People worry a lot about asteroids. Although the odds of
Earth being hit by a big one are slim, the consequences are terrifying:
tsunamis, climate change, even mass extinction. Fortunately, it doesn’t
happen often.

Four billion years ago it happened all the time. Our planet was still young
and the inner solar system was littered with asteroid-sized “planetesimals,”
the leftover building blocks of planets. Planetesimals, some big and some
small, hit Earth every single day. This so-called “Period of Heavy
Bombardment” (or PHB) lasted from about 4.5 to 3.8 billion years ago–a span
that couldn’t have been pleasant for terrestrial life.

Earth’s early pounding was one of the topics discussed this month when
experts and students from around the world met at “Perspectives in
Astrobiology,” a NATO Advanced Studies Institute in Chania, Crete, sponsored
jointly by NATO and the Marshall Space Flight Center. Science@N…’s Ron
Koczor was there, and this is his report:

Participants at the meeting noted something odd about the end of the Period
of Heavy Bombardment. From the earliest geologic records known (roughly 3.8
billion years old), there is fossil evidence indicating life on Earth. The
existence of apparently successful microbial life so soon after the end of
these cataclysmic bombardments suggests that life emerged on Earth during
the violent PHB itself.

How could that happen? Did comets or asteroids deliver life intact to Earth?
Maybe life rapidly evolved from organic building blocks deposited by comets.
No one knows. Astrobiologists would love to study rocks and chemical fossils
from that epoch, yet because of Earth’s wind, rain, earthquakes and plate
tectonics (normal environmental processes), the record has been completely
erased.

Scientists at the conference speculated that such a record does exist. Not
on Earth, but on the Moon.

When a large body strikes Earth, impact debris can be accelerated to orbital
speed and achieve Earth orbit. Four billion years ago Earth was probably
surrounded by debris ejected in this way. (The Moon itself is a big piece of
Earth that sundered when a Mars-sized planetestimal hit 4.5 billion years
ago.) During the Period of Heavy Bombardment, the Moon was considerably
closer to the Earth than it is now, perhaps 3 times closer. This placed the
Moon in an ideal position to sweep up some of the terrestrial debris.

Because the Moon lacks weather or tectonic activity, that debris might still
be there. While some has undoubtedly been destroyed by subsequent impacts of
asteroids or comets on the Moon, some might have survived in the lunar soil.
A recent study by Univ. of Washington graduate students John Armstrong and
Llyd Wells, in collaboration with Guillermo Gonzalez at Iowa State, suggests
that as much as 20,000 kg of Earth material could cover every 100 square
kilometers of the moon.

David McKay, an astrobiologist at NASA’s Johnson Space Center, notes that
“the Moon was in a unique position to be a collector of ejecta from Earth.
If we look in the right places, we could find a reservoir of materials for
study.”

The 400 kg or so of lunar rocks and soils already on the Earth thanks to
Apollo begs the question: Have such terrestrial materials been seen in any
of the returned samples? According to McKay: no. “I know of no reports of
such materials, but I think the reason may be that no one has actually
looked for them! This is an emerging area of research that was not
considered over the past 30 years since Apollo.”

Where should future astronauts look on the Moon for these ejected
terrestrial materials? There are several possibilities.

One place, according to John Armstrong, would be the Moon’s eastern (as seen
from Earth) limb. “The Moon’s rotation about its axis is synchronized with
its revolution around the Earth,” explains Armstrong. This means that the
same side of the Moon–its eastern limb–is always the leading edge as it
circles Earth. That leading edge would tend to sweep up more orbiting debris
than other areas.

“This is true for materials that are ejected into high Earth orbit. Another
possibility is that material is ejected and travels directly from Earth to
the Moon. In that case, it would be found anywhere on the Earth-facing
side,” said Armstrong. A third possibility is that the Earth material is
ejected into a longer-lasting solar orbit. In this case the material could
have spent thousands or millions of years in orbit and then impacted the
Moon in a more random pattern.

How could we distinguish an ancient Earth rock from a genuine Moon rock?
After all, the Moon itself is a very old piece of Earth.

According to Armstrong, that question is still under discussion. “There are
several possible chemical differences,” he says. One would be water. Earth’s
oceans formed after the Moon split off from our planet. While Moon rocks are
dry, some Earth rocks contain hydrated minerals–those which have water
incorporated into their molecular structure. Other differences could be the
presence of hydrocarbons or carbonates.

Perhaps a series of lunar missions would find such rocks: “We could develop
automated robotic techniques that scan thousands or millions of small rocks,
searching for Earth materials in lunar soils,” speculates McKay. “It would
be like looking for a needle in a haystack–a task that would be nearly
impossible if done by hand but easily done by robots. We simply need to know
which properties are the most useful for distinguishing Earth materials from
Moon materials and set the robotic instruments to sniff them out.”

It is often said that traveling makes you appreciate home that much more. In
this case, traveling to the Moon may be the only way we can ever understand
the early chaotic period of Earth’s formation.