  Extrasolar super-Earths (1-10 M$_{\earth}$) are likely to exist with a wide
range of atmospheres. Some super-Earths may be able to retain massive
hydrogen-rich atmospheres. Others might never accumulate hydrogen or experience
significant escape of lightweight elements, resulting in atmospheres more like
those of the terrestrial planets in our Solar System. We examine how an
observer could differentiate between hydrogen-rich and hydrogen-poor
atmospheres by modeling super-Earth emission and transmission spectra, and we
find that discrimination is possible by observing the transmission spectrum
alone. An Earth-like atmosphere, composed of mostly heavy elements and
molecules, will have a very weak transmission signal due to its small
atmospheric scale height (since the scale height is inversely proportional to
molecular weight). On the other hand, a large hydrogen-rich atmosphere reveals
a relatively large transmission signal. The super Earth emission spectrum can
additionally contrain the atmospheric composition and temperature structure.
Super-Earths with massive hydrogen atmospheres will reveal strong spectral
features due to water, whereas those that have lost most of their hydrogen (and
have no liquid ocean) will be marked by CO$_2$ features and a lack of H$_2$O.
We apply our study specifically to the low-mass planet orbiting an M star, Gl
581c ($M sin i$ = 5 M$_{\earth}$), although our conclusions are relevant for
super-Earths in general. The ability to distinguish hydrogen-rich atmospheres
might be essential for interpreting mass and radius observations of planets in
the transition between rocky super-Earths and Neptune-like planets.
