In a small rock, planetary scientists are reading important evidence of once-abundant water on Mars.

The rover Opportunity has examined a rock on Mars that dates back to a distant era when the Red Planet had plenty of water – and possibly an atmosphere with clouds and rain.

“A lot of water moved through this rock," said Steve Squyres of Cornell University, principal investigator for the Opportunity mission, run by NASA’s Jet Propulsion Laboratory.

The rock, given the name “Esperance” by scientists, has a combination of elements that indicate a clay-mineral composition, formed in a watery environment. “This is water you could drink," Mr. Squyres said, referring to a neutral, non-acidic composition like fresh water on Earth.

That’s important because such neutral water and a substantial surrounding atmosphere are conditions that are favorable for harboring life. Those conditions may have existed in Mars’ relatively early history, several billion years ago.

“The Esperance results are some of the most important findings of our entire mission," said Squyres, who is based in Ithaca, N.Y. "The composition tells us about the environmental conditions that altered the minerals.”

The finding isn’t the first evidence to emerge that Mars, now seen as a “hostile” place lacking in protective atmosphere, once had conditions hospitable to life.

Back in March, NASA reported important findings from drilling done by a more advanced rover, Curiosity.

Minerals in that rock sample “speak of abundant standing water, conditions neither too acidic or too alkaline for life, and the minerals that would have provided a ready energy source for microbes, if any had been there,” according to a Christian Science Monitor account of that discovery.

In contrast to the newer Curiosity, Opportunity’s longevity is part of the intrigue behind the latest evidence of a watery past on Mars.

Opportunity has chugged forward far longer than scientists expected, and is now nearing the 10-year mark in its exploratory mission.

It used a rock abrasion tool, spectrometer, and microscopic imager to examine the rock Esperance at an area of the planet called Cape York. Now Opportunity is trekking to a place called Solander Point, where researchers hope to use a large exposed cross-section of geological layering to learn about stages in the planet’s environmental history.

Much about the Martian past remains a mystery.

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“There are signs that in the distant past, billions of years ago, Mars was a much more inviting place,” NASA said in 2011 as the space agency was launching new missions to the planet. “It appears that in its youth, Mars was a place that could have harbored life, with a thicker atmosphere warm enough for rain that formed lakes or even seas.”

The exploring since then appears to be bearing out that view.