I found one of the interesting things in that NOVA special I linked to is the discussion of Mars' magnetic field, and what may have happened to it.
The Earth’s magnetic field is not even that strong.
from wiki:
“Earth’s magnetic field, also known as the geomagnetic field, is the magnetic field that extends from the Earth’s interior out into space, where it meets the solar wind, a stream of charged particles emanating from the Sun. Its magnitude at the Earth’s surface ranges from 25 to 65 microteslas.”
Gravity is even weaker insofar as a magnet can be used to pick something up from the surface of the planet. Of course, the other two forces are, “the weak force” and “the strong force” which can release nuclear explosions when disturbed.