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Life, Unbounded

Life, Unbounded

Discussion and news about planets, exoplanets, and astrobiology

The Moon has it all: Explosions, Water, and Clues to the Grand Tack

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Our high wilderness (Credit: T.A.Rector, I.P.Dell'Antonio/NOAO/AURA/NSF)

It's only 240,000 miles away, yet this high wilderness still surprises and delights with clues about the origins of the solar system, Earth's own water, and it even supplies the occasional brilliant explosion.

If you've been paying attention recently you'll have noticed that the Moon is getting a lot of press. One reason is that new investigations of the isotopic composition of volcanic lunar rocks, carbonaceous chondrite meteorites, and the Earth, are finding evidence that the mix of hydrogen and deuterium ('heavy hydrogen') is consistent between all three - indicating that they all share a common source of much of their water.

Until the past few years lunar science has tended to be of the mindset that the Moon is tremendously dry - not in the sense that it was missing great aquifers or subsurface lakes - but rather in the sense that the mineralogical content of the Moon included next to no traces of water embedded in it.

This has changed. The Moon is not awash in H2O, but it most certainly does contain water bound into minerals at levels ranging from parts per billion to several parts per million. It's very interesting, but also a bit of a challenge to understand. In part this is because the current picture of how the Moon formed involves a giant impact some 4.6 billion years ago between proto-Earth and another embryonic planetary object. In this scenario the Moon is simply re-coalesced from the pieces of that impact, forming from the disk of debris around the Earth following the collision.

Splat, crunch, you name it, this was a violent event (Credit: NASA)

It's possible that the energy of this collision might cause the loss of whatever water was around at that time - literally boiling it off to space and dissociating the molecules. This leads to a situation where Earth's water was eventually 'delivered' hundreds of millions of years later on, by incoming material from further out in the solar system. But that mechanism could not have put water into the Moon's deep interior because, once cooled, its rocky lithosphere acts like a thick sealant.

So, these new measurements of a common isotopic fingerprint between lunar water, terrestrial water, and the most primitive and ancient meteoritic rocks suggest a different picture. The Earth could have already had all of its water in place, and simply shared this with the Moon as both bodies emerged out of the orbital rubble - even with the extraordinary heat of collision and re-coagulation. It's also possible that much of the interior of the Moon is relatively unscathed leftover from its embryonic progenitor, with a water signature intact.

The isotopes also indicate where all the water might have originated from - a vast population of water carrying carbonaceous chondrites. But what business did these primitive rocks have in the inner solar system more than 4.6 billion years ago? This is where it gets even more interesting.

A rather bold theory has emerged in the last couple of years that tries to sort out the evolution of the terrestrial and giant planets in these very early stages - well before 4 billion years ago, and 20-80 million years before the final assembly of the Earth.

It's called the 'Grand Tack'. It's a little complicated, but in essence researchers asked how the giant planets, Jupiter in particular, might have behaved in the very, very young solar system. These large worlds should have formed before the smaller terrestrial worlds, siphoning up large amounts of primordial gas to make their great atmospheres. They find plausible arguments to suggest that Jupiter, interacting with the other giants and also with the mess of dust, rock, and remaining gas in the proto-planetary disk, could have migrated its orbit inwards to an astonishing 1.5 astronomical units - only 50% further from the Sun than the Earth is today.

Jupiter would then reverse course ('tack') and migrate back outwards due to gravitational interactions with Saturn. One consequence would be the destabilization of the orbits of a vast spread of cold, water rich, carbonaceous chondrites. These objects would have spanned from the distance of our present asteroid belt to far out in the young solar system - to where the giant planets are today. Some of these disturbed chunks would be cast inwards, intersecting and impacting with the still forming proto-Earth and providing it with its very specific isotopic flavor of water.

This is provocative stuff. If true it means our solar system actually has more in common with exoplanetary systems than we had perhaps thought - places where giant planets seemingly migrate their orbits with abandon.

It may not look very impressive, but that's a chunk of rock exploding as it impacts the Moon (NASA)

And today the Moon can still provide some real-time excitement when it meets up with bits of detritus from the early solar system. On March 17th an 88 pound rock slammed into the lunar surface and produced an explosive flash ten times brighter than any of the other 300 hundred lunar impacts recorded in the past 8 years (apart from the odd human-built spacecraft). It was caught in the act by an automated NASA telescope that's been monitoring the lunar surface for just such moments.

Probably about a foot across and moving at some 56,000 miles per hour this meteoroid exploded with the energy of approximately 5 tons of TNT, not huge, but enough to pop like a flashbulb. In fact it was bright enough that anyone who happened to be gazing at the Moon for that instant could have seen it with their unaided eyes.

The event coincided with a spate of meteors seen in Earth's northern skies, suggesting that we were also in the path of a cluster of material passing through the Earth-Moon system. It's a great reminder, once again, that the processes of planet formation and evolution are ongoing, even if a bit gentler than 4.5 billion years ago!

The views expressed are those of the author and are not necessarily those of Scientific American.

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