  GJ 229B was the first T-dwarf to be discovered in 1995, and its spectrum has
been more thoroughly observed than most other brown dwarfs. Yet a full
spectroscopic analysis of its atmosphere has not been done with modern
techniques. This spectrum has several peculiar features, and recent dynamical
estimates of GJ 229B's mass and orbit have disagreed widely, both of which
warrant closer investigation. With a separation of tens of AU from its host
star, GJ 229B falls near the border of the planet and stellar population
formation regimes, so its atmosphere could provide clues to formation processes
for intermediate objects of this type. In an effort to resolve these questions,
we performed retrievals on published spectra of GJ 229B over a wide range of
wavelengths (0.5-5.1 ${\rm \mu m}$) using the open-source APOLLO code. Based on
these retrievals, we present a more precise mass estimate of $41.6\pm3.3\, M_J$
and an effective temperature estimate of $869_{-7}^{+5}$ K, which are more
consistent with evolutionary models for brown dwarfs and suggest an older age
for the system of $>$1.0 Gyr. We also present retrieved molecular abundances
for the atmosphere, including replicating the previously-observed high CO
abundance, and discuss their implications for the formation and evolution of
this object. This retrieval effort will give us insight into how to study other
brown dwarfs and directly-imaged planets, including those observed with JWST
and other next-generation telescopes.
