Origin of the Moon


three failed theories for origin of the moon:

presently favored theory for origin of the moon:

Origin 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
     years.
 
 
 

                        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
                        (303) 546-6856.
 
 
 

     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
     stars.

     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
     moon.
 
 
 

                        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
     distant orbits.

     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
     impact event.

     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.

     Conclusions

     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 -- August 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.

     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, 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
     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.

     Editors: An image and animation of the moon-forming event are available
     from http://www.swri.org/press/impact.htm.
 
 
 

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 -- February 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
     moon.

     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.

     EDITORS: Download the image to support this story!