Vulcanism Shifted Moon’s Poles

I’ll be honest, I was all set to write a post about Craig Venter’s latest achievements in his endeavour to discover the absolute minimal requirements of a genome for life to still function. Then I realised that genetics is all I have written about in recent weeks, which isn’t terribly unreasonable given I’m a geneticist by day and studying genomics by night, but as Petrarch, the Father of Humanism, said: Sameness is the mother of disgust, variety the cure.

So it is that I turn my gaze upon the heavens, the moon to be exact. One of the puzzling things about the moon has long been that there isn’t nearly as much water up there as you might expect. Even Mercury, so close to the sun, has hundreds of times as much water as our parched moon. So where is it all? Was any there in the first place? If there was then where did it go and why? A paper published this week in Nature may have the answer.

It would be reasonable to suppose that what water is still there would be hidden up by the poles, the coldest and darkest areas as is the case here on earth. We already know that any water laying about on the surface was long ago blasted away and what remains would be concealed in the shadows of craters that never get exposed to the sun.

Researchers from the Planetary Science Institute in Tucson, Arizona, were looking at the south pole trying to find the deposits of ice expected. They found, though, that the largest deposit was about 6 degrees away from the pole. When they looked at the north pole data they found that the largest ice accumulation there was also shifted away from the pole by 6 degrees but in the opposite direction.

The clear inference from this is that the position of the poles has changed with time. This hypothesis would explain not just the non-polar ice deposits but the lack of water generally on the moon. Much of the water would have been blown away as the moon first settled down having been created a bit over 3 billion years ago. If the poles then shifted and presented a slightly different aspect of the moon to the force of the sun then this would have resulted in a second wave of dehydration.

To make the moon shift like that is no small thing and so the researchers set about trying to explain it. One theory which they dismissed was that of an asteroid impact. Though this could certainly make the moon move there aren’t any impact craters in the right sort of places that could explain the change satisfactorily. The theory they went for was one of change from within rather than being imposed by external agents, “From the direction and magnitude of the inferred reorientation, and from analysis of the moments of inertia of the moon, we hypothesise that the change in the spin axis was caused by a low-density thermal anomaly beneath the Procellarum region.” Put another way: volcanoes!

The Procellarum region is one of the many mares, or seas, that can be seen on the side of the moon that faces us. They are the smoother, flatter, darker areas and though Procellarum is the largest mare on the moon the one most of us has probably heard of is the Mare Tranquillitatis, or Sea of Tranquility. Given that Procellarum is the largest its actual name is Oceanus Procellarum, or Ocean of Storms.

Anyhoo, the mare of the moon were formed when it was young by intense volcanic activity and the researchers propose that it was the significant outpouring of liquid magma and its subsequent basalt formation that was capable of making the poles of the moon shift. The fact that this all happened early in the moon’s history is interesting because it means that what water remains has probably been sat there for a good 3 billion years or so. If we could sample this water and the volatiles it contains then this would give us a fantastic insight into the early composition of the solar system. If we ever wanted to establish a moon base its also likely we’d need to do so near to one of these icy deposits, so long as the astronauts don’t have a problem with drinking 3 billion year old ice.

The Oceanus Procellarum can be seen in the top left here running north-south. Image courtesy of Gregory Revera.

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