Astronomical Mystery #5: Revenge of the Mascons

The Moon. Astronomically, it’s quite nearby; close enough that it shows a disk even with the naked eye. Close enough that you can just about read a book by it at night, at least when it’s full. Close enough that its nature has been debated throughout history. Close enough that’s it’s even been physically-travelled to by actual, living human beings – the only celestial body for which this is true, aside from the Earth!

So, given all this closeness, you’d think we know the Moon pretty well, wouldn’t you?

Well, no, actually. We don’t. The Moon has its mysteries and some of them are rather baffling. In fact, the Moon’s mysteries are made all the more frustrating by the fact that it’s so nearby. One particular case is that of the ‘mascons’, or ‘mass concentrations’.

Basically, the Moon’s shape is broadly spherical, but its structure is lumpy. There is, in fact, some evidence that’s visible to the naked eye; the Moon’s so-called ‘seas’, the maria, are mainly located on the Earth-facing hemisphere. The maria, of course, aren’t bodies of water but are plains of frozen lava, darker in colour than the relatively-brighter, heavily-cratered lunar highlands. At some point billions of years in the past, the maria would have been true seas of a sort, albeit seas composed of red-hot molten rock. Those oceans of magma, however, quickly froze as they radiated their heat into space. These vast, volcanic outpourings were only a temporary phenomenon. The Moon is too small to have held much of its internal heat, and its mantle is solid and frozen. (Thanks to seismic data returned by remote landers and the Apollo missions, we actually have a relatively good knowledge of the Moon’s interior – although the keyword is ‘relatively’!)

Now, the side of the Moon that faces away from the Earth has far fewer maria. The ones that it does have are much smaller, and they seem to be confined mainly to the floors of big craters. It’s as if the far-side maria were actually produced when big impacts punched through the lunar crust, rather than through volcanic eruption. The reason for this appears to be partly that the Moon’s crust is thicker on the far side – it’s almost as if the entire Moon was actually several miles off-centre. In numbers, around 1% of the far side of the Moon is covered by maria, whereas 31.2% of the near side is covered in maria. The hemispheres have a clear, geological difference between them.

Yes, the Moon is weird.

But what, you may ask, does this have to do with the mascons? And what are they?

Well, in April 1972, Apollo 16 placed a small satellite in a close orbit around the Moon. This satellite, unromantically dubbed PSF-2, was intended to study the local environment around the Moon – charged particles, magnetic fields, that sort of thing. But the little satellite’s mission didn’t go to plan. The satellite’s orbit rapidly became unstable. It was originally meant to get no closer than 52 miles above the lunar surface. Instead, within two and a half weeks, PSF-2 was descending within six miles of the lunar surface. Six miles – that’s getting uncomfortably close to the height of some of the Moon’s bigger mountains! And all this instability developed in such a short space of time.

Needless to say, it didn’t take much longer for PSF-2 to crash headlong into the Moon’s rocky surface.

It turns out that the Moon’s crust contains some remarkable concentrations of matter. These dense lumps – the ‘mascons’ – also make the Moon’s gravitational field lumpy and uneven. The variance is surprisingly big, as much as half a percent. (For a gravitational field, this is a huge variation – for comparison, the Earth’s varies by less than one part in a thousand.) It would actually be easily-measurable to human astronauts, were any to land close to the relevant regions. Near the edge of a mascon, a pendulum or plumb-line would hang about a third of a degree off of vertical, with the tilt being oriented toward the mascon.

The biggest mascons are also located in the same area as the centres of the bigger lunar seas, so they are evidently related in some manner. That much at least, we do know. The positions of the mascons have been determined quite reliably, from measurements of satellite orbits. They have the effect of making low lunar orbits unstable, hence the unfortunate end of poor PSF-2. (The Apollo modules were sufficiently high so as not to be greatly-affected, thank heavens.) The mascons are presumably also much denser than normal lunar material, since they are massive enough to distort the Moon’s gravitational field. There is also a strong suspicion amongst planetary scientists that they were formed either through the volcanic upwellings of the maria themselves, or through large impact-events that ‘injected’ pockets of denser matter into the lunar crust, or some mix of both processes.

However, the truth is that we don’t really know.

Even within the above model, there are a lot of unknowns. For instance, there are demonstrably fewer mascons on the Moon’s far side, and those that exist are generally weaker too, despite the greater rate of impact on the far side. (The near face is shielded to an extent from impactors by the Earth.) Also, there do exist some mascons that aren’t obviously associated with maria. So, although we know of their existence, and we understand some of their properties, their full nature remains very much a mystery.

The Moon, it seems, guards her secrets closely.

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