 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. No, we had decided together to come to Mars—the way we decided together on everything. And that started the chain of events that led Val and me to end up as a madman's prisoners, on Mars. So, Mars. 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