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Transcript
Week #5 The Jovian Planets Introduction Jupiter, Saturn, Uranus, and Neptune are giant planets; they are also called the jovian planets. They are much bigger, more massive, and less dense than the inner, terrestrial planets. Their internal structure is entirely different from that of the four inner planets. Jupiter Jupiter, the largest and most massive planet, dominates the Sun’s planetary system. It alone contains two-thirds of the mass in the Solar System outside of the Sun, 318 times as much mass as the Earth (but only 0.001 times the Sun’s mass). Jupiter has at least 52 moons of its own and so is a miniature “planetary system” in itself. Jupiter Jupiter is more than 11 times greater in diameter than the Earth. From its mass and volume, we calculate its density to be 1.3 g/cm3. Jupiter, rather, is mainly composed of the light elements hydrogen and helium. Jupiter’s chemical composition is closer to that of the Sun and stars than it is to that of the Earth (see figure), so its origin can be traced directly back to the solar nebula with much less modification than the terrestrial planets underwent Jupiter Jupiter has no crust. At deeper and deeper levels, its gas just gets denser and denser, turning mushy and eventually liquefying about 20,000 km (15 per cent of the way) down. Jupiter’s “surface” (actually, the top of the clouds that we see) rotates in about 10 hours, though different latitudes rotate at slightly different speeds. Regions with different speeds correspond to different bands; Jupiter has a half-dozen jet streams while Earth has only one in each hemisphere. Jupiter Jupiter’s clouds are in constant turmoil; the shapes and distribution of bands can change within days. The most prominent feature of Jupiter’s surface is a large reddish oval known as the Great Red Spot. The bright bands are called “zones” and the dark bands are called “belts,” but the strongest winds appear on the boundaries between them. It is two to three times larger in diameter than the Earth. Jupiter emits radio waves, which indicates that it has a strong magnetic field and strong “radiation belts. Spacecraft to Jupiter Our understanding of Jupiter was revolutionized in the 1970s, when first Pioneer 10 (1973) and Pioneer 11 (1974) and then Voyager 1 and Voyager 2 (both in 1979) flew past it. The Galileo spacecraft arrived at Jupiter in 1995, when it dropped a probe into Jupiter’s atmosphere and went into orbit in the Jupiter system. The Cassini spacecraft flew by Jupiter, en route to Saturn, in 2000 –2001 (see figure). The Great Red Spot The Great Red Spot is a gaseous “island” a few times larger across than the Earth. It is the vortex of a violent, long-lasting storm, similar to large storms on Earth, and drifts about slowly with respect to the clouds as the planet rotates. We also see how it interacts with surrounding clouds and smaller spots. The Great Red Spot has been visible for at least 150 years, and maybe even 300 years. Sometimes it is relatively prominent and colorful, and at other times the color may even disappear for a few years. Jupiter’s Atmosphere Heat emanating from Jupiter’s interior churns the atmosphere. Pockets of gas rise and fall, through the process of convection. The bright bands (“zones”) and dark bands (“belts”) on Jupiter represent different cloud layers (see figure). Wind velocities show that each hemisphere of Jupiter has a half-dozen currents blowing eastward or westward. The Earth, in contrast, has only one westward current at low latitudes (the trade winds) and one eastward current at middle latitudes (the jet stream). Jupiter’s Atmosphere Jupiter’s winds were stronger than expected and increased with depth, which shows that the energy that drives them comes from below. Extensive lightning storms, including giant-sized lightning strikes called “superbolts,” were discovered from the Voyagers. The Galileo spacecraft photographed giant thunderclouds on Jupiter, which indicates that some regions are relatively wet and others relatively dry. Jupiter’s Interior Most of Jupiter’s interior is in liquid form. Jupiter’s central temperature may be between 13,000 and 35,000 K. The central pressure is 100 million times the pressure of the Earth’s atmosphere measured at our sea level due to Jupiter’s great mass pressing in. (The Earth’s central pressure is 4 million times its atmosphere’s pressure, and Earth’s central temperatures are several thousand degrees.) Because of this high pressure, Jupiter’s interior is probably composed of ultracompressed hydrogen surrounding a rocky core consisting of perhaps 10 Earth masses of iron and silicates (see figure). Jupiter’s Interior Jupiter radiates 1.6 times as much heat as it receives from the Sun. It must have an internal energy source It lacks the mass necessary by a factor of about 75, however, to have heated up enough to become a star, generating energy by nuclear processes (see Chapter 12). It is therefore not “almost a star,” contrary to some popular