This spectra demostrates that a rock (represented as McKittrick) sampled near Opportunity's landing site at Meridiani Planum has higher salt concentrations then a nearby soil patch (Tarmac). This spectra was taken with an alpha particle X-ray spectrometer which was able to assess the elemental composition of these two samples. What is important about this spectra is that it demonstrates "element fractionation" which usually occurs when watery brine slowly evaporates and the salt compounds fall out at different rates.