52 AUGUST 2019 • SKY & TEL-
Unruly Crater Rays
These long, bright streaks crisscross the lunar surface yet defy easy explanation.
C
rater rays are among the most
prominent, yet least studied, lunar
surface features. That’s due to their
unruliness. Ray lengths, widths, shapes,
distributions, and brightnesses vary
even for a single crater. The best-known
ray system radiates from Tycho, an
83-km-wide crater violently excavated
into the lunar highlands about 85 mil-
lion years ago. Like splashes of brightfl our, rays drape over highlands, maria,
mountains, and craters — so they must
have fallen out of the sky and must be
relatively young. Their great lengths,
averaging about six times the diameter
of the source crater, imply that signifi -
cant energy was required to emplace
them over such great distances.
As part of his investigation of the
physics of impact crater formation,SEANW
ALKE
R^ /S&TAUGUST 2019 OBSERVING
Exploring the Moon by Charles Wood
the pioneering lunar geologist Eugene
Shoemaker observed that rays them-
selves often contain numerous small
craters, often elongated in the same
direction as the ray. Shoemaker deduced
that rays are debris ejected during a
crater’s violent formation. The ejecta
followed ballistic trajectories at low
angles and relatively low velocity. Large
chunks account for the weak secondary
craters within the rays. You can easily
see secondary craters in the ray from
Copernicus that extends north from
the Carpathian Mountains (Montes
Carpatus) to east of Pytheas.
Rays are bright because of two fac-
tors. Rocks on the lunar surface darken
over time due to space weathering from
cosmic rays, the solar wind, and other
microscopic erosional processes; an
impact kicks up deeper, undarkened
material. Secondly, impacts in the lunar
highlands, like the one that formed
Tycho, eject anorthositic rocks rich in
light-hued aluminum compounds but
defi cient in darker iron and magne-
sium. So highland ejecta tend to make
brighter rays.
Over hundreds of millions of years,
rays slowly fade due to space weathering.
Rays made of mare lavas disappear in
about 750 million years, whereas those
that contain bright anorthosites remain
visible for about a billion years. So rays
are transitory features visible for a fi nite
period. Since every lunar crater formed
with bright rays, and in the Moon’s fi rst
half-billion years cratering was much
more intense than in later epochs,
bright rays must have crisscrossed the
Moon like psychedelic zebra stripes.
Rays are not single lines of bright-
ness but consist of a series of bright,tLong, meandering
crater rays lying atop
the lunar surface are
more easily seen
in this inverted
image.52 AUGUST 2019 • SKY & TELESCOPE
ThalesProclusPytheasPitatusTychoLongomontanusCopernicusKepler