In scientific contexts, mass is the amount of " matter " in an object (though "matter" may be difficult to define), but weight is the force exerted on an object's matter by gravity. At the Earth 's surface, an object whose mass is exactly one kilogram weighs approximately 9.81 newtons, the product of its mass and the gravitational field strength there. The object's weight is less on Mars, where gravity is weaker; more on Saturn, where gravity is stronger; and very small in space, far from significant sources of gravity, but it always has the same mass.