What it was
A spacecraft follows an icy body and samples the gas drifting off it. Among the molecules is glycine, one of the little links cells can string into proteins like beads on a thread. A comet can carry parts used by life without being alive.
Comets hold ice and dust from cold parts of the Solar System. All that water made them obvious suspects when scientists tried to trace Earth's oceans. But you need more than a wet rock. The water has to match, and enough of it has to arrive.
When and where
These bodies began far from the warm stretch where Earth grew. As young planets moved, their gravity could toss icy stuff toward the Sun. Scientists test those paths to work out how much comet material might have reached the growing Earth.
How we know
Water keeps a chemical fingerprint. Some hydrogen atoms are heavier than the usual kind; we call them deuterium (doo-teer-ee-um). Compare their share of the mix in comet water and ocean water, and you have a clue to whether they came from the same stock.
The match is messy. Some comets differ from our sea; others come closer. Readings can change with distance from a comet's solid center. Xenon, a gas that hardly reacts, gives a different clue. Its atom mix suggests comets brought some of Earth's air but probably little of the ocean.
Still a mystery
Nobody knows how much comet chemistry helped life begin here. Carbon-based molecules can form without cells, and finding them does not show the next step. Scientists must also check for traces from the spacecraft itself. Space chemistry and dirt from equipment can turn up in the same test.