Planetary Heat Losses / Brad Guth

Besides something nuclear going on, what's keeping the core and
subsequently the surface of a given planet like Earth so extra active
and getting warmer by the day?

Like Jupiter, does pressure alone give the necessary method of
sustaining a core at a given geothermal sustainable status?

What's the gravity or perhaps the vacuum at the center of Earth?

In other words, without an active and extremely fluid core, and
especially if without benefit of such a massive and nearby moon, why
wouldn't this planet of ours become that of a mostly monoseason and
somewhat icy environment?

It seems Mars is not only worse off than icy, as likely a planetology
that's dead to the core, but also having once been a mostly freshwater
environment (Mars w/o salt), and otherwise seemingly older than
Earth. What gives?

Venus is still very much alive and kicking from the inside out, and w/
o moon none the less, as it's losing geothermal energy at roughly 256
fold greater than Earth. What gives?
- Brad Guth