We present HST near-ultraviolet ( NUV ) transits of the hot Jupiter WASP-121b , acquired as part of the PanCET program .
Time series spectra during two transit events were used to measure the transmission spectra between 2280 and 3070 Å at a resolution of 30,000 .
Using HST data from 61 STIS visits , we show that data from HST ’ s Pointing Control System can be used to decorrelate the instrument systematic errors ( Jitter Decorrelation ) , which we used to fit the WASP-121b light curves .
The NUV spectrum show very strong absorption features , with the NUV white light curve found to be larger than the average optical and near-infrared value at 6- \sigma confidence .
We identify and spectrally resolve absorption from the Mg ii doublet in the planetary exosphere at a 5.9- \sigma confidence level .
The Mg ii doublet is observed to reach altitudes of R _ { pl } / R _ { star } = 0.284 \pm 0.037 for the 2796 Å line and 0.242 \pm 0.0431 in the 2804 Å line , which exceeds the Roche lobe size as viewed in transit geometry ( R _ { eqRL } / R _ { star } = 0.158 ) .
We also detect and resolve strong features of the Fe ii UV1 and UV2 multiplets , and observe the lines reaching altitudes of R _ { pl } / R _ { star } \approx 0.3 .
At these high altitudes , the atmospheric Mg ii and Fe ii gas is not gravitationally bound to the planet , and these ionized species may be hydrodynamically escaping or could be magnetically confined .
Refractory Mg and Fe atoms at high altitudes also indicates that these species are not trapped into condensate clouds at depth , which places constraints on the deep interior temperature .