Curiosity’s work at target named Rocknest last October helps advance the idea that a class of salts called perchlorates are common on Mars.
Curiosity’s analysis of Rocknest was discussed by Doug Archer (NASA – JSC) at the Lunar and Planetary Conference on March 19. Rocknest was the site of Curiosity’s first use of its Sample Analysis at Mars (SAM) mass spectrometer last October. The preliminary findings were first detailed in a NASA news conference last December. The initial results, now confirmed, indicated SAM had detected chlorinated hydrocarbons. The chlorine was likely derived from the perchlorate salts, but the source of carbon is unknown.
Perchlorate (ClO4) salts form when chlorine interacts with oxygen. Two models have been proposed for their formation on Mars. In the atmospheric formation model, chlorine gas is oxidized to form perchloric acid. The perchloric acid is then dehydrated, forming perchlorate salts. In the rock catalyst model, chlorine-rich rocks are bombarded by UV radiation. The UV light releases chlorine and oxygen, which then combine on the rock surface to form perchlorate.
The first definitive evidence of perchlorate salts was found in 2008 by NASA’s Pheonix lander, which landed at a latitude of 68N. Until recently, scientists were unsure whether perchlorates were restricted to the polar regions, or could be found worldwide. Curiosity’s findings and reinterpretation of old data by Archer show that the latter is more likely.
Curiosity’s samples at Rocknest are consistent with soil samples taken by the Viking landers in 1975. Archer noted that Sojourner, Spirit and Opportunity each detected elemental chlorine with their instruments. While those missions were unable to tell where the chlorine came from, the concentrations of chlorine were to Rocknest. Additional data collected by Mars Odyssey’s Gamma Ray Spectrometer detected a worldwide distribution of elemental chlorine. Elemental chlorine does not necessarily mean that perchlorates are present. However, its abundance is a clue that a large amount of free chlorine is available to form the salt.
The presence of perchlorate salts is a double-edged sword for Martian exploration. On one hand, the salts might be a food source for any Martian life. On Earth, extremophile bacteria reduce perchlorates as an energy source. Likewise, any Earth-like organisms on Mars may be able to use the salts as an energy source.
The findings also open up a new avenue for studying the past climate of Mars. The presence of perchlorates may make good indicator molecules for the red planet’s past climate. Mars is thought to have oscillated between wet and dry periods in the geologic past. Perchlorate salts are extremely soluble in water and only accumulate during dry periods. If the sediment record contains perchlorates, there is a good chance the sediments were deposited during an arid period.
Unfortunately, perchlorates also limit our ability to detect organic molecules on Mars. When the salts are heated by the mass spectrometer, they decompose to release free oxygen. The oxygen rapidly reacts with and destroys any organic molecules present, possibly leading to false negatives. If perchlorate salts are common in older Martian sediments, Curiosity may not be able to detect many organic molecules, as the SAM instrument heats samples past the decomposition point of perchlorates.
The new findings also reveal an additional environmental hazard faced by future Mars explorers. Perchlorate salts are toxic to humans, and can be absorbed through the skin or inhaled. Perchlorate salts interfere with the production of thyroid hormones, and may ultimately lead to thyroid tumors. Some evidence also suggests that prolonged exposure to perchlorates may also lead to the development of anemia.
Ultimately, the detection of perchlorate has implications for future exploration efforts. It provides a key molecule for determining the past climate of Mars and a possible source of food for Martian life. However, the presence of perchlorates also makes the search for organic molecules much more complicated. More results regarding perchlorates will likely be forthcoming as Curiosity continues to dig around Gale Crater.
IMAGE: Rocknest as imaged by Curiosity. Credit: NASA/JPL-Caltech/MSSS