  Strongly irradiated exoplanets develop extended atmospheres that can be
utilized to probe the deeper planet layers. This connection is particularly
useful in the study of small exoplanets, whose bulk atmospheres are challenging
to characterize directly. Here, we report the 3.4{\sigma} detection of C II
ions during a single transit of the super-Earth {\pi} Men c in front of its
Sun-like host star. The transit depth and Doppler velocities are consistent
with the ions filling the planet's Roche lobe and moving preferentially away
from the star, an indication that they are escaping the planet. We argue that
{\pi} Men c possesses a thick atmosphere with abundant heavy volatiles ($>=$
50{\%} by mass of atmosphere) but that needs not be carbon rich. Our reasoning
relies upon cumulative evidence from the reported C II detection, the
nondetection of H I atoms in a past transit, modeling of the planet's interior,
and the assumption that the atmosphere, having survived the most active phases
of its Sun-like host star, will survive another 0.2-2 Gyr. Depending on the
current mass of atmosphere, {\pi} Men c may still transition into a bare rocky
core. Our findings confirm the hypothesized compositional diversity of small
exoplanets, and represent a milestone toward understanding the planets'
formation and evolution paths through the investigation of their extended
atmospheres.
