"Follow the water" has long been the mantra of our scientific search for alien life in the Solar System and beyond. We continue seeking conditions where water can remain liquid either on a world's surface or elsewhere within a planetary body. This approach makes a lot of sense. Life as we know it requires water for the complex chemistry that enables growth and reproduction. Where there is water, we believe life has a chance.

"Basically, all active cellular systems live in watery environments," said John Hallsworth, an environmental microbiologist at Queen's University Belfast. "Without an aqueous milieu both inside the cell and outside, microbes can die, or, at best, manage to survive in an inactive state."

A good question to ask, then, is how much water at a minimum does life need? As a recent study in the journal Environmental Microbiology by Hallsworth and colleagues explains, the answer is not simply one of water quantity, but rather of its concentration.

Water, as we all know from brewing tea or making Kool-Aid from a powder mix, is excellent at incorporating other molecules into its liquid medium. More relevantly to life, water is a potent solvent that easily dissolves salts from the oceans and minerals on land. As a result, water can become disagreeable or even toxic to life, based on what dissolves in it. Life therefore needs "biologically available" water, extractable from its surroundings.

"Although it may sound counterintuitive, the quantity of water present in a potential habitat is not a meaningful measure of how biologically available water molecules are," said Hallsworth. "It is, however, the concentration of water molecules that acts as a determinant for life processes."

The measure of water's biological availability is called "water activity." Hallsworth said it works in cells much like a fuel mixture powers an engine. An engine might be designed to operate with a mixture of 95 percent ethanol and 5 percent water, and the quantity of water does not matter so much as the relative concentration of it.

"This engine may start and operate equally well whether there is 1 or 100 gallons of fuel in the fuel tank," said Hallsworth.

However, if the delicate ethanol-water ratio gets thrown off, "the engine will become less efficient and begin to labor." Should the fuel mixture ratio cross a critical threshold, "the engine may cease to work," analogous to how an organism would become inactive or die, he said.

In their new paper, Hallsworth and his fellows consider the minimum level of water activity tolerated by life here on Earth. The authors also explore what this threshold means for finding life elsewhere in the universe. The good news: Seemingly barren, dry locales in our solar system might boast a high enough water activity level for hardy microbes to chug along. In fact, some extraterrestrial environments are not that dissimilar from places on Earth where "extremophile" microbes survive and thrive in severe temperatures, pressures and low water activities.

"There is an ultimate water activity limit for even the most resilient life forms," said Hallsworth. "We therefore considered the types of aqueous milieux known to exist in the present-day Solar System, and found that some of these places resemble fertile microbial habitats on Earth."

How low can you go?
Although the vast majority of Earth's organisms live in high water activity environs, the planet certainly has its fair share that don't mind dryness. The minimum water activity for life looks to be around 0.600 - equivalent to 60 percent relative humidity, a metric we are accustomed to with weather. (For reference, pure water, which represents maximum water activity, has the arbitrary value of 1.)

All three domains of life on Earth have record-holder organisms that get by at or just above that 0.600 water activity level. For eukaryotes, the domain including humans and all other animals, a yeast called Xeromyces bisporus holds the top spot. This mold can eke out a living in relatively moisture-free, sugary environments and can cause food spoilage.

Researchers first discovered X. bisporus' amazing ability to grow at a water activity level of 0.605 in 1968. For decades, it, along with perhaps a dozen other fungi, occupied this austere realm of under 0.700 water activity. The other two domains of life, bacteria and archaea - both of which contain only single-celled creatures, known as prokaryotes - apparently crapped out at around 0.755 water activity.

This significant difference has confounded scientists with regard to the origin of life on Earth. It is widely reckoned that prokaryotic cells, which are simpler than eukaryotic cells, were the first kinds of life. Prokaryotes likely had the planet all to themselves for a couple billion years before the rise of eukaryotes.

Yet the earliest evidence for life points to it arising in salt-loaded environments with correspondingly low water activity. For instance, layers of microbes built up rocky, mound-like stromatolites in coastal environments, the fossils of which we study today. In the tidal areas where these microbe communities arose, salts would have accumulated on the stromatolite as water leftover from receding tides evaporated. In addition, the oceans a few billion years ago could also have been twice as salty as modern day. In short, primeval organisms somehow coped with loads of salt.

Recent studies by Hallsworth and other colleagues have offered a solution. Some prokaryotes, it turns out, can deal with such hyper-saline conditions, on par with the shockingly low water activity of the sugar-loving yeast, X. bisporus. Over the years, studies have even claimed that life operates below 0.600 water activity. However, subsequent work has found no evidence for such eyebrow-raising findings.

