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 the Earth.
|Surface gravity of familiar bodies (Earth = 1)|
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.