C’è acqua su Marte
articolo di Victoria Turk, 13 aprile 2015
The Curiosity rover has discovered evidence of liquid water on Mars. But extra-terrestrial hunters shouldn’t get too excited: While water is one condition for life as we know it, the researchers state that temperatures on the planet are still too low for microbes to survive.
That might sound a bit contradictory. We already know there’s ice on Mars, so how can there be liquid water if it’s still too cold for life? The answer, as some researchers had already theorised, is salt.
According to a new study in Nature Geoscience, perchlorate salts—salts from perchlorate acid—were found at the Gale crater, an area near the Red Planet’s equator. These salts lower the freezing temperature of water, so it can remain liquid in colder conditions.
The paper suggests that the salts absorb water vapour from the atmosphere in the Martian night, resulting in “liquid brines in the uppermost 5 cm of the subsurface,” which then evaporate during the day.In other words, despite having no rain, Mars still has its own water cycle.
Lead author Javier Martín-Torres from the Luleå University of Technology in Sweden told me in a phone call it was remarkable that they’d made the observation at the Gale crater, where conditions are driest and hottest. “If we see brines on the equator, there must be brines everywhere on the planet,” he said.
He explained that these brines form because the salts in question soak up so much water vapour that the combination becomes liquid.
“We don’t really see the water—we cannot see it, because we don’t have a camera to see the water,” said Martín-Torres. That’s because the liquid only appears at nighttime, when it’s too cold for most of Curiosity’s instruments to work and the rover goes to sleep. Instead, the researchers measured humidity, air temperature, and ground temperature using the Rover Environmental Monitoring Station (REMS) instrument, and found the results were compatible with the presence of brines at night. Perchlorate salts have previously been detected on Mars.
The researchers’ hypothetical water cycle for Mars. Image: Martín-Torres and Zorzano
It’s this dip in temperature that makes the liquid water possible. During the day, temperatures can be more than 100 degrees Celsius higher than at night, when it can be as cold as -100. Additionally, there’s 100 percent humidity at night—so water vapour can be taken from the air—but very little in the day.
However, the cold nights also mean that, despite the presence of liquid water, life on Mars doesn’t necessarily look any more likely. “When there are the conditions for liquid water on Mars, those conditions are such that the temperatures are so low that it’s not possible for life as we know it to reproduce or to metabolise,” Martín-Torres said.
But their observation is not just about dashing the hopes of alien-hunters; it could also have ramifications for planetary protection. At the moment, the Committee on Space Research (COSPAR) maps “special regions” on Mars where spacecraft should not land. These are regions thought to be most likely to have conditions to support life, and the idea is to avoid the risk of contamination with microbes from Earth. It’s known that microorganisms sometimes persist on spacecraft even after cleaning; if we accidentally take life to Mars and conditions support it, it could propagate.
This also means that all equipment heading to the planet has to be very thoroughly sterilised, and that’s really expensive—so much so that Martín-Torres thinks the rules should be relaxed in the wake of the new study’s conclusions on the unsuitability of Mars for life. “It seems like planetary protection policies should be revised,” he said. “They’re probably too conservative dealing with spacecraft on Mars.”
Still, some researchers hold out hope for life on Mars; notably the scientists who sent NASA’s Viking landers up to the planet back in the 1970s and whose assertions about potential signs of life are controversial. Martín-Torres said he thinks it’s very unlikely at least in the first five centimetres of the planet’s surface to have anything resembling what we call life on Earth.
“You never know, there could be a strange Martian microorganism that can survive these conditions,” he said. “But that’s really speculative.”