Far-infrared observations of SN 1987A from NASA's Kuiper Airborne Observatory (KAO) approximately 250 days after the event have yielded a detection of Fe II (25.99 micron emission at a level of 4.5 +/- 0.9 x 10^-18^ W cm^-2^, with a FWHM of 4000 +/- 600 km s^-1^. Less likely identifications of this feature are O IV (25.87 microns) and Fe V (25.92 microns). The minimum iron mass is found to be ~0.02 M_sun_, indicating that the emission originates in the heavy element mantle and not in the hydrogen-rich envelope. Since this mass is less than estimates based on near-infrared measurements or the optical light curve, the emission is probably optically thick. In this case, the flux measurement together with the observed line width suggests a temperature of 3500 +/- 1500 K for the mantle. The broad line width suggests that mixing of the ejected iron with lighter elements in overlying layers has occurred.
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Erickson et al. (1988) studied this question.