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We present the mid-infrared (5-12 $\mu$m) phase curve of GJ 367b observed bythe Mid-Infrared Instrument (MIRI) on the James Webb Space Telescope (JWST). GJ367b is a hot (T_eq=1370 K), extremely dense (10.2 +- 1.3 g/cm^3) sub-Earthorbiting a M dwarf on a 0.32 d orbit. We measure an eclipse depth of 79 +- 4ppm, a nightside planet-to-star flux ratio of 4 +- 8 ppm, and a relative phaseamplitude of 0.97 +- 0.10-- all fully consistent with a zero-albedo planet withno heat recirculation. Su ch a scenario is also consistent with the phaseoffset of 11 +- 5 degrees East to within 2.2$\sigma$. The emission spectrum islikewise consistent with a blackbody with no heat redistribution and a lowalbedo of A_B ~ 0.1, with the exception of one anomalous wavelength bin that weattribute to unexplained systematics. The emission spectrum puts fewconstraints on the surface composition, but rules out a CO2 atmosphere >~ 1bar, an outgassed atmosphere >~10 mbar (under heavily reducing conditions), oran outgassed atmosphere >~0.01 mbar (under heavily oxidizing conditions). Thelack of day-night heat recirculation implies that 1 bar atmospheres are ruledout for a wide range of compositions, while 0.1 bar atmospheres are consistentwith the data. Taken together with the fact that most of the dayside should bemolten, our JWST observations suggest the planet must have lost the vastmajority of its initial inventory of volatiles.

GJ 367b is a dark, hot, airless sub-Earth