accounts. upiter’s Magnetic Field The space missions showed that Jupiter’s tremendous magnetic field is even more intense than many scientists had expected (see figure). At the height of Jupiter’s clouds, the magnetic field strength is 10 times that of the Earth, which itself has a rather strong field. Jupiter’s Ring Though Jupiter wasn’t expected to have a ring, Voyager 1 was programmed to look for one just in case; Saturn’s rings, of course, were well known, and Uranus’s rings had been discovered only a few years earlier during ground-based observations. The Voyager 1 photograph indeed showed a wispy ring of material around Jupiter at about 1.8 times Jupiter’s radius, inside the orbit of its innermost moon. From the far side looking back, the ring appeared unexpectedly bright, probably because small particles in the ring scattered the light toward the spacecraft. Jupiter’s Amazing Satellites Four of the innermost satellites were discovered by Galileo in 1610 when he first looked at Jupiter through his small telescope. These four moons (Io, Europa, Ganymede, and Callisto) are called the Galilean satellites (see figure). One of these moons, Ganymede, 5276 km in diameter, is the largest satellite in the Solar System and is larger than the planet Mercury. Jupiter’s Amazing Satellites Through first Voyager-spacecraft and then Galileo-spacecraft close-ups (see figure) The four Galilean satellites, in particular, were formerly known only as dots of light. The Galilean satellites, range between 0.9 and 1.5 times the size of our own Moon Pizza-like Io Io, the innermost Galilean satellite, provided the biggest surprises. Scientists knew that Io gave off particles as it went around Jupiter, and other scientists had predicted that Io’s interior would be heated by its flexing. Voyager 1 discovered that these particles resulted from active volcanoes on the satellite, a nice confirmation of the earlier ideas. Eight volcanoes were seen actually erupting, many more than erupt on the Earth at any one time. Pizza-like Io Though the Galileo spacecraft could not go close to Io for most of its mission, for fear of getting ruined because of Jupiter’s strong radiation field in that region, Io’s surface has been transformed by the volcanoes, and is by far the youngest surface we have observed in the Solar System. Pizza-like Io Why does Io have so many active volcanoes? Gravitational forces from Ganymede and Europa distort Io’s orbit slightly, which changes the tidal force on it from Jupiter in a varying fashion. This changing tidal force flexes Io, creating heat from friction that heats the interior and leads to the volcanism. Europa, a Possible Abode for Life Europa, Jupiter’s Galilean satellite with the highest albedo (reflectivity), has a very smooth surface and is covered with narrow, dark stripes. The lack of surface relief, mapped by the Galileo spacecraft to be no more than a couple of hundred meters high, suggests that the surface we see is ice. Europa, a Possible Abode for Life Few craters are visible, suggesting that the ice was soft enough below the crust to close in the craters. Gravitational tidal heating like that inside Io provides the heat to soften the ice. Because Europa possibly has a liquid-water ocean and extra heating, many scientists consider it a worthy location to check for signs of life. We can only hope that the ice crust, which may be about 10– 50 km thick, is thin enough in some locations for us to be able to penetrate it to reach the ocean that may lie below. Giant Ganymede The largest satellite in the Solar System, Ganymede, shows many craters alongside weird, grooved terrain. Ganymede is bigger than Mercury but less dense; it contains large amounts of water ice surrounding a rocky core. But an icy surface is as hard as steel in the cold conditions that far from the Sun, so it retains the craters from perhaps 4 billion years ago. The grooved terrain is younger. Giant Ganymede Ganymede shows many lateral displacements, where grooves have slid sideways, like those that occur in some places on Earth (for example, the San Andreas fault in California). It is the only place besides the Earth where such faults have been found. Pockmarked Callisto Callisto, the outermost of Jupiter’s Galilean satellites, has so many craters (see figure) that its surface must also be the oldest. Callisto, like Europa and Ganymede, is covered with ice. A huge bull’s-eye formation, Valhalla, contains about 10 concentric rings, no doubt resulting from an enormous impact. Perhaps ripples spreading from the impact froze into the ice to make Valhalla. Saturn Saturn, like Jupiter, Uranus, and Neptune, is a giant planet. Its diameter, without its rings, is 9 times that of Earth; its mass is 95 Earth masses. It is a truly beautiful object in a telescope of any size. The view from the Cassini spacecraft, now in orbit around it, is breathtaking. Saturn The giant planets have low densities. Saturn’s is only 0.7 g/cm3, 70 per cent the density of water (see figure). The bulk of Saturn is hydrogen molecules