Until now, however, it has been impossible to observe water and molecular oxygen in the cold interstellar medium because its emission is absorbed by the Earth's atmosphere. Only by lifting the telescope above this obscuring canopy has SWAS made it possible to measure the abundance of these molecules in interstellar space for the first time. It's believed that some portion of the Earth's water was produced at a later stage in the evolution of our solar system. But the exact percentage is unknown. Those dust grains can stick to each other, and this might well be how comets are formed.
Comets contain a lot of ice.
The collision of a big dirty snowball like this with a young planet would certainly transport water. Melnick's goal is not so much to collect information about individual sources, but to observe a large enough sample of sources to be able to make broad statements about the nature and dynamics of the interstellar medium. I come in every day and get excited by what we've learned the night before, but it's just adding another piece to the puzzle.