惑星科学：月内部の非対称性を示す証拠

The detection of differences between the Moon’s interior in the near and far hemispheres is reported in Nature this week. The findings, based on gravitational field data from the NASA Gravity Recovery and Interior Laboratory (GRAIL) mission, indicate that temperature variations probably exist deep within the Moon. Such internal asymmetry could explain the contrast in the appearance of the Moon’s surface and the variation in volcanic activity between the near and far sides.  

The Moon’s nearside (that is, the side facing Earth) and farside have noticeable differences in geology, volcanism and crustal thickness. The nearside is dark and dominated by lava (indicating a higher concentration of volcanism), whereas the farside is more rugged. Some researchers have hypothesized that these differences may be explained by variation in the Moon’s internal structure, but observational evidence has been lacking.  

Ryan Park and colleagues analysed data from the NASA GRAIL mission to map the Moon’s gravitational response to its orbit around Earth, which can offer insights into the satellite’s internal structure. They found a 2–3% difference in the lunar mantle’s ability to deform between the nearside and farside. Park and colleagues then modelled the Moon’s structure and determined that these figures can be explained by a difference in mantle temperature of 100–200 kelvin between the two hemispheres, in which the nearside mantle is warmer than the farside. They hypothesize that this thermal difference could be sustained by radioactive decay of thorium and titanium within the Moon’s nearside, which could be a remnant of the volcanic activity that formed the nearside surface 3–4 billion years ago.  

The authors note that the methods used to probe the Moon’s interior could be used to measure differences in the structure of other planetary bodies such as Mars, Enceladus and Ganymede, specifically because they do not rely on a spacecraft landing on the surface.