QUESTION:
What actually Creates the Field of gravity on Earth or Mars?Is it something
to do with thier cores?Or is it Something else?
 

ANSWER from Bruce Jakosky on July 10, 2000:
Every object in the universe attracts every other object through a force
known as gravity.  The more massive the object, the stronger the force.  A
planet such as the Earth or Mars has a lot of mass concentrated in one
place, so it attracts other objects relatively strongly.  In addition, these
planets do not have a perfectly uniform distribution of mass throughout
their interior, so the pull of gravity is not perfectly uniform either.
Some areas, such as the Tharsis bulge on Mars, have extra mass so they
attract nearby objects, such as an orbiting spacecraft, more strongly.  We
can measure the variations in the gravity field by tracking an orbiting or
flyby spacecraft, and we can learn something from this about the interior
structure of the planet.