Phoenix Update: Pondering Perchlorates
Since we last checked in on Phoenix, the team has had made remarkable progress in investigating the lander’s local environment. The team has:
– Finished the mission-success panorama
– Officially detected water ice in TEGA
– Investigated the bizzarely clumpy and sticky nature of the landing site’s soil
– Observed changes in the ice deposits under the lander
– Continued to monitor the summer polar weather
– Received a mission extension from NASA through Sept. 30
And much more! But the discovery that I’d like to focus on today involves potential detection of an interesting chemical compound in a few select teaspoons of martian soil: Perchlorate salts.
The internet has been going batty over the past week or so, first with rumors that the Phoenix team had discovered “something really interesting” and “provocative”, then all of this week with confusion over the announcement of perchlorates. While there’s tons of information (and non-information) floating around about the discovery, I haven’t found any in-depth looks at perchlorates. So here’s mine.
What is a perchlorate?
A perchlorate is made of a chlorine atom bonded to four oxygen atoms (ClO4), making an anion. An anion is the negative half of a salt molecule, like the Cl in NaCl (table salt). So a perchlorate doesn’t exist on its own as a solid substance, only attached to a cation, like sodium (Na) or ammonium (NH3). Perchlorate can exist on its own when the solid salt has been dissolved in a liquid, which causes the cations and anions to split apart, making a solution.
Perchlorate is an oxidizer, which means that it tends to react with other compounds. This is especially bad when combined with organics, and the presence of such oxidizers on Mars may have implications for past or present life on Mars (see below).
How did Phoenix detect perchlorates?
Phoenix has 2 main instruments to investigate soil chemistry. The first is the wet chemistry lab on the Microscopy, Electrochemistry, and Conductivity Analyzer (MECA). MECA dissolves a sample of the soil in a water-based solution and does various tests to figure out what ions and anions are present. MECA detected perchlorate ions in 2 soil samples.
The second soil chemistry instrument is TEGA, or the Thermal and Evolved Gas Analyzer. TEGA bakes soil samples to high temperatures, and analyzes the gases that are released along the way. TEGA has also analyzed 2 soil samples. Interestingly, while TEGA did detect oxygen in one sample, but not the other, it did not detect chlorine in either. In other words, TEGA could not positively confirm the detection of perchlorates.
DID Phoenix detect perchlorates?
At this point, the results are too inconclusive for the team to make a concrete statement about the presence or abundance of perchlorates in martian soils. So, the answer is: we don’t know. All we can say is that some of the results are consistent with perchlorates.
How did perchlorates get into the martian soil?
The easiest way that perchlorates may have gotten into the soil samples is via Phoenix itself. Perchlorates are a common industrial compound on Earth, and, most notably, ammonium perchlorate is often used as a component of solid rocket fuel. While Phoenix’s retro rockets that landed it on the surface used a fuel that probably did not contain perchlorates, the vehicles used to launch Phoenix from Earth may have. Another wrench to throw in this cog is that the soil samples were taken from below the surface, and probably were not affected by the landing. Perhaps the scoop or MECA itself was contaminated earlier? These are all possibilities the Phoenix team is considering.
Alternatively, the perchlorates may have been formed on Mars. In the lab, perchlorates can be created by evaporating the right kind of acid. However, perchlorate salts also occur naturally in Earth in extremely arid environments, like the Atacama desert in Chile. One study has shown that these salts can form by exposing typical chloride salts (think NaCl) to sunlight or ultraviolet light for long periods of time (months). This is a pretty appealing case for Mars, since certain salts that often form with chloride salts have been detected all over the north polar region (ref: my thesis!), and the surface of Mars receives a ton of UV through the thin atmosphere.
What does this mean for life?
If perchlorates or other oxidizers are abundant in the surface soil layer on Mars, this is not a good sign for finding signs of past or present life on Mars near the surface. Prolonged exposure to oxidizing agents (not to mention ultraviolet and cosmic rays) would destroy almost all organic molecules, leaving little to no trace or organics for us to pick up.
However, if the primary oxidizing agent is perchlorate, the news might not be quite so dire. Perchlorate is one of the slightly more benign oxidizers, since it tends to react more slowly than most. Also, if it was formed by breakdown of chloride salts at the surface, it probably is only present near the surface. Lower soil layers may not be as much at risk, and the deeper subsurface might be free of oxidants all together.
So, we’ll have to wait and see what the Phoenix team learns from their results and future tests. We’ll keep you updated.