Initial Mapping of Mercury's Crustal Magnetic Anomalies: Relationship to the Caloris Impact Basin

Abstract:

78 low-altitude orbit passes of MESSENGER calibrated magnetometer data from August and September of 2014 have been applied to produce approximate maps of the crustal magnetic field covering latitudes of 50-80N and longitudes of 160-320E. Only anomalies with wavelengths < 215 km were mapped and amplitudes were adjusted for differences in spacecraft altitude using an equivalent source dipole technique. Maps of the radial field component show that the strongest large-scale anomalies are located in the western part of the mapped region just north and northeast of the 1550-km diameter Caloris impact basin centered at 164E, 30N. When adjusted to a common altitude of ~ 40 km, the strongest single anomaly (~170E, 60N; > 6 nT) lies over a smooth plains unit that extends north-northeastward from Caloris. A second anomaly (185E, 53N, > 5 nT) lies on the Odin Formation, interpreted as Caloris ejecta (e.g., Guest and Greeley, USGS, 1983). As previously reported by Johnson et al. (Science, 2015), a third anomaly (~ 212E, 61N, > 5 nT) also lies over a smooth plains unit, Suisse Planitia. Most smooth plains units on Mercury may have a volcanic origin (Denevi et al., JGR, 2013). However, as discussed by the latter authors, a subset of the smooth plains occur in an annulus around Caloris and could have an impact-related origin, involving fluidized basin ejecta deposition (Wilhelms, Icarus, 1976). A similar origin is widely accepted for the lunar Cayley smooth plains, which dominate the geology near the Apollo 16 landing site where the strongest surface magnetic fields were measured and which correlate best with orbital anomalies on the lunar near side (Halekas et al., JGR, 2001). Two of the remaining three anomalies (220E, 68N, > 4 nT; 234E, 77N, > 5 nT) lie over an older intermediate plains unit with an uncertain interpretation, possibly consisting of impact basin and crater ejecta as well as volcanic materials (Grolier and Boyce, USGS, 1984). In view of the proximity of the strongest anomalies to Caloris and their association with either Caloris ejecta or Cayley-like smooth plains units with a possible fluidized ejecta origin, it is proposed that at least some of the Mercurian anomaly sources consist of primary or secondary impact basin ejecta which acquired either thermoremanence or shock remanence or both shortly after deposition.