Explore! Ice Worlds! Background
Ice Is Found Throughout Our Solar System
The processes that formed our solar system a little over 4.5 billion years ago helped to distribute different types of ice. Close to the Sun, temperatures were too hot for water and other ices to condense. Instead, rocky materials and metals formed the smaller rocky planets. Farther out, beyond the asteroid belt, ices condensed 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 (including Pluto), and icy comets.
Ice Exists on Our Nearby Neighbors
The inner, rocky planets formed in a part of the solar system that was too hot for ices to condense. There are two primary sources for ice on Earth and our nearby neighbors: the planets themselves, and delivery by comets.
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 some of the terrestrial planets, as the surface cooled, water vapor in the early atmospheres eventually condensed and precipitated to form oceans. Each planet has a different history that influences whether or not it has ice.
Mercury: The Sun heats Mercury’s surface to temperatures that are far too hot for water to condense or for ice to exist . . . except in craters at its poles. These create permanently shadowed regions where the Sun never shines. Mercury doesn’t have an atmosphere to transfer heat to these dark craters, so the temperature remains extremely cold. NASA’s MESSENGER mission detected evidence of water ice at both poles, possibly delivered there by comet impacts.
Venus: Venus has a very dense atmosphere that contains about 97% carbon dioxide, a greenhouse gas that 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! 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. 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.
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. Earth has also had colder “ice ages” in the past, causing ice to expand across Earth's surface.
Find more information about Ice on Earth.
The Moon: Like Mercury, 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 and escape into space.
However, like Mercury, the Moon's poles have craters in permanent darkness. 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. Data collected by the Lunar Reconnaissance Orbiter and earlier lunar missions indicates that water ice, perhaps mixed with dust and rocks, exists in these permanently shadowed regions. Some estimates suggest that the Moon’s poles may hold as much water as it would take to fill the Great Salt Lake in Utah.
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. Over time, Mars lost much of its atmosphere. More information about Mars’ early climate is under Explore Life on Mars.
Because of the thin atmosphere and Mars' distance from the Sun, this planet is cold today. 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. Some of the water that was on Mars’ surface is now frozen under the surface and in the polar ice caps.
Water ice in a Mars crater, with some carbon dioxide ice on the crater edges. 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.
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. Like Earth, Mars' climate has fluctuated through geologic time. During colder times, its ice caps have expanded and glaciers extended farther across the martian landscape.
The Gas Giants and Their Moons Are Rich in Ice
The outer planets and their moons formed much farther from the Sun, in a region where water, carbon dioxide, and other “gases” condensed into ice. It is no surprise that the moons of the gas giants are rich in ice!
Europa has a youthful surface, an outer layer of water and ice, a rocky mantle, and probably an iron core. 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 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. Locked beneath a shell of ice, Europa harbors a liquid water ocean with an estimated volume twice that of all of Earth’s oceans. The ocean under the ice blanket is 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 potentially support life in Europa’s ocean.
Europa’s surface is covered by plates of ice floating on the ocean underneath. Image courtesy of NASA.
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!
The dark regions on Ganymede are heavily cratered, suggesting that they older. The lighter areas have grooves and ridges and a higher abundance of water ice. Image courtesy of NASA.
Callisto is mainly made of rock and water ice, although other ices like ammonia ice and carbon dioxide ice may be present. 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. (See All About Water for information on dissolved substances lowering 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 beautiful rings are 155,000 miles (250,000 kilometers) or more in diameter and less than half a mile (one kilometer) thick. The rings are made of particles ranging from the size of dust specks to large boulders, and they are more than 90% water ice!
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; Iapetus 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 than –274°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 has a large impact crater which provided its nickname of “The Death Star.” Image courtesy of NASA.
The icy surface of Enceladus. False-color image courtesy of NASA.
Enceladus caught the attention of scientists and the world with its spectacular icy 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. The material vented by Enceladus is what makes up an entire band of Saturn's rings (called the E ring)!
Plumes of water vapor and ice particles are jetted above the surface of Enceladus. 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.
Comets Are Made of Ice
Comets have been called "dirty snowballs." They are small celestial objects, made of ice, gas, dust, and a small amount of organic material, that orbit our Sun. There are about 1000 known comets and more are discovered each year.
Every comet has a nucleus of ice, gas, and dust between 1 and 10 kilometers (0.6 to 6 miles) in size. The ice is made of varying amounts of water, carbon dioxide, ammonia, and methane. The dust may contain hydrogen, oxygen, carbon, nitrogen, silica, and some metals. The nucleus may also have traces of hydrocarbons.
Nucleus of Comet Halley from the Giotto Project, European Space Agency. Astronomy Picture of the Day
As comets approach our Sun [within about 450 million kilometers (280 million miles)], they heat up and the ice begins to sublimate, transforming from a solid into a gas. The gas and dust forms an “atmosphere” around the nucleus called a "coma." Material from the coma gets swept into the tail.
As comets move close to the Sun, they develop tails of dust and ionized gas. Comets have two main tails, a dust tail and a plasma tail. The dust tail appears whitish-yellow because it is made up of tiny particles — about the size of particles of smoke — that reflect sunlight. Dust tails are typically between 1 and 10 million kilometers (about 600,000 to 6 million miles) long. The plasma tail is often blue because it contains carbon monoxide ions. Solar ultraviolet light breaks down the gas molecules, causing them to glow. Plasma tails can stretch tens of millions of kilometers into space. Rarely, they are as long as 150 million kilometers (almost 100 million miles). A third tail of sodium has been observed on Comet Hale-Bopp.
Comet Hale Bopp, taken by Joe Orman, showing the long, straight, blue plasma tail and the broader, shorter, whitish dust tail. [email protected]
Comets are surrounded by a broad, thin (sparse) hydrogen cloud that can extend for millions of kilometers. This envelope cannot be seen from Earth because its light is absorbed by our atmosphere, but it has been detected by spacecraft.
Comets travel around our Sun in highly elliptical (oval-shaped) orbits. The time it takes to make a complete orbit is called a comet's period. Comets are divided into short-period comets and long-period comets. Short period comets — such as Comet Halley — orbit our Sun in less than 200 years. Their orbital paths are close to the same plane of orbit as Earth and the other planets, and they orbit our Sun in the same direction as the planets. Based on these orbital characteristics, short-period comets are believed to originate in the Kuiper belt, a disk-shaped region extending beyond Neptune. The Kuiper belt contains small, icy planetary bodies, only a few of which have been imaged. These are the “leftovers” from early solar system formation. Occasionally the orbit of a Kuiper belt object will be disturbed by the interactions of the giant planets in such a way that it will have a close encounter with Neptune and either be flung out of the solar system or pushed into an orbit within our solar system.
Long period comets — such as Comet Hale-Bopp or Comet Hyakutake — take more than 200 years to orbit our Sun. Their orbital path is random in terms of direction and plane of orbit. Based on calculations from their observed paths, long-period comets are believed to originate in the Oort cloud, a round region that may extend 30 trillion kilometers (approximately 20 trillion miles) beyond our Sun. Oort cloud objects have never been imaged.