three failed theories for origin of the moon:
of the Moon home page: http://yso.mtk.nao.ac.jp/~kokubo/moon/kit/movie.html
1. Big Bang, New Moon
(this first piece, from 1999, includes figures that can be viewed at the above link)
Theoretical and computational
simulations at SwRI could soon explain how
the Earth came to have its orbiting neighbor.
by Robin Canup, Ph.D.
About 4.45 billion
years ago, a young planet Earth -- a mere 50 million years
old at the time -- experienced the largest impact event of its history. Another
planetary body with roughly the mass of Mars had formed nearby with an orbit
that had, by chance, placed it on a collision course with Earth. When young
Earth and this rogue body collided, the energy involved was 100 million times
larger than the much later event believed to have wiped out the dinosaurs. The
early giant collision destroyed the rogue body, likely vaporized the upper layers
of Earth's mantle, and ejected large amounts of debris into Earth orbit. From
this debris our moon coalesced, possibly on a time scale as short as one to 100
Dr. Robin Canup, a senior research scientist in the Boulder,
Colorado, office of the SwRI Instrumentation and Space Research
Division, specializes in models relating to the origin of the
Earth-moon system, the formation of terrestrial planets, and the
origin of planetary ring and satellite systems. Contact Canup at
This giant impact scenario
of lunar formation represents an important piece of
our overall understanding of the origin of the terrestrial, or Earth-like, planets
in the solar system, which include Mercury, Venus, Earth, and Mars. In turn,
understanding the origins of planets in our solar system is the key to determining
the likelihood of habitable planets in extrasolar systems.
A research group in the
Space Studies Department of the Southwest Research
Institute (SwRI) Instrumentation and Space Research Division studies the
origins of planetary bodies using both theoretical and computational methods.
The group includes the author, Institute Scientist Dr. William Ward, Space
Studies Department Director Dr. Alan Stern, Principal Scientist Dr. Harold
Levison, Senior Research Scientist Dr. Luke Dones, and Postdoctoral Researcher
Dr. Daniel Durda. Recently, research and computational facilities funded by
NASA, the National Science Foundation, and SwRI's internal research
program have been directed toward developing improved models of an
impact-triggered formation of the moon and examining the related
implications for the likelihood of similar planet-moon systems around other
Evolving theories of lunar origin
Compared to other moons
and their planets, the Earth's moon is unusual in
several respects. It is large relative to the Earth, with a density that is
abnormally low compared to the terrestrial planets, indicating that it lacks
high-density iron. Recent findings even suggest that the moon's core constitutes
only 2-4 percent of its total mass, compared to a terrestrial core with about 30
percent of the Earth's mass. The Earth-moon system also has an abnormally
large angular momentum per unit of mass, contained in both the Earth's spin
and the moon's orbit, compared to other planet-satellite systems.
SwRI researchers use the smoothed-particle hydrodynamics
method and N-body integrations to model the giant impact
scenario of lunar formation.
Prior to the 1970s, there
were three main theories regarding the origin of the
moon. The first involved a fission event, in which the moon broke off from a
rapidly spinning Earth. A co-formation theory proposed that the Earth and
moon formed contemporaneously as a gravitationally bound pair. The third
theory suggested that the moon formed as an independent planetary body that
was later "captured" by the Earth during a close pass. Each theory had
deficiencies. For example, it was difficult in both the capture and co-formation
models to account for the lack of a large lunar iron core, because both predicted
that the moon formed from the same mix of materials as the terrestrial planets,
which typically contain a more substantial abundance of iron.
One of the main scientific
objectives of the Apollo space program was to
differentiate between these theories to resolve the question of lunar origin.
However, the analysis of lunar samples raised new questions and challenges.
Relative to terrestrial samples, lunar material was discovered to be deficient in
volatile materials -- those that vaporize and escape easily when heated --
implying that the moon had undergone some intense thermal processing
compared to that experienced by the Earth. In addition, the abundance of
siderophile, or "iron-liking," elements in lunar rocks suggested that the moon
was derived from material that had once been part of the mantle of a larger
body with a sizeable iron core.
