Bulls-Eye Crater or Volcanic Vent?

A circular depression (700 m diameter) sits atop of a circular mound 3.7 km in diameter. It is either a perfectly placed impact crater on the hill's summit, or a volcanic vent. LROC NAC M181173832LR [NASA/GSFC/Arizona State University].

The origin of many features within the mare remain ambiguous. Crater or vent? Flow or tectonic feature? Since the mare are generally older than 3 billion years, small features are worn down and rounded from meteorite bombardment. Thus their true mode of origin is not easy to discern.

Cone or hill?
Broad view of the hill in question (located at 28.64°N, 359.65°E) and the adjacent plains. LROC NAC M181173832LR, resolution reduced by a factor of 4 [NASA/GSFC/Arizona State University].

To the south are the mare units of Palus Putredinis, and to the north is the ejecta from the crater Autolycus. The fact that the crater is centered on the summit, the symmetry of the topography and the presence of a crater on the east side that appears to have dark ejecta suggest the feature might be a volcanic cone. The hill stands about 250 m above the surrounding plains and has a slope of about 8°. The slope is steeper than expected for a low basaltic shield, more consistent with a cinder cone.

Vent or crater?
Regional view of the hills. The arrow shows the feature in the NAC images above. LROC WAC M117488146ME [NASA/GSFC/Arizona State University].

The grouping of hills seen in the WAC mosaic was originally mapped as Fra Mauro Formation (basin ejecta) and crater rim material by Hackmann (1966). Looking around the area south of Autolycus, there are several hills that have craters near their summit or are symmetric in shape, both attributes that might suggest a volcanic origin. Elsewhere on the Moon, there are fields of volcanic vents that look similar to these. This may be a volcanic complex that has not yet been recognized.

Map the entire NAC mosaic at full resolution and try and decide for yourself: basin ejecta or volcanic?

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Published by Jeff Plescia on 25 July 2012