Scientists have pinned down the birth date of the moon to within 100
million years of the birth of the solar system — the best timeline yet
for the evolution of our planet's natural satellite.
This new discovery about the origin of the moon may help solve a mystery about why the moon and the Earth appear virtually identical in makeup, investigators added.
Scientists have suggested the moon was formed 4.5 billion years ago by a
gigantic collision between a Mars-size object named Theia and Earth, a
crash that would have largely melted the Earth. This model suggested
that more than 40 percent of the moon was made up of debris from this
impacting body. (Current theory suggests that Earth experienced several
giant impacts during its formation, with the moon-forming impact being
the last.)
However, researchers suspected Theia was chemically different from Earth. In contrast, recent studies revealed that the moon and Earth appear very similar
when it comes to versions of elements called isotopes — more so than
might be suggested by the current impact model. (Isotopes of an element
have differing numbers of neutrons from one another.)
"This means that at the atomic level, the Earth and the moon are identical,"study
lead author Seth Jacobson, a planetary scientist at the Côte d'Azur
Observatory in Nice, France, told Space.com. "This new information
challenged the giant impact theory for lunar formation."
How the moon evolved
No one seriously disputed an impact as the most likely scenario for the formation of the moon,
Jacobson said. However, a virtually atomically identical moon and Earth
threw the exact circumstances of the collision into question, he said.
Now, by pinpointing when the moon formed, Jacobson and his colleagues
could help explain why the moon and Earth are mysteriously similar. The
scientists detailed their findings in the April 3 issue of the journal
Nature.
Efforts to date the moon-forming impact have proposed a range of ages.
Some have argued for an early event, about 30 million years after the
birth of the solar system, whereas others suggested that it occurred
more than 50 million years and possibly as much as 100 million years
after the solar system formed.
To help solve this mystery, Jacobson and his colleagues simulated the growth of the solar system's rocky planets — Mercury, Venus, Earth and Mars — from a protoplanetary disk of thousands of planetary building blocks orbiting the sun.
By analyzing how these planets formed and grew from more than 250
computer simulations, the researchers discovered that if the
moon-forming impact was early, the amount of material accreted onto
Earth afterward was large. If the impact was late, the amount would then
be small.
Past research had calculated the amount of material accreted onto Earth
after the moon-forming impact. These estimates are based on how on how
so-called highly siderophile or "iron-loving" elements such as iridium
and platinum show a strong tendency to move into Earth's core. After
each giant impact the nascent Earth sustained, these elements would have
leached from Earth's mantle and bonded with heavy, iron-rich material
destined to sink to Earth's heart.
Moon birth mystery
After the last giant impact that formed the moon,
the mantle should have been almost completely stripped of iridium,
platinum and their cousins. These elements are still present in the
mantle, but only in small amounts, which suggests only a small amount of
material accreted onto Earth after the moon-forming impact.
The researchers calculated the moon-forming impact must have occurred
about 95 million years after the formation of the solar system, give or
take 32 million years.
"A late moon-forming event, as suggested by our work, is very consistent with an identical Earth and moon," Jacobson said.
In addition, recent analyses propose that the impact that created the
moon required a faster, more energetic collision than previously
suggested. This makes sense if the impact took place relatively late
with an older protoplanetary disk, as the new findings suggest.
"Older disks tend to be dynamically more active, since there are fewer
bodies left in the disk to distribute energy amongst," Jacobson said.
These new findings raise an interesting new puzzle. While they suggest
the moon and the Earth formed together nearly 100 million years after
the solar system arose, evidence from meteorites from Mars suggests that
the Red Planet formed as little as a few million years after the solar
system was born.
"This means that Earth and Mars formed over dramatically different
timescales, with Mars forming much faster than the Earth," Jacobson
said. "How can this be? Is it just a matter of size? Location? What
about Mercury and Venus? Did they grow on similar timescales to the
Earth or on timescales more similar to Mars? I think these are some of
the really important questions that we, as a community of planetary
scientists, will be addressing in the future."