In 1976 and 1977, two
groups* proposed a new theory for lunar origin: the giant
impact scenario. The idea was that an off-center impact of a roughly
Mars-sized body with early Earth could provide Earth with its high initial spin,
needed to explain the current system's angular momentum, and eject enough
debris into orbit to form the moon. If the ejected material came primarily from
the mantles of the Earth and the impactor, the lack of a sizeable lunar core was
easily understood, and the energy of the impact could account for the extra
heating of lunar material required by lunar volatile depletions.
For nearly a decade,
the giant impact theory was heavily critiqued. The idea
that the moon was the result of a particular large impact event was considered
too arbitrary, and did not fit in well with the existing view of a quiescent planet
formation process. In 1984, a conference devoted to lunar origin prompted
critical comparison of the existing theories. The giant impact theory emerged
from this conference with nearly consensus support, enhanced by new models of
planet formation that suggested large impacts might indeed be common events
in the end stages of terrestrial planet formation. Such models demonstrated that
the relatively quiescent stage of planetary growth continued only until young
planets grew to sizes ranging from lunar to Mars-sized, and that the final stages
of growth were characterized by collisions among tens to hundreds of these
large, planet-sized bodies. In the course of the many impacts apparently
required to yield the final four terrestrial planets, it did not then seem so
unreasonable that one of the impacts would be of the type required to yield the
A time sequence computer simulation (beginning top left) shows a
potential moon-forming impact modeled using the
smoothed-particle hydrodynamics method. The mantles of the
Earth and impactor are represented by red particles that change to
orange when heated, while the iron cores are shown with blue
particles that change to green with increasing temperature. The
initial impact imparts a counterclockwise spin to the Earth, and part
of the impacting body temporarily re-coalesces before colliding
with the Earth a second time.
After the second hit, material primarily from the impactor's mantle
is sheared into a disk of debris; the total amount of iron left in orbit is
consistent with the moon's small core. The total time simulated by
this run is about a day. Simulations such as this one demonstrated
that a Mars-sized body colliding with the Earth with something
close to the current Earth-moon system angular momentum could
leave roughly a lunar mass worth of material in orbit. (Courtesy Dr.
Alastair Cameron, Harvard University)
Modeling the moon-forming impact
Clearly the impact of
a Mars-sized body with Earth cannot be reproduced
experimentally. For this scale of an event, researchers must rely on computer
simulations that can be compared with experimental results at small sizes and
then extrapolated to the larger scales of interest with relative confidence. For
modeling the moon-forming collision, impact energies of interest are high
enough to cause excessive heating and phase changes, and a hydrodynamic
treatment with an appropriate equation of state is required. The self-gravity of
the material involved in the impact must also be tracked explicitly because of
the large total mass and the deformation of the bodies that occurs during an
impact. Finally, a simulation must be able to both achieve high spatial
resolution at the point of contact between the two planets when they initially
collide, and track the dynamics and thermal properties of material ejected into
The method typically
used to simulate planet-scale collisions is the
smoothed-particle hydrodynamics (SPH) method. Using this technique, the
material in a simulation is represented by a finite number (typically 10,000 or
so) of discrete, overlapping particles that approximate a continuous distribution
of matter. Each particle has a mass, internal energy, and three-dimensional
spatial density probability distribution. At a given point in space, the local
density of material is then calculated by summing over the contributions from
the nearby particles. The evolution of the particles is tracked individually, so
that the computational resolution effectively follows the material wherever it
goes. At each time-step in the simulation, forces such as pressure and gravity
are calculated, and the particles are accelerated accordingly. The equation of
state relates pressure to internal energy and density, taking into account such
factors as latent heat of phase changes.
An SPH simulation of
a potential moon-forming impact requires months of
computational time on a single workstation. However, faster simulation speeds
and increased resolution with greater numbers of particles are possible through
the use of parallel computing. SwRI scientists are currently collaborating with
Dr. Erik Asphaug, of the University of California at Santa Cruz, in the
development of parallelized SPH methods.
Forming the moon
To model the accumulation
of the moon from the impact-ejected debris, SwRI
researchers track the interactions between particles as they orbit the Earth. This
is typically done by using an N-body integration method, which explicitly
calculates the gravitational force on each particle caused by every other particle
in the simulation at each time-step. When orbiting particles collide with low
enough impact energies, the result is a gravitationally bound aggregate. Such
aggregates continue to grow in size as they collide, forming larger and larger
bodies in a process called accretion.
