We have a somewhat surprising result from the Rosetta mission to comet 67P/Churyumov-Gerasimenko. For a long time it has been thought that the water from the Earth's oceans came from comets -- probably the same sort of comets that now make up the Kuiper belt. These are believed to have been scattered into the inner solar system in large numbers by migration of the planets Uranus and Neptune outward to their present locations.
67P/Churyumov-Gerasimenko belongs to a population of comets called the Jupiter family. These are short period comets with low orbital inclinations controlled by the gravitational influence of Jupiter. They originate in the Kuiper belt, and were perturbed by Neptune into more elliptical orbits that come closer to the sun, and then were perturbed by Jupiter into short-period orbits. The Jupiter family comets thus ought to be a nice sample of Kuiper belt material and, if the Kuiper belt is the origin of Earth's oceans, ought to have water with about the same deuterium/hydrogen ratio as the water of Earth's oceans.
Except 67P/C-G doesn't. It has about 3x as much deuterium as Earth ocean water.
Apparently, the Jupiter family comets have a wide range of D/H ratios. Comets 103P/Hartley and 45P/Honda–Mrkos–Pajdušáková, which are also Jupiter family comets, have D/H ratios close to that of Earth. Halley's comet, which has a high orbital inclination and originated in the Oort cloud, has a higher D/H ratio than Earth's, though not as high as 67P/C-G.
Note in the figure that Earth's water has a higher D/H ratio than that of the protosolar nebula, which supplied material for the four large planets. Earth's D/H is closest to that of many chondrites, which are a class of undifferentiated meteorites that provided the early material for the terrestrial planets, and to some, but apparently not all, Jupiter family comets.
These results for comet 67P/Churyumov-Gerasimenko were published online today (Dec 10, 2014) in Science:
67P/Churyumov-Gerasimenko, a Jupiter family comet with a high D/H ratio
Abstract
The provenance of water and organic compounds on the Earth and other terrestrial planets has been discussed for a long time without reaching a consensus. One of the best means to distinguish between different scenarios is by determining the D/H ratios in the reservoirs for comets and the Earth’s oceans. Here we report the direct in situ measurement of the D/H ratio in the Jupiter family comet 67P/Churyumov-Gerasimenko by the ROSINA mass spectrometer aboard ESA’s Rosetta spacecraft, which is found to be (5.3 ± 0.7) × 10−4, that is, ~3 times the terrestrial value. Previous cometary measurements and our new finding suggest a wide range of D/H ratios in the water within Jupiter family objects and preclude the idea that this reservoir is solely composed of Earth ocean-like water.