Lunar and Planetary Institute

Explore! Ice Worlds - Background

Ice in the Solar System Background

Ice Is Found Throughout Our Solar System

A long time ago, in our very own solar system . . . The processes that formed our solar system a little over 4.5 billion years ago helped to distribute the ices. Close to the Sun, it was too hot for water and other ices to condense. Instead, rocky materials and metals collected near the Sun to form the smaller rocky planets. Farther out, beyond the asteroid belt, the ices were able to condense in the colder reaches of space, forming the cores of Jupiter, Saturn, Uranus, and Neptune — the gas giants — and their moons. Beyond the gas giants, the Kuiper belt and Oort cloud are host to the leftovers of solar system formation, small icy rocky bodies (yes, including Pluto!), and icy comets.

Ice Exists on Our Nearby Neighbors

If the inner, rocky planets formed in a part of the solar system that was too hot for ices to condense, where did all the ice come from? There are two primary sources: first, the planets themselves, and second, delivery by comets (not unlike having pizza delivered to your home . . .).

As Earth, Venus, Mars, and Mercury evolved, they released gases from their interiors through volcanic activity. Volcanos on Earth continue to release gases today, including a lot of water vapor. On the early planets, these gases formed the planetary atmospheres. Atmospheres are important for maintaining relatively constant surface temperatures. On planets or moons without atmospheres that are close to the Sun, the surfaces in sunlight get very hot and the surfaces in darkness (nightsides) get very cold.

On some of the terrestrial planets, water vapor in the early atmospheres eventually condensed and precipitated to form oceans once the planetary surfaces cooled. Each planet has a different history that influences whether or not it has ice.

Mercury: Mercury's relatively small size did not provide sufficient gravitational attraction to "hold" an atmosphere. Because it was small, it cooled quickly, so volcanic processes stopped early in its history and did not replenish its atmosphere. In addition, Mercury is the closest to the Sun. Solar wind weathered away its atmosphere and the Sun continues to heat its surface to temperatures that are far too hot for water to condense or for ice to exist . . . except, possibly, in a few special places (foreshadowing!).

Venus:  Venus has a very dense atmosphere that contains ~97% carbon dioxide. Carbon dioxide is a greenhouse gas, a gas that can absorb solar radiation in the thermal infrared range of the spectrum. This thick blanket of gas traps the Sun's radiation and heats the planet's surface to a whopping 872°F (467°C). The surface of Venus is the hottest in the solar system — hotter even than Mercury, which is closer to the Sun! Venus is too hot to have any type of ice on it.

The surface of Venus is covered by its thick atmosphere of carbon dioxide.
Image courtesy of NASA.

Earth:  As Earth's surface cooled, water vapor in the early atmosphere condensed and precipitated, forming our oceans. Today Earth's atmosphere contains mostly nitrogen (78%), oxygen (21%), and minor quantities of other gases including carbon dioxide and water vapor. Our atmosphere has evolved; unlike Venus, a large amount of carbon dioxide has been removed from our atmosphere, dissolved in Earth's oceans, and precipitated as carbonate rocks. Over time, plants have contributed the oxygen through the process of photosynthesis.

Earth's atmosphere, like any planetary atmosphere, helps to moderate our temperatures so that the Sun's radiation does not cause the surface to get too hot on the daytime side or plunge to temperatures well below freezing on the nighttime side. The small amounts of greenhouse gases, such as water vapor and carbon dioxide, help to warm Earth even more, making it habitable. Earth's average temperature is about 59°F (15°C), but it ranges from –128°F (–89°C) to 136°F (58°C).

Not surprisingly, the ice on Earth is water ice because we have an abundance of water. Water ice is found where the temperatures are below the freezing point of water and there is enough precipitation for snow or ice crystals to fall or there is water that can freeze. Permanent ice is found on Earth's high mountains and in its polar regions, and sometimes in protected areas such as caves. During the winter months, seasonal temperatures get cold enough to allow snow to temporarily accumulate farther from the poles.


Snowflake. Image courtesy of


Antarctic mountain range. Image courtesy of the National Science Foundation's U.S. Antarctic Program. Glacier meets ocean. Image courtesy of the National Science Foundation's U.S. Antarctic Program.

The freezing point of carbon dioxide is –108°F (–78°C); pure ammonia's freezing point is –107°F (–77°C). These ices could exist in the coldest places on Earth, but the substances do not exist naturally in sufficient amounts.

Ice has not always been present on Earth's surface; during periods of geologic history Earth's climate has been warmer. Our climate also has been colder at times in the past, causing the ice to expand across Earth's surface.

