We present first results of the analysis of fine details present in GOMOS co-added transmittances. In particular, mesospheric sodium (around 589 nm) and the oxygen green line (at 557 nm) have been unambiguously identified and the Na concentration profile can be obtained by standard inversion techniques. Two other unusual spectral signatures have been observed, both exhibiting a strong altitude dependence, in absorption (511.7 nm) and in emission (601.2 nm), the first one perhaps being related to a transition c1Σu - b1Σg+ involving an excited state of molecular oxygen. The general usefulness of averaging limb transmittances data is discussed.