How Theia Helped Make Earth Habitable

How did Earth, uniquely among the Solar System’s rocky planets, come to support life? Amid so much cold, lifeless space, why did our world become warm, welcoming and capable of sustaining life?

The explanation is complicated and has many parts. One part lies in cosmochemistry, the interdisciplinary study of how chemical elements are distributed.

The Solar System is a dynamic environment in which everything moves. It was more disorderly still 4.5 billion years ago, when planets were continuing to form and planetesimals and planetary embryos sped through space, colliding with each other.

Somewhere within this turmoil, Earth acquired an unusually large share of carbonaceous chondrites, along with the amino acids and other chemicals that could enable life.

Cosmochemical research indicates that carbonaceous chondrites striking the young Earth supplied between 5% and 10% of its mass. Studies also suggest that the Theia impactor, which formed the Moon, contributed a substantial portion of this material.

To examine these proposals more closely, three researchers carried out dynamical simulations of the Solar System’s formation to determine whether they could reproduce this outcome.

The study is entitled “Dynamical origin of Theia, the last giant impactor on Earth.” Its lead author is Duarte Branco of the Institute of Astrophysics and Space Sciences at the Lisbon Astronomical Observatory in Portugal. The research is due to appear in the journal Icarus.

Carbonaceous chondrites and the early Solar System

A central distinction in cosmochemistry separates carbonaceous chondrites (CCs) from non-carbonaceous meteorites (NCs). This division places the Solar System’s meteorites into two populations and points to two separate reservoirs of material within the Solar System.

CCs formed at greater distances from the Sun, probably beyond Jupiter, and brought relatively abundant volatile substances with them, including water and organic compounds. NCs, which include iron meteorites, contain fewer volatiles.

To assess whether Theia could have brought CCs and volatiles to Earth, the team performed detailed Solar System simulations. These N-body calculations modelled the late stages of terrestrial planet growth.

The calculations started after the Solar System’s gaseous disc had dispersed, during the later phase of planetary formation. The remaining solid mass was apportioned between planetesimals and planetary embryos.

They incorporated CCs scattered inwards while Jupiter and Saturn were still growing and accreting material. Since planetary embryos are larger than planetesimals, they are more likely to interact with terrestrial planets and deliver CC material.

The researchers conducted three categories of simulation. The first, small only, contained small CC bodies alone: planetesimals. The second, large only, contained only large CC bodies, or planetary embryos. The third, termed the mixed scenario, included both CC planetesimals and embryos.

For 10 simulations in each scenario, they also modelled the dynamical instability of the giant planets. Astronomers refer to this as the “Nice model”, which describes giant planets moving from the orbits in which they originally formed.

The researchers aimed to establish how CCs and NCs were distributed across the Solar System, and why Earth incorporated more CCs than the other rocky planets, particularly Mars. They also investigated whether the Theia collision may have delivered much of Earth’s CC material.

Theia and Earth’s carbonaceous chondrites

One unambiguous finding was that giant-planet instability, and Jupiter’s move to another orbit in particular, strongly affected the amount of CC material accreted by Earth.

Adding giant-planet dynamical instability produced still more notable results. “The giant planet instability dramatically changed the evolution of the system causing a strong pulse of eccentricity excitement, which lead to a wave of collisions and ejections,” the authors write. Nevertheless, the system’s final configuration changed little.

The simulations place particular importance on the Theia impactor. Earlier work indicates that Theia may have been a carbonaceous body. If so, the collision may have been responsible for much of the material that made Earth life-supporting.

“In the mixed scenario with no giant planet instability, Earth's final impactor included a CC component in more than half of all simulations. In 38.5% of simulations, the final impactor was a pure CC embryo, and in another 13.5%, the impactor was an NC embryo that had previously accreted a CC embryo,” the researchers write.

Taken together, the simulations depict the early Solar System as containing two distinct planetesimal rings: an inner ring of rocky planetesimals and an outer ring of carbonaceous chondrites.

As the ice giants later migrated inwards, they drove CC material into the inner Solar System. Some became lodged in the asteroid belt, whereas the more massive objects were preferentially scattered into the orbital paths of the rocky planets.

“The late-stage accretion of the terrestrial planets involved a series of giant impacts between NC embryos and planetesimals, with occasional impacts of CC objects,” the authors explain.

This model accounts for several Solar System features. It explains the terrestrial planets’ masses and orbits, as well as the orbital arrangement of asteroids. It also agrees with the CC mass fractions of Earth and Mars, with Mars lacking Earth’s comparable concentration of CC material.

Were the small only simulation accurate, with CC material existing solely as planetesimals, Earth and Mars would have approximately the same CC mass fraction.

The team sought to demonstrate, consistently with earlier research, that Theia could have been Earth’s final major impactor and that it contained plentiful CC material. Their results appear to support that conclusion.

In the simulations, Earth’s last giant collision involved Theia, an object with higher CC concentrations that helped make Earth habitable. This outcome accords with existing scientific thinking.

Jupiter’s role in making Earth habitable

The work indicates that the final impact happened between 5 to 150 million years after gas dispersal, with a large proportion occurring within 20 to 70 million years. Although the timing of the Theia impact remains uncertain, these findings fall within those uncertainties.

The results also reinforce conclusions that CC embryos and planetesimals could have been accreted throughout Earth’s formation, although they were concentrated during its later growth stages.

“Within the context of this scenario, the last giant impactor on Earth contained a CC component in roughly half of all of the mixed simulations,” the authors write.

“In the majority of these (38% of simulations), Theia was a pristine CC embryo, and in the remainder of cases Theia was an NC embryo that had previously accreted a CC embryo.”

The study further shows that Jupiter was vital to the Solar System’s architecture. Besides truncating the asteroid belt, it helped determine the terrestrial planets’ final compositions by scattering CC material from the outer Solar System into the paths of rocky planets, especially Earth.

Countless conditions had to align for Earth to become the life-sustaining world it is today. How probable similar worlds are elsewhere remains unknown. An exoplanet may require more than a position within a habitable zone to sustain life.

There may be an overwhelming number of factors that must align, including migrating outer giant planets that transport carbon to rocky worlds in habitable zones.

This article was originally published by Universe Today. Read the original article.