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Water is vital for life as we know it. But not all water has life living in it. By combing through data from extreme environments, researchers have found the limits of what constitutes habitable water conditions on our planet. This could help us figure out what types of water on other planets would be more likely to host life.
The guiding principle in our current search for alien biology is "follow the water." But the new research suggests this target needs to be refined. "Should we follow the hot water or maybe the cold water?" asks Eriita Jones of the Australian National University, lead author of the study that appears in the latest issue of the journal Astrobiology.
On Earth, we know that life can survive in a wide variety of water temperatures and pressures, and yet there are watery places where no living things have been found. Jones and her colleague Charles Lineweaver have performed a comprehensive survey of just how far life has expanded into the available "water territory" on Earth.
"We try to quantify our understanding of the terrestrial biosphere better," Jones says.
Their results show that only 12 percent of the volume of the Earth where liquid water exists is known to host life. As for the rest of this volume, life presumably never found a way to adapt to the conditions there, despite having had several billions of years of evolution to work with. This may mean that some fraction of liquid water is strictly uninhabitable – both here and on other distant worlds.
To quantify what constitutes habitable water, Jones and Lineweaver plotted the range of water conditions on a pressure and temperature diagram. "This is a very natural way to parameterize any planet," Jones says.
Although we typically think of water being liquid between zero and 100 degrees Celsius, this is only true for pure water at Earth's sea level atmospheric pressure (about 14.7 pounds per square inch or 1014 millibar). If salt is present, water's freezing point drops below zero degrees and its boiling point rises above 100 degrees.
At high pressure, as well, water remains liquid above 100 degrees Celsius. In fact, the authors estimate that liquid water can exist to a maximum depth of 75 kilometers below the Earth's surface, where the temperature is more than 400 degrees Celsius and the pressure is 30,000 times that at the surface.
But could life live in this water? Probably not. The highest temperature known to support life is 121 degrees Celsius. Some biologists believe organisms might survive at even higher temperatures, but nothing has broken the record yet.
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