QUESTION:
Could it be possible that Mars is hollow and that there is a sun inside,
causing the polar caps to change shape?

ANSWER from Bruce Jakosky on June 23, 1997:
It is not possible that Mars is hollow and has an "inner" sun.  Such a state
would violate too many things that we understand about the planets. These
include the strength of rock (the hollow part would collapse), the way in
which planets generate volcanos, and our concept of how stars and planets
form.  This last, in particular, is significant, because there is an
underlying similarity of processes for formation of galaxies, stars, and
planets.

Similarly, everything we know about Mars suggests that the polar caps change
their shape because of the seasonal condensation and sublimation of CO2 from
the atmosphere.  Close-up images and other data from spacecraft confirm that
these processes are occurring.

One can always suggest a different process that might be capable of
explaining the observations.  Your suggestion of an "inner sun" is one, as
is the suggestion that the Face on Mars was carved by sentient, intelligent
beings.  However, in order to have a viable hypothesis, the suggestion must
be consistent with the observations and, if possible, be a better or simpler
explanation for them than any other hypothesis.  In the case of an inner sun
on Mars, it cannot explain many facts that we see about Mars, and it is not
consistent with what we see.  In the case of the face on Mars, again there
is a simpler explanation.  Always look for the explanation that is simplest
and most consistent with everything else that we know.

Bruce Jakosky