About this image
Upper reaches of Nirgal Vallis. This valley network is one of the longest on Mars and this image captures the sapping morphology (alcoves, stubby tributaries) associated with this channel. However, it is not clear how this channel formed (ground water sapping vs rain/snowmelt surface runoff). The last geomorphic process to occur is the one best preserved but it should be noted that earlier processes may have been modified and or wiped out.
Please see the THEMIS Data Citation Note for details on crediting THEMIS images.