A mountain on Mars that is almost as tall as Mount Denali in Alaska appears to be surprisingly light, scientists have reported.

For more than four years, the National Aeronautics and Space Administration's (Nasa) Curiosity rover has been exploring Mount Sharp, located within an ancient meteor impact crater known as Gale and rising more than 4.8km high. Now, measurements of tiny changes in gravity, recorded by the rover as it climbed in elevation, could help solve the question of how the mountain formed.