basins that later were filled in by lava flows. The lava covered the floor of the basins and formed flat plains. The planet Mars has some similar surface features, which are sometimes called maria.
mass: The mass of an object is the measurement of the amount of matter it contains. So long as no matter is added to or taken away from an object, its mass always stays the same. Weight is a measurement of how much gravity pulls on an object's mass. Weight is defined as the force of gravity multiplied by mass. On Earth, weight and mass are the same (at least at sea level). The mass of an object is the same on another planet, but its weight would change depending on the planet's gravity.
