Starship Asterisk*

The lecture video is embedded below but also available here in MP4 format.
Additionally, slides used in the lecture are embedded below but also are available here in Powerpoint format.
Questions after the lecture? Please ask them in here.
Wikipedia entries:

• Gas Giant
  Jupiter
  Io_(moon)
  Europa_(moon)
  Callisto_(moon)
  Ganymede_(moon)
  Uranus

How can astronomers be much surer of Ganymede with its mantle of ice having a rocky core, then Jupiter having a rocky core with its thick mantle of metallic hydrogen ?

Doug Ettinger, Pittsburgh, PA 02/14/2011

dougettinger wrote:How can astronomers be much surer of Ganymede with its mantle of ice having a rocky core, then Jupiter having a rocky core with its thick mantle of metallic hydrogen ?

There is lots of physical evidence supporting the idea that Ganymede has a liquid iron core, a rocky inner mantle, and an icy outer mantle. The bulk density is about twice that of ice, the moon has a magnetic field, and its moment of inertia has been measured.

The situation with Jupiter is much more complex, because there are fewer measurements to work from, and a good deal of uncertainty about the physical properties of materials at the very high pressures deep inside the planet.

Chris Peterson wrote:

dougettinger wrote:How can astronomers be much surer of Ganymede with its mantle of ice having a rocky core, then Jupiter having a rocky core with its thick mantle of metallic hydrogen ?

There is lots of physical evidence supporting the idea that Ganymede has a liquid iron core, a rocky inner mantle, and an icy outer mantle. The bulk density is about twice that of ice, the moon has a magnetic field, and its moment of inertia has been measured.

The situation with Jupiter is much more complex, because there are fewer measurements to work from, and a good deal of uncertainty about the physical properties of materials at the very high pressures deep inside the planet.

The moment of inertia of a rotating sphere is I = 2/5(m)(R 2). You know m from gravitational laws due to its orbit around Jupiter and you know R. And you know the densities of different materials. But how do you know whether the density is homogeneous or whether the heavier materials are concentrated in an inner core ? The rotation is affected by gravitational locking and should not help in postulating a differentiated core. I am not asking for a complete mathematical explanation; I would appreciate a brief conceptual explanation. Perhaps a tensor treatment (beyond my comprehension) is needed to explain an observed wobble and then its density distribution.

Doug Ettinger, Pittsbrurgh, PA 02/14/11

dougettinger wrote:The moment of inertia of a rotating sphere is I = 2/5(m)(R 2). You know m from gravitational laws due to its orbit around Jupiter and you know R. And you know the densities of different materials. But how do you know whether the density is homogeneous or whether the heavier materials are concentrated in an inner core ?

That is the definition of moment of inertia for a sphere of uniform density. Ganymede clearly does not have uniform density, given that its density is twice that of the material observed to make up its surface. Its actual density lies between that of ice and stone. As a spherical solid body, it is certainly differentiated, meaning that denser material is concentrated in the center. There are no known exceptions to this sort of differentiation, and no reason to think that Ganymede wouldn't be differentiated.

The actual moment of inertia was measured by the Galileo spacecraft. It is much too low for a uniform body- the low moment means that much of the total mass is concentrated within a small radius. Knowing the moment of inertia allows for different structural models to be tested. By using the magnetic field properties to estimate the iron core volume, and the moment of inertia and bulk density to predict the depth of the ice/stone boundary, a very reasonable inference can be drawn regarding the moon's interior.

Chris Peterson wrote:

dougettinger wrote:The moment of inertia of a rotating sphere is I = 2/5(m)(R 2). You know m from gravitational laws due to its orbit around Jupiter and you know R. And you know the densities of different materials. But how do you know whether the density is homogeneous or whether the heavier materials are concentrated in an inner core ?

That is the definition of moment of inertia for a sphere of uniform density. Ganymede clearly does not have uniform density, given that its density is twice that of the material observed to make up its surface. Its actual density lies between that of ice and stone. As a spherical solid body, it is certainly differentiated, meaning that denser material is concentrated in the center. There are no known exceptions to this sort of differentiation, and no reason to think that Ganymede wouldn't be differentiated.

The actual moment of inertia was measured by the Galileo spacecraft. It is much too low for a uniform body- the low moment means that much of the total mass is concentrated within a small radius. Knowing the moment of inertia allows for different structural models to be tested. By using the magnetic field properties to estimate the iron core volume, and the moment of inertia and bulk density to predict the depth of the ice/stone boundary, a very reasonable inference can be drawn regarding the moon's interior.

I am beginning to understand and am excited. If the R is smaller in I = 2/5(m)(R 2), then of course "I" is smaller. And "m" from gravitational equations can also be used to determine the bulk density. And the surface materials are known from other measurements. I could not find the moment of inertia for a hollow sphere with a certain thickness to determine the "I" for the lighter surrounding mantle.(?)

What are the newest assumptions regarding the reason for the measured magnetic field properties for a moon or planet? Is it assumed that the core is mostly ferretic, liquid, and moves with respect to the mantle to produce a magnetic field? Are all these asumptions required similar to the magnetic field produced by the Earth's core ?

Why does Ganymede's core stay heated and liquid for so long, if it was formed almost 4 billion years ago ? I am probably asking too many questions, but this last question is the most important.

Doug Ettinger Pittsburgh, PA 02/15/11

dougettinger wrote:Why does Ganymede's core stay heated and liquid for so long

Wikipedia: Ganymede: Origin and evolution

Yes, I should have checked Wikepedia first. It is difficult to believe that radiogenic and tidal heating could have supplied enough energy for about 4 billion years. I wonder if there is an estimated radiogenic heating table for temperatures vs. core size vs. mantle thickness ?

Your reference mentions that a Galileo flyby directly measured the moment of inertia to be 0.3105. How was this performed?

Doug Ettinger, Pittsburgh, PA

I recently bought a telescope and was able to see the red spot and moons easily! Jupiter is the brightest object in the night sky, aside from the moon, for the last month now. the weather is the most interesting aspect of Jupiter. im curious why the bands appear to rotate in different directions. im also curious why and how the anticyclonic storms last for hundreds of years. Great lecture, thanks for posting RN!! The Saturn lecture must be exciting to have been left out of this episode!