  Aims. The goal of this investigation is to determine the origin and surface
composition of the asteroid (121514) 1999 UJ7, the only currently known L4
Martian Trojan asteroid. Methods. We have obtained visible reflectance spectra
and photometry of 1999 UJ7 and compared the spectroscopic results with the
spectra of a number of taxonomic classes and subclasses. A light curve was
obtained and analysed to determine the asteroid spin state. Results. The
visible spectrum of 1999 UJ7 exhibits a negative slope in the blue region and
the presence of a wide and deep absorption feature centred around ~0.65
microns. The overall morphology of the spectrum seems to suggest a C-complex
taxonomy. The photometric behaviour is fairly complex. The light curve shows a
primary period of 1.936 d, but this is derived using only a subset of the
photometric data. The asteroid may be in a non-principal axis rotational state,
but our observational coverage is insufficient to draw definitive conclusions.
Conclusions. Although the observed spectral absorption is wider and deeper,
this finding may be compatible with the 0.7 microns spectral feature exhibited
by some Ch-type asteroids and could possibly be interpreted as diagnostic of
the presence of hydrated minerals. The inferred composition of 1999 UJ7 as a
primitive object can be consistent with a volatile-rich object originally
accreted beyond the snow line of the solar system and subsequently evolved to
reach the inner regions of the solar system.
