There is a computation spreadsheet on it available to those that enjoy scepticism, but anyway, Earth is the only possible source of the water, since Earth is coplanar to the orbits of all of them. (Kepler worked that out hundreds of years ago)
Wait a minute. Isn’t The Earth coplanar with almost all of the bodies in the solar system?
Huh? That's like saying Earth is the only possible source of iron or nitrogen. Water exists throughout our solar system and the universe.
The rings of Saturn are almost entirely composed of water ice, and Jupiter's moon, Europa, is completely covered with it.
Astronomers recently discovered the most water vapor ever detected, about 12 billion light years away surrounding a quasar, in the amount of about 140 *trillion* times all the water in our oceans.
In this environment it continues to produce this mass amount of water vapor according to astronomers. Astronomers found water is pervasive throughout the universe, going back to the early stages of the universe. In fact, there is water vapor and water in the form of frozen ice all over our Milky Way Galaxy.
Gas clouds observed in Orion region alone are said to generate enough water molecules in a single day to fill the Earth's oceans sixty times over. It's believed much of the water in the solar system was originally produced in a giant water-vapor clouds like the one observed in Orion.