The model-inferred abundances of perchlorate are far smaller than observations, however. If we assume the soil is mixed to an average depth of 2 m and the parent salt is Ca(ClO4)2 or Mg(ClO4)2, then we calculate concentrations of 4.3 × 10−8 wt. % and 4.0 × 10−10 wt. %, respectively. These concentrations are both many orders of magnitude lower than the observed abundance at the Phoenix landing site: 0.4–0.6 wt. % (Hecht et al., 2009). Also, the concentrations are far below what would be inferred from observations of perchlorate:nitrate ratios ∼1:60 measured in EETA79001 (Kounaves et al., 2014) and the Atacama perchlorate:nitrate of ∼1:1000. Distinct from the results for Earth where gas phase reactions were sufficient to reproduce data (Catling et al., 2010), we conclude that additional heterogeneous reactions must be present to account for the efficient formation of perchlorate on Mars, a hypothesis we discuss further in Section 7. But first we explore the sensitivity of deposition fluxes to model parameters.
perchlorate
observed abundance
0.4–0.6 wt. %
wt. %
soil
depth
2 m
m
Ca(ClO4)2
concentrations
4.3 × 10−8 wt. %
wt. %
Mg(ClO4)2
concentrations
4.0 × 10−10 wt. %
wt. %
perchlorate:nitrate
ratios
∼1:60
perchlorate:nitrate
ratios
∼1:1000