# Ice worlds' inner oceans--Emily Lakdawalla

1. Mar 12, 2015

### marcus

http://www.planetary.org/blogs/emily-lakdawalla/2015/03121716-ganymede-ocean.html

It's interesting how they infer ice layer from rotation rate and outer shape and density.
And then solve some equations to see if (within the ice layer) there could be a liquid ocean of a certain depth below surface, a certain salinity etc. Heating by tidal friction matters, of course.

Ceres is estimated to have an icy mantel something like 100 km thick (under a thin rubble crust) but no layer of ocean. It orbits the Sun by itself and isn't subject to tidal heating.

The ice mantle inference goes like this. Determine the size by inspection and the mass by observing interaction with other objects. If the density is substantially less than rock and it looks compact then there's water in its composition---calculate how much.
Now the question is, is it ice mixed with rock etc, or is it DIFFERENTIATED into layers.

That question is answered by comparing the FIGURE or shape (how flattened or oblate the ball is) with how fast it is rotating. The less differentiated, the more mass is farther out from the axis, experiencing more centrifugal force. The faster the spin, and the less differentiated, the more oblate, i.e. the more bulge at the equator.
So if it is spinning relatively fast and still not terribly oblate, it must be differentiated.

2. Mar 12, 2015

### phinds

Interesting.

Raises one question with me and that is, how could one infer salinity in a sub-surface ocean? Is there some reason oceans are always assumed to be saline to one degree or another?

3. Mar 12, 2015

### marcus

Have to see how Emily handles it. As I understand it, she has to present different cases. If this much salinity then you have this much ocean, soandso deep. I recall her presenting three cases: salinity A, B, C and each case has a different amount and depth of ocean. Because different melting point.

4. Mar 13, 2015

### Bandersnatch

http://www.nature.com/nature/journal/v519/n7542/full/nature14262.html

Last edited by a moderator: May 7, 2017
5. Mar 13, 2015

### Astronuc

Staff Emeritus
6. Mar 13, 2015

### Chronos

Salinity in oceans on earth is due to leaching of salts from rock. It appears reasonable to presume the same thing occurs on other worlds where liquid water exists.

7. Mar 13, 2015

### Bandersnatch

The layer of liquid water could be insulated from rock by a layer of ice.

8. Mar 13, 2015

### Chronos

Agreed, but, ice melts under pressure via a phenomenon known as regelation. Tidal heating would also tend to increase the temperature at the crust-ice interface.

9. Mar 13, 2015

### phinds

Good point. Thanks.

10. Mar 13, 2015

### phinds

But it seems reasonable to me (based entirely on ignorance) that it would be safe to assume that earlier in the life of those bodies they were warmer and the water might have been liquid all the way up and down.