Rosetta which landed last month on comet Churyumov-Gerasimeko (67P) detected water vapor. However, the data show something different that what scientists were expecting. For some time, it has been theorized that comets brought water to Earth during its formation. However, comet 67P has a composition of water that is different from that on Earth. The water on the comet has three times (5.3 ± 0.7) × 10−4) more of a chemical isotope which weights more than hydrogen which we call deuterium.
This find is significant because it presents the idea that comets brought water to Earth as a “too easy” explanation, if you will. More studies need to be done. It may be possible that water from extraterrestrial sources changed its composition as it entered Earth’s atmosphere or interacted with the compounds and isotopes on the surface. We do not know for sure.