Quadrant 38:
— Images and text provided by Howard Eskildsen
Eratosthenes to Gambart
At the top of the image the crater
Eratosthenes appears like a smaller version of
Copernicus. Though only 60 km in diameter,
Eratosthenes is structured like Copernicus with
central peaks, flat floor, terraced inner rim and
a continuous ejecta apron outside the outer
rim. However, it lacks rays and its features
appear softer and more subdued than those of
Copernicus. Why?
Eratosthenes is older and has had more time
for multiple meteor impacts to erode or garden
its surface. Similarly its rays have vanished due
to the erosive forces. It is estimated that it takes
about a billion years to erase a crater’s rays, so
Eratosthenes must be considerably older than
Copernicus. By using Apollo 14 rock samples
returned to Earth, scientists estimated the age
of Copernicus to be 800 million years. Wow!
No eyes existed on Earth to witness the cosmic
crash that created it. Eratosthenes then is much,
much older. It is estimated to be around 3.2
billion years old; only the most primitive forms of life
inhabited Earth when it was formed.
The two earliest lunar geological time periods are named
after these two craters. The Copernican period runs from
current day to approximately 1.1 billion years ago and
is defined by craters with bright rays. The Eratosthenian
period runs from 1.1 billion to 3.2 billion years ago and
is characterized by distinct craters without rays. Some
volcanic activity occurred during this time. It was slowly
ending but had not totally ceased during this period.
Eratosthenes is bounded on three sides by flat mare
plains of basaltic lava, including Sinus Aestuum. Could
encroaching lavas have partly buried part of its ejecta
and rays? A close look at higher resolution shows that
its ejecta is in fact littered over the mare basalts, so the
last layers of lava are older than Eratosthenes, though in
general they appear very close in age. As noted above,
those lavas and Eratosthenes are overlain by rays and
secondary craters from Copernicus, confirming that
Copernicus is younger than both features. In contrast,
poor Stadius is obviously older than the lava flows which
nearly obliterated it. Only portions of its outer rim still
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rise above the basalt.
Other volcanism is visible in the form of
pyroclastics (fragmented rocks and ash from
volcanic explosions) thought to have been the last
gasps of volcanism in the region. The pyroclastics
on this image appear to have been partly covered
by rays from Copernicus, so are obviously older.
Much later humans left an unintended mark (not
visible in telescopes) in the region southwest of
the pyroclastics. Surveyor 2 tumbled out of control
during a course correction rocket misfire and
crashed onto Mare Insularum.
Two craters appear to the left and below the
Surveyor 2 impact site. They are Gambart C (12.2
km) and Gambart B (11.5 km) and are classified
as simple craters since they lack central peaks
and have smooth inner rims. Simple craters are
generally less than 15 km in diameter whereas
craters over 20 km generally have characteristics
like Eratosthenes and Copernicus and are known as
complex craters.
Finally, other ruins of craters lie at the bottom of the
image. Gambart appears as an irregular ring inundated
by mare basalt. Mösting appears as the least worn crater
on the lower image, but Sömmering and Schröter were
Sky’s
Up
Sky’s
Up
hideously deformed long before they were covered by
basalts. Why? The explanation will follow in subsequent
quadrant descriptions.
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