Water is vital for life as we know it and is one of the key factors we associate with habitability and look for in search of life elsewhere in the universe. But not all water has life living in it. Researching extreme environments has helped to distinguish the limits of what constitutes habitable water conditions on our planet, which could help us determine what types of water are more likely to contain life on other planets. This suggests that the principle of looking for water in order to find life may need to be refined because not all water is habitable.
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.
Scientists have found that only 12 percent of the volume of the Earth where liquid water exists is known to host life. The remaining fraction of liquid water seems strictly uninhabitable – both here and possibly on other distant worlds. Although this is just taking into consideration the ability of our life form to live in these type of watery environments, not other alien life forms.
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. 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.
Conditions on Earth do not allow liquid water below a depth of about 75 km, which leaves a thin outer shell where liquid water can exist. This means that out of the entire Earth volume, 3.3 percent has the right conditions for liquid water but only 0.2% has the potential for habitable water.
The highest temperature known to support life--an Archaea named "Strain 121"--is 121 degrees Celsius. Some biologists believe organisms might survive at even higher temperatures, but nothing has broken the record yet. This means that the upper bound of habitable water, in terms of temperature, is about 121 degrees. At the other end of the thermometer, liquid water can be found on Earth at 89 degrees below zero in thin films, however, the coldest water temperature known to support active life is 20 degrees below zero, which is what the researchers take as their lower habitable boundary.
In terms of pressure limits, life has been found as far down as 5.3 km below the surface, where the pressure is 1500 times that at the surface. No one has yet dug deeper in search of life so we are not sure if life can survive in higher pressures than this. As for low pressure, life has been found high up in the atmosphere where the air is thin, but these microorganisms are typically dormant and are only revived when given the necessary nutrients. The authors therefore take the low pressure limit for active life to be one third of atmospheric pressure, which corresponds to the altitude at the top of Mt. Everest.
According to the above limits, life on our planet is restricted to a thin shell that roughly extends from 10 kilometers above the surface 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 which suggests that life and water are not equivalent and there seems to be a lot of water present that is hostile to life.
What is even more interesting is that nearly all of Earth's liquid water is located in habitable regions, so that only a small fraction of the water conditions on Earth are friendly to life. But this is slightly misleading because the only truly constraining factor in this analysis is the observation that life apparently can't survive above 122 degrees Celsius.
What does this mean for water on Mars?
Jones and Lineweaver are currently modeling the crust, mantle and core of Mars and using heat flow estimates to construct a Martian water phase diagram, like the one they made for Earth's water. The results will show at what depths potentially habitable water (as defined by the current study) might be found on Mars.
This sort of "habitable water" analysis could also be used for the liquid oceans that are thought to lie beneath the icy crusts of Jupiter’s moon Europa and Saturn’s moon Enceladus. And it may help characterize exoplanets for which a reasonable phase diagram can be estimated.
This may halp us focus our search for life. It also adds complexity to the idea that water and habitability are equivalent.
Another interesting point is that there may be more to the inhabitability of water besides just pressure and temperature. Perhaps there are regions that are too salty? or too high in certain chemicals? or too polluted?
One related recent discovery is that of methane-eating bacteria at "Lost Hammer":
there have been microbes found that live in water that is so salty it is liquid at subzero temperatures, contains no consumable oxygen, and has high amounts of bubbling methane. This spring is located in a highly unique spring--Lost Hammer--on Axel Heiberg Island in Canada's extreme North. Despite these incredibly harsh conditions, researchers from the McGill University have found life. This life consists of methane-consuming bacteria.
They were surprised not to find methanogenic bacteria that produce methane but instead to find these very unique anaerobic organisms that survive by essentially eating methane and probably breathing sulfate instead of oxygen. "The point of the research is that it doesn't matter where the methane is coming from," Whyte explained. "If you have a situation where you have very cold salty water, it could potentially support a microbial community, even in that extreme harsh environment."
It has been very recently discovered that there is methane and frozen water on Mars. Photos taken by the Mars Orbiter show the formation of new gullies, but no one knows what is forming them. One answer is that there could be that there are springs like Lost Hammer on Mars. There are places on Mars where the temperature reaches relatively warm -10 to 0 degrees and perhaps even above 0ºC and on Axel Heiberg it gets down to -50, easy. The Lost Hammer spring is the most extreme subzero and salty environment we've found and provides a model of how a methane seep could form in a frozen world like Mars, providing a potential mechanism for the recently discovered Martian methane plumes.