Sky & Telescope - April 2022 - 53

IMPACT DYNAMICS: S&T ILLUSTRATION, SOURCE: Z.
XIAO ET AL.; COPERNICUS SECONDARIES: NASA /
JSC / ASU
by high-velocity blocks of ejecta shot
out from near the center of the primary
crater. Because these projectiles impact
at high angles, the resulting secondaries
are nearly circular and difficult to
distinguish from small, primary craters.
Major discoveries about rays have
come from multispectral imaging.
By comparing the spectra of rays at
two near-infrared wavelengths, Jennifer
Grier and her colleagues at the
University of Arizona could identify
the maturity of a lunar ray. Maturity
describes the degree of space weathering
that affects the microscopic character
of minerals found in the ray. Over
millions of years, cosmic rays, solar
wind, and micrometeorites darken ray
material by creating iron grains and
glass-bonded soils on exposed surfaces.
Immature rays radiating from young
craters haven't been exposed to these
effects for long and still appear bright.
Space weathering reduces the brightness,
so all rays ultimately disappear.
The rate of fading depends on the
thickness of the ray. Small secondary
craters have shallow ray deposits, which
disappear faster than the thicker rays
produced by large cratering events.
Multispectral imaging provides a
second critical type of information
about rays. In 1985, Carle M. Pieters
and her colleagues at Brown University
imaged lunar rays at near-infrared
wavelengths to determine if some
are bright because they are immature
or because they contain intrinsically
bright lunar highland material.
The team discovered that the bright
ray from Copernicus that streaks north
across Mare Imbrium near Pytheas
contains abundant highlands material,
as well as some immature mare material
excavated by secondary cratering.
Lunar geologist B. Ray Hawke
and colleagues at the Pacific Regional
Planetary Data Center continued this
research, recognizing that some crater
rays are made of bright highlands material.
Other rays are the pulverized and
fractured target material now exposed
to space weathering, and some rays are
the product of both mechanisms. Thus,
there are compositional rays, immatuProjectile
Ejected
material
Melting and
vaporization
zone
Continuous
ejecta
deposit
Continuous
secondaries
Discontinuous
secondaries
pThe dynamics of crater excavation are illustrated above. At the point of impact, surface rocks
are melted and vaporized. Crustal materials are then pushed downward and away, flowing along
curved paths where some are ejected when reaching the surface. Boulders and debris near the
impact point are ejected first at high velocity and high angles, traveling farthest and forming discontinuous
secondaries and dunes. Forces pushing more downward excavate debris from greater
depths, ejecting it at lower angles and velocity. That debris is then deposited at the rim and as the
continuous ejecta deposit, leaving the deepest rocks at the top of the rim.
rity rays, and combination rays. But all
rays are really produced by secondary
craters and their deposits. Some craters
previously considered to be Copernican
in age have compositional rays and may
be considerably older.
The next time the Moon is full (or
nearly so), start with Tycho's long streaks
that cross Mare Serenitatis and
Mare Nectaris (at Rosse). Then jump
halfway across the Moon to the familiar
triangle of rays from Copernicus
(particularly the composition ray near
Pytheas), Kepler, and Aristarchus.
More challenging are the faint rays from
Langrenus near the Moon's eastern
limb. Look south for the 32-kilometerwide
Petavius B; its oblique-impact origin
is indicated by a ray-exclusion zone
to the crater's north. As a final challenge,
trace the bright ray from Glushko
Apollo 12 astronauts photographed this
oblique view showing many secondary craters
along the ray near Pytheas (middle) pointing
back towards Copernicus (top). South is up.
sk yand tele scope .o r g * APRIL 2022 53
at the western limb that passes northeast
of Krafft and Seleucus and continues
north of the Aristarchus Plateau - a
1,000-km-long ray!
¢Contributing Editor CHUCK WOOD observes
the lunar rays every time he turns
his telescope on the full Moon.
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Sky & Telescope - April 2022

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