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Alone among known planets, Earth has vast oceans on its surface and its landmasses are marked with lakes and extensive river drainage systems. Water is the biosphere's lifeblood, and without it, Earth would be just another dead world. If Earth life is a reliable indicator, then water is necessary for life, full stop.

That's why scientists are so interested in how Earth got its water.

For a long time, researchers examined the idea that Earth's water came from elsewhere in the solar system. The protoplanetary disk the planets formed in was massive, and it was dissected by a snowline. This is a line in the disk, defined by its distance from the sun, beyond which volatiles like water vapor can condense.

The idea is that water can condense, freeze, then be delivered to Earth by asteroids, meteorites, or comets. Astronomers know that comets are icy bodies, and there's growing evidence that asteroids hold water, either in frozen form or locked in minerals. This is the "late veneer hypothesis," which states that water was delivered to Earth after its core had already formed.

But a growing body of evidence and simulations shows that this picture may be incorrect. New research in The Astrophysical Journal Letters challenges the late veneer hypothesis by showing that the snow line may not accurately reflect reality. Rather than a single line that separates water sublimation as an on/off process, where it all sublimates on one side of the snow line and none sublimates on the other side, the reality is more nuanced.

The research is titled "Was Earth's Water Acquired Locally during the Earliest Phases of the Solar System Formation?" The lead author is Lise Boitard-Crépeau from the University Grenoble Alpes in France.

"There is consensus on the fact that molecular water was mostly formed on micrometer-sized dust grains at the very beginning of the solar system formation, in the molecular cloud and prestellar phase, where it remained frozen on the icy mantle enveloping the grains," the researchers explain.

Over time these grains formed rocks, asteroids, comets, and even the rocky planets. But as the sun commenced shining, conditions changed. The material in the proto-solar nebula disk warmed up, and water sublimated from the surface of the dust grains inward to the snow line, which is basically the condensation front of water.

As a result, conditions on the inside of the line changed. Any water that wasn't trapped inside larger bodies was dissipated, and the inner disk became depleted of water. This is where the rocky planets formed. Earth accreted out of this dry material, implying that water had to be delivered from beyond the snow line, and that's the late veneer hypothesis.

But in this new understanding, the line is more nebulous. The research is based on developments in quantum chemistry showing that there's no specific single temperature at which water binds itself to dust grains. Instead of a binary dividing line, the binding energies are subject to a Gaussian distribution of values.

"While the classically used snowline relies on a single condensation temperature, recent work in quantum chemistry shows that the binding energy (BE) of water on icy grains has a Gaussian distribution, which implies a gradual sublimation of water rather than a sharp transition," the authors write. The researchers computed the distribution of binding energies to understand how water ice on dust grains was spread across the proto-solar nebula (PSN) protoplanetary disk.

Using the distribution of different water binding energies, the researchers established a frost line that is actually several astronomical units wide. "The adoption of a water BE distribution does not generate a single snowline, inside which water ices are fully desorbed and remain 'completely dry,' as often assumed, but rather a water transition zone extending several astronomical units," the researchers write.

This all happened a long time ago, and aside from Earth itself, the only place we can look for evidence is in asteroids and meteorites. For the researchers' work to be accurate, it would need to duplicate Earth's water abundance and isotope ratio and the hydration patterns seen in meteorites. Chondrites remain unchanged since the solar system's early days and are an important evidentiary link with the deep past.

The research shows that while the bulk of water ice is desorbed farther out than about one astronomical unit, small percentages remain attached to dust grains inside that limit due to different binding energies. "This small fraction, between ∼ 0.04 and 2.5 wt%, can fully account for Earth's water content," the authors write. "In turn, terrestrial water could be mostly inherited from the dust grains that were in Earth's orbit, with no necessity of migration of outer ones."

So Earth's water abundance lines up with the researchers' results. But what about chondrites?

"Our model also successfully reproduces the observed water-equivalent content trend across chondrite groups," the authors write. Even though the parent bodies of these chondrites accreted at different times, their water-equivalent contents "likely represent lower limits of primitive water originally incorporated into silicate dust grains, the building blocks of chondrite matrices," they explain.

"In summary, in light of the above discussion, it is possible that the terrestrial water was inherited from the icy grains in the orbit of Earth, i.e., locally," the authors write. Their results also agree with the idea that Enstatite Chondrites (EC), a rare form of meteorite, are Earth's building blocks. ECs have a comparable isotopic ratio as Earth's water, and researchers think that they formed near Earth's orbit.

The timing of the late veneer hypothesis has always been tricky. How could the right quantity of water be delivered to Earth at the right time without disrupting the planetary accretion process? If Earth was gradually hydrated as it formed, that whole question can be side-stepped.

There are still some problems with the researchers' conclusions, and they point them out in their paper. For example, Earth's currently measured ratio of heavy water (deuterium) with regular water is equivalent to an elemental ratio. But the currently measured ratio may not represent the actual elemental ratio because of "various processes that cycle water between the surface and Earth's interior," the authors explain. Those processes could've altered the ratio.

The researchers have shown that water ice doesn't necessarily sublimate along a sharply defined line. Instead, a range of binding energies means that even inside the generally-agreed-upon snowline, some water can survive on the inside of rocks and larger grains. They've also shown that enough water can survive to account for Earth's water.

"In summary, these results suggest that a significant share of Earth's water could have originated locally, without requiring delivery from beyond the classical snowline," the authors conclude.

This won't be the end of the late veneer hypothesis, but if further work supports this research, the idea that Earth's water came from elsewhere and was delivered by comets and asteroids will grow weaker.