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Energy Flow Energy moves through Earth's systems in three different ways—by radiation, conduction, and convection. Energy from the sun reaches Earth through radiation, the movement of energy as visible light, ultraviolet rays, and other types of electromagnetic waves. Unlike conduction and convection, radiation does not require amedium. Thus, radiation is the only way that energy can travel through outer space. FIGURE 2: More than 99.98% of the energy that Earth emits to space originally comes from the sun. Solar energy is reflected and absorbed and then reemitted back out to space. Conduction is the transfer of heat energy through collisions of the atoms or molecules ofa substance. When you walk barefoot on hot ground, for example, heat moves by conduction from the ground to the soles of your feet. Air touching warm ground also is heated by conduction. Convection is the movement of matter caused by differences in density. Recall that density is the ratio of the mass to volume of a substance. Convection often moves thermal energy through a liquid or gas. For example, in a pot of simmering water, the water at the bottom of the pot is heated by conduction and becomes less dense. Because it is less dense, it rises and is replaced by downward-flowing water that is colder and denser. Energy is transferred by convection both within Earth's atmosphere and oceans and within Earth's interior.