Hypothetical Theia’s collision with early earth and the formation of our moon


The hypothetical ancient planet Theia

moon-formation-theories-debated_69202_990x742-640x480Theia /θə/ is a hypothesized ancient planet in the early Solar System that according to the giant impact hypothesis collided with the Early Earth around 4.533 billion years ago (BYa). Theia was about the size of Mars, and, had it not collided with Earth with a glancing blow, it could have destroyed Earth. In the end, Theia’s debris gathered together around Earth to form what was the early Moon. After the debris from the collision flew into space, some scientists think that it originally formed two moons which later merged to form the single Moon we know today. Others, however, believe that the two-moon hypothesis is not necessary to explain the difference in the faces of the near and far side of the Moon.


Theia’s Orbit

lagrangepoints_stripTheia was thought to have orbited in the L4 or L5 spots in the Earth’s orbit. It grew to a Mars-like size and, through gravitational perturbation from Venus, was moved around and eventually set on a collision course with Earth



The meaning of Theia’s name

titan goddess theia-1Theia (Thia, Thea, Euryphaessa) — Pre-Hellenic Titaness and a goddess of light. The name Theia alone means simply “goddess” or “divine”

Theia was the Titan goddess of shining, associated with shining light, shining metals or jewels. Her other name Euryphaessa means “wide-shining” and therefore she was connected with all that is shinning. In Pindar`s Isthmian Odes, Theia is described as the goddess of shining after whom the men honored gold as the most powerful shining object. She was also the goddess of sight, because ancient Greeks believed that eyes emitted beams of light which allowed them to see what they looked upon. She was one of the twelve titans and one of the six daughters of Uranus (the sky) and Gaea (the earth). Theia was also, like her sisters Phoebe and Themis, associated with prophecies. She had a shrine in Thessaly.

The mother of the Sun, the Moon, and the Dawn
In Greek mythology, Theia married her brother Hyperion (Titan god of light) and together they gave birth to Helios (the sun), Selene (the Moon) and Eos (the dawn).

Theia in Sciences
Theia’s mythological role as the mother of the Moon goddess Selene is alluded to in the application of the name to a hypothetical planet which, according to the giant impact hypothesis, collided with the Earth, resulting in the Moon’s creation.

Theia‘s alternate name Euryphaessa has been adopted for a species of Australian leafhoppers Dayus euryphaessa (Kirkaldy, 1907).

The Collision

This dynamical simulation shows how asteroids linger in the gravitational well of a Lagrange point of the Sun-Jupiter system. The principle of Sun-Earth Lagrange points is the same. Credit: Prof. Aldo Vitagliano/SOLEX.Theia orbited the Sun at around the orbit of Earth at the L4 or L5 Lagrangian points, but was perturbed by Jupiter and Venus into a collision with the early Earth. Fortunately, Theia struck Earth with a glancing blow and ejected many pieces of both Earth and Theia. These pieces either formed one body that became the Moon, or formed two moons that eventually merged to form the Moon. Had Theia struck Earth head-on, it would have led to the destruction of both planets, creating a short-lived asteroid belt between the orbits of Venus and Mars.


The Theories

Apollo_12_moon_rock_display Throughout history scientist have dealt with the question of how our Moon was formed. There have been several proposed theories and hypotheses. They all at some point have been disfavored because evidence has shown that they couldn’t possibly be the explanation of how our Moon formed. The first theory is the great capture theory, which states that the Earth captured the Moon, the second theory is the co-accretion theory it states that the Earth and Moon formed at the same time, the third theory is the fission theory which states that the Moon was part of the Earth and at some point in time it broke off. The last and final theory is the most commonly accepted by all scientists; it is the giant impact theory. This theory states that a Mars-sized object collided with our Earth and from the debris of the collision, our Moon formed becoming Earth’s only natural satellite. The Apollo missions also had a great deal in the discovery of which one was the most accepted theory. Every time one of these missions was performed, evidence was brought back that seemed to prove one of the other three theories wrong.

