Sky & Telescope - August 2023 - 52

AUGUST 2023 OBSERVING
Exploring the Solar System by Charles A. Wood
Mapping the Geologic Moon
T
New resources can enhance your lunar understanding.
he Moon was always considered an
astronomical object. After all, it's
located in the sky and is best observed
at night. But when, in 1962, President
John F. Kennedy decided that Americans
should go to the Moon by the end of
the decade, it then became a geologic
object. For astronauts to land on the
Moon and safely return, it was necessary
to know the nature and likely
condition of the lunar surface. This is
what geologists traditionally do - they
identify the composition of the terrain
as well as its origin and relation to the
surrounding materials.
To prepare for the Apollo landings,
the task of geological mapping was put
into the hands of Eugene Shoemaker,
Don Wilhelms, and their colleagues
at the United States Geological Survey
(USGS). They applied the fundamental
Lunar Geologic Timescale
Epoch
Billions of years 0
52 AUGUST 2023 * SKY & TELESCOPE
Copernican
(Present to 0.8)
1
Eratosthenian
(0.8 to 3.1)
2
3
Imbrian
(3.1 to 3.8)
terrestrial mapping concept of stratigraphy
(how the relative position of
strata relates to a terrain's age) to the
Moon to determine the relative ages
of surface materials. For example, the
impact crater Copernicus provides an
easy example of lunar stratigraphy -
the crater and its rays sit on top of older
materials. Using superposition relationships
as well as other methods to
determine the freshness of landforms,
USGS scientists mapped the geology of
the entire lunar nearside by 1971. They
were mostly right in determining that
maria are volcanic lava flows and bright,
highly cratered terrain is much older
and includes a great deal of fractured
material produced during the formation
of countless impact craters.
Amateur astronomers often are most
interested in individual landforms, such
as craters, mountains, domes, rilles, and
mare ridges. And some have adopted the
USGS geologic maps in order to better
understand the temporal relationship
of these landforms. The USGS system
divides the lunar surface into five stratigraphic
units, from youngest to oldest
(see below). These maps have been easy
to access since 2020, when the organization
compiled its 1970s lunar charts
into a Unified Global Geologic Map of
the Moon. This map was updated in a
variety of ways and, like its predecessors,
has become the most widely used
lunar geologic map.
Now there's another excellent
resource. Beginning with the success of
the Chang'e 1 spacecraft placed in orbit
in 2007, the Chinese Lunar Exploration
Program has grown to include
probes, rovers, sample returns, and
Nectarian
(3.8 to 3.9)
Pre-Nectarian
(3.9 to 4.5)
4
GEOLOGIC MAP OF THE MOON: JINZHU JI ET AL. / INSTITUTE
OF GEOCHEMISTRY / CHINESE ACADEMY OF SCIENCES / CC
BY 4.0; LUNAR GEOLOGIC TIMESCALE: TERRI DUBÉ / S&T
Sky & Telescope - August 2023

Table of Contents for the Digital Edition of Sky & Telescope - August 2023
