water shows ice/solid phase only below 273 K, much like in my kitchen freezer.

Right. The question is whether ice is fluid at that pressure, though.

[Nitpicker]

My simple estimation of the pressure at the core of Mimas is ~14.5 MPa. (Based on <mean density>*<surface gravity>*<mean radius>, which works quite well for Earth's core pressure of ~350 GPa.)

At that pressure in Mimas, the phase diagram for water shows ice/solid phase only below 273 K, much like in my kitchen freezer.

Right. The question is whether ice is fluid at that pressure, though.

If it is above 273 K, it would be liquid, so definitely fluid. And below that, maybe an exotic phase of ice would be (at least partially) fluid too.