The high-energy impact about 100 million years after our Solar System formed, resulted in the two colliding planetary objects completely merging to create the Earth-Moon system we have now. An international research team led by an Open University scientist has discovered that most of Earth’s water was present before the catastrophic impact and not delivered later by small asteroids and comets as previously thought. This suggests liquid water can survive giant impacts and could be abundant on exoplanets, making it more likely that life exists beyond the Solar System.
A collision between two proto-planets with unique isotopic compositions is thought to have created the Earth-Moon system. However, explaining why the Earth and Moon don’t then themselves have unique isotopic characteristics, as most planets in the Solar System do, has been challenging.
To resolve this, some scientists have proposed a high-energy collision model in which isotopes between the two were nearly equally mixed upon impact; any differences, then, may have resulted from subsequent impacts to the rocky planets, later in time.
To better understand the likelihood of this scenario, Open University researcher Richard Greenwood and co-authors compared the oxygen composition of lunar rocks brought back from all six of the NASA Apollo landings on the Moon with volcanic rocks from the ocean floor here on Earth.
The team detected only a small difference between the lunar and terrestrial rocks.
“Our analysis showed a 3- to 4-ppm (parts per million) difference between the oxygen isotopic concentrations of the lunar rocks and the terrestrial basalts, but no significant difference between the lunar samples and terrestrial olivine, a common mineral in Earth’s subsurface,” the scientists explained.
“This demonstrates how efficiently the two colliding planets mixed, and also places strict limits on the types of material that could have been added to the Earth afterwards.”
“If most of the water on Earth had arrived after the collision, we would expect the lunar and terrestrial rocks to have distinctly different oxygen compositions. This suggests that liquid water on Earth must have existed at an earlier stage, prior to the Moon-forming impact.”
“Because water is such a vital ingredient for life, we rightly see it as precious,” Dr. Greenwood said.
“Our research shows that water is also extremely resilient and can survive an event as catastrophic as two planets colliding.”
“What’s even more fascinating is that, because this worked for the Earth and the Moon, it must also work for planets beyond our Solar System. Exoplanets with water on their surfaces may be much more common than we previously thought. And where there’s water, there could also be life.”
The research is published in the journal Science Advances.
Richard C. Greenwood et al. 2018. Oxygen isotopic evidence for accretion of Earth’s water before a high-energy Moon-forming giant impact. Science Advances 4 (3): eaao5928; doi: 10.1126/sciadv.aao5928