The process by which the Earth formed left its crust lacking elements essential for life. A new study has proposed our planet's richness in these vital materials is a result of being hit by a large body – almost certainly the same impact thought to have produced the Moon. If true, the theory is significant for the prospects of extraterrestrial life.
The Earth is the only habitable planet we know. It also has a remarkably large satellite relative to its size. Astronomers have long wondered if these facts are connected. Many theories have been produced for how the Moon's great size could have facilitated our development, creating the conditions for a technological species to evolve. For example, it's angular momentum may stabilize the seasons.
Now Rice University graduate student Damanveer Grewal has pushed the idea of the Moon's importance further back. However, according to Grewal's theory, it is not the Moon itself that is so vital, but the process by which it was formed.
The dominant explanation for the Moon's formation is that a collision with an object the size of Mars threw material into orbit that eventually coalesced into the Moon, although the hypothesis faces competition from the idea there were many smaller impacts instead.
Light elements that easily turn to gasses can escape rocky planets close to the Sun, leaving them low in carbon and nitrogen, among other elements. In the quest to explain Earth's richness in these elements, so fundamental to life, astronomers have produced the “late veneer” theory, that it was delivered by meteorites after the planet's formation.
However, chondrite meteorites have 20 times as much carbon as nitrogen, while the Earth has a 40-1 ratio. Grewal argues in Science Advances the extra carbon came from an object large enough to have a planetary core that was rich in sulfur.
When Grewal modeled how carbon and nitrogen would move in a planet-sized object he found the core's sulfur content is crucial. Carbon is absorbed into low-sulfur cores, but excluded as the sulfur content rises. Meanwhile, Grewal said in a statement. "Nitrogen was largely unaffected,"
Using this information to model a wide variety of scenarios in which carbon, nitrogen, and sulfur could be delivered to Earth, Grewal said: “All the evidence – isotopic signatures, the carbon-nitrogen ratio and the overall amounts of carbon, nitrogen, and sulfur in the bulk silicate Earth – are consistent with a moon-forming impact involving a volatile-bearing, Mars-sized planet with a sulfur-rich core."
Since most planetary scientists are already convinced such an event occurred, causing the Moon's formation, the scenario is easy to believe. The cores of the colliding bodies merged, taking the incoming sulfur and nitrogen out of reach of the crust and mantle, but leaving the carbon behind. Grewal argues a steady rain of smaller meteorites could not produce the mix of elements we see.
Grewal and co-authors say the more spread out the sources of a planet's building blocks within the proto-planetary disk, the more likely it is there will be enough volatile elements to make life possible.
The big question with such research is what implications it has for the chances of life elsewhere. The answer to this depends on the likelihood of a collision as large as the one Grewal describes. It doesn't need to have come at the perfect angle to create a giant moon, but it has to have occurred during a narrow window of time in the planet's formation, possibly making it quite unlikely.