Cometary comae are a mixture of gas and ice-covered dust. Processing on the surface and in the coma change the composition of ice on dust grains relative to that of the nucleus. As the ice on dust grains sublimates, the local coma composition changes. Rosetta observations of 67P/Churyumov-Gerasimenko previously reported one of the highest D/H values for a comet. However, reanalysis of more than 4000 water isotope measurements over the full mission shows that dust markedly increases local D/H. The isotope ratio measured at a distance from the nucleus where the gas is well mixed is close to terrestrial, like that of other Jupiter family comets. This lower D/H has implications for understanding comet formation and the role of comets in delivering water to Earth.

中文翻译：

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彗星 67P/Churyumov-Gerasimenko 的近乎地球的 D/H


彗星彗发是气体和冰覆盖的尘埃的混合物。表面和彗形处理会改变尘埃颗粒上冰的成分（相对于原子核的成分）。随着尘埃颗粒上的冰升华，局部彗发成分发生变化。罗塞塔对 67P/Churyumov-Gerasimenko 的观测此前报告了彗星中最高的 D/H 值之一。然而，对整个任务中 4000 多次水同位素测量的重新分析表明，尘埃显着增加了局部 D/H。在距离气体充分混合的原子核一定距离处测得的同位素比值接近地球，就像其他木星家族彗星一样。这种较低的 D/H 对于理解彗星的形成以及彗星在向地球输送水方面的作用具有重要意义。