- Title: The Feeding Zones of Terrestrial Planets and Insights into Moon Formation
- Authors: Nathan A. Kaib & Nicolas B. Cowan
- First Author’s Institution: Department of Terrestrial Magnetism, Carnegie Institution for Science, Washington, DC
- Paper Status: Accepted for publication in Icarus
- Title: A Primordial Origin for the Composition Similarity Between the Earth and the Moon
- Authors: Alessandra Mastrobuono-Battisti, Hagai B. Perets, & Sean N. Raymond
- First Author’s Institution: Department of Physics, Technion, Israel Institute of Technology, Haifa
- Paper Status: Accepted for publication in Nature
Cover art by Dana Berry, Source: Robin Canup, SWRI
Earth’s moon is peculiar in our Solar System, and in many ways it is an amazing cosmic coincidence. Our moon is the largest known moon compared to the size of its host planet (sorry Pluto, you and Charon don’t count). Though she is creeping away from Earth a few centimeters every year, humans came about on this planet at the prime time to catch our moon at the exact distance to perfectly obscure the Sun during a solar eclipse. And some astrobiologists posit that humans may not be here at all if it weren’t for the tide pools our oversized moon synthesized on Earth that helped to render the first forms of life a few billion years ago. But how about what our moon is made of? Is its composition a cosmic coincidence or a likely result from the conditions of our early Solar System? Two recent studies investigated this question, and surprisingly arrived at completely contradicting results.
The Impact of Earth
The canonical theory of how our moon came to be is the giant impact theory: about 4.5 billions years ago during the late stages of planet formation in our Solar System, a Mars-sized body (which is referred to as Theia) delivered a glancing blow to the young Earth. The resulting debris from this collision then coalesced to form what is now our Moon. This theory does a good job of explaining why the spin of the Earth and Moon have similar orientations and why the Moon is depleted of iron, and has been able to properly recreate the Earth-Moon system using smoothed-particle hydrodynamic simulations.
Originally, this theory was also supported by the compositional similarities between the Earth and Moon. From analysis of lunar meteorites and lunar samples brought back by the Apollo missions, scientists have found that there are a number of stable isotopes that the Earth and Moon have nearly identical quantities of, whereas meteorites from other Solar System bodies such as Mars and Vesta have drastically different proportions of these isotopes. This was thought to be an indication that the Earth and Moon had a common origin – the Moon formed from a plethora of material that was ejected from the early Earth when it was struck by Theia. Studies simulating the Moon’s formation have found that this is not the case; after the impact the Moon forms primarily of the impactor Theia’s material rather than the proto-Earth’s material. This realization puts some holes in the giant impact theory, since it requires that the protoplanet Theia formed in a part of the planetary disk compositionally identical to Earth. Though some theories have been cooked up to explain the compositional similarity of the Earth and Moon (such as a fast-spinning Earth being struck by a large impactor, or an extremely high-velocity impactor), all so far require fine-tuning of initial conditions, which makes them unlikely scenarios.
The two papers of today’s post investigated the likelihood that a planet’s last major impactor is isotopically similar to the planet it hits, as is apparently the case for Theia and Earth. Both studies used the Mercury integration package (a common software used to study Solar System dynamics) to model the planet formation and late accretion processes of the early Solar System, and used an oxygen isotope called oxygen-17 as a gauge for the how similar planetesimals are in the simulation. Oxygen-17 is one of the best measured, and strikingly similar, stable isotopes evidencing Earth-Moon similarity with a difference of less than 0.016% in terrestrial and lunar rocks. However, the two studies came to contradictory conclusions: Kaib & Cowan (paper 1) found it very unlikely that Earth and Theia would form with similar compositions, and Mastrobuono-Battisti et. al (paper 2) concluded the opposite. Why did they differ? Here is a list of some of the differences between the two studies that may have led to conflicting conclusions:
1) Role of big planets? The behemoths of our Solar System, Jupiter and Saturn, play an important role on the formation of the inner rocky planets. Since orbital characteristic such as the eccentricity of these big planets may have changed since this time in the Solar System’s history, one cannot necessarily assume the orbital characteristics we see of these planets today. Both studies used a variety of configurations of Jupiter and Saturn that affected the accretion disk and feeding zones (the regions in the planetary disk from where developing planets grab material). Figure 1 shows how differing initial conditions of Jupiter, Saturn, and the accretion disk altered the types of planets that formed in the simulations of paper 1.
2) Where is the oxygen? The distribution of oxygen-17 in our Solar System’s planet-forming disk is unknown, and could potentially change the outcome of the analysis. Both studies used a linear distribution of oxygen-17 (the amount of oxygen-17 linearly changes with distance from the Sun in the initial disk), and paper 1 also investigated other possibilities: a bimodal distribution, a step function distribution, and a random distribution, though they found that these distributions did not affect their conclusions.
3) How much of the Earth went into the Moon? Though most simulations have the Moon being primarily composed of material from the impactor Theia, the percentage of the proto-Earth that gets mixed in is up for debate. Paper 2 was less stringent with their criteria for the impactor’s composition, because they allowed larger percentages of the initial planet to be mixed in with the impactor to form its moon. These percentages are consistent with Moon-forming simulations.
4) When is a planet the Earth? Paper 1 also considered only planets that were deemed “Earth analogs” and impactors that were “Theia-analogs” in their conclusions. Only planets that had a mass and orbital distance similar to Earth and impactors of Earth that were consistent with the predicted mass of Theia could lead to an Earth-Moon system. The compositional similarity of these bodies was then analyzed. Figure 2 shows the distribution of Earth and Theia analogs in the simulations of paper 1, as well as the distribution of planets deemed Venus- and Mars- analogs. Paper 2 considered all collisions between a planet and its last impactor in their conclusions, but also investigated the likelihood of collisions between Earth- and Theia-like bodies.
So what was determined by the two studies? Paper 1 found that less than ~5% of Earth-analogs were last struck by an impactor that was compositionally as similar to it as the Earth is to the Moon, and therefore the formation of a moon like Earth’s is a statistical outlier. Paper 2 determined that ~50% of all planets were last struck by an impactor compositionally consistent with what is seen in the Moon, assuming about a fifth of the impacted planet’s material was used in the synthesis of the moon. Both studies agreed that the feeding zones of terrestrial planets aren’t exclusive but rather shared among the inner planets, yet come to shockingly different conclusions from similar studies. The drastic differences most likely came from paper 1 only considering Earth-Theia analogs in their conclusions, and paper 2 allowing the Moon to contain a significant fraction of proto-Earth material. It seems that the origin of our nearest celestial neighbor may still be shrouded in mystery, and further analysis will need to be done to determine if our moon’s formation was a likely outcome or a cosmic coincidence.