The Martian surface today is a dry, frozen and inhospitable place but new evidence suggests that it was once a warm environment where water flowed freely, potentially supporting life.
Scientists have been unable to answer the mystery of what happened to all this water. Previous research has suggested that most of it was either lost into the atmosphere because of high winds after the planet's magnetic field collapsed or it became locked away as ice below the surface.
But a study published in the journal Nature suggests that much of the water may have been absorbed by rocks.
Scientists at Oxford University found that basalt rocks on Mars can hold around 25% more water than those on Earth, allowing them to suck up the majority of H2O from the Martian surface.
"The Earth's current system of plate tectonics prevents drastic changes in surface water levels, with wet rocks efficiently dehydrating before they enter the Earth's relatively dry mantle," said Jon Wade, lead author of the study.
"But neither early Earth nor Mars had this system of recycling water. On Mars, water reacting with the freshly erupted lavas that form its basaltic crust resulted in a sponge-like effect," he continued.
"The planet's water then reacted with the rocks to form a variety of water-bearing minerals. This water-rock reaction changed the rock mineralogy and caused the planetary surface to dry and become inhospitable to life."
Wade explained that the reason this process occurred on Mars and not on Earth was down to the fact that Mars is much smaller with a different temperature profile and contains more iron in its mantle – the geological layer between the crust and the inner core.
These features made the surface of Mars "more prone to reaction with surface water and able to form minerals that contain water," Wade said.
"Because of these factors the planet's geological chemistry naturally drags water down into the mantle, whereas on early Earth hydrated rocks tended to float until they dehydrate."