Press Release

SwRI, UCSC researchers identify the Moon-forming impact

By SpaceRef Editor
August 15, 2001
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Note: An image and animation of the moon-forming event are available from

The “giant impact” theory, first
proposed in the mid-1970s to explain how the Moon formed, has received a
major boost as new results demonstrate for the first time that a single
impact could yield the current Earth-Moon system.

Simulations performed by researchers at Southwest Research InstituteTM (SwRI)
and the University of California at Santa Cruz (UCSC) show that a single
impact by a Mars-sized object in the late stages of Earth’s formation could
account for an iron-depleted Moon and the masses and angular momentum of the
Earth-Moon system. This is the first model that can simultaneously explain
these characteristics without requiring that the Earth-Moon system be
substantially modified after the lunar forming impact. The findings appear
in the August 16 issue of Nature.

The Earth-Moon system is unusual in several respects. The Moon has an
abnormally low density compared to the terrestrial planets (Mercury, Venus,
Earth, and Mars), indicating that it lacks high-density iron. If the Moon
has an iron core, it constitutes only a few percent of its total mass
compared to Earth’s core, which is about 30 percent of its mass. The angular
momentum of the Earth-Moon system, contained in both the Earth’s spin and
the Moon’s orbit, is quite large and implies that the terrestrial day was
only about five hours long when the Moon first formed close to the Earth.
This characteristic provides a strong constraint for giant impact models.

Previous models had shown two classes of impacts capable of producing an
iron-poor Moon, but both were more problematic than the original idea of
a single Mars-sized impactor in the last stages of Earth’s formation. One
model involved an impact with twice the angular momentum of the Earth-Moon
system; this would require that a later event (such as a second large impact)
alter the Earth’s spin after the Moon’s formation. The second model proposed
that the Moon-forming impact occurred when Earth had only accreted about
half its present mass. This required that the Earth accumulated the
second half of its mass after the Moon formed. However, if the Moon also
accumulated its proportionate share of material during this period, it would
have gained too much iron-rich material — more than can be reconciled with
the Moon today.

The models developed by SwRI and UCSC use the modeling technique known as
smooth particle hydrodynamics, or SPH, which also has been used in previous
formation studies. In SPH simulations, the colliding planetary objects
are modeled by a vast multitude of discrete spherical volumes, in which
thermodynamic and gravitational interactions are tracked as a function of

The new high-resolution simulations show that an oblique impact by an object
with 10 percent the mass of the Earth can eject sufficient iron-free material
into Earth-orbit to yield the Moon, while also leaving the Earth with its
final mass and correct initial rotation rate. This simulation also implies
that the Moon formed near the very end of Earth’s formation.

“The model we propose is the least restrictive impact scenario, since it
involves only a single impact and requires little or no modification of
the Earth-Moon system after the Moon-forming event,” says the paper’s lead
author, Dr. Robin M. Canup, assistant director of the SwRI Space Studies
Department in Boulder.

UCSC Professor Erik Asphaug adds, “Our model requires a smaller impactor
than previous models, making it more statistically probable that the Earth
should have a Moon as large as ours.”

Modeling lunar formation is important to the overall understanding of the
origin of the terrestrial, or Earth-like, planets.

“It is now known that giant collisions are a common aspect of planet
formation, and the different types of outcomes from these last big impacts
might go a long way toward explaining the puzzling diversity observed among
planets,” says Asphaug.

The Moon is also believed to play an important role in Earth’s habitability
because of its stabilizing effect on the tilt of Earth’s rotational pole.

“Understanding the likelihood of Moon-forming impacts is an important
component in how common or rare Earth-like planets may be in extrasolar
systems,” adds Canup.

For more information, contact Maria Martinez at (210) 522-3305 or Dr. Robin
Canup at (303) 546-6856.

SpaceRef staff editor.