Comets are small celestial bodies that follow highly elliptical orbits around the Sun, remnants from the formation of the solar system. These objects are composed of a solid core, known as the nucleus, which contains rock, dust, and frozen volatile compounds. Water ice is one of their most significant components, leading scientists to nickname comets “dirty snowballs.”
Water Ice in the Comet Nucleus
The comet nucleus is a porous, irregularly shaped structure that acts as a reservoir of pristine material from the solar system’s earliest days. Water is held within this core in a solid state, primarily as amorphous ice, mixed with dust and other frozen gases. For some comets, water ice accounts for approximately 80% of the total frozen material.
This ice is mixed with other frozen volatiles like carbon monoxide, carbon dioxide, and methane. The nucleus is surprisingly dark, with an albedo lower than charcoal, due to a crust of non-volatile dust covering the ice beneath.
The Release of Water Vapor
As a comet travels toward the inner solar system, solar radiation heats its nucleus, initiating sublimation. Sublimation is the transition where a solid, such as water ice, turns directly into a gas or vapor without passing through a liquid phase. The low pressure in space prevents liquid water from existing stably, ensuring this direct transition.
This escaping water vapor, along with other gases and dust particles freed from the ice, forms a vast, temporary atmosphere around the nucleus called the coma. The Rosetta mission observed Comet 67P/Churyumov–Gerasimenko releasing the equivalent of two small glasses of water into space every second, even far from the Sun. Eventually, the pressure from the solar wind and sunlight pushes this material away, creating the characteristic long, bright tails visible from Earth.
Confirming Water Presence
Confirmation of water in comets relies on instruments that analyze the chemical composition of escaping gases. Spectroscopic analysis is a primary method, where scientists examine the light emitted or absorbed by the coma to detect the unique spectral signature of the water molecule (H₂O). Analyzing these spectral lines confirms the presence of water vapor and determines its production rate.
Space missions provide detailed, in-situ evidence. The Rosetta spacecraft, for instance, used the ROSINA spectrometer to directly measure water vapor streaming from Comet 67P. More recently, the James Webb Space Telescope (JWST) confirmed water vapor around Comet Read, an object located within the main asteroid belt, demonstrating water ice exists in diverse cometary types.
Comet Water and Earth’s Oceans
The water found in comets is of great interest to scientists studying the origin of Earth’s abundant liquid water. A leading theory suggests that comets and water-rich asteroids delivered a significant portion of our planet’s oceans during the early solar system. To test this hypothesis, scientists compare the water in comets to Earth’s oceans using the deuterium-to-hydrogen (D/H) ratio.
Deuterium is a heavier isotope of hydrogen, and the ratio of “heavy water” (HDO) to ordinary water (H₂O) provides a chemical fingerprint indicating the water’s origin. Many comets, particularly those from the distant Oort cloud, showed D/H ratios two to three times higher than Earth’s oceans, suggesting they were not the primary source of our water.
Several comets, including the Jupiter-family Comet 103P/Hartley 2 and Comet 12P/Pons-Brooks, have been found to possess a D/H ratio nearly identical to Earth’s water. This finding suggests that at least some populations of comets could have contributed to the planet’s water supply, supporting a scenario where Earth’s oceans are a mix of water delivered by both comets and asteroids.