Free Astronomy Magazine November-December 2014

MOON T he illustra- tions on the right show in dif- ferent ways the radioactive ele- ments and heat distribution in the region domi- nated by the Oceanus Procella- rum. The link with the recently discovered poly- gonal structure does not seem casual. [Jeffrey C. Andrews-Hanna et al.] T his photo- graph of the full Moon has been overlaid with the network of lava intrusions that in a distant past filled the fractures caused by the cooling of the lunar crust and by the activi- ty of a magma plume. It can be easily seen how this network af- fects a much larg- er area than just the Oceanus Pro- cellarum. [Koper- nik Observatory/ NASA/Colorado School of Mines/ MIT/JPL/GSFC] asteroid impacts, supposedly occurred in the first half billion years of the solar sys- tem’s history. At that time the lunar crust was still fragile and in the process of cool- ing, and below it there were vast reserves of magma ready to flood the huge craters created by the impacts. This would seem the most obvious reconstruction, and for most maria is certainly what really happen- ed, but not for all of them, or at least cer- tainly not for the Oceanus Procellarum, as shown by the results of a work by Jeffrey Andrews-Hanna (Colorado School of Mines), Maria Zuber (Mas- sachusetts Insti- tute of Technolo- gy) and a dozen other researchers, published in Na- ture in early Octo- ber. The team ex- amined the sur- face structure of the so-called ocean, using data collected during the GRAIL mission (Gravity Recovery and Interior Labo- ratory) that took place between Jan- uary and Decem- ber 2012, and that had as protago- nists two twin probes zooming on a polar orbit around the Moon, at an altitude of about 50 km and at a mutual dis- tance varying between 175 and 225 km. The purpose of the mission was to build a high resolution gravimetric map through precise and constant measurements of the distance between the two probes, expect- ed to vary according to the varying gravi- tational pull, which in turn varies with the masses distribution inside the Moon. In substance, from the slowdowns and ac- celerations degree of the individual probes overflying certain territories, the researchers were able to estimate the density of the materials hidden in the Moon’s depths and