Discovery of Perchlorate On Mars
In June 2008, the Wet Chemistry Laboratory (WCL) on board the 2007 Phoenix Mars Lander performed the first wet chemical analysis of martian soil. The analyses on three samples, two from the surface and one from 5 cm depth, revealed a slightly alkaline soil and low levels of salts typically found on Earth. Most unexpected though was the presence of ~ 0.6 wt % perchlorate (ClO4-), most likely as a Mg(ClO4)2 phase.
The extreme temperatures found on Mars typically lead to either crystallization or evaporation of water, making it difficult to imagine that water could be found in liquid form. The salts formed from perchlorates discovered at the Phoenix landing site act as “anti-freeze” and will substantially lower the freezing point of water. Based on the temperature and pressure conditions on present-day Mars at the Phoenix lander site, conditions would allow a perchlorate salt solution to be present in liquid form for a few hours each day during the summer.
The possibility that the perchlorate was a contaminant brought from Earth has been eliminated by several lines of evidence. The Phoenix retro-rockets used ultra pure hydrazine and launch propellants consisted of ammonium perchlorate. Sensors on board Phoenix found no traces of ammonium, and thus the perchlorate in the quantities present in all three soil samples is indigenous to the martian soil.
In 2006, a mechanism was proposed for the formation of perchlorates that is particularly relevant to the discovery of perchlorate at the Mars Phoenix lander site. It was shown that soils with high concentrations of natural salts could have some of their chloride converted to perchlorate in the presence of sunlight and/or ultraviolet light. The mechanism was reproduced in the lab using chloride-rich soils from Death Valley. In 2010, perchlorate was found at the 1000 ppb levels in a vast section of Antarctica, with implications that it is formed naturally and globally on Earth and probably on Mars. Recent isotopic studies have shown that natural perchlorate is produced on Earth by the oxidation of chlorine species through pathways involving tropospheric ozone or its photochemical products.
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