At any rate, all of the concrete evidence we have to date suggests that the water-activity range of 0.690 to about 0.600 represents "the biophysical fringe of Earth's functional biosphere," said Hallsworth.

A little bit goes a long way
This range of water activity encompasses scenarios where water need not exist in its life-friendly flowing form. From vapor to frost, water is a dynamic molecule over a relatively small temperature span.

Here on Earth, just a thin film collected on exposed rocks can serve as aqua aplenty for microbes. Previous research has suggested a water thickness of a mere three molecules might suffice for biological availability.

On cold, arid Mars, films of this thickness should exist on otherwise frozen ice in stages of pre-melt, as well as on minerals. Salt crystals, for instance, could absorb what little humidity there is in the martian atmosphere and develop a thin, briny coating. Water trapped within minerals, or even more promisingly, in underground melt-water deposits, could also provide microbes with a temporary water supply to reproduce. As conditions dry up again, microbes could go dormant, just as they do on Earth as water activity ebbs and flows. Furthermore, in its wetter, warmer past, Mars might have hosted bodies of saltwater well within Earthly life's water activity range.

Besides Mars, one of the Solar System's best bets for alien life is Jupiter's moon, Europa. Its planetary surface chemistry suggests that brines with acceptable water activity could form on the moon. Other factors, like extreme cold and low pressure, of course come into play on Europa as elsewhere. Europa's potentially warm, liquid interior, however, should be another story altogether from the surface environment, and one more conventional for life as we know it.

Numerous other locales in the Solar System could theoretically possess water activity levels amenable to extremophiles as defined by Earthly standards. Saturn's geyser-spewing moon, Enceladus, and even asteroids could have regions of sufficient water activity.

Hallsworth's paper touches on the debate about whether a life form could go through an entire life cycle without any source of water external to a cell. So long as a cell obtains a bit of water initially, it might then be able to develop all the way through a reproductive phase. The jury is still out on this, Hallsworth said, but if found to be so, life has even greater odds of clinging on where the conventional wisdom says it could not.

Spreading Earth's germs
Overall, with water activity not posing major limitations for life in a decent-sized portion of our Solar System, it certainly stands to reason that planetary bodies within other solar systems will have few problems, either.

With these possibilities in mind, part of Hallsworth's new paper addresses the issue of human explorers or robotic probes accidentally introducing Earthly life to other worlds. It's a subject space agencies take very seriously. NASA, for instance, has instituted a Planetary Protection program to try to ensure that no microbes hitch rides to Mars or elsewhere. Engineers construct rovers in sterilized "clean rooms" to reduce contamination.

"Where other conditions, such as temperature, are biologically permissive, the ubiquity of potential microbial habitats that are within the known water-activity range for life makes the implementation of planetary protection measures ever more pressing," said Hallsworth.

Get it tight, get it right
Water activity, it follows, is a phenomenon requiring firm understanding and careful monitoring. Yet for all the focus on "limits to life" research and in popular media, it's striking that water activity is often imprecisely measured, according to Hallsworth.

A big reason scientists overlook water activity is the historical accident of its reference maximum value being arbitrarily set at 1. Accordingly, determinations of water activity are expressed as a fraction of 1, such as the 0.755 and 0.605 figures mentioned previously. What are in actuality profound changes in water activity can, as a consequence, appear subjectively small.

Hallsworth's paper marshals evidence that a water activity change of 0.100 is equivalent to a whopping temperature change of about 95 degrees Fahrenheit (35 degrees Celsius). Obviously, such a swing would matter tremendously for, say, human beings, which perish outside of an internal body temperature range of perhaps a score of degrees Fahrenheit. A tendency has arisen in water activity studies, however, to measure it to a single decimal place.

"In most natural habitats, parameters such as temperature and water activity fluctuate continuously," he said. "This said, most microbiologists would consider it both unthinkable and unacceptable to measure temperatures of cultures or habitats to an accuracy of less than plus or minus 1 degree Celsius."

For genuine specificity, akin to how precisely biologists worry about temperature effects on an organism, water activity should be recorded to three decimal places, Hallsworth argued. Many examples exist of a thousandth of a difference in water activity, at the third decimal place, to matter to microbes, hence the importance of precision like 0.605.

"We now present evidence that the microbial cell is sensitive to changes in water activity at the third decimal place, and propose that cells are even more sensitive than this," said Hallsworth. "Unless determined and expressed to this level of accuracy, water-activity values can lack biological meaning."