Sky & Telescope - April 2023 QR - 52

APRIL 2023 OBSERVING
Exploring the Solar System by Charles A. Wood
Tracking Time on the Moon
Observing these craters can help you to recognize the age of other lunar features.
I
mpact craters, from tiny pits to giant
basins, are the dominant landforms of
the Moon. Just a handful of the largest
craters appear nearly unchanged since
they formed over a billion years ago.
The rest have been reshaped by subsequent
events. In fact, modification was
common in the first half-billion years
of lunar history. Craters large and small
were completely destroyed or otherwise
altered by other impacts in the earliest
lunar epoch. However, between about
3.8 and 1 billion years ago, this degradation
slowed as the impact rate rapidly
declined. In the last billion years, the
mixing of crater rays and mare soils
has become the dominant modification
process (S&T: July 2019, p. 52).
To show the effects of crater modification
over time, I've selected 10 craters
ranging from 40 to 100 km (25 to 62
miles) in diameter that formed between
85 million and 3.9 billion years ago.
Crater ages older than 2 billion years
are from a 2013 paper by Michelle
Kirchoff and colleagues at the
Southwest Research Institute
in Boulder, Colorado.
They used the standard
method of counting the
number of superposed
impact craters on
earlier craters. The
floors of these craters
generally show little
evidence of additional
modification, so the
counts should accurately
reflect their ages.
But at the same time the
craters older than 1 billion
years are statistically more
likely to have experienced later
2
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3
1. Tycho
Age: 0.1 * Diameter: 86 km
Depth: 4.7 km * Ray visibility: 5
52 APRIL 2023 * SKY & TELESCOPE
modifications. Did they?
The three youngest, large, dated craters
Tycho, Aristarchus, and Copernicus
show what fresh complex craters
look like - a " before " view to compare
with older, more battered craters. Each
of these so-called TAC craters has a high,
crenulated rim and a series of terraces.
Their broad, flat floors contain central
peaks and many small-impact melt
mounds and flows. Beyond their rims are
streamers of secondary craters and rays
extending hundreds of kilometers.
Harpalus is the next youngest crater
on the list. With an age of 1.1 billion
years, Harpalus has classic, complex
crater morphology, including radial
ejecta ridges and small secondary craters,
Crater Age Comparison
(AGE: BILLIONS OF YEARS; RAY VISIBILITY SCALE
OF 0 TO 5 STRONGEST)
4
5
but its rays are fainter than those of the
TAC craters, extending only about 150 to
200 km. Close examination shows that
Harpalus' inner terraces are somewhat
rounded. Harpalus is the same diameter
and depth as Aristarchus but has
been exposed to slow and steady erosion
roughly seven times longer.
The next three have 80-to-100-km
diameters, like Tycho and Copernicus,
but are several times older. Aristoteles,
Theophilus, and Geminus maintain
the continuous rims, terraces, and broad
floors as the TAC craters. Aristoteles has
significant radial ridges and secondary
craters, although its ray system is short
and faint. Its terraces are less rounded
than those of Harpalus, but most puz6
1
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8
10
6.
Theophilus
Age: 3 * Diameter: 100 km
Depth: 4.8 km * Ray visibility: 1
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