Dust From Beyond Neptune Carries Water Unlike Anything on Earth

Tiny grains of dust that drifted to Earth from the Kuiper Belt carry the chemical fingerprint of ancient water that looks nothing like the water in our oceans, according to ScienceAlert. A team led by planetary scientist Lindsay Keller of NASA's Johnson Space Center says the grains point to a large reservoir of "heavy" water far beyond the giant planets.

The researchers argue that this reservoir stayed largely cut off from material closer to the Sun, which would make it a rare surviving sample of the early Solar System.

Where did the dust come from?

The Kuiper Belt is a region of icy objects beyond Neptune, at roughly 30 astronomical units from the Sun, or 30 times the Earth-Sun distance. Few spacecraft have gone there, and ScienceAlert noted that landing a probe or bringing back samples is out of reach for now.

Some of its material comes to us anyway. Cosmic dust constantly falls on Earth, an estimated 5,200 metric tons every year. Keller and colleagues found 12 grains, each 10 to 30 micrometers across, in samples collected from the stratosphere by high-altitude NASA aircraft.

How can anyone tell a grain came from so far away?

The clue is radiation damage. Energetic particles from the Sun leave tiny tracks in the crystals of dust grains in space, and the more tracks a grain has, the longer it has been drifting. ScienceAlert reported that the 12 grains had track densities around 100 times higher than expected for dust from the asteroid belt or from Jupiter-family comets. That is consistent with a much longer journey, as far as the Kuiper Belt.

Their makeup supports the idea. The grains held about four times as much carbon as certain carbonaceous meteorites, which suggests they formed under different conditions, far from the Sun.

What does the water have to do with it?

Many of the grains contain clays such as saponite and serpentine, along with other minerals that form when liquid water reacts with rock. That means liquid water once existed inside the objects the dust came from. The Kuiper Belt is frozen today, but the researchers say heat from radioactive decay or collisions could have melted ice in the interiors of some bodies long ago.

That water is gone. The minerals, however, kept some of its oxygen. Oxygen comes in several isotopes, versions of the element with different numbers of neutrons, and their proportions act like a fingerprint of where a material formed. The team found the grains were unusually enriched in the heavier isotopes.

By reconstructing the chemical reactions that made the minerals, the researchers estimated the makeup of the water involved. It must have been unusually rich in heavy oxygen, which they read as the signature of an ancient reservoir in the outer Solar System.

Why it matters: what does this tell us about the Solar System?

Objects in the Kuiper Belt are thought to be among the least changed leftovers from the time the Solar System formed some 4.5 billion years ago. Because we cannot visit them, dust that falls to Earth is one of the few ways to study them directly.

The findings suggest that water in the outer Solar System was not simply the same as water nearer the Sun. Keller and his colleagues describe the result as compelling evidence for a major heavy water reservoir beyond the orbits of the gas giants that was largely isolated from inner Solar System materials.

For readers, the practical takeaway is about our origins: the finding adds a data point to the long-running question of where the water on planets came from and how different parts of the young Solar System were. It is a result from a small set of 12 grains, and further analysis would be needed to see how widely it applies.

Around Prague, based on reporting by ScienceAlert

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