Why were the U.S. aircraft carriers not at Pearl Harbor during the attack?

The U.S. aircraft carriers were not at Pearl Harbor during the Japanese attack on December 7, 1941, because they were deployed on specific missions to reinforce outlying Pacific garrisons.1

• The USS Enterprise was at sea returning from a mission to deliver Marine Corps aircraft and personnel to reinforce the garrison at Wake Island. Its return to Pearl Harbor had been delayed by weather.1
• The USS Lexington was engaged in a similar mission, ferrying Marine dive-bombers to Midway.1
• The USS Saratoga was in San Diego on the morning of the attack, where it was embarking a fresh complement of aircraft.1

These operations also meant that several heavy cruisers and destroyers were at sea with the carriers, sparing them from the destruction inflicted upon the battleships moored in the harbor.1