Our analysis led to the detection of a rare subtype (nine particles; Methods , Extended Data Table 2 ) whose spectra show highly significant features of phosphate species: in addition to the defining mass spectral peaks of Na-salt-rich water ice grains at 23 and 63 u (ref. 3 ), there were peaks corresponding to molecular masses of 125, 165 and 187 u forming a pattern we found to be unique to sodium phosphates at high concentrations within these ice grains (Fig. 1 and Methods ).
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