Researchers have suggested that massive impacts in Mars’ history might have caused a temporary wet climate.
The most prominent craters on Mars came into existence with impacts between 3.7 and 4.1 billion years ago. One such crater is the Argyre basin, which was created by a 100 to 200 kilometre rogue Solar System object nearly four billion years ago.
There is plenty of evidence that water once existed on Mars, as complex networks of river valleys and dried up lake beds are scattered across the now barren landscape. The era that these water features were created in coincides with the time of the large comet and asteroid impacts. This has led some scientists to theorise that the impacts could have inadvertently caused a temporary wet climate on Mars.
When a comet or asteroid smashes into a planet, it releases a colossal amount of kinetic energy. This energy could be used to heat the surface by hundreds of degrees and warm the atmosphere. This heat can cause any water in existence to be vaporised into the planet’s atmosphere, in a phenomenon known as a greenhouse effect. In this situation, most of the water would have existed in the atmosphere, making it a runaway greenhouse effect. Vaporising comets and asteroids could deliver an extra kick of greenhouse gases into the atmosphere, though the kinetic energy is the most important ingredient.
Eventually some of this water in Mars’ atmosphere was lost to space, as happened on Venus. However, this would reduce the heat in the atmosphere and the greenhouse state would collapse, possibly resulting in precipitation that caused the channels we see on Mars today. Alternatively, some liquid water could still have been present below the surface while the planet was in the runaway greenhouse state, and this could have flowed over the surface before being captured by the runaway greenhouse.
Mars eventually cooled down so much that liquid water was no longer viable on the surface, but it is possible that it could have existed for a period of a few hundred years.