and helium, reflecting Saturn’s formation directly from the solar nebula. Saturn is thought to have a core of heavy elements, including rocky material, making up about the inner 20 per cent of its diameter. Saturn’s Rings The rings extend far out in Saturn’s equatorial plane, and are inclined to the planet’s orbit. Over a 30-year period, we sometimes see them from above their northern side, sometimes from below their southern side, and at intermediate angles in between. When seen edge-on, they are almost invisible. Saturn’s Rings The rings of Saturn consist of material that was torn apart by Saturn’s gravity or material that failed to collect into a moon at the time when the planet and its moons were forming. Every massive object has a sphere, called its Roche limit, inside of which blobs of matter do not hold together by their mutual gravity. The forces that tend to tear the blobs apart from each other are tidal forces. Saturn’s Rings The radius of the Roche limit is usually 2½ to 3 times the radius of the larger body, closer to the latter for the relative densities of Saturn and its moons. The Sun also has a Roche limit, but all the planets lie outside it. The natural moons of the various planets lie outside their respective Roche limits. Saturn’s rings lie inside Saturn’s Roche limit, so it is not surprising that the material in the rings is spread out rather than collected into a single orbiting satellite. Saturn’s Rings Saturn has several concentric major rings visible from Earth. The brightest ring is separated from a fainter broad outer ring by an apparent gap called Cassini’s division. We know that the rings are not solid objects, because the rotation speed of the outer rings is slower than that of rings closer to Saturn. Radar waves bounced off the rings show that the particles in the rings are at least a few centimeters, and possibly a meter, across. Saturn’s Rings Each of the known rings was actually divided into many thinner rings. The number of these rings (sometimes called “ringlets”) is in the hundreds of thousands. The images from the Cassini spacecraft (see figure) surpassed even Voyager’s views of ringlets. Saturn’s Rings The outer major ring turns out to be kept in place by a tiny satellite orbiting just outside it. At least some of the rings are kept narrow by “shepherding” satellites that gravitationally affect the ring material, a concept that we can apply to rings of other planets. Density waves (see figure) were seen in the ring. Saturn’s Atmosphere Like Jupiter, Saturn rotates quickly on its axis; a complete period is only 10 hours, in spite of Saturn’s diameter being over 9 times greater than Earth’s. The rapid rotation causes Saturn to be larger across the equator than from pole to pole. This equatorial bulging makes Saturn look slightly “flattened.” Jupiter also looks flattened, or oblate, for this reason. Saturn’s Atmosphere Saturn has extremely high winds, up to 1800 km /hr, 4 times faster than the winds on Jupiter. Cassini is tracking the winds with higher precision than was previously possible. On Saturn, the variations in wind speed do not seem to correlate with the positions of bright and dark bands, unlike the case with Jupiter (see figure). Saturn’s Interior and Magnetic Field Saturn radiates about twice as much energy as it absorbs from the Sun, a greater factor than for Jupiter. One interpretation is that only ⅔ of Saturn’s internal energy remains from its formation and from its continuing contraction under gravity. The rest would be generated by the gravitational energy released by helium sinking through the liquid hydrogen in Saturn’s interior. The helium that sinks has condensed because Saturn, unlike Jupiter, is cold enough. Saturn’s Interior and Magnetic Field Saturn gives off radio signals, as does Jupiter, a pre-Voyager indication to earthbound astronomers that Saturn also has a magnetic field. The Voyagers found that the magnetic field at Saturn’s equator is only ⅔ of the field present at the Earth’s equator. Saturn’s magnetic field contains belts of charged particles (analogous to Van Allen belts), which are larger than Earth’s but smaller than Jupiter’s. (Saturn’s surface magnetic field is 20 times weaker than Jupiter’s.) These particles interact with the atmosphere near the poles and produce auroras (see figure). Saturn’s Moon Titan At 40 per cent the diameter of the Earth, Titan is an intriguing body for a number of reasons. Titan has an atmosphere that was detected from Earth preVoyager. Studies of how the radio signals faded when Voyager 1 went behind Titan showed that Titan’s atmosphere is denser than Earth’s. The surface pressure on Titan is 1½ times that on Earth. Saturn’s Moon Titan Titan’s atmosphere is opaque, apparently because of the action of sunlight on chemicals in it, forming a sort of “smog” and giving it its reddish tint. The Voyagers showed (see figure) several layers of haze. They detected nitrogen, which makes up the bulk of Titan’s atmosphere, as it does Earth’s. Methane is a minor constituent, perhaps 