---
title: "Origin of water on Earth"
chunk: 4/4
source: "https://en.wikipedia.org/wiki/Origin_of_water_on_Earth"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T03:51:05.225138+00:00"
instance: "kb-cron"
---

=== Comets ===
Comets are kilometer-sized bodies made of dust and ice that originate from the Kuiper belt (20-50 AU) and the Oort cloud (>5,000 AU), but have highly elliptical orbits which bring them into the inner solar system. Their icy composition and trajectories which bring them into the inner solar system make them a target for remote and in situ measurements of D/H ratios.
It is implausible that Earth's water originated only from comets, since isotope measurements of the deuterium to hydrogen (D/H) ratio in comets Halley, Hyakutake, Hale–Bopp, 2002T7, and Tuttle, yield values approximately twice that of oceanic water. This is also supported by analysis using the comparison of isotopic ratios for both carbon and nitrogen isotopes, which attribute only a few percent of the water present on earth to comet sources, indicating a much higher reliance on incoming asteroid matter. Using the cometary D/H ratio, models predict that less than 10% of Earth's water was supplied from comets.
Other, shorter period comets (<20 years) called Jupiter family comets likely originate from the Kuiper belt, but have had their orbital paths influenced by gravitational interactions with Jupiter or Neptune. 67P/Churyumov–Gerasimenko is one such comet that was the subject of isotopic measurements by the Rosetta spacecraft, which found the comet has a D/H ratio three times that of Earth's seawater. Another Jupiter family comet, 103P/Hartley 2, has a D/H ratio which is consistent with Earth's seawater, but its nitrogen isotope levels do not match Earth's.

== See also ==
Water on terrestrial planets of the Solar System

== Notes ==

== References ==

== External links ==
Dr. C's Ocean World: "How the Oceans Formed" (archived copy)
Nature journal: "Earth has water older than the Sun"