"Pathological" Martian craters: Evidence for a transient obliteration event?

We have detected three unusual, low-relief circular features, 1.2 to 2.1 la n in diameter, in the northwest Noachis highlands, which may be craters tha t have undergone isostatic deformation. They may shed light on the existenc e, nature, and timing of suspected widespread Martian erosion/obliteration events, and offer clues to a type of Martian terrain softening. Ln the surr ounding area, we find an anomalous deficiency of craters in the 3-11 km dia meter range and evidence that larger, older craters have undergone relief s oftening and infill. We discuss three different hypotheses to explain these features, two of which involve Martian ice. This region may have undergone a transient event in which a near-surface permafrost layer (several hundre d meters deep) underwent partial melting or softening. This would allow rel axation of kilometer-scale craters and softening of larger craters. Crater data presented here suggest that this event happened some time in mid-Marti an history. Whether the event was regional or related to global-scale event s is uncertain, though it may represent a class of events that also happene d in other Martian areas.