The Lunar Highlands: Old Crust, New Ideas *
Mankind first set foot on the Moon in the 1960’s and 70’s during the Apollo missions. The ideas that followed, as a result of rocks returned from these missions, have formed that basis of how we think early planetary crusts formed. The Moon is a uniquely place to study early crusts as such geological records are no longer accessible on Earth.
The classic hypothesis for lunar crustal formation is known as the ‘Lunar Magma Ocean’ hypothesis. This theory argues that as the moon crystallised, the dense minerals (such as olivine and pyroxene) sank forming the lunar mantle, whereas the light minerals (such as plagioclase) floated, forming the characteristic white-coloured, high albedo, crust we see at the surface of the Moon today.
Methods for analysing the chemistry of rocks has improved a lot over the last few decades; furthermore, the collection of meteorites thought to come from the Moon has expanded greatly, adding ~90 kg of rock to the ~382 kg of rock brought back by the Apollo missions (you can find out lots of great information about lunar meteorites here and through NASA here). Together this has resulted in scientists re-investigating the classic view of a one-stage plagioclase flotation model. For instance, within the lunar meteorite collection some samples are thought to have been launched from the Moon’s farside. Some of these rocks have been reported to be chemically different from their nearside equivalents. This has led some scientist to suggest that the magma which crystallised the early crust may have been compositionally different between the near- and farside. Additionally, precise dating of plagioclase-rich rocks using radiometric chronometers have shown that not all of the early-crustal material crystalized at the same time. This has led some scientists to propose that serial plagioclase-flotation events may have occurred during the Moon’s early history.
Such continuing debate among scientists illustrates how important the Apollo sample collection continues to be, particularly for placing the ever expanding lunar meteorite collecting in context. Furthermore, understanding the complexity of lunar crustal formation will ultimately help understand how our own planet formed.
* article written by John Pernet-Fisher
Full Citation: Pernet-Fisher, J. F., & Joy, K. H. (2016). The lunar highlands: old crust, new ideas. Astronomy & Geophysics, 57, 1-26. doi: 10.1093/astrogeo/atw039
Publisher’s website: http://astrogeo.oxfordjournals.org/content/57/1/1.26.short?rss=1
University of Manchester holdings: http://www.manchester.ac.uk/escholar/uk-ac-man-scw:296452 (download open access copy of the article)
Further Lunar Sample Resources
NASA Apollo curation labs http://curator.jsc.nasa.gov/lunar/index.cfm
LPI Lunar Sample resources http://www.lpi.usra.edu/lunar/samples/
Virtual Microscope http://www.virtualmicroscope.org/content/moon-rocks
Lunar Meteorite List http://meteorites.wustl.edu/lunar/moon_meteorites_list_alumina.htm
Articles about lunar science http://www.psrd.hawaii.edu/Archive/Archive-Moon.html