Sky & Telescope - June 2024 - 52

JUNE 2024 OBSERVING
Exploring the Solar System by Charles A. Wood
Carpenter
60°
Ch'5
Ap15
Ap17
L24
Copernicus
Cavalerius
Apex
Ap12
Bullialdus
Stevinus
Hainzel A
Tycho
-60°
-30°
Eratosthenes
Ap11
Ap14
Ap16
L16
Antapex
0°
0.7
0.8
0.9
1
30°
1.1
1.2
Craters Younger Than We Thought
A
New crater counts help to revise our understanding of lunar chronology.
mong the most critical data
required to understand lunar history
are the approximate times that
major lunar features formed. The most
accurate ages come from radiometric
analyses of lunar samples. Unfortunately,
samples from only 11 sites have
been returned to Earth by the lunar
programs of the U.S., the Soviet Union,
and China.
For the rest of the Moon, the standard
method used to estimate crater ages and
other formations is to count the number
of impact craters in an area of interest.
This technique assumes that craters
form all the time, and therefore older
surfaces have more craters and younger
regions have fewer. Impact craters have
formed throughout the last 4.5 billion
years, though not at a constant rate.
To determine how the crater production
rate (CPR) changed over time, scientists
count the number of impact craters
with diameters larger than 1 kilometer
that are present per square km. This
is called the N(1) value. For the areas
where lunar samples were returned to
Earth, their absolute ages were measured
52 JUNE 2024 * SKY & TELESCOPE
in laboratories. A graph of N(1) and age
data shows how the CPR decreased over
lunar history, with a very high cratering
rate about 4 billion years ago and a much
lower, near-constant rate for roughly the
last 3 billion years.
Astronomers establish an estimated
age (called the model age) for any feature
lacking radiometrically dated samples
by counting craters to derive their N(1)
values, then plotting them on a graph.
Having done so, scientists determined
hundreds of model ages for features. For
instance, the east side of Sinus Iridum
is about 3.35 billion years old, while the
Revised Crater Ages
(billions of years)
Crater
Carpenter
Cavalerius
Hainzel A
Bullialdus
Eratosthenes
Stevinus
Neukum
age
3.1
3.1
3
3.7
3.3
0.9
Lagain
age
2.4
2.5
2.5
3.6
3.3
1.0
pThis map illustrates the asymmetry of
lunar cratering rates, with red colors showing
higher-than-average rates and blues for lowerthan-average
ones. The two white circles mark
the apex or leading edge (left) and antapex or
trailing face (right) of lunar motion. Black and
yellow circles indicate 11 age-dated samples
used for chronology calibration; green circles
are craters listed in the table below.
west side is 350 million years younger.
Both values match that of adjacent
Mare Imbrium lavas, which flowed
downhill into Iridum.
As marvelous as the ability to ascertain
a model age is, there are uncertainties
that sometimes lead to disparate
estimates. For example, researchers often
count different numbers of craters in the
same area. This can be due to the difficulties
in deciding if a feature is a crater,
a degraded crater, or random topography.
They also sometimes disagree if a crater
is a primary impact or a secondary crater
that formed from the fallback of materials
ejected from a primary impact and
thus should be ignored. Another variable
arises when researchers develop different
mathematical fits to N(1) values and
absolute-age data. In extreme cases, this
Relative cratering rate
ANTHONY LAGAIN
Sky & Telescope - June 2024

Table of Contents for the Digital Edition of Sky & Telescope - June 2024
