Detection of escaping magnesium in the upper atmosphere of WASP-76b

Near-ultraviolet (NUV) observations can uniquely probe low-density, high-altitude gas in ultra-hot Jupiters (UHJs) via strong resonance lines of metals. WASP-76b is a well-studied UHJ with prior detections of refractory metals and complex day-night chemistry, making it a prime target for atmospheric escape diagnostics in the NUV. We present HST/STIS E230M (2274 - 3119) transmission spectroscopy of WASP-76b. While broad- (200) and narrow-band (4) transmission spectra do not show spectral features, a velocity-resolved single line analysis reveals clear excess absorption in both components of the Mg II doublet (2796 and 2803, k and h lines). The Mg II signal is localized to the blue wing (Doppler velocities [-50, -10] km s-1) and is recovered with narrower 0.5 bins; the inferred absorption exceeds both the optical continuum radius and the Roche-lobe scale, suggesting a highly extended upper atmosphere. We estimate a mass-loss rate of about10 12.14, g,s-1 from ATES. The present NUV data do not improve constraints on lower-atmosphere properties, and a comparison with upper-atmosphere forward models disfavor scenarios with extreme Mg enrichment. Together with the near-solar metallicity inferred from optical+IR spectra, this suggests that the Mg II absorption can arise in an escaping upper atmosphere without requiring extreme Mg enrichment, qualitatively consistent with hydrodynamic escape. Ongoing HST programs expanding the sample of UHJs focused on NUV observations will enable comparative tests of composition and kinematics in escaping atmospheres.