LOS ANGELES (Reuters) - NASA (news - web sites)'s two robotic rovers on Mars have begun scaling back their working hours as the approach of autumn on the Red Planet and dust on their solar panels slowly chokes off their power supplies, a NASA official said on Friday. The rovers Spirit and Opportunity, which landed on the Red Planet three weeks apart in January, are engaged in intensive searches for signs of water on opposite sides of Mars. Evidence of rocks or soil that formed in water would help validate scientists' theories that for the first half of its 4.6-billion-year...