Saturday, 20 August 2011

Is the Moon younger than we think?

Current theory suggests that the moon was formed early in the history of the solar system when a Mars sized planet (sometimes referred to as Theia) collided with the early earth, throwing a large chunk of the earth's crust into orbit about the planet. This then coalesced into a single body over a period of about 100 million years (0.1 billion years). The surface of the moon is thought to have solidified by about 4.5 billion years ago.

An initial collision with a Mars-sized body would have left the Earth with a ring of debris, that later coalesced to form the Moon.

This date was reached by examining the isotope ratios of minerals in volcanic rocks recovered from the moon by the NASA Apollo Missions between 1969 and 1972. This works roughly like this: Volcanic rocks form from the solidification of liquid magma. As the Magma cools, different minerals form at different temperatures, and each of these minerals has a known chemical composition at the time of formation. Some of these minerals contain unstable, radioactive isotopes, that decay to form other elements over time, elements that would not have been present when the minerals formed since they have different chemical properties to the original elements. Since radioactive isotopes decay at a known rate, it is possible to calculate the age of a mineral crystal by measuring the ratio of the original isotope to the decay product. The most familiar example is uranium, which decays to form lead over time, but the process also works with elements have both stable and unstable isotopes (hence isotope dating) as long as it is possible to measure the amount of the unstable isotope rather than the overall amount of the element (not hard if you have access to a mass spectrometer).

In this weeks edition of the journal Nature (17 August 2011), a paper by a team lead by Lars E. Borg of the Chemical Sciences Division at Lawrence Livermore National Laboratory details an examination of a piece of anorthosite (a type of volcanic rock) recovered by the Apollo 16 mission from the Descartes Highlands (roughly in the middle of the lunar disk when seen from Earth).

Apollo 16.

The (light) Lunar Highlands are generally considered to be the older part of the lunar surface; the (dark) Lunar Maria/Lowlands are thought to be later flood basalts. The piece of rock in examined by Borg et al. had large crystals, implying that it formed slowly deep beneath the surface of the moon (rock crystalizing at the surface would be exposed to the vacuum of space, causing it to solidify rapidly, forming small crystals), then was brought to the surface by some process, possibly the impact that formed the nearby Descartes Crater. This study was able to examine the presence of three product elements (lead, samarium and neodymium) giving a good correlation for the results.

Borg et al. derived an age of 4.3 billion years for the Descartes Highlands anorthosite, 200 million years (0.2 billion years) younger than the current estimated age of the surface of the moon. From this date Borg et al. deduce that the moon is either younger than current theories imply, or that it formed in a different way.

There are a couple of problems with this reasoning.

Current theory implies that the lunar highlands formed about 4.5 billion years ago, and that the Maria/Lowlands are younger flood basalts. This implies that while the surface of the moon had solidified 4.5 billion years ago, there was still liquid magma beneath the surface (where the anorthosite sample is thought to have formed). Current estimates for the age of the Lunar Maria rage from 3.5 to 4.2 billion years old; comfortably younger than the new date. A paper in the 24 July 2011 edition of Nature Geoscience by a team lead by Bradley L. Jolliff of the Department of Earth and Planetary Sciences at Washington University in St Louis reported the likely presence of shield volcanism in the highlands of the far side of the moon, implying that liquid magma may have been present beneath the surface even more recently.

Earlier this month the 4 August edition of the journal Nature contained a paper by Martin Jutzi and Eric Asphaug of the Earth and Planetary Science Department at the University of California Santa Cruz suggested a new theory of lunar formation. This theory suggested that the Lunar Maria formed earlier than the Highlands, which are the result of later accretion of material from a second Earth-orbiting satellite. This theory would allow for a younger date for the anorthosite minerals, although the large crystal sizes in the sample suggest it was formed bellow the surface of a reasonably large body, less likely on the smaller, unconsolidated object implied from Jutzi and Asphaug's theory.

Seen in this light the data from the Descartes Highland anorthosite tends to support the current theory of lunar development, rather than the new theory.

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