Water is vital for life as we know it. But not all water haslife living in it. By combing through data from extreme environments,researchers have found the limits of what constitutes habitable water conditionson our planet. This could help us figure out what types of water on otherplanets would be more likely to host life.
The guiding principle in our current search for alien biologyis "followthe water." But the new research suggests this target needs to berefined.
"Should we follow the hot water or maybe the coldwater?" asks Eriita Jones of the Australian National University, leadauthor 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 ofwater temperatures and pressures, and yet there are watery places where no livingthings have been found. Jones and her colleague Charles Lineweaver have performeda comprehensive survey of just how far life has expanded into the available"water territory" on Earth.
"We try to quantify our understanding of the terrestrialbiosphere better," Jones says.
Their results show that only 12 percent of the volume of theEarth where liquid water exists is known to host life. As for the rest of thisvolume, 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 strictlyuninhabitable ? both here and on otherdistant worlds.
Water diagram
To quantify what constitutes habitable water, Jones andLineweaver plotted the range of water conditions on a pressure and temperaturediagram.
"This is a very natural way to parameterize anyplanet," Jones says.
Although we typically think of water being liquid betweenzero and 100 degrees Celsius, this is only true for pure water at Earth's sealevel 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 itsboiling point rises above 100 degrees.
At high pressure, as well, water remains liquid above 100degrees Celsius. In fact, the authors estimate that liquid water can exist to amaximum depth of 75 kilometers below the Earth's surface, where the temperatureis more than 400 degrees Celsius and the pressure is 30,000 times that at thesurface.??
But could life live in this water? Probably not. The highesttemperature known to support life is 121 degrees Celsius. Some biologistsbelieve organisms might survive at even higher temperatures, but nothing hasbroken the record yet.
Jones and Lineweaver take the current limit of 122 degreesCelsius to be the upper temperature boundary for habitable water. At the otherend of the thermometer, liquid water can be found on Earth at 89 degrees belowzero in thinfilms. However, the coldest water temperature known to support active lifeis 20 degrees below zero, which is what the researchers take as their lowerhabitable boundary.
The researchers also looked at pressure limits. Life hasbeen found as far down as 5.3 kilometers below the surface, where the pressureis 1500 times that at the surface. Whether this is truly the highest pressurefor habitable water remains to be seen, since no one has yet dug deeper insearch of life.
"We have so far found life as deep as we have looked,"Jones says.
As for low pressure, life has been found high up in theatmosphere where the air is thin, but these microorganisms are typicallydormant and are only revived when given the necessary nutrients. The authorstherefore take the low pressure limit for active life to be one third ofatmospheric pressure, which corresponds to the altitude at the top of Mt.Everest. ?
Biosphere limits
According to the above limits, life on our planet isrestricted to a thin shell that roughly extends from 10 kilometers above thesurface down to 5 kilometers below (or to depths of 10 kilometers in the ocean).This leaves uninhabited 88% of the volume where water exists on Earth.
"It shows that life and water are not equivalent,"Jones says. "There may be a lot of liquid water that is hostile tolife."
Nearly all of Earth's liquid water is located in habitableregions. The point is that only a small fraction of the water conditionson Earth are friendly to life.
"Stated this way it sounds surprising and seems tosuggest that the 'follow the water' strategy for life search needsrethinking," says Chris McKay of the NASA Ames Research Center.
But he thinks this is slightly misleading. The only trulyconstraining factor in this analysis is the observation that life apparentlycan't survive above 122 degrees Celsius.
"None of the other worlds (save Venus) have surfacetemperatures that are hot enough to make this limit relevant," McKay says.
However, hotter temperatures can be found below the surface.Mars, for instance, may be too cold for liquid water on its surface, but thereis reason to believe that there is liquidwater underground.
Jones and Lineweaver are currently modeling the crust, mantleand core of Mars and using heat flow estimates to construct a Martian water phasediagram, like the one they made for Earth's water. The results will show atwhat depths potentially habitable water (as defined by the current study) mightbe found on Mars.
This sort of "habitable water" analysis could alsobe used for the liquid oceans that are thought to lie beneath the icy crusts ofJupiter's moon Europa and Saturn's moon Enceladus.? And it may help characterizeexoplanets for which a reasonable phase diagram can be estimated.?
"It may show us where to focus our search forlife," Jones says.
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