By Thomas

A new report in Science [1] is dealing with a striking and so far mysterious feature of the lunar surface:

An asymmetric distribution of large lunar basins.

Large in this case means, that the basins diameter exceeds 200 km and that the formation of the basin thinned the crust to an extend that could be picked up by NASA`s GRAIL mission which measured the thickness of the lunar crust.

If you count these large basins on the nearside and the farside of the Moon you`ll find 12 of them on each side (excluding the very old and very big South Pole-Aitkin basin). And that is what you expect for a Moon that is getting hit randomly by impactors like asteroids and comets.

So where is the asymmetry? It shows up when you look at the size of these basins:

The basins on the nearside are (in average) bigger than those on the farside (Fig. 1). Eight basins on the nearside are bigger than 320 km in diameter whereas there is only one of this kind on the farside.

Fig. 1: Global map of crustal thickness on the Moon derived from GRAIL gravity data. Even without measuring you can clearly see that nearside basins are (in average) much bigger than the farside basins. Figure from [1].
Fig. 1: Global map of crustal thickness on the Moon derived from GRAIL gravity data. Even without measuring you can clearly see that nearside basins are (in average) much bigger than the farside basins. Figure from [1].
A possible explanation is that bigger (or faster) impactors favourably hit the nearside. But there is no reasonable mechanism which could explain this behaviour of the impactors. So it could only happen by chance. A chance that is lower than 2%. And that is not a very satisfactory scientific explanation.

As the impactor properties couldn`t provide an answer the team that published the paper had a look on the target properties, meaning the surface of the Moon. And once again, another striking feature of the Moon’s surface, the Procellarum KREEP Terrain (PKT) on the nearside, plays a major role. The origin of the PKT itself is still not a closed issue (I wrote about that here). However it already existed at the time when most of the big basins were formed. And one of the PKT`s main properties is that it is enriched in heat-producing elements (due to their radioactivity). The presence of these elements might have warmed the nearside crustal and upper-mantle rocks (or at least big parts of it) considerably compared to their farside counterparts.

What effect would that have on basin forming impact events?

In the modelling on which the report is based, the team found that in the initial stage of the basin forming impact there is little effect caused by the target temperature. This stage, which is mainly characterized by growing of the crater and ejection of material from within the growing crater, is mainly affected by the impactor properties (speed and mass).

However the target rock properties come into play in the next stage, when the formed crater is modified by subsequent processes. In this stage the mantle below the crater is uplifted (as there is no or much less crust above it, holding it down). On the warm nearside this uplift is much stronger and occurs over a wider area. The basic principle behind this is that warmer rocks can be more

Fig. 2: The real basin size distribution on the nearside (red solid line) respectively farside (blue solid line). The dotted red line shows the distribution as it would be if it weren`t for the described target effects. Figure from [1].
Fig. 2: The real basin size distribution on the nearside (red solid line) respectively farside (blue solid line). The dotted red line shows the distribution as it would be if it weren`t for the described target effects. Figure from [1].
easily elastically deformed than colder rocks. The uplifting mantle prevents the crater rim and surrounding crustal material from collapsing into the crater (“This is my spot!”). On the farside, where this isn`t the case, a lot of crustal material

collapses into the crater. Therefor those basins look much smaller for the GRAIL mission as it detects the thickness of the crust and it cannot distinguish between normal crustal material and crustal material that fell back into the basin. If one accounts for this target related effects on the crater formation the size distributions of the lunar basins look pretty similar on the near- and farside (Fig. 2).

This gives a neat explanation of the observed asymmetry of the large lunar impact basins and similar process might account for weird basin size variations on other planets, especially Mars. It also might mean that we have overestimated the impact flux recorded in the lunar surface as the production of the big nearside basins can be explained with smaller impactors than previously thought.

“P.S.: I came across the paper through a fairly new blog. The site adds (on a daily basis) links to recently published papers that are related to cosmochemistry. So if you are interested in that field it might be worthwile checking it out: http://cosmochemistry-papers.com/

[1] Miljkovic et al., 2013. Asymmetric Distribution of Lunar Impact Basins Caused by Variations in Target Properties. Science 342, 724-726.