Not only was there once water on Mars - some of it was warm enough to support life, an analysis of meteorites shows.
Hydrothermal fractures around Martian impact craters may once have been a habitable environment for microbial life, with water temperatures ranging from 50°C to 150°C.
Microbes on Earth can live in water at similar temperatures, after all - famously, for example, in the volcanic thermal springs at Yellowstone Park. They use the reactions during mineral formation to gain energy and elements essential for survival.
So could the same have happened on Mars? To find out the temperature of ancient water on the planet, the University of Leicester team examined Mars meteorites on Earth for the effects that water had had on them.
"While the orbiters and rovers are studying the minerals on Mars, we also have meteorites from Mars here on Earth. They come in three different groups, the shergottites, the nakhlites and the chassignites," says reader in planetary science Dr John Bridges.
"Of most interest for the question of water on Mars are the nakhlites, because this group of Martian meteorites contains small veins, which are filled with minerals formed by the action of water near the surface of Mars."
Altogether, eight nakhlite Martian meteorites are known, all with small but significant differences in their alteration minerals.
It appears that the first newly formed mineral to grow along the walls of the vein was iron carbonate, which would have been formed by CO2-rich water around 150°C. When the water cooled to 50°C, it would have formed clay minerals, which were then followed by an amorphous phase with the same composition as the clay.
It seems likely that the heating was caused by meteorite strikes on Mars.
"The driving force heating the water might have been an impact into the Martian surface," says Bridges. "And you only have to look at a map of Mars to see how numerous those are on the Martian surface."