Quadrant 29:
— Images and text provided by Howard Eskildsen
Lava flows raise questions
Once again the predominant features on
this image are the broad, flat lava plains of
Mare Imbrium as well as Sinus Aestuum.
They cover parts of mountain ranges and
older craters and are the bed upon which later
craters formed. Obviously the partially buried
features existed prior to the lava flows, while
the fresh craters and their associated scars
appeared after the basaltic lava hardened. So
relative time scales can be readily discerned
from the image, though the actual time
involved cannot. Also, the partially buried
features raise questions as to what else might
lie beneath the basalt “seas.”
At the lower part of the image both the
Carpathian and the Apennine mountains
dip downwards toward a gap that was
overflowed by basaltic lava, and suggests that
they are connected beneath the plains and
are part of the same outer ring of Imbrium.
Also, on the upper right of the image other
highlands near Beer and Feuilee dip below
the basalt plains and perhaps connect beneath
the surface with the peaks near Euler. Indeed this area
does look like it could once have been a great molten
sea, but there is no way to tell from this image if it was
or not, however unlikely that might be.
The answer finally came a few decades ago from the
crater Pytheas when it was photographed by Apollo 17.
Pictures of its walls revealed that many layers of basalt
had been laid down through its 2.5 km depth and must
have taken very long periods of time for each layer to
be emplaced, solidify and weather before the next layer
formed. Other satellite and earth-based imaging have
revealed differing elemental composition of various
mare regions as well, so there were many lava flows
of many different compositions that produced the final
flat-looking surface. Additionally, Apollo lunar samples,
spectroscopic studies and other dating techniques reveal
that the Imbrium basin was excavated about 3.85 billion
years ago with the most intense lava flows occurring
between about 3.7 and 3.3 billion ago. Some of the
youngest lava flows may be around 2.5 billion years
ago. Wow! Life was only in its earliest stages on Earth
when the final flows cooled on the moon.
It is apparent that very few large craters have
38
appeared in the past 2-3 billion years on the
moon.