We can determine the surface composition of a star by studying its spectrograph. If a exoplanet transits the star, then there is a slight change to the spectrograph, which lets us determine the composition of the planet's atmosphere.
When the planet just become visible from behind it's star, then the star's light will:-
- come from the star (known composition)
- travel through the planets's atmosphere (known composition)
- reflect from the planet's surface (unknown composition)
- travel through the planet's atmosphere again (known composition)
Is it possible (either currently, or theoretically) to use this method to determine the bulk surface composition of an exoplanet? This might be very difficult to achieve as the light reflected from the planet will be swamped by direct light from the star.