Mars is too cold for liquid water at its surface, yet evidence abounds liquids are reshaping its surface. The most likely solution – in both senses of the word – involves magnesium perchlorate dissolved in water. When replicated in the lab, this produces an effect equivalent to pressurizing pure water more than on the ocean bottom. The finding could explain some puzzling features we have seen on Mars, and probably has yet to be resolved implications for the possibility of life on Mars.
The Phoenix Lander found calcium and magnesium perchlorate salts in Martian soil. Scientists attempting to explain the patterns on Mars that look like dry river beds have pondered the possibility the Red Planet sees occasional outbursts of brines – water laden with these chemicals instead of sodium chloride as would be more common on Earth.
"The surface temperatures on Mars may reach a high of about 20°Celcius at the equator and as low as -153° Celsius at the pole. With an average surface temperature of -55° Celsius, water itself cannot exist as a liquid on Mars, but concentrated solutions of perchlorate could survive these low temperatures," Dr Lorna Dougan of the University of Leeds noted in a statement.
In Nature Communications, Dougan reports on studies conducted on water with varying magnesium perchlorate Mg(ClO4)2 concentrations.
Impurities lower the melting point of a substance compared to the pure version, so magnesium perchlorate solution has a lower melting point than distilled water. At a concentration of 44 percent by weight, Dougan found brine stays liquid down to 206 K (-67 °C or -88 °F), a temperature even Mars usually exceeds.
The presence of dissolved ions is known to alter the structure of water molecules. However, there are so many molecules that can be dissolved in water that the effects of many of these are unstudied. Dougan found magnesium perchlorate at 44 percent concentration replaces hydrogen bonds between water molecules with cation-water-anion bonds, reshaping the water molecules as if they were under pressures of two billion pascals – almost 2,000 times the atmospheric pressure at sea level.
"We found these observations quite intriguing,” Dougan said. “The magnesium perchlorate is clearly a major contributing factor on the freezing point of this solution and paves the way for understanding how a fluid might exist under the sub-freezing conditions of Mars.”
Liquid water is considered essential for life. It certainly doesn't need to be pure – our oceans are very salty – but we don't know how water filled with magnesium perchlorate rather than predominantly sodium perchlorate would change things. Dougan suggested examining life in extreme conditions, such as deep sea species that survive under enormous pressures, to get a better handle on how Martian organisms, if they exist, might adapt to their environment.