Thermal infrared and visual observations of a water ice lag in the Mars southern summer

We present thermal infrared and visual evidence for the existence of water ice lags in the early southern summer. The observed H₂O‐ice lags lay in and near a chasma and appears to survive between 6–8 sols past the sublimation of the CO₂. Possible sources of the H₂O that compose the lag are (1) atmospheric H₂O that is incorporated into the seasonal cap during condensation, (2) cold trapping of atmospheric water vapor onto the surface of the cap in the spring, or (3) a combination of the 2 processes where water is released from the sublimating cap only to be transported back over the cap edge and cold trapped. We refer to this later process as the “Houben” effect which may enrich the amount of water contained in the seasonal cap at 85°S by as much as a factor of 15. This phenomenon, which has already been identified for the northern retreating cap, may present an important water transport mechanism in the Southern Hemisphere.