Lunar dust is key to understanding changing faces of the Moon

As many people may have observed this week, during the night of April's Supermoon (Monday 26), the Moon often seems to change its appearance: it can look larger, brighter or redder, for example, due to its phases, its position in the solar system or smoke in Earth's atmosphere.

Another factor in its appearance is the size and shape of Moon dust particles, the small rock grains that cover the Moon's surface. Researchers at the National Institute of Standards and Technology (NIST) are now measuring tinier Moon dust particles than ever before, a step toward more precisely explaining the Moon's apparent colour and brightness. This in turn might help improve tracking of weather patterns and other phenomena by satellite cameras that use the Moon as a calibration source.

NIST researchers and collaborators have developed a complex method of measuring the exact three-dimensional shape of 25 particles of Moon dust collected during the Apollo 11 mission in 1969. The team includes researchers from the Air Force Research Laboratory, the Space Science Institute and the University of Missouri-Kansas City.

The researchers have been studying the Moon dust for several years, but with the advent of X-ray nano-computed tomography (XCT) they can now examine the shape of particles as small as 400 nanometres (billionths of a metre) in length.

The research team developed a method for both measuring and computationally analysing how the dust particle shapes scatter light. Follow-up studies will include many more particles and more clearly link their shape to light scattering. Researchers are especially interested in a feature called 'albedo', moonspeak for how much light or radiation it reflects.

"The procedure [for measuring the Moon's nano dust] is elaborate because it is hard to get a small particle by itself, but one needs to measure many particles for good statistics, since they are randomly distributed in size and shape," said Ed Garboczi, a NIST Fellow.

"Since they are so tiny and because they only come in powders, a single particle needs to be separated from all the others. They are too small to do that by hand, at least not in any quantity, so they must be carefully dispersed in a medium. The medium must also freeze their mechanical motion, in order to be able to get good XCT images.

"If there is any movement of the particles during the several hours of the XCT scan, then the images will be badly blurred and generally not usable. The final form of the sample must also be compatible with getting the X-ray source and camera close to the sample while it rotates, so a narrow, straight cylinder is best."

The procedure involved stirring the Apollo 11 material into epoxy, which was then dripped over the outside of a tiny straw to get a thin layer. Small pieces of this layer were then removed from the straw and mounted on dressmakers' pins, which were inserted into the XCT instrument.

The XCT machine generated X-ray images of the samples that were reconstructed by software into slices. NIST software stacked the slices into a 3D image and then converted it into a format that classified units of volume, or voxels, as either inside or outside the particles. The 3D particle shapes were identified computationally from these segmented images. The voxels making up each particle were saved in separate files that were forwarded to software for solving electromagnetic scattering problems in the visible to the infrared frequency range.

The results indicated that the colour of light absorbed by a Moon dust particle is highly sensitive to its shape and can be significantly different from that of spherical or ellipsoidal particles of the same size.

"This is our first look at the influence of actual shapes of lunar particles on light scattering and focuses on some fundamental particle properties," said co-author Jay Goguen, Space Science Institute. "The models developed here form the basis of future calculations that could model observations of the spectrum, brightness and polarisation of the Moon's surface and how those observed quantities change during the Moon's phases."

The authors are now studying a wider range of Moon dust shapes and sizes, including particles collected during the Apollo 14 mission in 1971. The Moon dust samples were loaned to NIST by Nasa's Curation and Analysis Planning Team for Extraterrestrial Materials program.

The researchers latest findings are published in the journal, IEEE Geoscience and Remote Sensing Letters.