Based on reviews and trailers, The Martian is apparently based on a traditional honor code. This operates at two levels. At one level, you don't leave a comrade behind. Even if he's injured, you try to take him with you. Shared risk.
Over and above the crew or military unit itself are those in power. Because we're responsible for getting you into that risky situation, we're responsible for attempting to get you out of that risky situation.
This appeals to an instinctive sense of social obligations. But here's the snag: from a secular standpoint, it's ludicrous that people would move heaven and earth to save one human life, one smidgen of matter, stranded some 50 millions miles away (give or take). Even if the rescue mission is successful, Mark Watney will still be dead in a few decades. He will slip into oblivion, like the billions who lived and died before him. Just a blip between one nothingness and another.
Perhaps the best case an atheist can make is that since everything we do is equally meaningless, we might was well do something preposterously pointless rather than mundanely pointless.