Kuiper Belt water: 12 dust grains point to a hidden ice reservoir

What is the Kuiper Belt?

The Kuiper Belt lies beyond Neptune, more than 30 astronomical units from the Sun. One astronomical unit is the Earth-Sun distance, about 150 million km. Unlike the asteroid belt between Mars and Jupiter, it is mostly water ice, frozen methane and ammonia mixed with rock and dust. Pluto lives there too. Because the material has stayed so far from the Sun, it was never heated and reworked as much as material nearby, so astronomers treat it as a deep freezer from more than 4.5 billion years ago.

How the dust gave away its origin

The team used electron microscopy, chemical analysis and precise measurements of oxygen isotopes. Isotopes are versions of an element with the same number of protons but different numbers of neutrons. Oxygen-16 is the common one, while oxygen-17 and oxygen-18 are heavier.

To guess where the grains came from, the researchers looked at tracks left by high-energy particles from the Sun. Earlier work suggested that a dense scattering of these marks hints at an origin in the Kuiper Belt, since material out there is exposed for a very long time. The 12 grains showed many such marks.

Their oxygen was also clearly different from known meteorites. The original water no longer exists in the grains, so the researchers worked backwards from the minerals to reconstruct the liquid that formed them. Their results suggest that water had an oxygen-18 to oxygen-16 ratio up to about 15 percent higher than Earth's ocean water.

Similar traces had turned up before in one primitive meteorite and in material linked to comet 67P/Churyumov-Gerasimenko, but it was unclear whether they were typical of the outer Solar System. These 12 grains make that case more convincing.