We report the first detection in the atmosphere of Mars of the nightside O2(a1Δg) emission at 1.27 μm from limb observations of the OMEGA imaging spectrometer on board Mars Express (MEX). The emission, detected in three cases out of 40 observations, is due to recombination in a downwelling air parcel of O atoms produced by photodissociation of CO2 on the dayside in the upper atmosphere (O + O + M→ O2* + M), and not from ozone UV photodissociation, as is often seen on the dayside. Observed vertical profiles and total retrieved vertical intensities are compared with models. When detected, the emission is 10 times larger than previous predictions, at ∼240 kR. This can be explained in the frame of a general circulation model (GCM) of Mars. As predicted by the GCM, all positive observations were obtained at high latitudes, during the winter night. The model is validated, which simulates the large Hadley cell characterizing the meridional circulation, ascending from the summer pole and descending to the winter pole. This new emission is tracing uniquely a downward advection transport mechanism, and therefore its detailed study will provide important constraints on the overall aeronomy and dynamics of Mars. The impact on long-term stability of methane is examined. It is found that recycling through the mesosphere will not decrease significantly the overall lifetime of CH4 (∼300 years), because the descent of air is confined to high latitudes and winter seasons. These observations are demonstrating a new diagnosis of the aeronomy and atmospheric dynamics of Mars.