A bright vein of mineral deposited by water has been found by NASA's Mars Exploration Rover Opportunity in the surface of Mars, an evidence that water was present long time ago. The vein was informally named "Homestake."

The Opportunity rover, which landed on Mars nearly eight years ago, has discovered the thin, bright mineral vein, apparently gypsum, along the rim of a huge crater called Endeavour, and researchers believe this mineral was deposited by liquid water billions of years ago.

"This is the single most powerful piece of evidence for liquid water at Mars that has been discovered by the Opportunity rover," said Steve Squyres of Cornell University, Opportunity's principal investigator.

Close examination revealed that the vein was 0.4 to 0.8 inch wide and 16 to 20 inches long, protruding slightly higher than the bedrock on either side of it. After an analysis of the vein with Opportunity's cameras and X-ray spectrometer last month, researchers concluded that it is gypsum, a hydrated calcium sulfate that on Earth is used to make drywall and plaster of Paris.

The discovery of the gypsum vein is intriguing for scientists who are interested in whether or not Mars was ever capable of supporting life. Previous findings of ancient wet environments were very acidic but the gypsum formation is consistent with a more neutral and therefore benign pH, researchers said.

"It is a mystery where the gypsum sand on northern Mars comes from," said Opportunity science-team member Benton Clark of the Space Science Institute in Boulder, Colo. "At Homestake, we see the mineral right where it formed. It will be important to see if there are deposits like this in other areas of Mars."

It could have formed in a different type of water environment, one more hospitable for a larger variety of living organisms," Clark said.

According to NASA, Opportunity and its rover twin, Spirit, completed their three-month prime missions on Mars in April 2004. Both rovers continued for years of extended missions and made important discoveries about wet environments on ancient Mars that may have been favorable for supporting microbial life.

Spirit stopped communicating in 2010. Opportunity continues exploring, currently heading to a sun-facing slope on the northern end of the Endeavour rim fragment called "Cape York", NASA said.