Featured Image - 07/15/2008
The Ages of the Moon
This spectacular Apollo 15 metric frame taken from lunar orbit by Astronaut Al
Worden aboard the Apollo 15 Command Module Endeavour shows Mare
Serenitatis. You can immediately see the difference between this
relatively young mare surface and some of the other Apollo Featured Imagesk: This mare surface is smooth, and the craters
are smaller. Even though this area of the Moon lacks some of the large impact
craters and structural features seen elsewhere on the Moon, it's every
bit as interesting. Lunar scientists can actually use images like
this to determine the relative ages of lunar geologic units.
Figure 1. Apollo Image AS15-M-0576, showing a
region in western Mare Serenitatis [NASA/JSC/Arizona State
How can we determine the age of a surface just from looking at a picture?
The process is time-consuming, but the results are worth it! It works
like this: Because of the Lunar Orbiter and Apollo missions, we
have some basic information about the stratigraphy of the lunar
surface as well as accurate age dates obtained from the samples
returned by the Apollo astronauts. The Apollo samples were
collected by highly-trained astronauts on the lunar surface, so we can
use their field experiences at the Apollo landing sites to relate the
samples to specific geologic units on the lunar surface. We can then
count the craters on these age-dated surfaces to compute the
size-frequency distrubtion (the number and size of craters per unit
area) of craters produced as a function of time. Comparing the absolute age of the Apollo samples to the
size-frequency distribution in the age-dated regions near the Apollo
landing sites has allowed lunar scientists to derive calibrated
size-frequency distribution curves that can be used to estimate the
relative ages of other lunar regions. In principle, one simply counts
the number and size of impact craters on the surface in an unknown
area of the Moon and compares it to one of the known regions. If the
size-frequency distribution is the same, then an absolute date can be
assigned to that particular area.
Figure 2. Cumulative crater size-frequency curves
for the some of the Moon's major time-stratigraphic and geologic units,
reproduced from Chapter 7 of Wilhelms (1987). The dashed curves are average
frequencies of impact craters of the pre-Nectarian (pNc), Nectarian
(Nc), Imbrian (Ic) and the Copernican/Eratosthenian periods (CEc) [United States Geological Survey].
Astronauts Harrison Schmitt and Gene Cernan sampled Mare Serenitatis
directly during the Apollo 17 mission in 1972. Although most of the
mare basalts they sampled were about 3.8 billion years old, using this
crater counting technique it is now thought that the full range of
basalt ages in Mare Serenitatis is 2.4 billion to 3.8 billion years old.
Similar crater counts have been performed for all lunar maria, however the Apollo astronauts only sampled a handful of mare units.
Unfortunately, the Apollo missions only visited middle-aged to ancient
mare basalts so we do not know the absolute ages of the younger mare.
If one assumes that the impact rate was the same from about 1
billion years to 3.8 billion years ago, age estimates can be made for
these younger basalts. However, there is good reason to believe that
the number of impacts has been declining since about 3.8 billion years
ago. Thus, we must obtain samples of a few of the youngest mare
basalt units to complete the age-dating job begun during Apollo.
The Apollo missions only photographed about twenty percent of the Moon
at high resolution and only about half of that coverage is suitable
for crater counting due to Sun angle differences (it is very difficult
to spot craters in images taken near lunar noon). Right now the
Japanese mission Kaguya is mapping the whole Moon at 10 meters (33
feet) per pixel and the Indian Chandrayaan-1 mission to be launched
this fall will take images with twice as much resolution. Early in 2009
year the United States Lunar Reconnaissance Orbiter will enter lunar
orbit and detect features down to a size of 50 cm (1.6 feet) as it
scouts potential human exploration sites on the lunar surface.
For more information, read:
Wilhelms, D. E. (1987) The Geologic History of the Moon. United States Geological Survey Professional Paper 138. United States Government Printing Office.
Stöffler, D., Ryder, G., Ivanov, B. A., Artemieva, N. A., Cintala, M. J., Grieve, R. A. F. (2006) Cratering History and Lunar Chronology. Reviews in Mineralogy & Geochemistry, 60, pp. 519-596.