Free Astronomy Magazine November-December 2014

MOON impact, given that such an occurrence would have left a circular −or elliptical, depending on the incidence angle− footprint, not only on the surface but also in the underlying layers. What appeared to be, with its 3200 km diam- eter (and about 4 mil- lion square km of sur- face area), one of the largest impact basins in the solar system is in- stead something else. According to Andrews- Hanna and Zuber’s team, those segments can only represent a system of deep frac- tures created as a re- sult of the cooling of the lunar crust and subsequently filled by molten material inject- ed by a giant magma plume active in that re- gion −material that be- sides filling the frac- tures also completely inundated all less ele- vated lands and spread well beyond the cur- rent ocean boundaries. The most eastern areas of the great structure were subsequently al- tered by the formation and overlapping of other maria −these in- deed created by falling asteroids. In less re- mote times (hundreds of millions, rather than billions of years), the ap- pearance of the ocean changed again with the formation of smal- ler, more “classic” shap- ed craters. In looking for evidence to their quent geological events: in the first instance (previous page top) the flow of lava runs along the northern edge of the Mare Frigoris; in the second (previous page bottom) we can see it extend- ing to the south- ern region of the Oceanus Procellarum. [NASA/Colorado School of Mines/ MIT/JPL/GSFC] R ight, a series of cylindrical projection maps (centred on the visible hemi- sphere of the Moon) which through different techniques and indices show the structure discov- ered thanks to the gravimetric data collected as part of the GRAIL mission. [Jeffrey C. Andrews- Hanna et al.]