Mars: Early Mars had a climate that was warmer and wetter than today; its atmosphere was thicker and water flowed across the surface. Mars may even have had oceans. As the interior of Mars cooled, volcanism declined and the atmosphere of Mars thinned. Today's atmosphere is made of 95% carbon dioxide, 3% nitrogen, and small amounts of other gases, including water, oxygen, and methane. The atmospheric pressure on the surface of Mars is about 1/100 that of Earth's atmospheric pressure at sea level. Because of the thin atmosphere and Mars' distance from the Sun, Mars is cold. Its temperatures range from –193°F (–125°C) to 23°F (–5°C), well under the freezing point of water and also cold enough to freeze carbon dioxide.


Water ice in a 35-kilometer-wide crater in the northern hemisphere of Mars.

Because of the low atmospheric pressures, liquid water at the surface of Mars would evaporate into water vapor. So what happened to all that water that used to be on the surface of Mars? Some did evaporate into space. But much is frozen under the surface and in the polar ice caps. Mars has water ice!

Water ice in a 22-mile-wide (35-kilometer-wide) crater in the northern hemisphere of Mars. The light dusting of ice on the crater edges is carbon dioxide ice.Image courtesy of the European Space Agency (ESA/DLR/FU Berlin (G. Neukum).

Mars also has another type of ice — carbon dioxide ice — which is familiar to us as "dry ice." In the winter, the carbon dioxide in the atmosphere condenses and falls to the ground as carbon dioxide ice. In the summer, much of this changes from the solid form back into gas (sublimates).

Carbon-dioxide snow on the surface of Mars. Carbon dioxide gas in the atmosphere condenses and precipitates in the cold winter temperatures. Image courtesy of NASA.

Carbon-dioxide snow on the surface of Mars.

Mars has ice caps at both its poles. The north pole ice cap is about 600 miles (1000 kilometers) across — about the width of Montana! The southern ice cap is about 1/3 this size. Both ice caps are made mostly of water ice, but the southern ice cap has a permanent cover of carbon dioxide ice. The ice caps grow each winter as carbon dioxide ice is added to them, and decrease each summer as the carbon dioxide sublimates back to the atmosphere.

Northern ice cap on Mars. Southern ice cap of Mars
Northern ice cap on Mars. Much of the ice cap is water ice, with a thin layer of
carbon-dioxide ice on the surface.
Image courtesy of NASA.

Southern ice cap of Mars. This ice cap has two layers. A top layer of carbon dioxide ice, about 27 feet (8 meters) thick,
lies over a thick layer of water ice.
Image courtesy of NASA.

Like Earth, Mars' climate has fluctuated through geologic time, sometimes getting warmer and sometimes getting colder. During colder times, its ice caps expanded and glaciers extended farther across the martian landscape.

The Moon and Mercury Are Surprising Places to Expect Water  

Our Moon has no atmosphere; as it spins on its axis, its surface experiences temperatures ranging from 225°F (107°C) in sunlight to –243°F (–153°C) in the dark. Ice and water cannot exist under these conditions; they would evaporate. Why, then, is NASA exploring the Moon's surface to see if water ice exists?

The Moon's poles have areas of permanent light and permanent darkness. Sunlight reaches the north and south polar regions at low angles of incidence. Because the Moon's axis of spin is tilted at a very small 1.5° to its orbit around the Sun, this low angle of incidence does not change during the year (as it does on Earth, causing seasons). Deep craters at the poles never receive sunlight. They are permanently shadowed and permanently cold! These are the cold-storage pits of the lunar surface. They are cold enough to trap volatiles — elements that evaporate readily at standard temperature and pressure — like water.

Maps of the lighting conditions at the Moon's north pole (left) and south pole (right). Areas
colored with blue (15–30%), purple (0–15%), and black (0%) receive low levels of illumination.
Image courtesy of P. Spudis.

Radar data collected by the Clementine spacecraft suggest that water ice, perhaps mixed with dust and rocks, exists at the lunar south pole. Spectrometers onboard the Lunar Prospector spacecraft detected hydrogen — one component of water — at the lunar poles. Based on the presence and distribution of the hydrogen, scientists hypothesize that extensive water ice exists at the poles. Several spacecraft will provide more definitive data about the presence of water ice. NASA's Lunar Reconnaissance Orbiter (LRO) and India's Chandrayaan-1 spacecraft carry radar instruments to map the extent and distribution of materials at the poles in far greater detail than previous missions. NASA's Lunar Crater Observation and Sensing Satellite (LCROSS) mission will help confirm the presence of water ice by impacting the lunar surface in a permanently shadowed crater. The resulting plume will be analyzed for water ice and vapor and other materials by instruments on the LCROSS shepherding spacecraft and LRO, and by telescopes on Earth. If there is ice at the lunar poles, there are still many questions about how it got there, its composition, how fast it accumulates, and how much dust and rocks are mixed with it.

