  The Kepler and TESS missions revealed a remarkable abundance of sub-Neptune
exoplanets. Despite this abundance, our understanding of the nature and
compositional diversity of sub-Neptunes remains limited, to a large part
because atmospheric studies via transmission spectroscopy almost exclusively
aimed for low-density sub-Neptunes and even those were often affected by
high-altitude clouds. The recent TESS discovery of the hot, dense TOI-824b
($2.93\,R_\oplus$ and $18.47\,M_\oplus$) opens a new window into sub-Neptune
science by enabling the study of a dense sub-Neptune via secondary eclipses.
Here, we present the detection of TOI-824b's hot day side via Spitzer secondary
eclipse observations in the $3.6$ and $4.5\,\mathrm{\mu m}$ channels, combined
with a reanalysis of its interior composition. The measured eclipse depths
(142$^{+57}_{-52}$ and 245$^{+75}_{-77}$ ppm) and brightness temperatures
(1463$^{+183}_{-196}$ and 1484$^{+180}_{-202}$ K) indicate a poor heat
redistribution ($f>$ 0.49) and a low Bond albedo (A$_{B}<$ 0.26). We conclude
that TOI-824b could be an "exposed Neptune mantle": a planet with a
Neptune-like water-rich interior that never accreted a hydrogen envelope or
that subsequently lost it. The hot day-side temperature is then naturally
explained by a high-metallicity envelope re-emitting the bulk of the incoming
radiation from the day side. TOI-824b's density is also consistent with a
massive rocky core that accreted up to 1% of hydrogen, but the observed eclipse
depths favor our high-metallicity GCM simulation to a solar-metallicity GCM
simulation with a likelihood ratio of 7:1. The new insights into TOI-824b's
nature suggest that the sub-Neptune population may be more diverse than
previously thought, with some of the dense hot sub-Neptunes potentially not
hosting a hydrogen-rich envelope as generally assumed for sub-Neptunes.
