Sky & Telescope - April 2022 - 52

APRIL 2022 OBSERVING
Exploring the Solar System by Charles A. Wood
 Unlike most other lunar features, crater rays
are best seen at or near full Moon.
period were believed to have visible rays.
The best-known rays are the nearly
Mare
Serenitatis
Pytheas
Seleucus
Krafft
Glushko
Kepler Copernicus
Langrenus
Petavius B
Rosse
Pitatus
Tycho
Aristarchus
Moon-girdling ones radiating from
Tycho. These are best seen at full Moon,
which led some to theorize that rays are
flour-like deposits of bright highlands
material with no topographic expression.
But following Baldwin, Shoemaker
recognized that embedded within these
bright rays were many small, elongated
craters. He interpreted these as secondary
craters, created by low-velocity
impacts of ejecta from the primary
crater and consistent with rays being
fragmented and pulverized rocks.
High-resolution images from NASA's
Crater Rays -
Mysterious No More
Understanding these bright features can add further
enjoyment to your lunar observing.
W
ell into the 20th century, the
first sentence of many articles
describing lunar rays stated that their
origins were enigmatic. This was due
to our poor understanding about these
bright, filamentary streaks radiating
from many young lunar craters. That
perception began to change with Ralph
Baldwin's transformative 1949 book,
The Face of the Moon. Baldwin was the
first to convincingly make the case
for the impact origin of lunar craters
by asteroids and comets (rather than
volcanic eruptions), with the rays being
material ejected during crater formation.
A decade later, legendary lunar
scientist Eugene Shoemaker published
the first ballistic analysis of lunar
impacts and the resulting emplacement
of ray-forming ejected rocks. Since then,
Apollo 12 astronauts collected samples
52 APRIL 2022 * SKY & TELESCOPE
of a ray from Copernicus, and scientists
have even studied rays on other worlds.
It's safe to say that rays are no longer
completely mysterious, though some of
their features aren't fully understood.
Both Baldwin and Shoemaker
noticed that rays are most conspicuous
from craters like Copernicus and
Tycho having youthful characteristics,
such as crisp rims and a lack of
superposed impact craters. They also
recognized that Eratosthenes and
other craters have fainter rays, while
most have no visible rays at all. This
implies that rays disappear over time, a
fact that Shoemaker and his colleagues
at the U.S. Geological Survey used to
identify the youngest epoch of lunar
history - the Copernican era spanning
from about 1.1 billion years ago to the
present day. Only craters formed in this
Lunar Orbiter and subsequent spacecraft
revealed that many secondary
craters occur in clusters and chains,
indicating that they were formed by
clumps of debris, rather than solid
boulders. Additionally, the horizontal
component of the ejecta's velocity
carried smaller particles, creating
downrange deposits of dunes and
herringbone patterns of fine, bright
debris. In most cases these secondaries
and other ray deposits are too small to
observe telescopically. However, when
examining the floor of Pitatus, you
can see Tycho ejecta as V-shaped white
splotches with clusters of secondary
craters on their uprange sides.
Careful studies show that material
excavated and ejected from the initial
impact event changes in thickness and
character with increasing distance
from the originating crater. The nearest
ejecta is thick, contributing to the
height of the crater rim itself. It then
rapidly decreases farther from the rim,
reaching the level of the original terrain
at about one crater radius away. This
inner annulus is called the continuous
ejecta deposit. Beyond that is an area of
abundant secondary craters in clusters
and chains - the zone of continuous
secondaries. Then, stretching hundreds
of kilometers from larger craters is a
zone of discontinuous secondaries, often
isolated craters rather than clusters of
secondary impacts. These are formed
SEAN WALKER / S&T
Sky & Telescope - April 2022

Table of Contents for the Digital Edition of Sky & Telescope - April 2022