Water is a valuable resource for future human exploration and habitation on the Moon. The presence of water will reduce the cost of transporting water to the Moon (at $10,000 per pound!). Beyond the need for drinking water, it can be separated into its two components — hydrogen and oxygen — and used to make propellant for spaceflight. The oxygen can also be used for the production of breathable air.

Where did this ice on the Moon come from? Unlike the terrestrial planets, our Moon's geologic evolution did not produce water vapor or water. Scientific data suggest that our Moon formed when a giant impactor, half the size of Earth, struck Earth. The impact spewed debris into orbit around Earth. This material eventually clumped together — accreted — to form our Moon. The impact and accretion were violent processes; they drove off any water or gases or other easily evaporated materials. Our Moon is a dry place! So where, then, did the water come from? Comets! Comets striking the surface of the Moon delivered water ice that became trapped in the permanently shadowed craters. This process is not unique to the Moon. Comet impacts occur across our solar system, delivering water ice and other substances to all the planets and moons.

Mercury is too hot to have any form of ice . . . or is it? Mercury also lacks an atmosphere, and it is very close to the Sun. Like the Moon, however, Mercury's axis is tilted only a small amount; at 0.1°, it is tilted even less than the Moon. And like the Moon, Mercury has deep craters at its poles that are permanently shadowed — and permanently cold. These cold dark craters could trap water and store it as ice. In the coming years NASA's MESSENGER mission will provide more information about whether or not water ice exists at Mercury's poles.

The Gas Giants and Their Moons Are Rich in Ice

Based on the scientific models of how our solar system formed, it is no surprise that the moons of the gas giants are rich in ice!

Jupiter's Moons:  Europa's crust of water ice floats on top of a saltwater ocean. The crust may be many miles (kilometers) thick and its surface is very smooth. It does not have many craters, suggesting that the surface is relatively young and active; the processes that cover or remove craters are continuing to happen.

The icy shell of Europa is one of the smoothest surfaces in the solar system. The reddish streaks may be from salts such as magnesium sulfate, that were left behind by evaporating water. Image courtesy of NASA.

Europa is far from the Sun and its surface temperature is a chilling –260°F (–160°C) at the equator and –370°F (–220°C) at its poles. At these temperatures the water ice is very hard and rock-like. The ocean under the ice blanket is kept heated by the constant tidal forces: Europa gets pulled and stretched in different directions by the gravitational attraction of Jupiter and its moons, generating heat. The presence of liquid water could mean that life is supported in the sea of Europa.

Surface of Europa Surface of Europa showing thick plates of ice that float on the ocean underneath.
Image courtesy of NASA.

Artist's drawing of what probes, sent aboard spacecraft missions to Europa, may find at the sea floor of the moon's salty ocean. Image courtesy of NASA. Artist’s drawing of what probes, sent aboard spacecraft missions to Europa, may find at the sea floor of the moon’s salty ocean.

Ganymede is a mixture of rock and water ice. Scientists suggest that it has a water ocean beneath its crust, between layers of ice. Its
surface has large amounts of water ice. This
frozen moon even has polar ice caps!

Ganymede's surface has two types of terrain. The dark regions are heavily cratered, suggesting that they are very old.The lighter areas are younger (though still old).These regions have grooves and ridges, and a higher abundance of water ice. Image courtesy of NASA.
Callisto Callisto is composed mainly of rock and water ice, although other ices like ammonia ice and carbon dioxide ice may be present. Water ice occurs at the surface of Callisto. Like Europa and Ganymede, a salty ocean may exist under the crust; some scientists hypothesize that a small amount of ammonia in the water may keep it from freezing (remember from the All About Water Background Information that dissolved substances lower the freezing temperature of water?)

Callisto's surface is heavily cratered, suggesting that it is very old. Image courtesy of NASA.

Saturn's Rings and Moons:  Saturn's rings are one of the most remarkable features in the solar system. They are 155,000 miles (250,000 kilometers) or more in diameter and less than half a mile (one kilometer) thick! The rings are composed of particles ranging from the size of dust specks to large boulders, and they are more than 90% water ice!

Saturn's Rings Temperatures of Saturn's rings taken by Cassini-Huygens instruments.
Saturn's rings. Image courtesy of NASA.
Temperatures of Saturn's rings taken by Cassini-Huygens instruments. Red areas are –261°F(–163°C); green are about –297°F(–183°C); and blue are –333°F(–203°C). Image courtesy of NASA.

Saturn has over 60 moons, most of which appear to be composed primarily of water ice with varying amounts of rocky material:  Mimas and Tethys are composed almost completely of water ice; Iaeptus and Rhea each appear to be about 25% rocky material; and Dione, Enceladus, and Titan are each about 50% rocky material. All these bodies are heavily cratered. Most have surface temperatures less than274°F (170°C), well below the freezing point of water and other ices. Water ice at the surfaces of these moons is rock-hard.

