Bill, here is an equation which will work for almost all celestial bodies:
Pressure = (Mass^2 * G) / (Volume * Radius)
Earth Mass is: 5.98E10^24 [kg]
Earth Radius is: 6.37E10^6 [m]
Newton's Constant G is: 6.67E10^-11 [m^3/kg*s^2]
V = (4 * pi * R^3) / 3 = 1.08E10^21 [m^3]
M^2 * G =...