Like a chameleon in the night sky, the Moon often changes its appearance. It may appear larger, brighter, or redder, for example, due to its phases, position in the solar system, or smoke in Earth’s atmosphere. (However, it’s not made from green cheese.)
Another factor in its appearance is the size and shape of moon dust particles, the small grains of rock that cover the moon’s surface. Researchers at the National Institute of Standards and Technology (NIST) are now measuring particles of moon dust smaller than ever before, a step towards a more accurate explanation of the Moon’s apparent color and brightness. This in turn could help improve the tracking of weather patterns and other phenomena by satellite cameras that use the Moon as a source of calibration.
NIST researchers and collaborators have developed a complex method of measuring the exact three-dimensional shape of 25 moon dust particles collected during the Apollo 11 mission in 1969. The team includes researchers from the Air Force Research Laboratory, Space Science Institute and University. of Missouri-Kansas City.
These researchers have been studying moon dust for several years. But as described in a new journal, they now have nan X-ray tomography (XCT), which has allowed them to examine the shape of particles as small as 400 nanometers (billionths of a meter) in length.
The research team developed a method to measure and analyze by computer how dust particle shapes scatter light. Follow-up studies will include many more particles and more clearly relate their shape to light scattering. Researchers are particularly interested in a function called “albedo”, the language of the moon for the amount of light or radiation it reflects.
The recipe for measuring the moon’s nanopust is complicated. You have to mix it with something first, like making an omelet, and then light it on a stick for hours like a roast chicken. Straws and seamstress pins are also involved.
“The procedure is elaborate because it is difficult to get a small particle on its own, but you have to measure many particles for good statistics, because they are randomly distributed in size and shape,” said Ed Garboczi, member of NIST.
“Since they are so small and only come as powders, only one particle needs to be separated from all the others,” Garboczi continued. “They’re too small to do it by hand, at least not in quantity, so they have to be carefully scattered in a rack. The media must also freeze their mechanical movement, in order to be able to get good XCT images. is particle movement during the several hours of the XCT scan, then the images will be very blurry and generally unusable. The final shape of the sample should also be compatible with the proximity of the x-ray source and the camera. while it is spinning, so it is better to use a narrow and straight cylinder. “
The procedure involved stirring the Apollo 11 material in epoxy, which was then poured over the outside of a tiny straw to form a thin layer. Small pieces of this layer were then removed from the straw and mounted on seamstress pins, which were inserted into the XCT instrument.
The XCT machine generated X-ray images of the samples which were reconstructed by software in slices. NIST software stacked the slices into a 3D image, then converted it to a format that categorized units of volume, or voxels, such as inside or outside the particles. The 3D particle shapes were identified by computer from these segmented images. The voxels constituting each particle were recorded in separate files which were transmitted to software to solve the problems of electromagnetic scattering from the visible to the infrared.
The results indicate that the color of light absorbed by a moon dust particle is very sensitive to its shape and can be significantly different from that of spherical or ellipsoidal particles of the same size. That doesn’t mean too much to researchers – yet.
“This is our first look at the influence of the actual shapes of lunar particles on the scattering of light and focuses on some fundamental properties of the particles,” said co-author Jay Goguen of the Space Science Institute. “The models developed here form the basis of future calculations that could model observations of the spectrum, luminosity and polarization of the moon’s surface and how these observed quantities change during the phases of the moon.”
The authors are now studying a wider range of shapes and sizes of moon dust, including particles collected during the Apollo 14 mission in 1971. The moon dust samples were on loan to NIST by the planning team at NASA’s conservation and analysis for the extraterrestrial materials program.