Newly discovered perchlorates, or salts, have opened the possibility of a wet, habitable red planet
Just in the past few years, orbiter cameras and Mars landers have gathered evidence that watery liquid does exist on the Red Planet, at least during some part of the day or some part of the year. The presence of water in such an inhospitable environment—freezing cold, with low atmospheric pressures that drive rapid evaporation—is a bit of a puzzle. But a number of lines of research indicate that perchlorates, a form of salt found in Martian soils by the Phoenix lander in 2008, may play a key role in sustaining liquids on Mars.
Phoenix’s discovery, and the subsequent identification from a Mars Reconnaissance Orbiter camera of seasonal surface markings resembling fluid streaks, has significantly reframed the discussion of where, and how, water might exist on Mars today—and with it, perhaps, some form of microbial life. During a scientific workshop this week on the present-day habitability of Mars, Chris McKay of the NASA Ames Research Center in Moffett Field, Calif., noted that perchlorates had become, unofficially at least, “the theme for this conference.”
“I would say it is probably the most important astrobiological discovery since Viking—the discovery of perchlorate,” McKay added. (The Viking landers were NASA’s landmark 1970s life-detection missions.) For starters, perchlorates would have reacted at high heat to destroy any organic compounds in the soil samples analyzed by the Vikings, potentially explaining their nondetection of biological molecules, even if Martian soils do record the past presence of life.
Perchlorate salts also appear to greatly extend the environmental conditions under which brines could remain liquid on Mars today. “They do seem to depress the freezing temperature to the point where you could have stable liquid on the surface,” said Selby Cull of Bryn Mawr College.
What is more, salts such as perchlorates provide a way for Martian soils to take up water from the atmosphere. When the Martian atmosphere becomes more humid during its daily cycle, perchlorates can undergo a process called deliquescence, pulling water from the air and essentially transforming from a crystal to a droplet. As the humidity decreases again, the salts effloresce, releasing the water vapor back into the atmosphere. Intriguingly, the process is asymmetrical—preliminary laboratory data presented at the conference demonstrated that perchlorate salts deliquesce and effloresce at very different humidity levels. Once a perchlorate has taken in water via deliquescence, which occurs as moderate humidities, it will retain that water until the humidity drops to very low levels. “The salt really likes to maintain its aqueous state,” said Raina Gough of the University of Colorado at Boulder. The ready uptake of water by perchlorate salts points the way to the regular emergence of pockets of liquid under typical Martian conditions. “We’ve expanded the region where you can have an aqueous phase to lower temperatures and lower humidities,” Gough said.
But liquidity is not enough to guarantee habitability. Martian salts may be resistant to efflorescence, and may form brines that remain liquid to very low temperatures, but many of those same brines are too dissimilar to pure water to enable life as we know it to grow. (Perchlorates are generally poisonous, to make matters worse.) “Briny water on Mars may or may not be habitable for microbes from Mars or from Earth,” acknowledged Alfred McEwen of the University of Arizona.
All the same, McEwen and his colleagues have recently found what appears to be the best evidence for present-day flowing liquid on Mars. In 2011 the researchers used the Mars Reconnaissance Orbiter’s HiRISE camera to identify several sites where warm slopes are periodically darkened by linear streaks. Flowing briny water was, and still is, the best explanation for the seasonal appearance of the so-called recurring slope lineae—although McEwen noted that non-perchlorate salts, with less extreme properties, could be in play.
The presence of various salts on Mars, and the apparent ability of those salts to permit the flow of liquids even in Mars’s harsh climate, has opened new doors to the possibility of a wet, perhaps even habitable Mars. “I’m struck by how different this discussion is than just a few years ago,” said David Paige of the University of California, Los Angeles, the convener of the conference. “There are clearly places that water activity does appear to be occurring on Mars.”
Not long ago, McEwen noted, a hypothetical mission seeking out Mars life would have had no obvious target for exploration. Such a search would need to find the proverbial needle in a planet-size haystack. “Now we have some very strong ideas about where to go and what to look for,” he said.
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