Scattered Moonlight: Research by members of the Manchester Astronomical Society

Our friends over at the Manchester Astronomical Society have a new research paper out about using moonlight to understand the mineralogy and geological makeup of the surface of the Moon.

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Polarimetry of moonlight: a new method for determining the refractive index of the lunar regolith. By Andrew Fearnside, Philip Masding and Chris Hooker

In a paper recently published in the planetary science research journal Icarus , a team of three amateur astronomers from Manchester, and Oxford, present a new method for remotely measuring the refractive index of the lunar surface using polarized moonlight.

Umov’s Law correlates the degree of linear polarisation of scattered moonlight to the brightness of the scattering lunar surface. This Law is well known, but its interpretation has remained unclear ever since it was discovered at the turn of the 20th century. To tackle this issue, the team developed mathematical and computer models of light scattering by lunar dust grains. These models predicted a new way to interpret Umov’s Law which allows astronomers to new way to remotely measure the chemistry of the lunar surface regolith.

For many astronomers have believed that Umov’s Law can be used to measure variations in regolith grain size. This is now challenged. The new interpretation is it is not grain size variation that is captured by Umov’s Law, but that variation in the refractive index of lunar dust grains is revealed using Umov’s Law. This allows different rock minerals to be detected without the complicating influences of other regolith grain parameters, such as grain size variation and space weathering.

The team tested their predictions by conducting polarimetric experiments on terrestrial mineral grain samples and lunar regolith simulant materials. These experiments confirmed the theoretical predictions.  This confirmed that, using Umov’s Law, a simple formula can now be used to generate refractive index maps of the lunar surface from telescopic measurements.  An example is shown in Figure 1 for a highly volcanic region of the Moon known as the ‘Marius Hills’ next to Marius crater. This region is known to be peppered with volcanos.

Moonlight

Figure 1: The refractive index map of the Marius Hills and immediately surrounding mare. The local terrain relief is shown with vertical scale exaggerated and a refractive index map draped over it. This map covers an area of about 200 x 200 km. The colour side bar shows the refractive index (i.e. rock density/type) linear scale. Image: Andrew Fearnside

Refractive index is indicative of rock type. The dominant minerals on the Moon are plagioclase, pyroxene and olivine (find out more about lunar rocks and minerals here).

  • Plagioclase is the least dense and has the lowest refractive index of the three (about 1.5 to 1.6). It dominates the lunar highland regions.
  • Pyroxene and olivine are each more dense and have higher refractive indices (about 1.65 to 1.85). They dominate in the lunar mare regions.

The team found that the Marius volcanic region (and others) comprises material of unusually low refractive index (i.e. low density), especially where lunar volcanos are present near Marius crater (see Figure 1). These results throw light on the on-going debate over the geology and mineralogy of this area, and indicate that volcanic domes there may be formed from rocks rich in plagioclase phases.

Currently more polarimetry data is being collected to extend mapping coverage and allow a refinement of data and analysis. Interested readers are encouraged to contact Dr Andrew Fearnside for more information and involvement in this program is welcomed.

To contact the Manchester Astronomical Society:

Godlee Observatory
Sackville Street Building
The University of Manchester
Manchester
M1 3BU
United Kingdom
By Phone: 0161 306 4977
By Email: enquiry@manastro.org

For details of upcoming talks at the society see http://www.manastro.co.uk/

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Full paper citation:

Fearnside A.Philip Masding P. and Hooker C. (2016) Polarimetry of moonlight: A new method for determining the refractive index of the lunar regolith. Icarus Volume 268, April 2016, Pages 156–171 doi:10.1016/j.icarus.2015.11.038

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About Katherine Joy

Hello! I am Katherine Joy. I am part of the University of Manchester Isotope Geochemistry and Cosmochemistry group. More details about my research interests can be found at http://www.seaes.manchester.ac.uk/people/staff/profile/?ea=katherine.joy
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