Surface gravity is the local gravitational field strength at the surface of an astronomical body. It determines, for example, how much a person would weigh if they were to stand on that object. In the case of a gas giant or a star, the surface gravity is calculated as if there were a solid surface at the top of the atmosphere.

 

Local gravitational field strength is given by g, the force acting on a mass of 1 kilogram at the surface, according to the formula g = GM/r ², where M is the mass of the body, r its radius, and G the gravitational constant (= 6.67 × 10^–11 newton. meter squares, Nm²). Thus greater mass means greater surface gravity, but greater diameter means lesser surface gravity. For the Earth, g has a value of about 9.8 newtons per kilogram (N/kg). For Mars, by contrast, g is only about 3.7 N/kg, or 0.38 times as much. g is also known as the acceleration due to gravity. The table below compares the surface gravity of various bodies to that of Earth.

 

Surface gravity of familiar bodies (Earth = 1)
object	surface gravity
Mercury	0.378
Venus	0.907
Earth	1
Moon	0.166
Mars	0.377
Jupiter	2.36
Europa	0.135
Ganymede	0.145
Saturn	0.916
Titan	0.138
Uranus	0.889
Neptune	1.12
Pluto	0.059
Sun	29.9

 

The surface gravity of a world is expected to strongly influence what, if any forms of life, may appear on it. For more on this, see alternative forms of life.