Interior and
Geological Activity

Before the Apollo missions we knew almost nothing about the interior of the Moon. The Apollo missions left seismometers on the lunar surface that have allowed us to deduce the general features of the Lunar interior by studying the seismic waves generated by "moonquakes" and occasional meteor impacts.

The Structure of the Interior

Our present picture of the Moon's interior is that it has a crust about 65 km thick, a mantle about 1000 km thick, and a core that is about 500 km in radius. A limited amount of seismic data suggests that the outer core may be molten. There does appear to be some amount of differentiation, but not on the scale of that of the Earth. It has no magnetic field to speak of, but magnetization of Lunar rocks suggests that it may have had a larger one earlier in its history. Although there is a small amount of geological activity on the Moon, it is largely dead geologically (the energy associated with the Earth's seismic activity is about 10^14 times larger than that of the Moon). Most Lunar seismic activity appears to be triggered by tidal forces induced in the Moon by the Earth.

Geological History of the Moon

The weight of the evidence is that the Moon was active geologically in its early history, but the general evidence suggests that the Moon has been essentially dead geologically for more than 3 billion years. Based on that evidence, we believe the chronology of Lunar geology was as follows:
  1. The Moon was formed about 4.6 billion years ago; maybe hot or maybe cold. The surface was subjected continuously to an intense meteor bombardment associated with debris left over from the formation of the Solar System.

  2. By about 4.4 billion years ago the top 100 km was molten, from original heat of formation and from heat generated by the meteor bombardment.

  3. By 4.2 billion years ago the surface was solid again.

  4. As the intense meteor bombardment associated with debris left over from the formation of the Solar System continued, most of the craters that we now see on the surface of the Moon were formed by meteor impact.

  5. The fracturing and heating of the surface and subsurface by the meteor bombardment led to a period of intense volcanic activity in the period 3.8-3.1 billion years ago. Meanwhile, the meteor bombardment had tapered off because by this time much of the debris of the early Solar System had already been captured by the planets.

  6. The lava flows associated with the volcanism filled the low areas and many craters. These flows solidified to become the flat and dark maria, which have little cratering because most of the original craters were covered by lava flows and only a few meteors of significant size have struck the surface since the period of volcanic activity. The regions that were not covered by the lava flows are the present Highlands; thus, they are heavily cratered, and formed from different rocks than the seas.

  7. The volcanism stopped about 3.1 billion years ago: the Moon has been largely dead geologically since then except for the occasional meteor impact or small moonquake, and micro-meteorite erosion of the surface.
Thus, Lunar surface features, particularly in the Highlands, tend to be older than those of the Earth, which remains to this day a geologically active body.