The atmospheric re-ioniser was a terraforming device that was intended to re-ionise the atmosphere of Mars, making it more habitable to humans. Following the abandonment of the project, according to one account, the actions of the Eighth Doctor in the 23rd century had made Mars breathable to humans. (AUDIO: Deimos / The Resurrection of Mars)
Though it worked in theory, a lack of funding and the Great Recession caused the terraforming project to be abandoned. The re-ioniser was still functioning, and stood as a signifier of the excesses that led to the Recession inside Deimos Moonbase.
In the 23rd century, when the Ice Warriors were awoken from their catacombs on Deimos, they attacked the base to make Mars as it was long before. The Eighth Doctor and Professor Boston Schooner had sabotaged the device and fled to the passenger rocket to evacuate. (AUDIO: Deimos)
However, an artificial gravity eddy brought the rocket back down to Mars. Lord Slaadek threatened the lives of the crew if the Doctor didn't co-operate in repaitring the re-ioniser. By testing the device on the rocket, the Ice Warriors killed all 600 people on board anyway.
When the process started on Mars, the Doctor tried interfering. Unable to reverse the effects, the Doctor brought the re-ioniser "to a very sudden halt with a very loud bang" by reducing a critical feedback in the ionisation beam, giving everyone about five minutes to escape. At the last minute, the Doctor also made the process alter Mars into an Earth-like atmosphere rather than that of the Mars of long before.