Moon Formation: Zinc Makes Collision Theory More Plausible

Bash the Earth up and get a moon- that’s the theory that scientists have been looking at for a long time. However, the signs of violence are very subtle and very difficult to detect. It turns out that this cataclysmic event leaves behind a trail of only slight imbalance in the different varieties of zinc. Yes, boring – completely boring – to a layman, but to a scientist, it’s like adrenaline.

The Big Smashup?

Written down in Zinc

Planetary expert Frederic Moynier and his group at Washington University in St. Louis have claimed that they have discovered this tiny imbalance of zinc in the rocks of the Earth and that of the Moon. The theory goes like this: in the catastrophic event, under very hot conditions, heavier zinc atoms sunk in and condensed, while lighter ones escaped before condensation. These two isotopes (different atoms of the same element which differ only by their weights and not by their chemical properties) of zinc on the Moon will, thus, bear a ratio of their abundance different from what it is on Earth. In simpler language, say, the heavier zinc atoms outnumber the lighter atoms by a ratio 5:1 on Earth. On the moon, this ratio will be slightly (and not wildly) different – it can be say 6:1. Given that the other elemental abundances match very well, it is this tell-tale difference between the zinc ratios that should give us hints of its fiery origin.

Volatile situation

The moon rocks are chemically similar to those on Earth, except that the volatile elements are gone! This would be expected if a large body slammed into the Earth releasing a huge chunk of material that eventually formed the moon. Most of the Earth would retain whatever elements it had, but the Moon wouldn’t.

This loss of volatiles will also be accompanied by a process due to which heavy zinc atoms would outnumber the lighter zinc atoms! And this is precisely what the team found by analyzing the moon rocks.

In order to make sure that this depletion of the lighter zinc species is not a local aberration, i.e. located around only one point on the Moon’s surface, the team analysed rocks from different moon missions. These missions had picked up rocks from all over the moon and all the samples recorded the same heavy to light zinc ratio. This observation cannot be explained by any other theory and gives credence to the collision theory.

Sometimes, the smallest clues are the way to solve a great mystery. The jury is still undecided on this, however.

The technical study appears here:

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Debjyoti Bardhan

Is a science geek, currently pursuing some sort of a degree (called a PhD) in Physics at TIFR, Mumbai. An enthusiastic but useless amateur photographer, his most favourite activity is simply lazing around. He is interested in all things interesting and scientific.