The Moons Jupiter & Saturn Earth 1 Mars 2 Jupiter 63 Saturn 47 Uranus 27 Neptune 13 Pluto 3 Moons of the Planets Galileo (1610) found the first four moons of Jupiter. Total 156 (as of Nov. 8, 2005) Shortened Version Feb 15, 2011 Jupiter s Galilean satellites are easily seen with Earth-based telescopes The four Galilean satellites orbit Jupiter in the plane of its equator All are in synchronous rotation The orbital periods of the three innermost Galilean satellites, Io, Europa, and Ganymede, are in the ratio 1:2:4 Data from spacecraft reveal the unique properties of the Galilean satellites The Galilean satellites formed like a solar system in miniature The two innermost Galilean satellites, Io and Europa, have roughly the same size and density as our Moon They are composed principally of rocky material The two outermost Galilean satellites, Ganymede and Callisto, are roughly the size of Mercury Lower in density than either the Moon or Mercury, they are made of roughly equal parts ice and rock 1
The Galilean satellites probably formed in a similar fashion to our solar system but on a smaller scale Io is covered with colorful sulfur compounds ejected from active volcanoes Io How can a small body like Io remain hot enough inside to produce such vigorous volcanic activity????? The energy to heat Io s interior and produce the satellite s volcanic activity comes from tidal forces that flex the satellite This tidal flexing is aided by the 1:2:4 ratio of orbital periods among the inner three Galilean satellites Tidal Heating Why Io is hot inside Io is pulled by the gravity of Europa and Ganymede, plus Jupiter. A resonance of 2:1 with Europa and 4:1 with Ganymede keeps Io s orbit slightly elliptical. (Remember what a resonance is?) Jupiter keeps trying to make the orbit more circular. The gravity pull causes Io to bend and flex. This flexing produces heat that keeps the interior molten. What s this for? 2
Io s Volcanoes Io s Volcanoes At least 16 volcanoes are known, and 4-8 are erupting at any one time Io s volcanoes erupt about 500 cubic km of lava each year more than 100 times as much as all of Earth s volcanoes Eruptions on Io can shoot gas and particles up to 300 km (190 miles) above the surface Some of the eruptions are powered by sulfur dioxide (SO 2 ) gas, which is also found in Earth s volcanoes. Much of Io s surface is covered with frozen sulfur (yellow and black) and SO 2 ice (white). Io s volcanic eruptions produce a thin and variable atmosphere, mostly of SO 2. Auroras are seen in this atmosphere. Jupiter s magnetic field makes electric currents flow through Io The Io torus is a ring of electrically charged particles circling Jupiter at the distance of Io s orbit Interactions between this ring and Jupiter s magnetic field produce strong radio emissions Io may also have a magnetic field of its own 3
Europa Europa Europa has no impact craters. It s icy surface shows an intricate network of crossing cracks, similar to cracks in the Arctic ice pack on Earth. There is very little vertical relief (no mountains or deep valleys). Europa s surface is very young. Europa is covered with a smooth layer of ice that may cover a worldwide ocean While composed primarily of rock, Europa is covered with a smooth layer of water ice The surface has hardly any craters, indicating a geologically active history As for Io, tidal heating is responsible for Europa s internal heat Minerals dissolved in this ocean may explain Europa s induced magnetic field Other indications are a worldwide network of long cracks and ice rafts that indicate a subsurface layer of liquid water or soft ice 4
Europa s cracked and colored crust Amazing structures in the icy crust of Europa Broken and tilted icebergs floating in a frozen sea 5
Broken and tilted blocks of ice Remains of impact craters? The flowing ice has filled them in. Europa is heated by gravitational effects of Ganymede and Jupiter (like Io), and therefore it has an interior layer ( mantle ) of liquid water. Could anything be living in the global ocean of Europa? Microbes? More advanced creatures?? NASA is considering a mission to Europa, to land and drill through the ice crust to get access to the liquid ocean below. Liquid water may also lie beneath the cratered surfaces of Ganymede and Callisto Ganymede Ganymede is highly differentiated, and probably has a metallic core It has a surprisingly strong magnetic field and a magnetosphere of its own While there is at present little tidal heating of Ganymede, it may have been heated in this fashion in the past An induced magnetic field suggests that it, too, has a layer of liquid water beneath the surface 6
