Pools of salty liquid water could be covering as much as 40 percent of the surface of Mars suggests a new study published today; a finding that would normally give some hope of finding somewhere suitable for life to thrive on our martian neighbour, however, the study also suggests that the properties and temperatures of the brines means they cannot sustain terrestrial life.
The question of life on Mars and whether it has ever existed either long in its past or is possibly active today is as old as our knowledge of the planet itself and still remains unanswerable.
Water is key to life as we know it, but with a thin, tenuous atmosphere, many studies have shown that stable liquid water is unable to persist on the martian surface. However a common substance that we often take for granted could help create liquid substances that can last for some time under the planet’s harsh conditions; salt.
A wide range of salts have been detected on Mars, with carbonate and sulfate salts being the most abundant. When added to water to form a salty mixture, a brine lowers the freezing point of the water so it becomes stable against evaporation for longer.
On Earth, salt loving (halophilic) and cold loving (psychrophilic) microbes populate the kind of habitats that occur on Mars, prompting some scientists to ask whether these transient liquid water environments could be hospitable for life on the Red Planet.
One such study by Eriita Jones at the University of South Australia is in agreement with this suggestion. Her study suggests that microbial life could be supported in nightly brines that are likely widespread through the shallow loose surface deposits known as Martian regolith.
Now, a new study presented by a team of US astronomers and headed by Edgard Rivera-Valentín at the Lunar and Planetary Institute, Houston, Texas, agrees that liquid brines on Mars may be more common, and can last longer, than previously thought, but their results in respect to habitability paint a very different picture than past studies such as those by Jones.
To investigate where the brines could form and for how long, Rivera-Valentín and colleagues combined an experimentally validated thermodynamic model with results from the Mars Weather Research and Forecasting (MarsWRF) general circulation model (GCM).
Two specific components were studied; brine formation through the transition from a solid crystalline salt into an aqueous solution, a process known as deliquescence, and the stability of melt-induced brines against freezing, evaporation and boiling.
The team found that up to 40 percent of the martian surface, at all latitudes down to the equator, could host stable brines for up to six consecutive hours and for up to 2 percent of the entire Martian year.
The authors also found that brines in the subsurface could last up to 10 percent of the martian year at a depth of 8 centimetres.
But, say the authors of the new study, the martian brines are not suitable for life to persist, because their water activities and temperatures fall outside the known tolerances for terrestrial life. This is contrast to studies such as those by Jones who advocate more favourable life sustaining conditions in the salty solutions.
Our results, write the authors in the research paper published today in Nature Astronomy, indicate that although high-water-activity solutions may be stable on present-day Mars, the corresponding brine temperature is systematically below 210 K (-63.15 degrees Celsius), which is well below the temperature limits for life.
Water activity is essentially a measure of how available the liquid water molecules are to biological processes and while salts help to stop a liquid from evaporating it also lowers the ‘activity’ of the water.
In the hyperarid conditions of Mars, coupled with its crippling freezing temperatures, it is just too much for microorganisms to survive, argue the authors and as such it means these brines cannot be classified as ‘Special Regions’ according to Planetary Protection policies, as they cannot sustain terrestrial life.
Special Regions on Mars are defined as environments able to host liquid water that simultaneously meets certain temperature and water activity requirements that allow known terrestrial organisms to replicate and therefore could be habitable, note the authors in their paper.
Although the brines do not fit the criteria for Special Regions that doesn’t mean to say we shouldn’t explore them say Rivera-Valentín and colleagues. Indeed, the locations of the stable brines could be targets for future Martian exploration, more so now since the risk of biological contamination from Earth is negligible, say the team.