Modified from http://www.lpi.usra.edu/education/explore/ice/background/iceSolarSystem/
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 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, beginning near the outer asteroid belt, 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 or icy asteroids (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 likely did not provide sufficient gravitational attraction to "hold" an atmosphere. Because it was small, it cooled quickly, so volcanic processes may have 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 degrees F (467 degrees 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.
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 degrees F (15 degrees C), but it ranges from -128 degrees F (-89 degrees C) to 136 degrees F (58 degrees 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.
The freezing point of carbon dioxide is -108 degrees F (-78 degrees C); pure ammonia's freezing point is -107 degrees F (-77 degrees 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 the 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 degrees F (-125 degrees C) to 23 degrees F (-5 degrees C), well under the freezing point of water and also cold enough to freeze carbon dioxide.
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!
Mars also has another type of ice -- carbon dioxide ice -- which is familiar to us as "dry ice." Because Mars is so cold, in the winter the carbon dioxide in its 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).
Mars has ice caps at both its poles. The north pole ice cap is about 600 miles (1000 km) 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, sometimes getting warmer and sometimes getting colder. During colder times, its ice caps expanded and glaciers extended farther across the Martian landscape.
Asteroids: Some asteroids may also contain ice and water. The Dawn mission is exploring Vesta, a very dry asteroid, although some scientists believe that beneath Vesta's surface ice could exist. A possible place for the presence of surface ice is near the north pole that has been in darkness for two Earth years. The gamma ray and neutron detector should be able to detect this if a significant amount of water ice is present. The Dawn mission will also investigate the largest asteroid, Ceres, which scientists believe from its density and shape to contain a vast store of water and water ice.
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 degrees F (107 degrees C) in sunlight to -243 degrees F (-153 degrees 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 degrees 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.
Radar and spectroscopic data collected by several spacecraft, including the Lunar Reconnaissance Orbiter, suggest that large amounts of water ice, perhaps mixed with dust and rocks, exist at the lunar south pole. NASA's Lunar Crater Observation and Sensing Satellite (LCROSS) mission impacted the lunar surface in a permanently shadowed crater. The resulting plume has been analyzed for water ice and vapor and other materials by instruments on the LCROSS shepherding spacecraft and LRO, and by telescopes on Earth. Cold temperatures and thermal dynamics of the plume suggest it is possible for water ice, delivered by comets, to exist near and within some of the Moon's polar craters.
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 degrees, 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. NASA's MESSENGER mission, currently orbiting Mercury, has found radar-bright areas at Mercury's poles that suggest water ice exists there as well.
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! (Additional information, activities and resources about the rings and moons are available in the YSS topic
Moons and Rings: Our Favorite Things.)
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 absent of high topography, but it is crossed by ridges and covered by pits and domes and features called "chaos terrain." 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.
Europa is far from the sun and its surface temperature is a chilling -260 degrees F (-160 degrees C) at the equator and -370 degrees F (-220 degrees 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, especially if its rocky core is similarly heated, producing hydrothermal vents. (Check out the YSS topic Got Life? for more information.)
Ganymede is the largest moon in the solar system, actually larger than Mercury, and is made mostly of water ice with a rocky core. Scientists suggest that it has a water ocean beneath its crust, sandwiched between thick layers of ice. The surface of Ganymede is older than Europa's, but still has evidence for active geology in its past, and maybe present. This frozen moon even has polar ice caps! The most interesting thing about Ganymede is probably that it has its own magnetic field, meaning its core is likely still molten.
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. Interestingly, Callisto does not seem to be differentiated.
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 km) 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 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 than-274 degrees F (-170 degrees C), well below the freezing point of water and other ices. Water ice at the surfaces of these moons is rock-hard.
Enceladus caught the attention of scientists and the world with its spectacular ice geysers. The Cassini spacecraft flew through a plume and sampled water vapor and ice particles and minor components of other molecules. The material vented by Enceladus is what makes up an entire band of Saturn's rings (called the E ring)!
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 degrees F (-179 degrees C). Titan has lakes of liquid hydrocarbons at its surface and a terrain that contains mountainous features and dunes composed of ice. Deposits of water ice and hydrocarbon ice occur at its surface. This moon probably has a methane cycle that forms clouds and even methane rain! Titan is similar to Ganymede, indicating it may have an "ice-water sandwich" under its surface.
Moons of Uranus and Neptune: Uranus' moons are ice-rock conglomerates made of about half-ice and half-rock; in addition to frozen water, the ice may include ammonia and dry ice. Some of the moons appear to have younger surfaces that may have gone through recent geologic activity. Miranda's surface is bizarre, with 20-km deep canyons and valleys and ridges. The surfaces of some of these moons may have been covered with flows of frozen water, perhaps from heating from gravitational tidal interactions, or from the heat of impacts.
Neptune's moons are also thought to contain large amounts of ice. Triton, Neptune's largest moon, is the only large moon in our solar system that orbits in the opposite direction of its planet's rotation -- a retrograde orbit. Triton is one of the coolest objects in our solar system; it is so cold that most of Triton's nitrogen atmosphere is condensed as frost, giving its surface an icy sheen that reflects 70 percent of the sunlight that hits it.
Triton is geologically active. Nitrogen has erupted from its surface in geysers, and there is volcanic activity from water ice rather than from liquid rock. It has a sparsely cratered surface with smooth volcanic plains, mounds and round pits formed by icy lava flows of frozen water. Its crust is frozen nitrogen, water and dry ice; frozen water makes up much of this cold moon's interior.
Comets
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.
Most 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 km), 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. Recently, several small asteroids in the main asteroid belt have been observed to produce comet-like tails when they approach the sun; these are now known as main belt comets.
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. For further information about comets, please check out the YSS topic Small Bodies / Big Impacts.
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.
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.
