---
title: "Origin of water on Earth"
chunk: 2/4
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"
---

While the majority of Earth's surface is covered by oceans, those oceans make up just a small fraction of the mass of the planet. The mass of Earth's oceans is estimated to be 1.37 × 1021 kg, which is 0.023% of the total mass of Earth, 6.0 × 1024 kg. An additional 5.0 × 1020 kg of water is estimated to exist in ice, lakes, rivers, groundwater, and atmospheric water vapor. A significant amount of water is also stored in Earth's crust, mantle, and core. Unlike molecular H2O that is found on the surface, water in the interior exists primarily in hydrated minerals or as trace amounts of hydrogen bonded to oxygen atoms in anhydrous minerals. Hydrated silicates on the surface transport water into the mantle at convergent plate boundaries, where oceanic crust is subducted underneath continental crust. While it is difficult to estimate the total water content of the mantle due to limited samples, approximately three times the mass of the Earth's oceans could be stored there. Similarly, the Earth's core could contain four to five oceans' worth of hydrogen.
While not considered to be one of the major sources of water on the planet, it should also be noted that biological processes such as photosynthesis and respiration are key factors in the hydrologic cycling of water on earth. These processes may have also had a more vital role on the early earth, and as such, they could impact the amount of available water present in a given location at a given time. This is due to one of these aforementioned processes consuming water, and the other generating water, and the balance between them is therefore what truly determines their impact. If photosynthesis occurs in higher degrees than respiration, earth's water inventory would decrease, while if respiration occurs in higher degrees than photosynthesis, earth's water inventory would increase. 

== Hypotheses for the origins of Earth's water ==

=== Within Earth itself ===
Miozzi, et al., (2025) propose that a hydrogen-infused magma ocean  could have presented on Earth itself at ambient temperatures over 4000 K. The experiments of Miozzi, et al., (2025) find that copious amounts of hydrogen can dissolve into a magma melt at 4000 K, with a lesser dependence on pressures, from 16 to 60 GPa. Further, the reduction of iron oxide by hydrogen leads to the production of significant amounts of water, along with the formation of iron blebs.  

=== Extraplanetary sources ===
Water has a much lower condensation temperature than other materials that compose the terrestrial planets in the Solar System, such as iron and silicates. The region of the protoplanetary disk closest to the Sun was very hot early in the history of the Solar System, with temperatures ranging from 500-1500 Kelvin or 200-1200 Celsius, and therefore, it is not feasible that oceans of water condensed with the Earth as it formed. Further from the young Sun where temperatures were lower, water could condense and form icy planetesimals, which accumulated to form the Oort cloud. The boundary of the region where ice could form in the early Solar System is known as the frost line (or snow line), and is located in the modern asteroid belt, between about 2.7 and 3.1 astronomical units (AU) from the Sun. It is therefore necessary that objects forming beyond the frost line–such as comets, trans-Neptunian objects, and water-rich meteoroids (protoplanets)–delivered water to Earth. However, the timing of this delivery is still in question.
One hypothesis claims that Earth accreted (gradually grew by accumulation of) icy planetesimals about 4.5 billion years ago, when it was 60 to 90% of its current size. In this scenario, Earth was able to retain water in some form throughout accretion and major impact events. This hypothesis is supported by similarities in the abundance and the isotope ratios of water between the oldest known carbonaceous chondrite meteorites and meteorites from Vesta, both of which originate from the Solar System's asteroid belt. It is also supported by studies of osmium isotope ratios, which suggest that a sizeable quantity of water was contained in the material that Earth accreted early on. Measurements of the chemical composition of lunar samples collected by the Apollo 15 and 17 missions further support this, and indicate that water was already present on Earth before the Moon was formed.