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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, rifles, 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 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, zlooking 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-filled 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 filled 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. Magma must have also risen up along fractures underlying Archimedes, explaining how mare lava partially filled the crater's floor and buried its central peaks.

Shoemaker noticed that the 59-km-wide crater Eratosthenes, which interrupts the Montes Apenninus, has small secondary craters and faint rays visible at full Moon on the nearby plains of Mare Imbrium. So Eratosthenes must have formed after the last lava flows in this area. Moreover, because it maintains some rays (which erode away rather quickly), it's probably younger than other craters on Mare Imbrium that lack ray patterns.

Are any nearby craters likely to be younger than Eratosthenes? There's one obvious candidate: Copernicus. Based on the brightness and extent of its rays and secondary craters, 96-km-wide Copernicus is the youngest large landform in this area. Its ejecta clearly lie atop those of Eratosthenes. Ages determined by counts of small craters superposed on Copernicus' wide apron of debris indicate that this impact occurred 800 million to 1 billion years ago. This is ancient by Earth standards--but relatively recent for events on the Moon.

Finally, are any features in this southeast Imbrium quadrant even younger than Copernicus? Many small craters a few kilometers in diameter appear younger, because they display crisp rims that would have been smoothed and eroded over time.

But can we use the law of superposition to prove that they're younger than Copernicus? Yes! During full Moon, point your telescope to the tract of mare just north of Eratosthenes. They're a challenge to see, but under high magnification you might notice about a half-dozen dark splotches. At the center of each is a 3- to 5-km-wide impact crater gouged into bright ray material from Copernicus. These small pits, known as dark halo craters, excavated underlying mare lavas and spread them as pulverized debris on top of the ray.

Within this eyepiece-wide view of southeast Imbrium, a careful observer can identify landforms created during 4 billion years of lunar history, from battered pre-basin ruins to very young dark halo craters cutting through bright rays. Using the benchmarks explored here, now have a look at every other feature in the field of view and try to decipher where it fits in this lunar sequence.

The Moon October 2015

Phases

LAST QUARTER October 4,21:06 UT

NEW MOON October 13,0:06 UT

FIRST QUARTER October 20,20:31 UT

FULL MOON October 27,12:05 UT

Distances

Apogee 252,518 miles

Perigee 222,739 miles

October 11,13" UT diam. 29' 40"

October 26,13h UT diam. 33'3"

Librations

Byrd (crater)        October 1
Short (crater)       October 22
Mare Humboldtianum   October 27
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Title Annotation:OBSERVING: Exploring the Moon
Author:Wood, Charles A.
Publication:Sky & Telescope
Article Type:Column
Date:Oct 1, 2015
Words:900
Previous Article:Asteroid occupations.
Next Article:Millions together: the stars gather close in the southern wing of Cygnus the Swan.
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