February 11, 2011
How Old is That Mare?
left image from Maurice Collins
The answer to the title question is, "It depends on where you are." Like all maria, Mare Crisium is a vast pile of lava that was
erupted over a considerable period of time. Harry Hiesinger and colleagues have used LRO WAC mosaics to count impact
craters to estimate ages for different parts of Mare Crisium (left). The oldest crater-dated lavas are 3.6 billion years old, and
the youngest seen are 2.7 b.y. Of course, ages can only be obtained for lavas that are still visible; we have no evidence of the
ages for the earliest Crisium eruptions whose lavas are probably completely buried in the center of the basin. It might be
expected - as at Serenitatis - that younger lavas filled the middle of the basin, the lowest area, and older lavas are only visible
around the edges where they were too elevated to be covered. That is partially true here, with old ages of 3.61, 3.50, 3.60 and
3.59 b.y. occurring along the west, north, east and southeast shores. But ages of 3.4 to 3.5 b.y. are visible in a broad swath
across the center from north to south. And young ages of about 3.0 b.y. occur along the south, east and northeast edges, with an
east-west band of 2.8 to 3.0 b.y. crossing the mare center. I placed one of Maurice Collins' visualizations of LRO altimetry data
on the right to look for possible correlations of age groups with topography. But there are no consistent correlations. These
unsatisfying results demonstrate that the Moon often fails to abide by our simple models of how it should work. This gives an
added meaning to Heinlein's comment that the The Moon is a Harsh Mistress.
Chuck Wood
Related Links
Rükl plate 27
Hiesinger, van der Bogert, Reiss and Robinson] (2011) Crater size-frequency distribution measurements of Mare Crisium,
42nd Lunar and Planetary Science Conference
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