A nascent 15 km wrinkle ridge in Jenner crater. Wrinkle ridges help lunar scientists understand the stresses affecting a region. LROC NAC M115693540LE, image width is 1350 m [NASA/GSFC/Arizona State University].
Jenner is a 75 km crater located in Mare Australe at -42°, 96° E. This small ridge is a one of several wrinkle ridges within Jenner crater. Wrinkle ridges often form in mare units due to compressional stress. The floor of Jenner crater was completely covered by lava, and the weight of the lava may have resulted in a slight sinking of the crater floor, resulting in compression and buckling of the mare deposit. The mare was thick enough that only its central peak and a few terraces remain unburied! But how are scientists sure that this crater is filled with lava and not impact melt?
Context image of Jenner crater. Today's Featured Image is located in the white box. Image width is 100 km [NASA/GSFC/Arizona State University].
Our first clue are the wrinkle ridges that commonly form in mare, but this is not enough evidence. Other useful observations are the volume and texture of the crater floor. Tycho crater (86 km diameter) is an example of a crater with impact melt covering most of its floor. However, Tycho's floor has a rougher, more chaotic texture than Jenner. Jenner's floor is smooth, much more similar to a mare surface than to the impact melt deposits within Tycho. The smoking gun comes from mineralogical data. Clementine multispectral data of Jenner shows a mafic signature, indicative of a mare unit. Looking at these variables, Jenner is more similar to other mare flooded craters, such as Archimedes crater, than to Tycho crater.
Look at the crater floor in more detail in the full NAC!