Fuel and oxidizer and the vehicle's "brains"; fires twice, once to insert the vehicle-spacecraft stack into low Earth orbit and then again to accelerate the spacecraft out of Earth orbit and on its way towards Mars.
Two interstage adaptors connect the first stage of the Atlas with its Centaur upper stage. The Centaur's has a restartable RL-10 engine from Pratt & Whitney Rocketdyne. This engine uses liquid hydrogen and liquid oxygen and can provide up to about 22,300 pounds (99,200 newtons) of thrust. The Centaur can control its orientation precisely, which is important for managing the direction of thrust while its engine is firing. It carries its own flight control computer and can release its payload with the desired attitude and spin rate.