The Major Theories and their Flaws

1. The Capture Theory: The Moon was formed somewhere else and was later captured by the gravitational field of the Earth. The moon had an independent orbit before it got caught up by the earth’s gravity. This theory fails to explaine why the earth and the moon have a shared chemistry. Both bodies have the same collection of oxygen isotopes.
Flaw: This doesn’t seem to go up for any other celestial body in our solar system. So how can the earth and the moon match so perfectly?

2. The co-accretion theory (also known as the Condensation Theory): The Moon and the Earth condensed together from the original nebula that formed the Solar System. The moon formed just like the earth, from the beginning when the Protoplanetary disk accreted out of the dust from our solar system, more or less at just where it is right now. The earth and the moon would have been a binairy system from the very start.
Flaw: This theory might explain the shared isotopes but it doesn’t explain the moon’s particular orbit.

3. George Darwin’s Fission Theory: The Moon was once part of the Earth and somehow separated from the Earth early in the history of the Solar System. The present Pacific Ocean basin is the most popular site for the part of the Earth from which the Moon came. According George Darwin’s theory the moon was spinning so fast that it left the ocean behind.
Flaw: This would explain the isotopes but does not fit with our current understandings of the moon, the earth or the ocean.

The main reason why the Apollo Missions were launched was to explore the three theories that NASA believed were how the Moon was formed.  Their goal was to choose the right theory, but not one of the three theories seemed to be consistent with the projects (Apollo Missions) on how the Moon or any of the other planets formed. One piece of evidence that proved that none of the theories were favored was that the Moon rocks were depleted in elements that vaporize quickly; therefore, the diminution in these elements cause rocks to be very dry. As the Moon heated up it lost almost all of its H2O. It got to a point where it heated up hotter than planet Earth. Since all three original theories were disfavored, the Apollo missions found some evidence that dispute these theories; therefore, another theory came along: giant impact theory. With the help of the Apollo missions amongst many others that have been launched today the most accepted theory is the giant impact theory.

4. Giant Impact Hypothesis:

impact theory

The Giant Impact Theory

The giant impact hypothesis has been a widely accepted theory for how the Earth-moon system formed. In the giant impact scenario, the moon forms from debris ejected into an Earth-orbiting disk by the collision of a smaller proto-planet, with the early Earth. One of the challenges to the longstanding theory of the collision, is that a Mars-sized impacting body, named Theia, whose composition likely would have differed substantially from that of Earth, likely would have left Earth and the moon with different chemical compositions, which they are not.

The lunar rock samples that the astronauts from the Apollo 11, 12 and 16 missions on the moon took back to earth were very similar in composition to the earth.

Flaw: The Giant impact theory of A Mars-size body called Theia, collided with the earth with just the right speed and angle for the moon to be formed by the debris, would explain the moon’s orbit, but again, how did the two end up with the same make up of oxygen isotopes?

If however, after colliding, the two similar-sized bodies then re-collided, forming an early Earth surrounded by a disk of material that combined to form the moon, the re-collision and subsequent merger left the two bodies with the similar chemical compositions seen today.


Moon Made by Giant Impact with Earth – New Evidence | Video Credit: Science/AAAS 28 aug. 2014
Samples from Apollo missions containing isotopic oxygen supporting the theory that a Mars-sized object (called Theia) slammed into Earth setting off a series of conglomeration events that ultimately created the Moon. (Full Story)


Theia’s date with destiny: possible conditions leading to a Giant Impact – Billy Quarles (SETITalks)
SETI Institute 13 jan. 2015 – Billy Quarles | NASA Ames

The Giant Impact theory is the leading explanation for the Moon’s origin, but mysteries remain in the conditions leading up to the event. Collisions were common during the turbulent infancy of the Solar System and led to a small set of terrestrial planets.

Dr. Quarles presents a numerical model that considers the penultimate orbits of the Solar System, when five terrestrial planets are present. From this model, he indicates which starting parameters for Theia (the proto-Moon) result in a late Giant Impact consistent with physical dating constraints.

