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GLASS beads in soil from the Moon may help explain why there was an explosion
in the number of species on Earth around 570 million years ago. Scientists in
California say the beads hint that bombardment of the Moon and Earth by
asteroids and comets stepped up around the time of the so-called Cambrian
explosion, and might have spurred the rapid evolution of life.

It’s impossible to know how often asteroids and comets smashed into the Earth
in prehistoric times because erosion and the shifting of the continents have
erased most craters. “The Earth is very fickle in terms of preserving evidence
of these impacts,” says Paul Renne of the University of California at Berkeley.
But the relative calm on the Moon has preserved craters for billions of years,
and knowing lunar impact rates may indicate how often the Earth was hit.

Scientists have estimated past impact rates on the Moon by counting craters
in lava flows sampled during the Apollo missions and dated back on Earth. The
findings suggest the rate has gradually decreased since the Moon formed. But
Renne’s Berkeley colleague Richard Muller devised a more precise way to pin down
the rate by dating tiny glass beads found in a soil sample returned by Apollo
14.

The beads, roughly 200 micrometres across, formed when small impacts melted
the soil. Lunar soil contains radioactive potassium, which decays to form
gaseous argon-40. On melting, the gas would have escaped before the glass
solidified. But any argon-40 produced afterwards would be trapped and accumulate
in the beads, allowing the impact to be dated.

A single soil sample also contains glass beads formed by impacts hundreds of
miles apart, because “lunar gardening” mixes the soil over huge areas. “The
surface is continually being bombarded, ejecting material at high speed, and
there’s no atmosphere with friction to stop it flying over a very large area,”
says Renne. So the proportion of beads with different amounts of argon-40 gives
a measure of how impact rates changed over time.

The results suggest that the number of impacts decreased over the past 3.5
billion years to a low point between 400 and 600 million years ago. But since
then, they have increased nearly fourfold. “That was completely surprising,”
says Rene. It’s possible that the rates increased because large asteroids in the
Solar System collided and broke up over time, boosting the number of small
objects hurtling around the Sun.

The researchers say the impact rate on Earth would also have increased at
that time—potentially coinciding with the Cambrian explosion. Biologists
have speculated that asteroid impacts triggered this diversification by wiping
out some animals and creating new ecological niches. Palaeobiologist Simon
Conway Morris of Cambridge University says there’s little evidence on Earth to
back this speculation. “Nonetheless, this lunar link is intriguing. The Cambrian
explosion is still a bit of an enigma.”

Astronomer David Hughes of the University of Sheffield argues that although
small asteroids pockmark the Moon, they burn up in Earth’s atmosphere. “The
results are interesting, but you can’t compare what they’ve been finding on the
Moon with what we find on Earth.”