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A small fraction of the lunar highlands is somewhat depleted in craters. This is largely due to blanketing by ejecta from huge impacts, though in some regions there might be ancient lava flows, over 4200 Ma old, since broken up by impacts and mixed with ejecta. Some features here and there on the Moon are consistent with more recent volcanism. For example, crater densities on parts of Mare Imbrium are so low that volcanism might have persisted here until about 2500 Ma ago. Even today there is the rare observation of possible gas or ash emission in tiny quantities. However, these transient lunar phenomena (TLPs) have many possible causes, e.g. meteor strikes and moonquakes - volcanism is not generally required. Two possible exceptions are an 8 second flash observed by an amateur astronomer in 1953, and a possible venting of lunar gases (on the near side) seen by Lunar Prospector. But overall, from about 4200 Ma ago, lunar volcanism was dominated by lava infill of mare basins, and this was almost entirely over by 3200 Ma.

The lunar cratering rate through lunar history

It was explained in Section 6.2.5 how the radiometric ages of lunar surfaces with different crater densities have been used to deduce the cratering rate throughout its history, with the outcome shown in Figure 6.14. The cratering rate was very high early on, and as a result most of the lunar surface became saturated. There has since been little resurfacing in what we see now as the highland regions. The cratering rate declined steeply as the supply of debris left over from planetary formation diminished, with 3900 Ma ago marking the end of the heavy bombardment, possibly marked by a narrow peak, as outlined in Section 6.2.5. The rate continued to decline until about 2000 Ma ago, since when it has not varied much.

7.1.8 Lunar Evolution

We can outline a plausible lunar evolution in broad terms as follows, on the assumption that just after its formation the Moon had a uniform composition broadly similar to that of the Earth's mantle today, though depleted in volatiles and enriched in refractories.

(1) A small lunar core formed by 25-30 Ma after the Moon formed (at about 4500 Ma), depleting the mantle in siderophile elements.

(2) Impact melting helped create a magma ocean several hundred kilometres deep. A thin skin solidified and gradually thickened, but not before a crust rich in anorthosite formed, overlying a mantle rich in olivine. The crust was in place by about 4400 Ma.

(3) By about 4000 Ma the magma ocean had solidified throughout its depth, with the whole surface nearly saturated with impact craters, including some large basins. Further impact basins formed, but with a rapidly declining impact rate even the youngest (Orientale) is about 3800 Ma old.

(4) By about 4000 Ma ago the heat from radioactive decay had raised temperatures to the point (about 2000 K) where there was an extensive asthenosphere. Isolated pockets of partial melt formed in the rising legs of convection cells, and the asthenosphere migrated deeper as radiogenic heating declined.

(5) The partial melt supplied basalt magma to infill some of the large impact basins, notably on the near side where the basins are at lower altitude. Because of slow, deep convection the magma did not reach the basins until hundreds of million years after they formed.

(6) As the rate of radioactive heat generation subsided, the interior cooled and the lithosphere thickened, to about 300 km by perhaps 3600 Ma ago, and it continued thickening until magma became only very rarely available from about 3200 Ma ago.

Note that the magma ocean might not have been planet-wide at any one time. The degree of isostatic compensation varies considerably across the Moon in a manner indicating that the crust and upper mantle might have become rigid at different times in different regions.

Question 7.1

If the lunar highlands had a peridotite composition, how would this modify our view of lunar evolution?

Question 7.2

(a) If the mare infill had been derived from the highlands through gradation, how would the composition of the infill differ from that observed?

(b) Were a large impact basin to be excavated in the Moon today, why is it likely that any subsequent infill would only be through gradation?

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