A most anticipated analysis in the mission of the space probe Rosetta concerned the water present on the comet 67P/Churyumov-Gerasimenko. The reason is that scientists are looking for a definitive answer to the problem of the origin of water on Earth. Comets are among the main candidates but the analyzes revealed by ESA and published in the journal “Science” show that the water analyzed by Rosetta is different from that on Earth.
That of water is, at least in appearance, a very simple molecule consisting of two hydrogen atoms and one oxygen atom. However, a small amount of hydrogen is actually an isotope which has an extra neutron and is called deuterium. The water containing a deuterium atom and one of normal hydrogen is called heavy water.
On Earth, on average a water molecule every about 3,200 is of heavy water. In other parts of the solar system this ratio is different so its analysis is used to decide if the water in two celestial bodies have the same origin. The ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) instrument of the Rosetta space probe analyzed the steam coming from the comet 67P/Churyumov-Gerasimenko when the spacecraft reached it. The result is that the deuterium is about three times that on Earth.
The comet 67P/Churyumov-Gerasimenko is part of a group of comets known as the Jupiter family comets but there are other groups and this makes the water research more complicated. In fact, so far only the water of the comet 103P/Hartley 2, subject to spectral analysis by the Herschel space telescope in 2011, was found to have a deuterium/hydrogen ratio close to that of the water on Earth.
The comet 103P/Hartley 2 is also part of the Jupiter family comet. The fact that two comets of the same family contain very different percentages of heavy water is surprising. Kathrin Altwegg, the principal scientist in charge of ROSINA and also the lead author of the article in “Science”, has suggested that these comets were formed in an area of the young solar system wider than they previously thought.
In any case, these results rule out the possibility that the comets of the Jupiter family comet can all contain the same water of the Earth. This leaves open the possibility that the water has been brought to Earth mainly by asteroids. This may have happened during the late heavy bombardment, a period between 4.1 and 3.8 billion years ago when a huge amount of asteroids hit the Earth and the other planets of the inner solar system.
Asteroids generally contain little water, especially when compared with comets. However many measurements in meteorites fallen on Earth show that the deuterium/hydrogen ratio in the water they contain is close to that of the Earth. The research will continue but these new data seem to give a plausible answer.