Expert: Jayendra Upadhye - 7/14/2009

Question
Hello,

I understand how we can determine the content of a planet's atmosphere with spectral analysis but how can we determine the composition of a planet's interior?

Answer
Hi,
In my language there is a saying.
That to judge all rice, all one needs to do is inspect a grain from the sack.
What I am implying here is that astronomers know that the relative abundance of elements is almost uniformly same over a wide are of the universe. The larger the sample. the more is the homogeneity or sameness, till when one begins going into deep field looking over truly vast tracts of space, the astronomers speak out in wonder about isotropy. (sameness everywhere in all directions).
Knowing this, astronomers indirectly arrive at the bulk density of a planet by
1 - Albedo & orbital params about the size and mass of the planet.
2 - Surface features tell us if it is rocky or all gas, or all ice.
If the density matches (if size estimates are beyond question and orbital params tell us the mass, then we can reach a fair guess with regards to density.
If it (it normally does) fall in the range specified by the relative abundance figure, then knowing the size, we can predict if it has a frozen core or not by observing the bow shock in the solar wind caused by its van Allen belts. If there is a magnetic field, then there is a convecting core. Meaning a magma layer would by implication, and volcanoes and outgassing and presence of water and all the rest as corollaries.
Geologists if they see water, will even reach estimates of the erosion processes and plate tectonics, and salinity of the sea and age of the planet and chemistry of its atmosphere.
Presence of methane will imply vegetative and vegetable eating animals.
Presence of Ozone will imply Oxygen, and so on.

One can call it much or one can call it a pittance.
Depends on ones point of view.

regards
Jayen