The Wet Chemistry Laboratory (WCL) on the Phoenix Lander provided in situ measurements of the composition of soluble salts in the martian soil. Soluble sulfate was present at 1.3 ± 0.5 wt.% (Kounaves et al., 2010b), along with cations of sodium, potassium, calcium and magnesium. The most surprising result was the presence of perchlorate (ClO4-) at an inferred concentration in the soil of ∼0.5 wt.% (Hecht et al., 2009; Kounaves et al., 2010a). The dominance of Mg(ClO4)2 is consistent with simulations of evaporation and freezing at the Phoenix landing site (Marion et al., 2010); however, further analysis of data from the WCL suggests that Ca(ClO4)2 may be the dominant form of perchlorate (Kounaves et al., 2012). The perchlorate-sensitive electrode in the WCL experiment was also sensitive to nitrate, but it was 1000 times more sensitive to perchlorate. Thus, the methodology precluded the detection of nitrate because the signal would have required a mass of nitrate that exceeded the mass of the sample (Hecht et al., 2009). Recently, the MSL mission has also confirmed the presence of perchlorate using pyrolysis as part of the SAM experiment (Steininger et al., 2013). Specifically, pyrolysis showed release of chloromethane and O2 from heated soil samples at the Rocknest location, which is consistent with the decomposition of perchlorate (Sutter et al., 2013). If all of the evolved O2 was released from perchlorate, then the samples contained a comparable amount of perchlorate to the samples at the Phoenix landing site (Leshin et al., 2013). Furthermore, reanalysis of the Viking thermal volatilization experiments suggest ⩽1.6% perchlorate at both Viking 1 and Viking 2 landing sites (Navarro-Gonzalez et al., 2010); however, this has been subject to some debate (Biemann and Bada, 2011). Native perchlorate has also recently been measured in the martian meteorite EETA79001, albeit at a level <1 ppm by mass (Kounaves et al., 2014). Given the various locations of possible detection, perchlorate appears to be ubiquitous on the martian surface.
Soluble sulfate
1.3 ± 0.5 wt.%
wt.%
soil
perchlorate (ClO4-)
∼0.5 wt.%
wt.%
perchlorate-sensitive electrode
sensitive to nitrate
1000 times
times
Viking 1 and Viking 2 landing sites
perchlorate
⩽1.6%
%
martian meteorite EETA79001
Native perchlorate
<1 ppm by mass
ppm by mass