Keep Out — Fredric Brown

For fifty years he had tried to colonize Mars and all his efforts had failed. Besides this dome which had been built for us there was only one other outpost, another glassite dome much smaller and less than a mile away. It had looked as though mankind could never spread to the other planets of the solar system besides Earth for of all of them Mars was the least inhospitable; if he couldn't live here there was no use even trying to colonize the others.