Free Astronomy Magazine November-December 2014

MOON non-as teroidal hypothesis, An- d r ew s - H a n n a , Zuber and col- leagues model- led the gravita- tional gradient of the entire re- gion by simulat- ing the presence of crystallized la- va in correspon- dence of the poly- gon's perimeter. The results obtai- ned through this procedure satis- factorily matched the gravity data recorded by the probes, confirm- ing that the frac- tures system de- marcating the maria of the western lunar hemisphere is consistent with a thermal stress produced by the differential cooling of a wide re- gion relative to its surrounding areas. Cou- pled to the thermal stress there was also a large scale magmatic activity, triggered by a heat flux hotter than the average lunar subsurface temperature. It is still not yet clear what mechanism could have activated the plume of magma that existed before the Oceanus Procellarum formed, such as to make it flood the frac- tured surface, but researchers favour two possible solutions: a primordial asteroid impact, whose traces were completely eras- ed by subsequent events, or, alternatively, an abnormal accumulation of heat deep inside our satellite, produced by the decay of considerable quantities of radioactive elements. It is clear at this point that not all large scale lunar volcanism can be related to asteroid im- pacts suffered by our satellite during its “in- fancy”. On the contrary, a sig- nificant part of that volcanism has different origins, linked to internal heat m e c h a n i s m s whose nature and space-time extent are not yet sufficiently known. n J eff Andrews- Hanna (picture above) had al- ready brilliantly exploited the po- tential of the gravitational gra- dient by discov- ering in 2008 the origin of a large impact basin on Mars. In the side animation, the last three orbits of the Ebb and Flow probes be- fore their pre- dicted crash into the Moon on 17 December 2012. [NASA]