  Brown dwarf spectra contain a wealth of information about their molecular
abundances, temperature structure, and gravity. We present a new data driven
retrieval approach, previously used in planetary atmosphere studies, to extract
the molecular abundances and temperature structure from brown dwarf spectra.
The feasibility of the approach is first demonstrated on a synthetic brown
dwarf spectrum. Given typical spectral resolutions, wavelength coverage, and
noise properties precisions of tens of percent can be obtained for the
molecular abundances and 10s-100s K on the temperature profile. The technique
is then applied to the well studied brown dwarf, Gl 570D. From this spectral
retrieval the spectroscopic radius is constrained to be 0.75 - 0.83
$R_{\mathrm{J}}$, $\log(g)$ to be 5.13 - 5.46 and $T_{\mathrm{eff}}$ to be
between 804 and 849 K. Estimates for the range of abundances and allowed
temperature profiles are also derived. The results from our retrieval approach
are in agreement with the self-consistent grid modeling results of Saumon et al
(2006). This new approach will allow us to address issues of compositional
differences between brown dwarfs and possibly their formation environments,
disequilibrium chemistry, missing physics in current grid modeling approaches
as well as a many other issues.
