Did comets bring water to Earth?

Observations of Hartley 2 have revealed the first comet with water similar to that on our home planet

October 05, 2011

Not only the impacts of asteroids, but also comets may have provided Earth with large parts of its water. This is a result of new measurements performed by ESA's space observatory Herschel that were led by scientists from the Max Planck Institute for Solar System Research (MPS) in Germany. The researchers were able to identify 103P/Hartley 2 as the first comet who’s water is characterized by a similar ratio of deuterium to hydrogen as Earth's water. Approximately one year ago, Hartley 2 had passed Earth in a distance of only 18 million kilometers thus allowing for the highly sensitive measurements.

„The water molecules within the coma emit a characteristic radiation in the far infrared“, says Hartogh. This also holds true for the heavier cousin of water: water moleclues in which one hydrogen atom has been replaced by a deuterium atom. „From this characteristic radiation we can determine the ratio of deuterium to hydrogen“, he adds. However, since the heavy water is very rare, its radiation intensity is extremely weak. Nevertheless, with Herschel's HIFI instrument, the most sensitive detector for water vapor, the researchers were able to detect the molecule with an astonishingly good signal-to-noise ratio.

„Our measurements showed that within the comet’s water there is one deuterium atom to every 6200 hydrogen atoms“, says Hartogh. This ratio is very close to that on Earth. „Comets like Hartley 2 therefore have to be taken into account when looking for bodies that delivered water to Earth“.

However, the new results also raise new questions. Until now, scientists assumed that the distance of a body's origin from the Sun correlated to the deuterium-to-hydrogen ratio in its water. The farther away this origin lies from the Sun, the larger this ratio should be. With a „birth place“ within the Kuiper belt and thus well beyond the orbit of Neptune, Hartley 2, however, seems to violate this rule. „Either the comet originated in greater proximity to the Sun than we thought“, says Hartogh. „Or the current assumptions on the distribution of deuterium have to be reconsidered.“ And maybe Hartley 2 is a so-called trojan that originated close to Jupiter and could never overcome its gravitational pull.



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