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surface gravity
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 kg 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 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/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) | |
|---|---|
| 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.
Related entry
• gravity
Related category
• ASTRONOMICAL QUANTITIES
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