Self-Sacrifice
One of the main themes of the movie is self-sacrifice, whether it be for others, or for the higher purpose towards which everyone on the team is working. Astronauts tend to be a strong example of this, because as Watney points out, every time they go into space they know it might be the last time, but they sacrifice their fear and their personal safety for the cause of knowledge and human advancement. The movie also looks at sacrifice in a different way; Annie believes that it would be better to sacrifice Watney, whom they have already declared dead and mourned publicly, than to admit to America that they accidentally left an astronaut on Mars. Her belief is that whilst she cares deeply about the team as an entity, she cannot afford to care about individuals more than the team and the mission itself, and therefore it is Watney's job to sacrifice himself if it is the best thing for the program.
Isolation and Loneliness
Solitude can sometimes be quite uplifting, but being marooned alone on another planet millions of miles away from earth is an isolating experience. Mark Watney is separated from everyone, everything and every place that he knows; in fact he is essentially the last man standing because he is separated from everything that is human. Sometimes he is able to overcome his loneliness and focus on a scientific question; sometimes he feels as though he is going a little crazy. Most of the time he has an extreme version of homesickness, feeling bereft and hopeless. As he is marooned for longer, the battle to maintain his sanity is visible to us. It takes an extraordinary effort to maintain control of his mind, which just goes to demonstrate what a powerful and controlling experience loneliness can be, even to the most brilliant of people whom we believe should be able to "think" themselves out of any difficulty.
Space Exploration
As the characters are astronauts and NASA scientists, one of the main themes is that of space exploration and the inherent danger of it. It is also an optimistic look at space exploration, because already, in a time that is in the future, but the near future, man is on Mars, and this prospect is presented as something that is perfectly reasonable and likely. Astronauts are shown to be people of diverse skills, expert scientists who are also space explorers. The scope of the space program is shown to be far greater than we can ever comprehend or imagine. For the most part, the reason and intent behind the mission still remains positive and intact despite the disaster that has resulted in an astronaut stranded on Mars.
Spirit of Survival
Prior to The Martian, the widely accepted worst case scenario for being marooned somewhere was the ubiquitous desert island; after all, we've all been challenged to come up with five essential items should we ever find ourselves shipwrecked somewhere sandy and deserted. The scenario faced by Mark Watney is worse than the desert island scenario because at least on a deserted island there is an air of familiarity; the air, the sky, the ocean, and to a botanist, any kind of trees or plant life. This movie shows us that mankind has a spirit of survival so ingrained that even being marooned on Mars is survivable with determination and perseverance. Mark is not upbeat all the time by any means, but every time Mars throws up a challenge he figures out a way to live up to it. It is ultimately perseverance and the spirit of survival that enables him to survive this experience.
Homesickness
Mark's homesickness is both a blessing or a curse whilst he is lost on Mars, but by and large, it is a blessing because his memories and thoughts of home keep it alive for him, and in turn he keeps himself alive so that he will be able to return. Nostalgia for home hits him at random and unconnected moments, and sometimes the memories and mental pictures are almost overwhelming, because he misses them so much, but they do keep him focused on the goal that he has for each day and his homesickness gives him a focus that he needs in order to get back to his loved ones and home one day.
