Liquid water could have carved Mars’ famous gullies

Mars’ mysterious gullies could have been carved by liquid water produced when the planet was more heavily tilted on its axis, researchers in the US believe. During these periods, increased solar radiation at the planet’s poles would vaporise carbon dioxide, raising atmospheric pressure and allowing liquid water to form.

Mars has ice at each pole and beneath the surface elsewhere, but is thought to be devoid of liquid water because its atmospheric pressure is below water’s triple point (612kPa). Under these conditions, ice sublimates rather than melting when warmed. But in 1999 observations by NASA’s Mars Global Surveyor mission revealed what appeared to be river gullies on steep slopes.