This water could be some of the oldest on the planet and may even contain life. Also, the similarity between the rocks that trapped it and those on Mars raises the hope that comparable life-sustaining water could lie buried beneath the Red Planet's surface, scientists believe.
Researchers from the universities of Manchester, Lancaster, Toronto and McMaster analysed water pouring out of boreholes from a mine 2.4 kilometres beneath Ontario, Canada.
The hydrogen and methane come from the interaction between the rock and water, as well as natural radioactive elements in the rock reacting with the water. These gases could provide energy for microbes that may not have been exposed to the Sun for billions of years.
The crystalline rocks surrounding the water are thought to be around 2.7 billion years old.
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Using ground-breaking techniques developed at the University of Manchester, the researchers show that the fluid is at least 1.5 billion years old, but could be significantly older.
"Our finding is of huge interest to researchers who want to understand how microbes evolve in isolation, and is central to the whole question of the origin of life, the sustainability of life, and life in extreme environments and on other planets," Ballentine said.
Before this finding, the only water of this age was found trapped in tiny bubbles in rock and is incapable of supporting life. The water found in the Canadian mine pours from the rock at a rate of nearly two litres per minute.
"Our Canadian colleagues are trying to find out if the water contains life right now. What we can be sure of is that we have identified a way in which planets can create and preserve an environment friendly to microbial life for billions of years," said Dr Greg Holland of Lancaster University, lead author of the study.
"This is regardless of how inhospitable the surface might be, opening up the possibility of similar environments in the subsurface of Mars," Holland said.