From: Amara Graps (amara@amara.com)
Date: Fri Feb 23 2001 - 16:26:25 MST
From: Ian Goddard <Ian@Goddard.net>
>A question about this here (hick-up) moonshine...
>One thing I notice as I review Appolo images is that the
>lunar surface appears to have a specular-reflection effect
>that causes some areas of lunar surface to reflect brighter
>than other areas relative to sun & viewer positions (surface-
>luminosity differential is also a function of ground slant).
http://www.hq.nasa.gov/office/pao/History/alsj/a11/a11.1093226_bm.jpg
That moon image is interesting ...!
I don't know exactly why that effect, but I would guess that several
things are occuring, some already discussed in this thread:
* The "opposition" effect is probably occuring (dominating).
* The moon is dusty (dust scatters light, different direction though),
* There are glassy particles on the surface (refracting light)
Hal pointed to some literature already about the "opposition
effect". Saturn's rings display this effect, for example. When the
illumination angle is zero, that is, when the viewing is from
precisely the same direction as the incoming sunlight, then we no
longer see the particles shadowing one another, and the object
"surges" in brightness. This is one way that scientists have
determined Saturn's rings thicknesses.
The following some background on the other light-scattering effects.
There is a large subfield in astronomy and planetary science that is
concerned with the optical properties of objects observed: i.e.
planetary surfaces, rings, comets, asteroids, etc. I'm not sure of
the correct name of the subfield, but usually when you hear one of
these scientists using words like: "forward-scattered",
"back-scattered", "Mie scattering", "phase-angles", "I/F", then they
are very likely working an "inverse problem": starting with an
image, and trying to infer the object's particle size, composition,
density. In addition, the optical properties of the object affect
the object's dynamics (if it is small), because there is a force
called: "radiation pressure force", which is when photons skrike a
particle (a dust grain, say), and transfers momentum to the grain,
affecting its motion.
When a photon (in the broader view: an "electromagnetic wave", it
does not necessarily have to be in visible wavelengths) hits an
object, then several things can happen: the light can be scattered
or absorbed. In the mathematical description of the scattering
process, an "optical efficiency factor Q" is determined, which
depends on the particle's refractive index, size, some other
factors, and on the wavelength of light.
To find the radiation pressure felt by a real particle in
interplanetary space, one uses Mie's solution to Maxwell's equations
(so-called "Mie theory") to compute the optical efficiency factors
corresponding to the different optical properties of the various
wavelength regions and then one integrates these over the energy
distributed in the solar spectrum.
Back to that dusty moon image.
I would be interested in seeing a similar scene in both
forward-scattered and back-scattered light. I'll bet that you would
see it brighter in forward-scattered light. That is, take an image
of the moon's horizon in both lighting angles, The forward-scattered
image would be brighter, because there are dusty particles
encompassing a layer lifted off of the Moon's surface. David
Criswell's levitating moon dust (see below).
"Small" and "large" bodies, are well distinguished by their
scattering properties, because dust tends to be highly
forward-scattering whereas the parent bodies are mostly
back-scattering. The Sun radiates nearly all of its energy in a
narrow wave band around 0.6 microns so that the transition from
geometric optics to Rayleigh scattering takes place in the
micrometer size range. Therefore, if you take an image of an object
in back-scattered light (Sun behind the camera), and you take it
again in forward-scattered light (Sun behind you), and you find that
it is much brighter in forward-scattered light, then the object
contains a large number of micrometer-sized particles. For example,
Jupiter's ring is about ten to twenty times brighter in
forward-scattered light than back-scattered light. Most images of
circumplanetary dust were taken when the spacecraft was in the
planet's shadow, with illumination phase angles of ~180 degrees.
On the glass beads on the lunar surface
I looked into Paul Spudis' chapter; "The Moon" in _The New Solar
System_ by K. Beatty et al, and Spudis gave a nice description of
tiny (~0.03 mm across) beads of glass, that were found at virtually
all of the Apollo landing sites. They're volcanic-like, in nature,
created by a process cooling quickly. The color comes from a high titanium
content with some coated with zinc, lead, sulfur and chlorine.
A photo in the book (pg. 131) shows some lovely orange glass
"moon beads". (I want some)
On "electrostatic dust transport"
(copying what I wrote last summer)
Electrostatic dust transport on the Moon was first envisioned by
T. Gold in 1955 and then later, after the Surveyor 5,6,7 spacecraft
missions, the topic was embraced again by D.R. Criswell and co-workers
to explain some features seen in the lunar images. In those images, a
western horizon glow was observed following the local sunset. The most
likely explanation for the glow in the images is forward-scattered
sunlight by a cloud of dust particles, less than about 10 microns in
radii, *migrating away* from the sunlit hemisphere of the moon. The
cloud is created from tiny particles, levitated above (a few to a few
tens of centimeters) the lunar surface, the levitation caused by
repulsive forces between the charged particles and the extremely low-
electrically- conductive lunar surface.
reference:
Criswell, D.R., chapter "Horizon-glow and the motion of lunar dust",
in _Photon and Particle Interactions with Surfaces in Space_,
ed. R.Grard, Reidel, 1973. and many more papers....
BTW, we had a discussion here on the list last September 2000 about
the Heiligenschein (thread had "Heiligenschein" in the subject), if
anyone wants to look into that too.
Amara
********************************************************************
Amara Graps email: amara@amara.com
Computational Physics vita: finger agraps@shell5.ba.best.com
Multiplex Answers URL: http://www.amara.com/
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"Whenever I see an adult on a bicycle, I do not despair for the
future of the human race." -- H. G. Wells
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