  We present spectroscopic observations of the dust- and gas-enshrouded,
polluted, single white dwarf star SDSSJ104341.53+085558.2 (hereafter
SDSSJ1043+0855). Hubble Space Telescope Cosmic Origins Spectrograph
far-ultraviolet spectra combined with deep Keck HIRES optical spectroscopy
reveal the elements C, O, Mg, Al, Si, P, S, Ca, Fe, and Ni and enable useful
limits for Sc, Ti, V, Cr, and Mn in the photosphere of SDSSJ1043+0855. From
this suite of elements we determine that the parent body being accreted by
SDSSJ1043+0855 is similar to the silicate Moon or the outer layers of Earth in
that it is rocky and iron-poor. Combining this with comparison to other heavily
polluted white dwarf stars, we are able to identify the material being accreted
by SDSSJ1043+0855 as likely to have come from the outermost layers of a
differentiated object. Furthermore, we present evidence that some polluted
white dwarfs (including SDSSJ1043+0855) allow us to examine the structure of
differentiated extrasolar rocky bodies. Enhanced levels of carbon in the body
polluting SDSSJ1043+0855 relative to the Earth-Moon system can be explained
with a model where a significant amount of the accreted rocky minerals took the
form of carbonates; specifically, through this model the accreted material
could be up to 9% calcium-carbonate by mass.
