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Some scientists have suggested that most of it arrived via other asteroids that crashed into Earth later in smaller collisions. But for that idea to hold weight, asteroids would have to carry water. Comets aren't a good possibility for this scenario because the water they hold tends to be of a slightly different nature, with atoms in a different configuration, or isotope, than most of the water on Earth.

Though the recent measurements can't tell anything about the isotope ratio of the water on 24 Themis, the fact that there is water there at all is an encouraging sign.

This statement is completely misleading and nonsensical. Different 'configuration or isotope'? Water is configured as water is, 2 Hydrogens, one Oxygen. Water does not have isotopes. Hydrogen might, Oxygen might. But all water molecules, whether they are constructed with H-1, H-2, H-3, O-16, or other Oxygen isotope, ALL behave the same chemically. This article is useful for composting only.