Expert: Jayendra Upadhye - 8/17/2004

Question
This article states that there is a theory that Mercury is shrinking and that the core would be frozen. How would it be frozen since it's the closest planet to the Sun?

http://www.msnbc.msn.com/id/5725497/

Answer
Hi Charles,
Mercury's dayside temperature is 1000 degrees F.
Converted to Degrees centigrade, that is not enough to even melt its surface that is exposed to the sun!
Else we would NOT be seeing its cratered "history" of a billion years, molten "terra-forming" would have erased that, as it happened in the lunar Maria.
It has a metallic core.
This can be surmised by :-
1) - Its possession of a magnetic field
2) - High density almost matching that of the earth's (And
    the earth DOES have a metallic core and a magnetic
    field).

Obviously, such a core if it has a megnetic field, is in a state of flux, and not compeletely frozen, as in the case of Mars , which is a dead world, in more than one sense!

The "scarps" or areas of surface subsidence (upto 1 km) as observed by Mariner 10 (it had a resolution of 1 mile, so any feature visible to it HAD to be at least 1 mile up/down/across), are supposed to be proof of a shrinking mercury core.

Scientists also point out that just as on the moon, a core the size of mercury's, would have completely cooled off, about 2 billion years ago. WHY it has not done so, as shown by its active magnetic field, is a mystery.

Apart from the contradiction inherent in the two paras above - This is my personal view not published anywhere so use liberal pinches of salt! :) - There is one more possibility so far NOT discussed anywhere, and that is due to the fact that mercury has become "face locked" (3-to-2 spin-orbit coupling)with the sun.
It is the only solid planet in the solar system whose surface is undergoing temperature gradients of 1000 deg f / 550 deg c.
Plus the LONG days / nights allow for the temperatures to affect subsurface layers too.

Naturally then, even with a dead core, one would expect to see subsidence creasing or random faults!

But that was all about the scarps, and MY pet reasons for them!

Your answer is plain and simple! Though so near, yet too far! There is no way for its surface temperatures to prevent the core from cooling. You see, for that to happen, mercury would have to be totally face locked one side always pointing to the sun (which is only partly so now) and a billion years would have to elapse to allow the temperature to SOAK all the way to the core!
[For heat ransfer through non-conducting mantle and crust is a painfully slow process]
Use this article about mercury temperatures:-
http://www.space.com/reference/mercury/climate.html
Jayen