I agree with Doctor Stephen Hawking about the subject of extraterrestrial colonization. In order for humanity to survive minor cosmic events, such as asteroid or cometary collisions with Earth, we need to prepare a second seat of civilization. We are in constant peril of extinction as long as we keep all of our eggs in one basket. While some may think colonizing Mars will corrupt the red planet, I don't feel this will be the case at all.
Of all of the worlds in the Solar system, Mars is the best candidate for a future colony. It has an atmosphere, albeit thin and loaded with carbon dioxide, it lies close enough to the habitable zone of our system to allow for future terraforming, and it is rich in resources necessary for a young colony. Mars in many ways is a more viable candidate for colonization than the Moon, since a lunar colony would have to operate under hard vacuum and constant meteoric bombardment. While it is true that Mars' thin atmosphere doesn't stop all of the meteorites, such is also true of the Earth.
It wouldn't be easy to colonize either terrestrial body, but Mars' colonization has other advantages over the lunar option. Evidence of geothermal activity on Mars means there is available power just below the surface, while the Moon is stone cold. Near the equator, the temperature rises above freezing for brief periods, and images show evidence of water erosion as well as freeze and thaw cycles. The day is close to an Earth day, while a lunar day is two weeks long. While the energy from sunlight is just a quarter of that on Earth, experiments have show many plants can adapt to such lower light levels, as many already have, yet two weeks of lunar darkness is asking a bit much of anything more than lichen.
The steps to colonization are not easy, nor is terraforming. The first stages to colonization may be the introduction of lichen to transform the atmosphere and create soil, as well as various extremophilic organisms to release oxygen from the abundant iron oxide on the surface. Geothermal sources must be tapped to provide power for subsurface water extraction and energy production. Unlike the Moon, human colonization can begin immediately, since current technology an already derive our basic needs from the Martian environment. While the distances involved may be daunting, so were the first crossings of the Atlantic and Pacific oceans. Over time, we will get better at it. Practice makes perfect and there is nothing like a few dozen years of space travel to get things right.
Since the practical obstacles are surmountable, there must arise the moral ones. Through poorly thought out international treaties, the United States of America agreed never to colonize other worlds for anything more than research purposes. I feel this was a terrible mistake, but that aside, the moral argument given is that we must keep other worlds pristine and untouched by human corruption. These are high and mighty words lacking in any substance whatsoever. If gold, silver, copper, or even iron show up in abundance on any of these other worlds, pristine would go right out the window. As soon as extraction of such resources becomes economically feasible, the first waves of explorers will be out there. Every capable nation will empty their prisons and heavy lift boosters will be bound for the first extraterrestrial gold rush. Is it moral? Probability not, but it is reality. It happened in the past, and there is no reason it won't happen in the future. The New World colonies arose out of the need to expand territory, gain resources, and export surplus population. Do we want the surface of Mars marred by open pit mines and slag heaps? No, at least not by the more altruistic among us, for the rest of us, take a picture, because if two hundred years, if the human race is still around, you won't recognize Mars. However, it won't be because the surface will become ugly, quite the contrary. As people emigrate, first to pressurized shelters and caves and eventually to a terra formed surface, the priorities of resource exploitation will give way to those of colonial sustainment. The greening of Mars will be in full swing and nothing will stop it.
Nevertheless, is it right? Should we, as the apologists for humanity so often cry, pollute the rest of the universe with our presence? I say we would not be polluting the universe, but bringing our own brand of beauty to it. We are small, very small. We are so insignificant in the scale of the universe; we could never hope to pollute the smallest fraction of it. Our chance of bothering anything outside our own Solar system is more remote than lighting striking us twenty-five times in the same week in five different places. We are so puny that one small comet can wipe us out in a single impact. So small in fact, that the death of all pollinators would kill us in five to ten years. It is morally wrong to consider remaining on Earth and jeopardizing something truly divine.
Be fruitful and multiply.