NASA's Curiosity rover on Mars has detected perchlorate compounds that are able to lower the freezing point of water, allowing it to remain in liquid form near the surface. Based on the data collected from the rover, researchers believe that liquid salt water may actually exist close to the surface.
The rover discovered salty perchlorate compounds in the Martian soil that, under the right conditions, can absorb water vapor from the atmosphere and lower the freezing point of water, allowing the water to remain as a liquid in the form of brine (concentrated salt water.) Scientists believe that this brine could form both at the surface and a few centimeters below it as well.
The process is likely only to occur when the air on Mars is at its coldest either during the night or on cold winter mornings.
"Perchlorates are not only oxidants, but they also form highly hygroscopic (water-absorbing) salts and are strong freezing-point depressors when added to water," Professor Morten Bo Madsen, a member of Curiosity's science team from the Niels Bohr Institute in Copehagen, Denmark, said.
After the sun sets, some water vapor condenses on the planet's surface as frost. The freezing point of the water is lowered when absorbed by perchlorates in the soil, thus turning the frost into a liquid. Because the soil is porous, the liquid water can seep down and precipitate somewhere else below the surface.
"This can explain mobility of salts just below the surface, and with this study we now know that this can occur presently and hence is not just a relic from the past," said Morten Bo Madsen. However, this liquid water is transient, forming within the uppermost 5cm (2in) of the surface overnight, and drying out again at sunrise.
"This is not about the detection of salts, but about the impact these salts have on the possibility to have liquid water on Mars," Dr Javier Martin-Torres, senior research scientist at the Instituto Andaluz de Ciencias de la Tierra (CSIC-UGR) in Granada, Spain, and lead author of the paper, said. "If you look at the phase diagram of pure water, you will find at the pressure and temperatures of Mars it is impossible to have liquid water on the surface. This has been well known for a long time."
Perchlorates were first discovered by NASA's Phoenix spacecraft back in 2008, although scientists has speculated about their presence since experiments on board NASA's Viking crafts first detected an oxidizing compound in the Martian soil back in 1977.
"The perchlorate is only present in very small amounts (between 0.5 and 1 per cent by weight), as is the case on Earth," said Professor Morten Bo Madsen. "Perchlorates are rare in nature, but do occur at very low concentrations in very arid areas such as the Atacama desert."
"This is the first time that we find conditions for the formation of brines on Mars. There have been hypotheses and laboratory studies supporting this possibility, but even these works were pointing to this only occurring at high altitudes," said Dr. Martin-Torres.
"The temperatures for formation are very low, and only reached (at least in the Equator, where Curiosity is) at night time, and especially in winter-time. It is really a surprise that we find brine conditions at the Equator, which is the driest and hottest region of the planet, but the fact that we find them there tells us that brines must be everywhere in the planet where there are perchlorates."
"The transition of the existing salts within the soil, from hydrated or frozen states, into small liquid droplets could only be directly detected either by measuring electrical conductivity changes or calorimetric changes of that droplet or tiny salt patch within the soil," says Martin-Torres. "This kind of contact science requires a sub-surface probe and instruments that currently do not exist within the MSL platform."
"In general, the findings of this study have broad implications for studies on the availability and the history of water on Mars, for preserving plausible organic products, for studying the corrosive interaction of these brines with spacecraft materials, and, of course, for other geological processes related to water and climate on Mars today," he concludes.
