 Why have Val and Ron joined a mission on Mars?
THE HUNTED HEROES By ROBERT SILVERBERG The planet itself was tough enough—barren, desolate, forbidding; enough to stop the most adventurous and dedicated. But they had to run head-on against a mad genius who had a motto: Death to all Terrans! "The surest way to die out here on Mars is to give up." I reached over and turned up the pressure on her oxymask to make things a little easier for her. Val's usually about the best wife a guy could ask for, but when she wants to be she can be a real flying bother. Nothing but what had stopped us could stop a sandcat: sand in the delicate mechanism of the atomic engine. But no; she blamed it all on me somehow: So we were out walking on the spongy sand of the Martian desert. "Maybe there isn't any uranium in this sector at all. When Val's tired and overwrought there's no sense in arguing with her. Behind us somewhere was the comfort of the Dome, ahead nothing but the mazes and gullies of this dead world. Remember—we're doing this for Earth. We're heroes." "Heroes, hell!" We had wandered fruitlessly over the red sands all day, both of us listening for the clicks of the counter. Even though the Martian gravity was only a fraction of Earth's, I was starting to tire, and I knew it must have been really rough on Val with her lovely but unrugged legs. "Heroes," she said bitterly. She had been just as much inflamed by the idea of coming to Mars to help in the search for uranium as I was. We knew the pay was poor, but we had felt it a sort of obligation, something we could do as individuals to keep the industries of radioactives-starved Earth going. And we'd always had a roving foot, both of us. No, we had decided together to come to Mars—the way we decided together on everything.
 to restore Earth's depleted fuel reserves
 to locate a new source of fuel  to restore Earth's depleted fuel reserves  to determine if the planet can be colonized  to identify the source of recent astronaut murders