Based on this model,
the first question to be addressed was, why would a swarm
of debris orbiting close to the Earth yield a single large moon when we find
systems of multiple moons and rings around the gas giant planets? For a
pre-lunar debris swarm, most of the accretion simulations predict the formation
of one large moon orbiting at a characteristic distance of about 3-5 Earth radii
(12,500-20,000 miles) from the center of the Earth. The moon's current
distance is about 60 Earth radii (240,000 miles). However, we know that the
tidal interaction between the Earth and moon that gives rise to our twice-daily
oceanic tides has also caused the lunar orbit to expand. Thus the moon was
much closer to the earth when it formed, appearing more than 10 times larger in
the sky than it does today.
Putting it all together: The devil is in the details
While the models of lunar
accumulation easily account for why the Earth has
only one moon, they also suggest that the accretion process is very inefficient,
with always less than half of the initially orbiting debris incorporated into the
final moon. This, in turn, means that a more massive initial debris cloud -- one
containing at least two times the lunar mass -- must have been produced by the
impact to yield a lunar-sized moon. For the past two years, SwRI staff have
collaborated with Dr. Alastair Cameron, a professor at Harvard University in
Cambridge, Massachusetts, to identify which sizes and velocities of impactors, as
well as which impact angles, are capable of producing sufficiently massive debris
disks to form the moon.
One class of impacts
that characteristically places sufficient amounts of
material into orbit involves bodies with three times the mass of Mars and more
than twice the current system angular momentum. These impacts yield an Earth
with a moon of the correct size, but leave the system spinning too rapidly. From
the basic laws of physics, it is known that the angular momentum of the
Earth-moon system has been very nearly conserved over the age of the solar
system, with a small amount (less than 10 percent) lost to interaction with the
sun. Thus, for these extremely high angular momentum impacts, one must
invoke some mechanism to significantly slow the Earth's spin after the
moon-forming event, such as perhaps a second massive impact.
Another class of possible
impacts that could produce an appropriately sized
moon yields the correct total angular momentum, but results in an Earth that is
only 60 percent of its current mass. This scenario is also problematic: If the
Earth continued to accumulate solar-orbiting material in large amounts after
the moon was formed, it would have been difficult to prevent the moon from
becoming contaminated with iron-rich material imported in such collisions as
well. Researchers have thus yet to identify a set of impact conditions that yields
the Earth-moon system in the most widely accepted paradigm for lunar origin.
Fortunately for the impact
theory, there are still many avenues to explore. SwRI
researchers are using an algorithm developed by Levison to conduct the first
highly accurate simulations of the final stages of terrestrial planet formation
that will explicitly track the dynamics of the largest impact events. To date,
results from this work suggest that Earth-like planets could experience several
large impact events and that the final masses and angular momentum of the
Earth-moon system may be the combined result of more than one impact.
Simulations have also shown that Earth probably had a significant spin prior to
the moon-forming event, and this could affect the ejecta yield predicted by the
SPH simulations that, to date, have assumed Earth was not rotating prior to the
These results suggest
that new regions of parameter space need to be modeled
using impact and accretion simulations to develop a consistent, plausible model
for the origin of the Earth-moon system. Such research can then be applied to
models of the formation of the Pluto-Charon system -- also believed to be the
result of an impact event -- as well as to the formation of planets and moons in
solar systems around other stars.
As we detect other planetary
systems that are quite different from our own, the
question that drew many astronomers and planetary scientists into the field even
as children arises: Are there other Earths? By learning more about the moon, we
become increasingly aware of its interactions with our planet. In particular, we
now know that the presence of our large moon acts to stabilize the variation of
the Earth's rotational axis, a fact that was first discovered in 1974 by SwRI
Institute Scientist Dr. William Ward, who was then at the Harvard Center for
Astrophysics. Were it not for the moon, the influence of the giant planets in our
system would cause Earth's obliquity -- the angle between the Earth's equator
and the plane of its orbit, whose current value is 23.5 degrees -- to vary wildly
with values as extreme as 0 to 80 degrees. Such variation would probably cause
extreme climatic changes that would render the planet uninhabitable. Thus
having a large moon may be one of the key characteristics necessary for a
habitable Earth-like planet -- making it all the more important to resolve
questions about the origin of our Earth and moon.
