 Why have Val and Ron joined a mission on Mars?
The planet itself was tough enough—barren, desolate, forbidding; enough to stop the most adventurous and dedicated. "The surest way to die out here on Mars is to give up." 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. 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. 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. No, we had decided together to come to Mars—the way we decided together on everything. The geiger kept up a fairly steady click-pattern, but never broke into that sudden explosive tumult that meant we had found pay-dirt. I longed to lie down on the soft, spongy Martian sand and bury myself. I felt almost guilty for having dragged her out to Mars, until I recalled that I hadn't. I stopped, slipped out of the geiger harness, and lowered myself ponderously to the ground. She slid down beside me, curled up, and in a moment she was fast asleep, sprawled out on the sands. Maybe we shouldn't have come to Mars after all. I watched her, sleeping peacefully, a wayward lock of her soft blonde hair trailing down over one eyebrow, and it seemed hard to believe that we'd exchanged Earth and all it held for us for the raw, untamed struggle that was Mars. It had been spun on us only a short time ago, I realized. I thought I knew everyone on sparsely-settled Mars. Val and I helped each other to our feet as best we could, considering our arms were still tightly bound against the sides of our oxysuits. Earth needed radioactives, and the only way to get them was to get out and look. The great atomic wars of the late 20th Century had used up much of the supply, but the amount used to blow up half the great cities of the world hardly compared with the amount we needed to put them back together again.
 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