Like a chameleon of the night sky, the Moon often changes its appearance. It
might look larger, brighter or redder, for example, due to its phases, its
position in the solar system or smoke in Earth's atmosphere. (It is not made
of green cheese, however.)
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 color 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.
These researchers have been studying moon dust for several years. But as
described in a new journal paper, they now have X-ray nano computed
tomography (XCT), which 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 for both measuring and computationally
analyzing 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 recipe for measuring the Moon's nano dust is complicated. First you need
to mix it with something, as if making an omelet, and then turn it on a
stick for hours like a rotisserie chicken. Straws and dressmakers' pins are
involved too.
"The procedure 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," NIST Fellow Ed Garboczi
said.
"Since they are so tiny and because they only come in powders, a single
particle needs to be separated from all the others," Garboczi continued.
"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 color 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. That
doesn't mean too much to the researchers -- yet.
"This is our first look at the influence of actual shapes of lunar particles
on light scattering and focuses on some fundamental particle properties,"
co-author Jay Goguen of the Space Science Institute said. "The models
developed here form the basis of future calculations that could model
observations of the spectrum, brightness and polarization 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.
Reference:
Somen Baidya, Mikolas Melius, Ahmed M. Hassan, Andrew Sharits, Ann N.
Chiaramonti, Thomas Lafarge, Jay D. Goguen, Edward J. Garboczi. Optical
Scattering Characteristics of 3-D Lunar Regolith Particles Measured Using
X-Ray Nano Computed Tomography. IEEE Geoscience and Remote Sensing Letters,
2021; 1 DOI:
10.1109/LGRS.2021.3073344
Tags:
Space & Astrophysics