Don’t call it b56: geology and geophysics of a Caloris-size impact basin and implications for Mercury’s global tectonics

The knowledge of Mercury's crater record has significantly evolved during the last decade thanks to the MESSENGER mission, which enabled the implementation of updated global crater catalogs by allowing the identification of a wealth of previously unknown impact structures. Particularly, recent re-examination of MESSENGER data led to the identification of tens of previously unreported large (D>150 km) impact basins. These large basins have not yet been studied in detail despite their significant geological and geophysical importance. Here, we present geological, topographical, structural, compositional, and geophysical investigations of one (maybe the largest) of these basins. The basin has not yet an International Astronomical Union (IAU) name but it has been reported as b56 or Lennon-Picasso basin in the published literature. The basin has an approximate diameter of more than 1500 kilometers and is centered at about 15S, 50E, located north-westward of Rembrandt basin. However, the most external structures associated to this basin extend fairly more than 1500-km form its center, suggesting a diameter closer if not higher than 2000 km, thus equivalent if not bigger than Caloris basin, known as the largest Mercury’s basin. Geology, tectonics, and composition of this basin have been investigated by using photointerpretation and remote sensing techniques applied to all the available data. Visible imagery from MDIS camera has been integrated with Digital Elevation Models (DEM), spectral data (color mosaics and data from MASCS), and gravity data from the Radio Science (RS) experiment onboard of MESSENGER into Geographic Information System (GIS) environment. DEM and slope maps highlight the annular terraced floor of the basin, suggesting that this may have formed as a peak ring or multiring structure. The basin is defined by several major lineaments (up to thousands of km long) consisting of a series of tectonic structures encompassing a broadly circular topographic low corresponding to the internal floor of the basin. This central region shows a gravity anomaly and a low crustal thickness with respect to the surrounding areas. The impact triggered volcanic activity since the floor presents the typical characteristics of other volcanic infilled Mercury’s basins: a smoother texture and lower albedo with respect to the surroundings regions, radial faults, wrinkle ridges and concentric circular lobate scarps, and spectral signatures from MASCS data. Most importantly, our results show that the tectonics associated to this basin, along as that of many other recently discovered large basins, might have had a regional and global significance, which has been so far overlooked. Indeed, these impact-related structures have been so far included in current estimates of Mercury’s contraction, most likely leading to its overestimation. After presenting the above mentioned lines of evidence we will discuss the implications of the tectonics of Mercury’s large basins for the planet global tectonics and contraction estimates.