Ice in the Solar System: From Lakes to Comets

From the Lecture Series: A Field Guide to the Planets

By Sabine Stanley, Ph.D., John Hopkins University

Ice in the Solar System is of various types and materials: from ice giants to the water ice particles orbiting Saturn, to the type that is found on comets. The existence of ice in some places indicates the existence of water. Some planets have oceans of water reaching all over, even though they might not look like it. Mars is the perfect example.

A 3-D render of an astronaut near a crater with water.
Ice is found everywhere in the Solar System in various forms. (Image: iurii/Shutterstock)

The eight famous planets of the Solar System are located in its first 1% space close to the Sun. In the order of distance from the Sun, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune are the main planets. However, the Solar System is filled with dwarf planets, moons, comets, particles, and ice. Ice in the solar is not limited to the water ice we are used to seeing on Earth.

The category called ices refers not only to water but also to ammonia, methane, and other materials that are volatile due to their low melting and evaporation temperatures. Moreover, these ice materials might be solid, liquid, or gaseous. Still, we are not the only ones with water ice.

Water and Ice in the Solar System

There are numerous places in the Solar System where water or frozen water can be found. Some particles orbiting Saturn are made of pure water ice; Mercury has water ice in its dark pole craters, and Earth has oceans and different types of water. Earth is the only planet withstanding liquid surface water, but not the only place with water.

Mercury planet landscape.
Even Mercury hosts water ice amid its extreme heat. (Image: Jurik Peter/Shutterstock)

Even Mercury hosts water ice amid its extreme heat. Almost all the moons in the outer Solar System are covered in shells of frozen water ice. Mars also has frozen water, despite its rusty-red surface. It has a north pole ice cap around 1000 kilometers across and almost equal to the Greenland ice sheet in volume. Topographic evidence suggests that the northern hemisphere of Mars used to be covered with vast oceans of liquid surface water, billions of years ago. Dried riverbeds, as well as sediment layers and hydrated minerals in the craters, are some of these pieces of evidence. These craters were filled with standing water a long time ago.

This is a transcript from the video series A Field Guide to the Planets. Watch it now, on The Great Courses Plus.

Liquid water is more difficult to find. For example, three of Jupiter’s moons, namely, Europa, Ganymede, and Callisto, have subsurface water oceans. Titan and Enceladus, two of Saturn’s moons, also have subsurface water oceans. Even Pluto is likely to have a subsurface water ocean, but these oceans are all from several kilometers to hundreds of kilometers below the surface. On Earth, Lake Vostok in Antarctica is located four kilometers under the East Antarctic Ice Sheet, with a width of almost 250 kilometers. The subsurface oceans in the outer Solar System may stretch all over the host’s surface. The ice giants, however, are vast water reservoirs.

Learn more about Neptune: Windy with the Wildest Moon.

Uranus and Neptune

Uranus and Neptune are the last two main planets, mostly made of water, sometimes in forms that are not even found on Earth. Both planets look blue, but that is due to methane. Water in Uranus and Neptune is locked under the surface, as ionic water, with broken-up water molecules.

Ionic water turns into the superionic phase under the extreme pressure and heat in depths of Uranus and Neptune. The hydrogen and oxygen in the H2O water molecule rearrange themselves into a new structure under this pressure. The oxygen atoms bond into a network and the hydrogen atoms flow freely through the cage-like oxygen structure. This new form of water is the focus of many studies and experiments.

Learn more about Uranus: A Water World on Its Side.


Comets are masses of particles drawn toward the Sun from different places of the Solar System. Some long-period comets originate from the Oort Cloud, 50,000 times farther than Earth. They always have a solid part, and many are covered in dust and ice. Sometimes, the solid part is merely a rubble pile of icy rocks, loosely held together by weak self-gravity of the comet.

An illustration of a bright comet with large dust and gas trails.
Comets are usually covered in dust and ice. (Image: solarseven/Shutterstock)

In 2014, the Rosetta mission landed on the nucleus of a comet, numbered 67P. The landing was not very smooth, and there were some unexpected bounces. Comets usually have occasional eruptions and explosions, as they get closer to the Sun. The heat makes their surface vaporize and causes the eruptions. As a result, some parts of the comet fly off the surface, forming the tail seen from the Earth.

Water is the essential element of life, but it is not found only on Earth. Even the smallest objects in the Solar System comprises different forms of water and ices.

Learn more about Comets, the Kuiper Belt, and the Oort Cloud.

Common Questions about Ice in the Solar System

Q: What planets have ice on them?

Ice in the Solar System is found in numerous elements. Even Mercury has some ice in the pole craters. However, the biggest ice giants are Uranus and Neptune. They are composed of a variety of ices, not just the solid water ice that we know from Earth.

Q: Does Jupiter have ice?

Jupiter cannot host ice on its surface since it is a gas giant. The hydrogen does get as dense as metal near the core of Jupiter, but nowhere on the planet is there a solid surface for ice to form. However, the core used to be a rocky-icy one.

Q: Which planet can float in water?

Saturn. Even though the ice in the Solar System can be found almost everywhere, Saturn has no ice. The planet is made up of gas, and its density is lower than water. Unlike Jupiter that forms ‘metal hydrogen’ around the core and exceeds water density, Saturn does not.

Q: Is there water on Mercury?

As strange as it may seem, there is ice on Mercury, despite its closeness to the Sun. Ice in the Solar System is not a rare phenomenon, and Mercury has some ice in areas that remain cold enough due to the eternal shadow over them and the lack of atmosphere.

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