NASA's Curiosity rover has found mineral veins at a site on Mars, offering clues about multiple episodes of fluid movement in the area.
The mineral veins were found at the site called "Garden City" on the slopes of a 5-km-high mountain known as 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: dark on both edges and white in the middle," said Linda Kah, a Curiosity science-team member at the University of Tennessee, Knoxville.
"These materials tell us about secondary fluids that were transported through the region after the host rock formed," she said.
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 sulfate 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 said.
Mud that formed lake-bed mudstones Curiosity examined near its 2012 landing site and after reaching Mount Sharp must have dried and hardened before the fractures formed.
The dark material that lines the fracture walls reflects an earlier episode of fluid flow than the white, calcium-sulfate-rich veins do, although both flows occurred after the cracks formed.
Garden City is about 39 feet 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.
The Curiosity mission spent about six months examining the first 33 feet of elevation at Pahrump Hills, climbing from the lower edge to higher sections three times to vertically profile the rock structures and chemistry, and to select the best targets for drilling.
Analysis is still preliminary, but the three drilled samples from Pahrump Hills have clear differences in mineral ingredients.
The first, "Confidence Hills," had the most clay minerals and hematite, both of which commonly form under wet conditions. The second, "Mojave," had the most jarosite, an oxidised mineral containing iron and sulfur that forms in acidic conditions. The third is "Telegraph Peak."
Examination of Garden City has not included drilling a sample.
