For a video explaining the new crater catalog,
Craters tell tales about moon’s past
By Camille M. Carlisle
The moon’s face can hide its age but not
Data from NASA’s Lunar Reconnaissance Orbiter describe a moon with a
more complex history than previously
thought and highlight what could be the
oldest lunar regions, planetary scientists
report in the Sept. 17 Science. Two papers
present measurements of the chemical
composition of the moon’s surface, and a
third details findings from a comprehensive crater catalog. The results support
previous theories about the moon’s past
and suggest targets for future missions.
Craters’ density and arrangement can
help scientists determine the age of a
planetary body’s surface. In general, fewer
craters mean a younger surface, because
fewer meteorites have had time to hit it.
Scientists have linked crater density to
age for locations on the moon using radio-
metric dating of samples from Apollo
missions. Ages for other surfaces can be
estimated by comparing their crater num-
bers with the numbers in dated regions.
This image, generated with data from the Lunar Reconnaissance Orbiter, shows the
lava-smoothed Oceanus Procellarum (right) and heavily cratered highlands (left).
crater densities that we observe mean,”
The study also confirms previous work
by Robert Strom of the University of Arizona in Tucson and colleagues suggesting that the moon and other solar system
bodies have suffered bombardment from
two separate asteroid populations. The
survey found that the highlands, unlike
younger, smooth areas called maria,
have a higher density of large craters,
indicating that impactors tended to be
larger during the moon’s early history.
The results match exactly what would
be expected if the moon were hit first by
objects from the main asteroid belt during
a period called the Late Heavy Bombard-
ment, roughly 3. 9 billion years ago, and
then later by smaller near-Earth asteroids
that had been ejected from the belt, says
Strom. “It’s a very good paper,” he says.
“This is further confirmation to me that
there are two crater populations.”
In the two other papers in Science, a
team reports on silica-rich materials
that point to past lunar volcanism. These
include silicic materials like quartz, with
high silicon and oxygen content.
Highly silicic minerals have never
before been directly detected on the
moon, says study coauthor Timothy
Glotch of Stony Brook University in New
York. While astronomers have known
since the 1970s that the lunar highlands
largely comprise silica- and calcium-rich
feldspar minerals, the new data distinguish between different types, such as the
known calcium-rich feldspars and newly
discovered sodium-rich deposits.
The distribution of highly silicic materials suggests that there were once thick
magmatic flows on the moon, with melts
spreading above and below the surface.
The prevalence of silicates runs
counter to evidence from rock samples
brought back by moon missions, in which
silicates were rare, says study coauthor
Paul Lucey of the University of Hawaii
at Manoa. The minerals’ commonness
raises their importance in understanding the moon’s early history.