Irregular Rimless Depression - Candidate for an Explosive Volcanic Vent on Mercury (Natural Colors; credits: Lunexit)

This Region of high reflectance was just barely seen on the limb during MESSENGER's 2nd flyby, but without enough detail to characterize it as anything other than a bright spot.
A more favorable viewing angle reveals this bright spot to be an irregular rimless depression approx. 30 Km across surrounded by highly reflective material. Its features are distinctly different from those of impact craters and, though its origin remains ambiguous, it is suspected to be volcanic, possibly the site of an explosive Volcanic Vent. The high-reflectance halo surrounding this enigmatic feature is distinct in color and may represent a pyroclastic deposit greater than 150 Km in diameter.

Date Acquired: September 29, 2009
Instrument: Narrow Angle Camera (NAC) of the Mercury Dual Imaging System (MDIS)
Scale: The irregular rimless depression in the center of the image is approx. 30 Km (about 19 miles) across