This is not a question that requires an immediate answer, but it will come up as a future debate. NASA’s latest Mars mission, Maven, is designed to orbit the red planet and help explain the processes that removed the original atmosphere. We know Mars had one because we can find traces of it in meteorites that originated from there long ago. We also see clear evidence that Mars once had abundant water, and that implied an atmosphere to retain it. Without it liquid water would now rapidly disappear. Mars is smaller and has a weaker gravity than Earth, so escape velocity for gas molecules is lower. It has negligible or no magnetic field to protect it from the solar wind, which may have eroded the atmosphere over time. Maven will measure the current rate of loss and extrapolate backwards to learn the possible timescale of atmospheric depletion.
Then comes the question of whether we do anything about it. The process of changing planetary habitats to render them more suitable for Earth life-forms is known as ‘terraforming,’ and it is assumed that engineers of the future might develop appropriate technologies. This assumes no current life on Mars, though there may be records of extinct life-forms. Earth microbes might be genetically engineered to thrive in the Martian environment and to reproduce en masse and release some of the gases locked into the Martian surface. It has also been suggested that asteroids of ice might be diverted to bombard the now-arid planet with fresh supplies of water. Ethical issues will undoubtedly be raised by some environmentalists, especially the ones who regard human beings as a form of pollution, and would really like to see the universe altered by humans as little as possible. On the other side will be those of the ‘can do’ attitude who want to see humans settle on other planets. My own view is that I would like to see us do it. Since I am unlikely to be around then, I might feature it in a science fiction story for young adults instead…
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Madsen Pirie 
I do not think Mars would ever support an Earth like atmosphere. The eco balance of life dependant on nitrogen, oxygen and carbon dioxide would be a massive challenge to establish. The gravitational pull is considerably less than Earth and you rightly state that the retention of an atmosphere is largely dependant on this. It would require trillions of gallons of water being imported from space in a rock like state of frozen ice and the impact on the planet’s landscape would be daunting. Earth produces it’s own heat from within and this can be seen in deep mines and caves etc. Mars seems to have no internal source of heat and would be as cold as our moon. It would take thousands or even millions of years to see any results and even this is unlikely to provide a temperate climate at the distance Mars orbits around the sun. Our technological advances seem to move almost exponentially but mankind’s efforts would be better spent on protecting what we have inherited and passing this on to our descendants. Continue to explore by all means but we are all Earthlings, not Martians.