...can be found here:

The three petals are connected to the base of the tetrahedron with hinges. Each petal hinge has a powerful motor that is strong enough to lift the weight of the entire lander (The Rover plus Lander is about 530 kilograms, which would weigh almost 1,200 pounds on Earth but only 437 pounds on Mars. The Rover alone is 170 kilograms, weighing 375 pounds on Earth and 140 pounds on Mars.) Having a motor on each petal ensures that the lander can place the rover in an upright position no matter which side the lander comes to rest on after the bouncing and rolling subsides on the surface of Mars.