* Dr. William Hartmann
and Dr. Don Davis (both of the Planetary Science
Institute), and Dr. Alastair Cameron (Harvard University) and Dr. William
Ward (currently at SwRI, but then at the Jet Propulsion Laboratory), first
proposed the giant impact scenario in 1976 and 1977.
Published in the Spring
1999 issue of Technology Today, published by
Southwest Research Institute.
2. SwRI, UCSC researchers identify the Moon-forming impact
Boulder, Colorado --
15, 2001 -- 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.
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, under funding from the National
Science Foundation and NASA, 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 time.
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
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.
Editors: An image and
animation of the moon-forming event are available
3. New theory links moon's current orbit to its formation via a giant impact
Moon's unusual inclination long-questioned by researchers.
Boulder, Colorado --
15, 2000 -- The mysterious tilt of the moon's
orbit is probably a natural consequence of the moon's formation from a giant
collision with early Earth, according to a new study by scientists at Southwest
Research Institute (SwRI).
The moon's orbit can
be traced backwards in time to reveal that when the moon
formed near the Earth, its orbit was inclined by approximately 10 degrees
relative to the Earth's equator. Most other planetary satellites in the solar
system have orbital inclinations smaller than 1 or 2 degrees. The cause of the
moon's large orbital tilt has long been a mystery.
"The inclination problem
had been one of the last remaining obstacles for the
impact hypothesis of moon formation," says SwRI Institute Scientist Dr.
William R. Ward. The widely favored "giant impact theory" proposes that a
Mars-sized body collided with Earth 4.5 billion years ago, creating a hot disk of
debris from which the moon accumulated. Previous models of the moon's
formation from such a disk predict that the lunar orbit should have been nearly
aligned with the Earth's equator, with only about a 1 degree tilt.
The new theory, published
in the February 17 issue of Nature, proposes that the
moon acquired its large tilt soon after it formed because of a gravitational
interaction with debris left over from the impact event. Modeling results
presented in the paper, authored by Ward and SwRI planetary scientist Dr.
Robin M. Canup, show that the moon could have acquired its 10 degree tilt as a
consequence of the moon-forming impact.
To yield a lunar-sized
moon, the giant impact must place about two lunar
masses of material into an Earth-orbiting disk, according to Canup. In the
model, debris particles in the inner regions of such a disk are prevented from
coalescing by Earth's gravity, which tends to pull objects apart. Instead, the
moon rapidly coalesces at the outer edge of the debris disk, at a distance of about
14,000 miles from the Earth. "The newly formed moon would have likely
co-existed for some time with an inner disk of gas and debris left over from the
impact," says Canup.
After the moon coalesced,
its gravity would generate waves in the inner disk.
The gravitational interaction of the moon with these waves would, in turn,
modify the lunar orbit. The waves are launched at certain locations in the disk
where the motions of disk particles are in resonance with the motion of the
The waves generated at
one such resonance -- where the orbital period of the
moon is approximately three times that of the disk particles -- are called
"bending waves," which corrugate the surface of the disk. The gravitational
attraction between the moon and these rippled waves in the disk then acts to
amplify the tilt of the moon's orbit.
Ward and Canup's model
simulated the interaction of the moon and the inner
debris disk, assuming that the moon formed in an orbit with only a 1 degree tilt.
They found that the interaction of the moon with the bending waves it generates
in such a disk can amplify the lunar inclination to values as high as 15 degrees
before the disk dissipates. The required tilt of about 10 degrees can be achieved if
the disk contained at least 25 to 50 percent of a lunar mass and persisted from
decades to as long as a century. These values are consistent with those predicted
by other models of the impact event.
"This theory explains
the moon's anomalous orbital tilt as a natural
consequence of its formation from a giant impact event," says Ward. "Rather
than producing conflicting evidence, the lunar inclination may now represent
an additional corroboration of the impact event," agrees Canup.
the image to support this story!