Sky and Telescope - October 2015 - 52

OBSERVING

Exploring the Moon

A Corner of Imbrium
In just one telescopic view, you can retrace more than
4 billion years of lunar history.
Most "Exploring the Moon" columns describe a single
type of lunar landform, such as basin rims, crater ﬂoors,
rilles, or mare ridges. Yet when looking through a telescope's eyepiece, you don't see just one feature in isolation
but rather everything in that portion of the Moon. The
fun then comes in deciphering which processes created
the diﬀerent landforms you see - and in what sequence.
In fact, this is the stratigraphic approach that legendary astrogeologist Eugene Shoemaker and his colleagues
developed in the late 1950s, when modern mapping of
the Moon began in preparation for the Apollo landings.
Stratigraphy is based on the law of superposition, namely,
the rock units on top must be younger than the ones
below. For example, sometimes craters sit atop other features and therefore are obviously younger. More often,
however, a crater's deposits - secondary craters, rays, or
Archimedes

Timocharis
Lambert

Pytheas

MARE IMBRIUM

Ap
es

i
nn

Eratosthenes

NASA / LRO

Copernicus

SINUS
AESTUUM

100 km

The array of features in the southeast corner of Mare Imbrium
holds the key to understanding the region's geologic history.
For example, why does Archimedes have truncated rim deposits
and a ﬂat, lava-covered ﬂoor?

October 2015 sky & telescope

other ejecta - deﬁne the overlap relationships.
Shoemaker ﬁrst applied this stratigraphic interpretation to the southeast corner of the Imbrium basin,
stretching from Copernicus to Archimedes. And it's a
good starting point for budding lunar explorers to learn
to read the Moon's history, because it's well placed, dramatic, and reveals a fascinating story.

A Big, Ancient Splat
Start your observational investigation with a broad overview, looking over the entire Imbrium region. You'll see
an oval outline of mountain chains that surround and
contain Mare Imbrium. The Alps, Caucasus, Apennine,
and Carpathian ranges are the remnant rim of the giant
Imbrium impact basin, which formed about 3.85 billion
years ago when a huge asteroid collided with the Moon.
Because its lava-ﬁ lled interior and mountainous rim
cover so much territory, this basin must be one of the
oldest lunar features.
But what did that primordial projectile smash into?
Some pre-Imbrium material is visible at the edges of
the eyepiece view. For example, the lower-right corner
of the photo at left shows hints of old craters that were
smashed through, covered, and bulldozed by Imbrium
ejecta that ﬂowed across the land like an immense mudslide. These ruined features are the oldest landforms
visible in this area.
What happened after this enormous basin formed
is clear: it ﬁ lled with mare lava ﬂows. But a question
that careful visual observations solved was: when did
those ﬂows erupt? Shoemaker recognized that the crater
Archimedes, 81 kilometers (50 miles) across, provides
critical clues about the sequence of events after the
basin's creation.
Although Archimedes' rim is relatively fresh, its outer
deposits of ejecta have been completely covered by Mare
Imbrium's lava ﬂows. Shoemaker reasoned that the
basin's formation erased all previous topography in the
impact zone. Then lavas erupted repeatedly and ﬂowed
across the basin's ﬂoor. Later, Archimedes and other craters formed on this fresh lava plain. Later still, another
round of Imbrium lavas ﬂowed across the surface,
surrounding and covering the ejecta from Archimedes.

Sky and Telescope - October 2015

Table of Contents for the Digital Edition of Sky and Telescope - October 2015