1 per cent. A greenhouse effect is present, making some scientists wonder whether Titan’s surface may have been warmed enough for life to have evolved there. Saturn’s Moon Titan The temperature near the surface is only about 180°C (93 K), somewhat warmed (12°C) by a combination of the greenhouse effect and the anti-greenhouse effect. This temperature is near that of methane’s “triple point,” at which it can be in any of the three physical states—solid, liquid, or gas. So methane may play the role on Titan that water does on Earth. Saturn’s Moon Titan Scientists wanted to know whether Titan is covered with lakes or oceans of methane mixed with ethane, and whether other parts are covered with methane ice or snow. Also, using filters in the near-infrared, first the Hubble Space Telescope and then ground-based telescopes have been able to penetrate Titan’s haze to reveal some structure on its surface (see figure), though no unambiguous lakes were found. Saturn’s Moon Titan Titan is so intriguing, and potentially so important, that the lander of the Cassini mission was sent to plunge through its atmosphere. So while the lander of Jupiter’s Galileo mission went into the planet itself, this lander, known as Huygens, penetrated the clouds around Saturn’s largest moon when it arrived on January 14, 2005. At higher levels, the Huygens probe was buffeted by strong winds that ranged up to 400 km /h, though the winds calmed near the surface. Saturn’s Moon Titan As the Huygens lander drifted downward on its parachute for 2½ hours, it measured the chemical composition of Titan’s atmosphere. It then imaged the surface below it when it got sufficiently far below the haze. The shape of a shoreline is visible (see figure). No liquid substances were seen (there was never a glint of reflection of the type that occurs off a shiny body, for example), even though methane or ethane had been expected. The dark material is thought to be tar-like and has probably settled out of the atmosphere. Saturn’s Moon Titan A final triumph came when the probe survived for 90 minutes on the surface, sending back pictures of ice blocks. These pieces of ice are rounded, apparently also revealing the past presence of flowing liquid. Later analysis of infrared images seems to show a 30-km-wide structure that may be a volcanic dome. Such an ice volcano could be caused by the energy generated by tidal stresses within Titan caused by its elliptical orbit. It would release methane to the atmosphere. So the methane long measured in Titan’s atmosphere might not be from a methane-rich hydrocarbon ocean after all. Saturn’s Other Satellites So many of Saturn’s other moons proved to have interesting surfaces when seen close up from Cassini that we show a variety of images (see figures). Also, as was the case for Jupiter, dozens of small, irregular moons continue to be found from space images and from Earth-based telescopes. Uranus The two other giant planets beyond Saturn—Uranus (pronounced “U´ran-us”) and Neptune—are each about 4 times the diameter of (and about 15 times more massive than) the Earth. Like Jupiter and Saturn, Uranus and Neptune don’t have solid surfaces. Their atmospheres are also mostly hydrogen and helium, but they have a higher proportion of heavier elements. They reflect most of the sunlight that hits them, which indicates that they are covered with clouds. Some of the hydrogen may be in a liquid mantle of water, methane, and ammonia. At the planets’ centers, a rocky core contains mostly silicon and iron, probably surrounded by ices. Uranus Uranus was the first planet to be discovered that had not been known to the ancients. The English astronomer and musician William Herschel reported the discovery in 1781. Actually, Uranus had been plotted as a star on several sky maps during the hundred years prior to Herschel’s discovery, but had not been singled out as anything other than an ordinary star. Uranus revolves around the Sun in 84 years at an average distance of more than 19 A.U. Uranus is apparently surrounded by thick clouds of methane ice crystals (see figure), with a clear atmosphere of molecular hydrogen above them. The trace of methane gas mixed in with the hydrogen makes Uranus look greenish. Uranus Uranus is so far from the Sun that its outer layers are very cold. Studies of its infrared radiation give a temperature of -215°C (58 K). There is no evidence for an internal heat source, unlike the case for Jupiter, Saturn, and Neptune. Uranus Rotation - for its axis of rotation is roughly perpendicular to the other planetary axes, lying only 8° from the plane of its orbit (see figure). Uranus Sometimes one of Uranus’s poles faces the Earth, 21 years later its equator crosses our field of view, and then another 21 years later the other pole faces the Earth. Polar regions remain alternately in sunlight and in darkness for decades. Spring comes once in every 84 years Uranus Voyager 2 reached Uranus in 1986. It revealed most of our current understanding of Uranus, its rings, and its moons. (the figure (below) shows a more recent view.) Its moon Miranda, for example, though relatively small, has a surface that is extremely varied and interesting (see figure, right). Uranus’s Atmosphere Even though Voyager 2 came very close to Uranus’s surface, as close as 107,000 km (a quarter of the distance from the Earth to the Moon), it saw very little detail on it. Thus Uranus’s surface is very bland. Uranus’s clouds form relatively deep in the atmosphere. A dark polar cap was seen on Uranus At lower levels, the abundance of methane gas (CH4) increases. Thus most of the light that is reflected back at us is blue-green. Uranus’s Rings In 1977, astronomers on Earth watched as Uranus occulted (passed in front of ) a faint star. Predictions showed that the occultation would be visible only from the Indian Ocean southwest of Australia. The scientists who went to study the occultation from an instrumented airplane turned on their equipment early, to be sure they caught the event. Surprisingly, about half an hour before the predicted time of occultation, they detected a few slight dips in the star’s brightness (see figure). They recorded similar dips, in the reverse order, about half an hour after the occultation. Uranus’s Rings The dips indicated that Uranus is surrounded by several rings, some of which have since been photographed by Voyager 2 and with the Hubble Space Telescope. Eleven rings are now known. They are quite dark, reflecting only about 2 percent of the sunlight that hits them. They are very narrow from side to side; some are only a few km wide. Uranus’s Rings Voyager provided detailed ring images (see figure). Interpreting the small color differences is important for understanding the composition of the ring material. Quite significant was the single longexposure, backlighted view taken by Voyager. Study of these data has shown that less of the dust in Uranus’s rings is very small particles compared with the dust in the rings of Saturn and Jupiter. Uranus’s Interior and Magnetic Field Voyager 2 detected Uranus’s magnetic field. It is intrinsically about 50 times stronger than Earth’s. Since Uranus’s field is so tilted, it winds up like a corkscrew as Uranus rotates. Uranus’s magnetosphere contains belts of protons and electrons, similar to Earth’s Van Allen belts. Voyager also detected radio bursts from Uranus every 17.24 hours. Thus Uranus’s interior rotates slightly more slowly than its atmosphere. Neptune Neptune is even farther from the Sun than Uranus, 30 A.U. compared to about 19 A.U. Neptune takes 164 years to orbit the Sun. Its discovery was a triumph of the modern era of Newtonian astronomy. Mathematicians analyzed the amount that Uranus (then the outermost known planet) deviated from the orbit it would follow if gravity from only the Sun and the other known planets were acting on it. The small deviations could have been caused by gravitational interaction with another, as yet unknown, planet. Neptune A year later, the French astronomer Urbain Leverrier independently worked out the position of the undetected planet. The French astronomers didn’t test his prediction right away either. Leverrier sent his predictions to an acquaintance at the observatory in Berlin, where a star atlas had recently been completed. The Berlin observer, Johann Galle, discovered Neptune within hours by comparing the sky against the new atlas. Neptune Neptune has not yet made a full orbit since it was located in 1846. But it now seems that Galileo inadvertently observed Neptune in 1613 and recorded its position, which more than doubles the period of time over which it has been studied. We have used the positions he measured to improve our knowledge of Neptune’s orbit! Neptune’s Atmosphere Neptune, like Uranus, appears greenish in a telescope because of its atmospheric methane (see figure, top). Some faint markings could be detected on Neptune even before adaptive optics systems became available on Earth (see figure, bottom). It was thus known before Voyager that Neptune’s surface was more interesting than Uranus’s. Still, given its position in the cold outer Solar System, nobody was prepared for the amount of activity that Voyager discovered. Neptune’s Atmosphere As Voyager approached Neptune, active weather systems became apparent (see figure, left). An Earth-sized region that was soon called the Great Dark Spot (see figure, center) became apparent. Though colorless, the Great Dark Spot seemed analogous to Jupiter’s Great Red Spot in several ways. For example, it was about the same size relative to its planet, and it was in the same general position in its planet’s southern hemisphere (see figure, right). Putting together a series of observations into a movie, scientists discovered that it rotated counterclockwise, as does the Great Red Spot. Thus it was anti-cyclonic, which made it a highpressure region. Neptune’s Atmosphere Clouds of ice crystals, similar to Earth’s cirrus but made of methane, form at the edge of the Great Dark Spot as the high pressure forces methane-rich gas upward. But