  New near- and far-ultraviolet (NUV and FUV) HST spectra of Mrk 231, the
nearest quasar known, are combined with ground-based optical spectra to study
the remarkable dichotomy between the FUV and NUV-optical spectral regions in
this object. The FUV emission-line features are faint, broad, and highly
blueshifted (up to ~7000 km/s), with no significant accompanying absorption. In
contrast, the profiles of the NUV absorption features resemble those of the
optical Na I D, He I, and Ca II H and K lines, exhibiting broad blue-shifted
troughs that overlap in velocity space with the FUV emission-line features and
indicate a dusty, high-density and patchy broad absorption line (BAL) screen
covering ~90% of the observed continuum source at a distance less than ~2 - 20
pc. The FUV continuum emission does not show the presence of any obvious
stellar features and is remarkably flat compared with the steeply declining NUV
continuum. The NUV (FUV) features and continuum emission have not varied
significantly over the past ~22 (3) years and are unresolved on scales ~40
(170) pc. These results favor an AGN origin for the NUV - FUV line and
continuum emission. The observed FUV line emission is produced in the
outflowing BAL cloud system, while the Balmer lines arise primarily from the
standard broad line region seen through the dusty BAL screen. Our data are
inconsistent with the recently proposed binary black hole model. We argue
instead that Mrk~231 is the nearest example of weak-lined "wind-dominated"
quasars with high Eddington ratios and geometrically thick ("slim") accretion
disks; these quasars are likely more common in the early universe.
