sciencemag.org SCIENCEPHOTO: ERNEST WRIGHT, NASA/GSFC SCIENTIFIC VISUALIZATION STUDIOINSIGHTS
PLANETARY SCIENCEWhen Earth
got pummeled
The frequency of impacts
on Earth’s surface increased
about 290 million years ago
PERSPECTIVES
By Christian Koeberl1,2C
ollisions and impact processes have
been important throughout the his-
tory of the solar system, including that
of Earth. Small bodies in the early so-
lar system, the planetesimals, grew
through collisions, ultimately forming
the planets. Recognizing the remnants of
impact events on Earth is difficult because
terrestrial processes either cover or erase the
surface expression of impact structures in
geologically short timespans. Because Earth
and the Moon are subjected to the same flux
of impactors, the latter’s crater record serves
as a proxy for that of Earth. On page 253 of
this issue, Mazrouei et al. ( 1 ) report that infra-
red images of the Moon taken by the Lunar
Reconnaissance Orbiter Diviner instrument
can be used to estimate the ages of young
lunar craters. They find the impact rate in-
creased within the last ~500 million years.
Impact cratering is a high-energy event
that occurs at more-or-less irregular inter-
vals, although over long periods of time, an
average cratering rate can be established
( 2 ). The terrestrial rock record, in the form
of crustal rocks, extends back to only about
89% of Earth’s history, to ~4.0 billion years
ago , with the oldest rocks showing very lim-
ited exposures in Greenland and Canada.
The importance of the process of impact
cratering on a planetary scale has only
recently been recognized in the geologi-
cal sciences. During the past few decades,
planetary scientists and astronomers have
demonstrated, with the aid of many space
missions, that the surfaces of Earth’s moon ,
Mercury, Venus, Mars, the asteroids, and
the moons of the outer gas planets are all
covered (some surfaces to saturation) with
meteorite impact craters.Impact craters on the lunar surface persist
over billions of years and help to constrain Earth's
impact record. Orientale (shaded topographic
map shown) is a large and relatively young impact
basin on the Moon.224
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