Earth’s Wellspring.

Scientists came up with an apparently sound theory: Comets from the outer solar system delivered water to the inner solar system after the planets formed, crashing into the rocky planets. PHOTO (COLOR): COMET 67P Material shed by Comet Churyumov-Gerasimenko during its trip past the Sun doesn't match the isotopic levels in Earth's water, suggesting comets have contributed little to our planet's overall composition. But beyond the snow line - the boundary where the temperature was low enough for water to condense into ice - frozen water could readily mix with the rocky bits that would eventually merge to form the outer solar system bodies, producing planets, asteroids, and comets teeming with water and other volatiles. Other, Earth-based observations showed that many comets have much higher D/H ratios than Earth's oceans, dampening the prospect that they delivered most of our planet's water. [Extracted from the article]