Why is the Martian atmosphere so thin?

India's sojourn to the Red Planet has a very specific objective: of trying to decipher how Mars continues to lose its atmosphere. Incidentally, the Martian air is mostly composed of carbon dioxide and there is very little oxygen. The ISRO scientist, M Viswanatham, is flying a special instrument called Lyman Alpha Photometer, a device that will hopefully unfold how the Martian atmosphere is constantly stripped from the planet. It's an important mission since, if ever Mars has to be colonized, providing air would be essential. (For more on India's Mars Mission, click here.)