Giant Steps – Walking on the Moon



To the outside world science can seem to be full of unanswered questions. “More research needed” are the usual last words on a host of unsolved mysteries. So sometimes it’s great to have a few certainties, like for example, well, how the Moon was formed. Of course it was all down to a Mars-sized planet that hit the Earth a sort of slow glancing blow and then, hey presto! the Moon formed from the left over debris. Nice and easy, sorted! Well at least that’s how it seemed for a couple of decades or more. But then, as usual, doubts started to creep in. The Moon should have been made mainly from the material that came from the smaller impacting planet. And yet, as discussed in a previous entry, a wide range of evidence indicated that the Moon and Earth were essentially identical. All of a sudden the classic formulation of the so-called giant impact hypothesis was starting to look a bit wobbly.

Then along to the rescue came a couple of papers in the journal Science.

The classic giant impact hypothesis held that the angular momentum of the Earth-Moon system has remained unchanged since the lunar-forming impact. If true this meant that the event was relatively low energy and so there was only scope for limited mixing between the two colliding bodies. But Cuk and Stewart proposed a much more energetic scenario, leading to a system with high angular momentum. This fast spinning Earth-Moon system was later slowed down by an orbital resonance between the Sun and Moon. If angular momentum was no longer a constraint, much more energetic collisions would be possible and these would result in much greater mixing between the colliding bodies. Based on this possibility Robin Canup suggested a moon-forming scenario involving the collision between two almost identically-sized bodies.

So now our nice simple picture of a leisurely moon-forming collision was in the dustbin.

To paraphrase Fagin in Oliver, it was time to review the situation. Step in the UK’s preeminent scientific body, the Royal Society. At a two day meeting being held this week at their London HQ, and a later satellite meeting at Chicheley Hall, the whole moon-forming event is up for discussion. I was there yesterday and caught the first day. It was a lot of fun, a chance to see science in action.

The meeting kicked off with a review of the current state of play by one of the meeting’s organisers Professor David Stevenson. This was followed by a review of the role of giant impacts in terrestrial planet formation given by Professor Alessandro Morbidelli. One of the group of scientists who formulated the celebrated Nice model for early solar system evolution, Professor Morbidelli provided a fascinating glimpse into the world of dynamical modelling. He was open both about where things fit the data and where more constraints are required. The heart of the morning session was taken up by presentations of the new lunar-forming models by Dr Robin Canup and then subsequently Professor Sarah Stewart. It was clear from Professor Stewart’s talk that the angular momentum loss mechanism remains critical to these new high-energy collisions and that it is an area under active investigated. Dr William Ward was due to give a talk on the evolution of the disc of material from which the moon eventually formed. Unfortunately, he was unable to attend and instead his presentation was bravely presented by Dr Canup. This is a really complex and important subject and Dr Canup did a superb job in presenting this material.

After a “networking lunch” the meetings co-organiser Professor Alex Halliday discussed the isotopic differences and similarities between the Earth and Moon. He pointed out that the Moon is extremely volatile-depleted compared to the Earth, but indicated that the mechanism involved in this depletion remains poorly understood. Further isotopic constraints on the origin of the Moon were then given by Professor Nicolas Dauphas and after tea Professor Hugh O’Neill compared and contrasted the compositions of the Earth and Moon. Dr Richard Carlson then used age measurements of the lunar crust to try and place constraints on the timing of lunar formation. Finally, Professor Alberto Saal presented recent hydrogen isotope measurements of lunar volcanic glasses which indicate that the interior of the Moon is far from being dry as once thought and in fact may contain as much water as is present in various types of volcanic rocks erupted on the Earth today.

The origin of the Moon was thought to have been fairly well sorted out, but clearly, as these presentations demonstrate, this is not the case. A key outstanding question relates to the apparently volatile-poor composition of the Moon. If the Moon is part of the Earth, then how and when did this loss of volatiles take place? Or is this volatile depletion selective, effecting elements such as Na and K, but not water, as the apatite and volcanic glass data indicates. And then there is the question of whether the high angular momentum produced by energetic collisions really can be lost at a later stage.

Interestingly, quite a few of the participants seemed to feel that getting a sample back from Venus would help to illuminate the problem by providing geochemical and isotopic evidence from an additional inner solar system planet. While I’m not convinced it would provide much useful evidence to solve the origin of the Moon, it would be a great sample to have nonetheless. But that as they say is another story.

Blog Image Credit: Apollo 17 astronaut Eugene Cernan – the last man on the Moon! (image: NASA)

Richard Greenwood September 2013


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