Sky's Up Summer 2016 - Page 38

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.