On impact, all the ice in the cometary chunks will instantly vaporize. At the same moment, a large amount of ice held as permafrost deep in the Martian soil will flash into steam. In fact, the comet will vaporize an amount of permafrost equal to hundreds to thousands of times the mass of the comet itself. Comets will therefore be directed to impact in higher latitudes, where permafrost is concentrated. Humans who are not genetically engineered will probably always require oxygen masks to breathe comforably on Mars, but it will be a shirt-sleeve environment.
Geologic evidence suggests that Martian water has been liberated, perhaps many times, in the past.
As Mars cooled ground water must have been frozen and been permanently trapped. A tenth to a third of the total water released by Mars' great volcanoes must remain in the soil as permafrost. Oceans will fill in the basins and impact craters, producing a Mars that looks something like Figure 4. In a replay of the early processes of life on Earth, the plants will pioneer the planetary ecosphere. Ultimately, the plants will create conditions hospitable for animal life.