Planetary Interiors

I. Type of information

A. Measured diameter of a planet

B. Mass of the planet as determined from its effect on satellites

C. Mean density of the planet as determined from A&B above. This is compared to known materials:

Ice - 1000 kg m-3
Igneous rock - 3000 kg m-3
Iron-rich rocks - 6000 kg m-3
Iron - 8000 kg m-3

Some planetary examples:

Rocky planets with significant iron cores -
Mercury - 5420, Venus - 5250, Earth - 5520

Rocky planets with small iron cores:
Mars - 3940, Moon - 3340

Ice-rich mixtures:
Pluto - 2030, Triton - 2070, Callisto - 1790

D. Moment of Inertia - the concentration of mass to the center of a body.

I = KMr², where decreasing K indicates concentration of mass to the center of a body. I is determined from the wobble of a rotating planet caused by an external torque.

Some examples:

homogeneous sphere K = 0.4

a sphere with a core radius = 1/2 r and core density = twice the density of the remainder of the planet , K = 0.347

Moon 0.4
Mars 0.37
Earth 0.33
Jupiter 0.26

E. The gravitational field has been determined globally for the Earth, Moon, Venus, and Mars.

F. The moon is the only other planet than Earth with a radial seismic velocity profile. For the Earth lateral variations have also been determined.

A sample of important results:

The Earth has compositional layers
The Earth has a liquid outer core

G. Heat flow , where is the temperature gradient at the surface and k is thermal conductivity. For Earth typical is 20 K/km, and q is 60-80 milli-watts/m².

A sample of important results derived from heat flow:

The Earth has a convecting interior.
Radioactive elements are concentrated in the crust.
On Earth ~65% of heat loss from the interior is caused by the formation and cooling of plates, ~20% is conduction through the lithosphere, and ~15% is caused by radioactivity in the continental crust.

II. Chemical boundaries in a planet's interior:

A. Crust - the uppermost chemical layer.

On Earth,, two types - continental (~40 km thick) and oceanic (7 km). For other planets this is typically what came to the top through melting and never went back down.

B. Mantle - whatever is below the crust. Usually considered to be homogeneous and convecting. For Earth it appears to have a roughly chondritic composition.

C. Core - dense element that separated out to center of a planet. For terrestrial planets the composition is Fe-Ni and perhaps some minor light elements.

III. Mechanical boundaries

A. Lithosphere - the uppermost mechanical layer. There are actually different types of lithosphere:

1. Elastic lithosphere - the layer able to support large elastic stresses for a long time. On Earth typically 30-40 km thick.

2. Thermal lithosphere - the upper thermal boundary layer where heat is transported by conduction rather than convection. On Earth it is ~100 km thick.

3. Seismic lithosphere - on Earth, the depth to the low-velocity p-wave zone.

B. Asthenosphere - perhaps existing only on Earth, a region of low viscosity and low seismic velocity, perhaps caused by water in the upper mantle.

C. Mesosphere - from the asthenosphere to the core.

D. The Core - both a chemical and mechanical boundary.