NASA's Curiosity rover has found "two-tone" mineral veins by climbing a layered Martian mountain that offer clues about favourable conditions for microbial life on ancient Mars.
The two-tone veins are seen at an area called "Garden City" on lower Mount Sharp.
They appear as a network of ridges left standing above the now eroded-away bedrock in which they formed.
Individual ridges range up to about 2.5 inches high and half that in width, and they bear both bright and dark material.
"Some of them look like ice-cream sandwiches. These materials tell us about secondary fluids that were transported through the region after the host rock formed," said Linda Kah, a Curiosity science-team member at the University of Tennessee, Knoxville.
These episodes occurred later than the wet environmental conditions that formed lake-bed deposits the rover examined at the mountain's base.
Veins such as these form where fluids move through cracked rock and deposit minerals in the fractures, often affecting the chemistry of the rock surrounding the fractures.
Curiosity has found bright veins composed of calcium sulphate at several previous locations.
"The dark material preserved here presents an opportunity to learn more.
"At least two secondary fluids have left evidence here. We want to understand the chemistry of the different fluids that were here and the sequence of events. How have later fluids affected the host rock?" Kah added.
Garden City is about 12 metres higher than the bottom edge of the "Pahrump Hills" outcrop of the bedrock forming the basal layer of Mount Sharp, at the centre of Mars' Gale Crater.
NASA's Mars Science Laboratory Project is using Curiosity to find if the planet has hosted microbes ever and the changes from those environments to drier conditions that have prevailed on Mars for more than three billion years.