Torn Away: The Moon's Violent Birth
posted: 07:07 am ET
01 September 2000
Two new studies bolster the popular theory that the moon formed from debris after a rogue planet smacked into Earth about 4.5 billion years ago.
One fresh line of evidence for the so-called "big whack" comes from the oldest record of Earth'socean tides ever found – 3.2-billion-year-old
rocks from South Africa. Pinstriped sand-and-silt layers in the rocks were deposited by daily, fortnightly and monthly tidal cycles.
The layers reveal the primeval moon’s orbit was nearly circular, as it is today, consistent with the "giant impact theory," said Virginia Tech
geologist Ken Eriksson, who led the study in September’s issue of the journal Geology.
Earth's Offspring? The Collision Theory
One theory has emerged over competing explanations of lunar birth: themoon formed when a big object whacked Earth.Click to learn more.
If the moon formed elsewhere, then was captured intact by Earth’s gravity, the moon’s orbit would have been extremely elliptical, and tidal rock
layers would not have shown such normal cycles, he said.
Eriksson said the South African rocks also indicate that 3.2 billion years ago, the moon orbited Earth in perhaps a 20-day month and was 25-percent
closer to Earth than it is today. But Earth would have spun faster, so there would have been about 550 days in a year.
The second study
In the second study, German geochemists determined Earth and moon each separated into a core, mantle and crust, 50 million to 100 million years after
the solar system formed some 4.56 billion years ago.
The study, published in the September 1 issue of the journal Science, adds evidence the moon-forming impact also happened 50 million to 100 million
years after the solar system's birth. Only after the impact did oceans of molten rock cool so Earth and moon could fully develop internal layers.
Carsten Munker of the University of Munster said Earth’s core probably started forming soon after the solar system’s 50 millionth birthday. Then a
planet-sized object hit Earth, which still might have been covered by an ocean of magma, or molten rock.
Impact debris was hurled into orbit around Earth, and a portion of the fragments consolidated to form the moon, which was so hot a magma ocean covered
it. Then, Earth and moon cooled, with the moon developing a core, overlying mantle and uppermost crust, while Earth also completed its core, mantle
and crust, Munker said.
A compelling theory
"The impact theory is becoming immensely popular," said University of Utah geologist Marjorie Chan. "Impacts capture people’s imaginations. People
like those theories because they are glitzy, 'gee-whiz,' 'wow.' A spectacular collision is more exciting than the Earth just rotating around and
capturing a moon somehow."
In the early 1990s, Chan and a student found what were then the world’s oldest confirmed "tidal rhythmites" – 900 million-year-old rocks from Big
Cottonwood Canyon near Salt Lake City, Utah.
But in their new study, Eriksson and Edward Simpson, a geologist at Pennsylvania’s Kutztown University, identified 3.2 billion-year-old,
tide-deposited rock layers near Barberton, in eastern South Africa.
Eriksson first found the layers of sand and silty shale – which were mildly metamorphosed into quartzite and slate – in the 1970s. He suspected they
were tidal rocks. He said the new study proved it by showing how the pinstripe sediment patterns coincided with daily, twice-monthly and monthly tidal
Just as today, rising flood tides deposited light-colored, sandy layers on shallow coastal sea floors and estuaries. When waters went slack, fine silt
and clay particles formed thinner dark, muddy layers.
Two sand layers and two silt layers represent a single day’s two high tides and two low tides. The thicker sand layer – up to an inch (25 millimeters)
thick – was laid down by the day’s highest high tide, which had a faster current to carry more sand.
The rocks also display twice-monthly and monthly tide patterns from spring and neap tides.
Spring tides happen twice monthly when Earth, moon and sun are aligned, triggering the month’s highest high tides with faster currents that deposited
thicker sand layers.
Neap tides happen twice monthly when the moon is completely unaligned, producing the month’s lowest high tides with slow currents that deposited the
thinnest sand layers in the 3.2 billion-year-old rocks.
Layers of sand and mud in this South African rock formation reveals the cycles of tides and the path of the moon's orbit 3.2 billion years ago. The
scale on the rock is 3.5 inches long (9 centimeters).
"There were tides today and tides back then, which indicates the moon was in orbit around the Earth at the time," Eriksson said from Australian
National University in Canberra, where he is on a fellowship. "Our work indicates a near-circular orbit, which favors the impact model for the
formation of the moon."
If the moon formed elsewhere and was captured intact, its speed before capture would have made its orbit around Earth highly elliptical and
asymmetrical, Eriksson said.
If that had happened, the rocks would have some sand layers from the few days each month when the moon was close enough to cause tides, then a thick
layer of mud representing the lack of tides when the moon was far from Earth.
In the other study, Munker and colleagues studied ancient Earth rocks and meteorites that originated on the moon. They looked for unusual amounts of
zirconium 92 produced by the radioactive decay of niobium 92.
Such decay virtually stopped about 50 million years after the solar system’s birth because almost all of the niobium 92 was used up by then, Munker
said. The study found that unlike asteroids, ancient Earth and moon rocks lacked unusual zicronium 92.
That suggests rocks from Earth and the moon did not solidify from magma until all the niobium 92 was gone, namely, at least 50 million years after the
solar system was formed. That, along with earlier research on the decay of hafnium 182 to tungsten 182, implies a big collision formed the moon at
least 50 million years after the solar system’s birth, and only later did Earth and the moon develop all their distinct layers.
The moon’s oldest rocks cooled and solidified about 4.44 billion years ago, or about 100 million years after the solar system was born. So the moon
must have formed sometime between 50 million and 100 million years after the solar system was born, Munker said.
Munker conducted the study with Munster geochemists Stefan Weyer, Klaus Mezger, Mark Rehkamper and Frank Wombacher, along with planetologist Addi