Date: 18 May 1979
This color image shows a thin layer of water ice frost on the Martian surface at Utopia Planitia. It was taken by the Viking 2 Lander camera 2 on 18 May 1979, almost exactly one Martian year (687 days) after the spacecraft imaged frost in the same spot.
The layer is thought to be only a couple thousandths of a centimeter thick. It is speculated that dust particles in the atmosphere pick up tiny bits of water. When it gets cold enough for carbon dioxide to solidify, some of it attaches to the dust and ice and it falls to the surface. The view is looking towards the south, southeast -- the long boulder to the right is roughly one meter across.