this Great Dark Spot had disappeared when the Hubble Space Telescope photographed Neptune a few years later, so it was much less long lived than Jupiter’s Great Red Spot. Neptune’s Interior and Magnetic Field Voyager 2 measured Neptune’s average temperature: 59 K, that is, 59°C above absolute zero (214°C). This temperature, though low, is higher than would be expected on the basis of solar radiation alone. Neptune gives off about 2.7 times as much energy as it absorbs from the Sun. Thus there is an internal source of heating, unlike the case of Uranus, which otherwise seems like a similar planet. Neptune’s Interior and Magnetic Field Why is the average density of Uranus and Neptune higher than that of Jupiter and Saturn? Their densities show that Uranus and Neptune have a higher percentage of heavy elements than Jupiter and Saturn. Voyager detected radio bursts from Neptune every 16.11 hours. Thus Neptune’s interior must rotate with this rate. On Neptune and Uranus, as on the Earth, equatorial winds blow more slowly than the interior rotates. By contrast, equatorial winds on Venus, Jupiter, Saturn, and the Sun blow more rapidly than the interior rotates. Neptune’s Interior and Magnetic Field The field, as for Uranus, turned out to be both greatly tipped and offset from Neptune’s center (see figure). Astronomers favor the explanation that the magnetic field is formed in an electrically conducting shell outside the planets’ cores. The fields of Earth and Jupiter, in contrast, are thought to be formed deep within the core. Neptune’s Rings Before Voyager’s arrival, astronomers wanted to know whether Neptune has rings, like the other giant planets. There was no obvious reason why it shouldn’t. Some observations from Earth detected dips during occultations of stars, similar to those produced by Uranus’s rings, while others didn’t. It was thought that perhaps Neptune had incomplete rings, “ring arcs.” As Voyager came close to Neptune, it radioed back images that showed conclusively that Neptune has narrow rings. Further, it showed the rings going all the way around Neptune. The material in the densest of Neptune’s rings is very clumpy. Neptune’s Rings The clumps had led to the incorrect idea of “ring arcs.” The clumpy parts of the ring had blocked starlight, while the other parts and the other rings were too thin to do so (see figures). The rings can now be studied from the Hubble Space Telescope and with ground-based telescopes having adaptive optics (see discussion in Chapter 3). Neptune’s Rings The fact that Neptune’s rings are so much brighter when seen backlighted tells us about the sizes of particles in them. The most detectable parts of the rings have at least a hundred times more dust-sized grains than most of the rings of Uranus and Saturn. Since dust particles settle out of the rings, new sources must continually be active. Probably moonlets collide and are destroyed. Neptune’s Moon Triton Neptune’s largest moon, Triton, is a little larger than our Moon and has a retrograde (backward) orbit. It is massive enough to have a melted interior. Its density is 2.07 grams /cm3, so it is probably about 70 per cent rock and 30 per cent water ice. It is named after a sea god who was a son of Poseidon. It is denser than any jovian-planet satellite except Io and Europa. Even before Voyager 2 visited, it was known that Triton has an atmosphere. The scientists waited eagerly to learn if Triton’s atmosphere would be transparent enough to see the surface. It was. The atmosphere is mostly nitrogen gas, like Earth’s. Neptune’s Moon Triton Triton’s surface is incredibly varied. Much of the region Voyager 2 imaged was near Triton’s south polar cap (see figure). The ice appeared slightly reddish. The color probably shows the presence of organic material formed by the action of solar ultraviolet light and particles from Neptune’s magnetosphere hitting methane in Triton’s atmosphere and surface. Nearer to Triton’s equator, nitrogen frost was seen. Neptune’s Moon Triton Many craters and cliffs were seen. They could not survive if they were made of only methane ice, so water ice (which is stronger) must be the major component. Since Neptune’s gravity captures many comets in that part of the Solar System, most of the craters are thought to result from collisions with comets. Triton’s surface showed about 50 dark streaks parallel to each other as is readily visible near the bottom of the figure. They are apparently dark material vented from below. The material is spread out by winds. Neptune’s Moon Triton A leading model is that the Sun heats darkened methane ice on Triton’s surface. A couple of these “ice volcanoes” were erupting when Voyager 2 went by. This heating vaporizes the underlying nitrogen ice, which escapes through vents in the surface. Since the streaks are on top of seasonal ice, they are all probably less than 100 years old. Much of Triton is so puckered that it is called the “cantaloupe terrain.” It contains depressions 30 km in diameter, crisscrossed by ridges (see figure).