Two types of terrain are found on the icy surface of Ganymede: areas of dark, ancient, heavily cratered surface regions of heavily grooved, lighter-colored, younger terrain Callisto has a heavily cratered crust of water ice The surface shows little sign of geologic activity, because there was never any significant tidal heating of Callisto However, some unknown processes have erased the smallest craters and blanketed the surface with a dark, dusty substance Magnetic field data seem to suggest that Callisto has a shallow subsurface ocean Jupiter has dozens of small satellites that have different origins As of early 2004, Jupiter has a total of 63 known satellites In addition to the Galilean satellites, Jupiter has four small inner satellites that lie inside Io s orbit Like the Galilean satellites, these orbit in the plane of Jupiter s equator The remaining satellites are small and move in much larger orbits that are noticeably inclined to the plane of Jupiter s equator Many of these orbit in the direction opposite to Jupiter s rotation The icy surfaces of Saturn s six moderate-sized moons provide clues to their histories As of early 2004, Saturn has a total of 31 known satellites In addition to Titan, six moderatesized moons circle Saturn in regular orbits: Mimas, Enceladus, Tethys, Dione, Rhea, and Iapetus They are probably composed largely of ice, but their surface features and histories vary significantly The other, smaller moons include shepherd satellites that control the shapes of Saturn s rings and captured asteroids in large retrograde orbits Enceladus (Moon of Saturn) Geysers of erupting liquid water announced in March 2006, discovered by the Cassini spacecraft This is probably the source of the E ring 7
Enceladus (Moon of Saturn) Geysers of erupting liquid water announced in March 2006, discovered by the Cassini spacecraft Enceladus (Moon of Saturn) Geysers of erupting liquid water announced in March 2006, discovered by the Cassini spacecraft Finding warm spots on the surface of Enceladus Titan: Largest Moon in the Solar System Orbital period around Saturn is 16 days Thick atmosphere made mainly of N 2 (nitrogen) with a small amount of CH 4 (methane), which is a hydrocarbon. Many other hydrocarbons also present. Titan Surface pressure is about 1.5 bars Surface temperature -180 C (= -290 F) Titan would be colder, but it has a greenhouse effect that warms the atmosphere. (Which gas is a greenhouse gas on Titan?) The atmosphere is nearly opaque with organic smog, produced by sunlight acting on methane and nitrogen. We can see the surface indistinctly at certain wavelengths where the atmosphere is partly transparent. Titan has a thick, opaque atmosphere rich in methane, nitrogen, and hydrocarbons The largest Saturnian satellite, Titan, is a terrestrial world with a dense nitrogen atmosphere A variety of hydrocarbons are produced there by the interaction of sunlight with methane These compounds form an aerosol layer in Titan s atmosphere and possibly cover some of its surface with lakes of ethane 8
Titan is being explored by the Cassini spacecraft as it orbits Saturn. Huygens Probe Jettisoned, December 25, 2004 The Huygens probe was detached from Cassini and entered Titan s atmosphere in January 2005. It descended through the atmosphere, making scientific measurements, and landed on Titan s surface, where it continued to make measurements for a few hours. Huygens Descent into Titan s Atmosphere January 14, 2005 Organic Chemistry on Titan N 2 -atmosphere with a few % CH 4 The chemistry in the atmosphere of Titan may resemble that which was at work on Earth 4 billion years ago, before the appearance of life. Titan is a large prebiotic chemistry laboratory on a planetary scale. But it s very cold on Titan s surface (-180 C), and there is no liquid water Titan s Atmosphere and Surface Studying the chemistry on Titan today may be a way to sample Earth s prebiotic chemistry 4 billion years ago. 9
Huygens finds flow channels on Titan Images from the descent Probe cameras Hydrocarbon dust forms in atmosphere Dust settles on land (H 2 O ice) Titan s Surface Liquid methane may emerge from springs, forming channels as it flows downhill Methane (CH 4 ) rains on Titan, washing hydrocarbon dust into streams and lakes Titan s Surface Islands in a stream channel formed by flowing liquid methane Titan landscape from Huygens lander, January 14, 2005 Rounded boulders in foreground are about 4-15 cm across. The boulders are probably frozen H 2 O. Titan Summary So many moons, so little time Titan has a thick atmosphere in which there is the active formation of complex organic chemicals. These may be the chemicals that formed on the Earth preceding the origin of life on our planet. We study Titan to learn about the chemistry of our own planet, and about other worlds where the chemistry of life exists. 10