He also finds that the likely semimajor axis of Theia, at the epoch when the simulations begin, depends on the assumed mass ratio of the Earth-Moon progenitors (8/1, 4/1, or 1/1). The low eccentricities of the resulting terrestrial planets are most commonly produced when the progenitors have similar semimajor axes at the epoch when the model starts.

Additionally, Dr. Quarles will show that perturbations from the giant planets can affect the dynamical evolution of the system leading to a late Moon Forming Giant Impact.


Moon was produced by head-on collision – The key to reconstructing the giant impact was a chemical signature revealed in the rocks’ oxygen atoms. By University of California – Los Angeles Astronomy.com |  Published: Friday, January 29, 2016


Theia’s date with destiny: possible conditions leading to a Giant Impact  – SETI Institute 13 jan. 2015
Moon Made by Giant Impact with Earth – New Evidence | Video | Full Story Credit: Science/AAAS 28 aug. 2014
Faceoff! The Moon’s oddly different sides Astronomy, August 2014, 44-49.
The origin of the Moon. DOI: 10.1098/rsta.2014.0289 Published 11 August 2014
The giant impact hypothesis: past, present (and future?) DOI: 10.1098/rsta.2013.0249 Published 11 August 2014
New approaches to the Moon’s isotopic crisis 
Geochemical arguments for an Earth-like Moon-forming impactor. DOI: 10.1098/rsta.2013.0244 Published 11 August 2014
Isotopes as tracers of the sources of the lunar material and processes of lunar origin Earth used to have TWO moons… but one was destroyed in a giant lunar collision. Mail online. 2011-08-04.
Alternate Theories of Moon Formation – CLOE feb 2010
STEREO Hunts for Remains of an Ancient Planet near Earth. NASA.  2009-04-09.
Join STEREO and Explore Gravitational “Parking Lots” That May Hold Secret of Moon’s Origin NASA April 9, 2009
Twin Spacecraft May Reveal Secret of Moon’s Origin. NASA  04-09-09
NASA scientist Jen Heldmann describes how the Earth’s moon was formed – NASA Oct 7th, 2008
The Theia Hypothesis: New Evidence Emerges that Earth and Moon Were Once the Same. The Daily Galaxy. 2007-07-05

More info
Apollo Program wiki
Apollo 11Apollo 11 lunar sample display wiki
Apollo 11 transcripts at Spacelog
Apollo 11 Detailed mission information by Dr. David R. Williams, NASA Goddard Space Flight Center
Apollo 11 & 12 Recovery” written by Bob Fish for the USS Hornet Museum
Apollo 12 wiki
Apollo 12 Science Experiments at the Lunar and Planetary Institute
Analysis of Surveyor 3 material and photographs returned by Apollo 12 (PDF) 1972
Examination of Surveyor 3 surface sampler scoop returned by Apollo 12 mission (PDF) 1971
Apollo 12 “Pinpoint For Science ” – NASA Space Program & Moon Landings Documentary on YouTube
Apollo 16 wiki
Apollo 16 Science Experiments at the Lunar and Planetary InstituteNASA Lunar Science Institute | Solar SystemTheia (Mythology) | Greek Goddess TheiaHow the Moon was formed (Video’s)History of Solar System formation and evolution hypothesesResearch links
The Apollo Missions | NASA
Exploration Research (SSERVI)NASA GRAIL Mission
Sungrazer Project
STEREO website
Lunar Reconnaissance Orbiter (LRO)
Southwest Research Institute (SwRI), San Antonio, Texas
Solar TErrestrial RElations Observatory — STEREO home page
Planets Glossary JPL
JPL: Jet Propulsion Laboratory
Origins Program Factsheet (pdf)
PDS: The Planetary Data System
Lunar Quest Program
LADEE: Lunar Atmosphere Dust Environment Explorer
NASA’s Future: US Space Exploration Policy

2 thoughts on “Hypothetical Theia’s collision with early earth and the formation of our moon

  1. An interesting post is worth a comment. I think that you should write more on this topic, it might not be a taboo subject but generally people do not speak enough about such topics. To the next. Cheers


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