NASA’s long-lived Mars Opportunity rover has joined the more recently landed and better equipped Curiosity rover in finding evidence for a chemically neutral form of water in rocks favorable for the emergence of  biological activity during the red planet’s early history.

NASA's Opportunity rover has explored Mars' since 2004. Image Credit/NASA

Both discoveries were associated with clay minerals, which form readily in neutral water on Earth.

Opportunity’s discovery, made by boring into a rock called Esperance on the rim of Endeavour Crater at the small rover’s January 2004 landing site on  Meridiani Planum was announced on Friday. The news came from Opportunity mission project scientist Steve Squyres, of Cornell University, and other members of the mission science team.

It joins a similar announcement from March made by NASA’s Curiosity science team. Curiosity bored into a rock atYellowknife, characterized as a stream bed in Gale Crater. Curiosity landed in Gale in early August 2012.

Map illustrates landing sites for NASA spacecraft. Curiosity, also known as the Mars Science Laboratory, landed in Gale Crater on Aug. 6, 2012. Image Credit/NASA

Opportunity, a golf cart sized, solar powered rover, had found similar evidence of water/rock interactions in Meridiani Planum in the past. However, each of those suggested a highly acidic form of water rich in sulphur and not conducive to the rise of biological activity.

“What we have here is very different chemistry,” Squyres, told a June 7 news briefing. “This is water you can drink. This water is probably much more favorable in its level of acidity for things like pre-biotic chemistry, the kind of chemistry that could lead to the origin of life.”

Opportunityfollowed a rover twin onto the Martian surface during January 2004, for what were envisioned to be 90 day missions. Spirit, the twin, landed in Gusev crater, and explored until 2010.

Curiosity, equipped with a nuclear power source, is in the earliest stages of a two-year mission and now headed toward the base of Mount Sharp, a mountain that likely holds many clues to the evolution of the Martian environment and if and when conditions became favorable for life.

Meanwhile, Opportunity’s mission appears far from over.

It is now headed toward another exploration site on the rim of Endeavour crater, Solander Point.

“Opportunity is in remarkably good health, considering her age,” said John Callas, the Opportunity project at NASA’s Jet Propulsion Laboratory. “I don’t think anyone would have imagined this rover would have lasted this long when we started almost 10 years ago.”

Odyssey’s roll to Solander Point should conclude by Aug. 1. There, Odyssey will look for gentle slopes that will ensure its solar panels are illuminated by sunlight during the coming Martian southern hemisphere winter. That will permit the venerable robotic explorer to continue working.

Curiosity’s nuclear power source allows it to function even in darkness.