May 9, 2013
Moon’s Water Supply Came From Earth, Not From Comet Impacts
John P. Millis, Ph.D. for redOrbit.com - Your Universe Online
But the rocks brought back by the lunar explorers revealed that the Moon, while still dry compared to Earth, contained a surprising amount of water. Furthermore, the composition of the Moon led scientists to the current Big Impact hypothesis of how the Moon was created.
It is believed that a massive body, perhaps the size of Mars, collided with the still-forming Earth, ejecting massive amounts of material into space. Earth´s gravity kept it in orbit around our planet until, over time, the matter coalesced into the Moon that we see today.
While it has become clear that the Moon itself came from the Earth, the question of where its water came from is still a mystery. The creation event occurred early in the formation of Earth, so scientists have not been certain of the presence of water even on Earth´s surface at that time.
Even more puzzling is the fact that this impact would have released a significant amount of energy, offering this puzzle: How would water, if present in the first place, have survived to be used in the Moon´s formation?
A research team — led by Alberto Saal, associate professor of Geological Sciences at Brown University, in collaboration with Malcolm Rutherford also from Brown, Erik Hauri of the Carnegie Institution of Washington, and James Van Orman of Case Western Reserve University — believe that they have found clues to the answer.
Looking at Melt inclusions — tiny dots of volcanic glass trapped within crystals called olivine — the team was able to determine the ratio of hydrogen to deuterium (hydrogen atoms with an extra neutron). This fraction acts as a fingerprint, since water throughout the solar system will have different ratios depending on their distance from the Sun when formed.
What they found was that the ratio not only matched that of Earth, but was aligned with carbonaceous chondrites — meteorites originating in the asteroid belt near Jupiter. This is a departure from previous hypotheses that suggested that the Moon's water was brought by comets, which would have a significantly higher percentage of deuterium having formed much further from the Sun.
“The measurements themselves were very difficult,” Hauri said in a statement, “but the new data provide the best evidence yet that the carbon-bearing chondrites were a common source for the volatiles in the Earth and Moon, and perhaps the entire inner solar system.”
But would we have expected the ratio to be the same for both the Earth and Moon?
According to Saal, “the simplest explanation for what we found is that there was water on the proto-Earth at the time of the giant impact. Some of that water survived the impact, and that´s what we see in the Moon.”
Of course, there is still the matter of how the water survived the impact in the first place. “The impact somehow didn´t cause all the water to be lost,” Saal said. “But we don´t know what that process would be.”
“Our work suggests that even highly volatile elements may not be lost completely during a giant impact,” said Van Orman. “We need to go back to the drawing board and discover more about what giant impacts do, and we also need a better handle on volatile inventories in the Moon.”