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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 transitevents were used to measure the transmission spectra between 2280 and 3070Angstroms at a resolution of 30,000. Using HST data from 61 STIS visits, weshow that data from HST's Pointing Control System can be used to decorrelatethe instrument systematic errors (Jitter Decorrelation), which we used to fitthe WASP-121b light curves. The NUV spectrum show very strong absorptionfeatures, with the NUV white light curve found to be larger than the averageoptical and near-infrared value at 6-$\sigma$ confidence. We identify andspectrally resolve absorption from the Mg ii doublet in the planetary exosphereat a 5.9-$\sigma$ confidence level. The Mg ii doublet is observed to reachaltitudes of $R_{pl}/R_{star}=0.284\pm0.037$ for the 2796 Angstrom line and$0.242\pm0.0431$ in the 2804 Angstrom line, which exceeds the Roche lobe sizeas viewed in transit geometry ($R_{\rm eqRL}/R_{star}$ = 0.158). We also detectand resolve strong features of the Fe ii UV1 and UV2 multiplets, and observethe lines reaching altitudes of $R_{pl}/R_{star}\approx0.3$. At these highaltitudes, the atmospheric Mg ii and Fe ii gas is not gravitationally bound tothe planet, and these ionized species may be hydrodynamically escaping or couldbe magnetically confined. Refractory Mg and Fe atoms at high altitudes alsoindicates that these species are not trapped into condensate clouds at depth,which places constraints on the deep interior temperature.

The HST PanCET Program: Exospheric Mg II and Fe II in the Near-UV transmission spectrum of WASP-121b using Jitter Decorrelation