Decoding episodes of past temperature spikes on the Martian surface using jarosite

The overwhelming presence of jarosite (KFe₃(SO₄)₂(OH)₆) on the Martian surface makes its chemical and isotopic signatures important because they carry a lot of information about the planet's past. In this study, using first principles density functional theory we have calculated: (i) the relative thermodynamic stability of jarosite in the presence of relevant dopants; and (ii) explored the possibility of suitably exploiting the thermal decomposition products of jarosite to deduce past Martian temperature conditions. Previous studies report that pure jarosite undergoes thermal decomposition under Martian conditions at 18 °C, which is well above the present-day average surface temperature of Mars. However, the presence of other elements in the jarosite matrix, namely, Na and Al is expected to influence this stability. Moreover, the planet has been known to have undergone several episodes of intense heating in the past, whereby the temperatures may have soared past the present day temperature conditions, potentially threatening jarosite stability. Previous estimates of the past temperatures on the Martian surface are found to be quite broad. To fine-tune this estimate and provide more precise temperature constraints, we have devised a thermometer based on the temperature-dependent distribution of Fe isotopes between the Fe-bearing decomposition products of jarosite. Our proposed thermometer has the potential to record past (post jarosite formation) temperature peaks on the Martian surface. Additionally, we propose that the time elapsed since then can be deduced using a relevant well-established chronometer, i.e., the ⁴⁰Ar/³⁹Ar dating method on the K-bearing end-product of jarosite.