 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. 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. I stopped, slipped out of the geiger harness, and lowered myself ponderously to the ground. 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. 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. And that started the chain of events that led Val and me to end up as a madman's prisoners, on Mars. With every source of uranium mined dry on Earth, we had tried other possibilities. Project Sea-Dredge was trying to get uranium from the oceans. So, Mars. There's not much uranium on Mars, and it's not easy to find or any cinch to mine. It's a stopgap effort, just to keep things moving until Project Sea-Dredge starts functioning. Enter the Geig Corps: volunteers out on the face of Mars, combing for its uranium deposits. For a puzzled moment I thought it was our Dome, the settlement where all of UranCo's Geig Corps were located, but another look told me that this was actually quite near us and fairly small.
 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