A juvenile squid from the Celebes Sea, between Borneo and The Philippines - one of the strange new forms of life found to be living at up to a depth of 2.8 km.
Credit: Woods Hole Oceanographic Institution
SYDNEY: The strategy of 'following the water' in the search for extraterrestrial life may need some tweaking, suggests a new study in the journal Astrobiology.
Put simply: there's an awful lot of places where water could exist - either on the surface of the Earth, or deep within it - yet life is largely concentrated in a small sliver of this.
Eriita Jones and Charley Lineweaver of the Australian National University in Canberra have estimated the volume of the Earth where liquid water can exist, and calculated that life inhabits as little as 12% of it.
Limited range
After compiling observations of life in extreme environments, the researchers found that liquid water in only a limited range of temperatures and pressures can support life.
These observations could help focus the search for life elsewhere in our Solar System. "We suspect that these limits that we find on Earth are the fundamental limits of any water-based life," says Lineweaver.
Hence, they suggest that the search for life on another planet or moon can be narrowed down to those regions with just the right temperatures and pressures.
Lack of nutrient and energy supply
Life on Earth has turned up in environments ranging from deep ocean hydrothermal vents to Arctic ice cores.
Liquid water, however, can exist under a much wider range of conditions, from the high temperatures and pressures of Earth's mantle to the atmosphere, where it exists only transiently as thin films on particulates, according to the study. A lack of nutrient and energy supply in those regions seems to prevent life from developing there.
"Life has tried and tried to inhabit all this water," says Lineweaver, noting that approximately four billion years of evolution has given rise to life in only some conditions. "I think we should listen to what life has to say about its own limits."
Not just any liquid water
Lineweaver and his graduate student Jones, were initially interested in life-supporting environments on Mars, but soon realised they should take advantage of knowledge about life on our own planet.
By looking at Earth first, says Lineweaver, "we can test the [widely held] idea of 'if there is water then there is life'. We're refining that rule," he says. "It's not just any liquid water."
All life on Earth requires water at some point, and H2O is pervasive in the Solar System, says Lineweaver. Therefore he thinks following the water is still a sensible approach.
Limits of life
However, a significant amount of water seems to be too hot, too cold or too salty to be inhabitable, so the pursuit can be more directed.
This study is the first to have provided so much detail into the limits of life in liquid water, according to geologist Jonathan Clarke, vice president of Mars Society Australia.
"'Follow the water' has been and still is a good first approximation," said Clarke. "Like any good approximation, it needs refining, and this is what they've done."
Change in strategy?
Clarke doesn't believe this report will drastically alter the strategy in the search for extraterrestrial life, but agrees that understanding Earth first narrows the target.
"There's not much else we can do, really," he says. "It's like forensic science: you have a general description of the suspect, and it might be the wrong one, but you have to start somewhere."
As the mathematicians would say:
Water is (currently thought to be) a necessary but not a sufficient condition for life.
Guess I was led to believe almost the opposite...
I've heard when explorers set down in the Marianas trench - deepest point on earth - some fish scurried away. I've been to Death Valley and see life swimming in pools of salt water as it crystallized in the sun, have seen pictures of life teeming on deep sea vents, under polar caps, and much more.
Sure you might be able to find some rare instances of water without life, but for the most part, liquid water = life. Additionally, for the most part, no liquid water = no life. You're still substantially in favorable odds by looking for liquid water.
Pure water is not enough
As the article says, water is not enough. There is a lot of life near the surface of the ocean at the microscopic level, but it varies based on the nutrients available. That's because the nutrients tend to get gobbled up near the source of the nutrients. There's also a large volume of mostly empty water between the top and bottom. If there were enough nutrients, the entire ocean would be wriggling. Life at the bottom is living off the concentrated layer of stuff which can't fall any further. There are critters which pass through the large empty spaces but it seems relatively empty -- darned if I have any idea how whales find squid down there.
Water isn't the guide, but the article misses what is
We really are hamstrung by our narrow definition of life, colored by the form life takes here on Earth. Someday we will realize that life really is a chemical mechanism that reduces thermodynamic chaos while maximizing information content, and as such really only needs an environment where suitable solvents are at or near their triple points. So, I expect we will not only find life where water is at its triple point, but also at places like Mars where hydrogen peroxide is at its triple point, and similarly Titan with methane and Io with salt.
So...
I hate to be a naysayer but this just rings a bit hollow to me. Is this article saying that 88% of water on Earth is effectively sterile ? Sounds distinctly unlikely.
I can readily accept that complex (multi-cellular) life is more fussy and will tend to congregate around readily available nutient and energy supplies.
At the end of the day, if we detect life on another planet at present, it will be from atmospheric compounds. We're hardly going to be able to check the pressure and selenity of the water there.
So water isn't a guarantee of life ? Not really that much of a revelation.