Messaline was an Earth-like planet with a similar atmospheric composition, with a roughly 80:20 ratio of nitrogen and oxygen and high ozone levels. The main differences were the occasional radiation spikes on the surface and the lack of native life.
Messaline was colonised by humans and Hath in July 6012 of the New Byzantine Calendar. It was a barren world, so terraforming was planned to make the planet hospitable for the two species. However, when the mission commander died, there was a power vacuum, resulting in the two species engaging in a seven-day battle, although approximately one hundred forty generations passed during the week. (TV: The Doctor's Daughter)
When the Tenth Doctor destroyed the terraforming device and released the terraforming gases into the air, a peace was established between the two races as they realised what the battle actually was about. The colonisation of the planet continued as planned. Subsequently its terrain was transformed with the earlier release of gas from the terraforming device. (TV: The Doctor's Daughter)