Mimas Tethys
Mimas .. "The Death Star." Mimas has a large crater on its surface which was caused by an impact event. Image courtesy of NASA.

Tethys, another icy moon of Saturn. Image courtesy of NASA.


Dione against Saturn and its rings.
Image courtesy of NASA.

Enceladus caught the attention of scientists and the world with its spectacular geysers. The Cassini spacecraft flew through a plume and sampled water vapor and ice particles and minor components of other molecules. Scientists suggest that the water collects in heated, pressurized chambers under Enceladus' surface and periodically erupts at the surface — an icy geyser. The material vented by Enceladus is what makes up an entire band of Saturn's rings (called the E ring)!

Enceladus Plumes of water vapor and ice particles are jetted above the surface of Enceladus.

Plumes of water vapor and ice particles are jetted above the surface of Enceladus. Image courtesy of NASA.

The icy surface of Enceladus. False-color image courtesy of NASA.



Titan, the largest moon of Saturn, is a geologically complex body with a thick nitrogen-rich atmosphere. Far from the Sun, its temperatures remain at a chilly –290°F (–179°C). Titan has lakes of liquid hydrocarbons at its surface and a terrain that contains mountainous features composed of ice. Deposits of water ice and hydrocarbon ice occur at its surface.

First view of Titan's surface The pebbles are made of dirty water ice. Image courtesy of NASA.


Comet Hyakutake. Image by Jim Martin, Huntsville AL, Courtesy of NASA.

Comets have been called the "dirty snowballs" of our solar system! Every comet is made of the same basic ingredients — ice and dust. However, comets vary in how much of the ice is water ice and how much is ice made of other substances, including methane, ammonia, carbon dioxide, carbon monoxide, sulfur, and hydrogen sulfide. Comets also vary in the different types of trace elements and hydrocarbons that are present.

Comets have long elliptical orbits that carry them from the chilly outreaches well beyond Neptune to nearer our Sun. As comets approach the Sun (within about 280 million miles or 450 million kilometers), they heat up and their ice begins to sublimate — change from a solid directly to a gas. The gas and dust forms an "atmosphere" around the nucleus called a "coma." Material from the coma gets swept into tails that are millions of miles long.


For over 4.6 billion years, since the formation of our solar system, comets have been colliding with planets and moons and asteroids, delivering their water ice to these bodies. Comets may be the source for water ice on the Moon and Mercury, and they certainly have added water to other celestial bodies, including Earth.

Scientists Can Look for Ice in the Solar System without Leaving Earth

The above discussion about where water ice might be found in our solar system reveals some of the ways that scientists are testing for its presence. If scientists cannot go to a planet to explore or send a lander that will return samples, they can examine the surface using a variety of detectors onboard spacecraft or on Earth-based telescopes. One of the primary ways of detecting water is to analyze the spectrum of light reflected from a planetary surface. Spacecraft detectors may probe surfaces using the Sun's reflected light, or they may use radar to bounce radio waves off the surfaces.

Different materials reflect and absorb different — and characteristic — wavelengths of light. Some of these wavelengths are visible to our eyes (red, orange, yellow, green, blue, purple) and some are invisible to us (for example, infrared and ultraviolet wavelengths). Scientists can compare the spectra from the surface of a planet to spectra of known substances to determine what materials occur on the planet. Water has a characteristic spectral "fingerprint," especially in the infrared. Other substances have their own unique spectral fingerprints. Spectra can be collected by spectrometers onboard orbiting spacecraft or by telescopes viewing the planet or planetary body from Earth.

A spectrometer onboard the galieleo spacecraft detected the presence of water ice on Europa.

A spectrometer onboard the Galileo spacecraft detected the presence of water ice on Europa (lines A–D) and Ganymede (line E) The curves C, D, and E match the unique spectral fingerprint of water ice, measured in the laboratory (line F). They indicate regions of the moons that are dominated by ice. Image courtesy of NASA/JPL.

Other wavelengths of light, such as radio waves and gamma rays, can provide additional clues. Different surfaces reflect radio waves in different ways. Radar can detect the characteristic signatures of ice and soil mixed with ice. Other instruments onboard spacecraft, such as gamma-ray spectrometers, can detect the abundance of hydrogen (and other elements), which is a component of water molecules. The presence of hydrogen may be interpreted to indicate the existence of water on a planet. Scientists have interpreted water ice to be present in deep craters near the Moon's poles based on radar and gamma-ray spectrometer data.

Investigate the water ice on our own planet further in Ice on Earth.

Last updated
October 19, 2009


Back to top