Unit 5 - The Gas Giant Planets (Chapters 14 - 16)

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Earth Sciences
Earth Sciences 1086F/G
Phil Mc Causland

Introduction April-21-14 8:52 PM - The gas giants are sometimes known as the Jovian planets (after Jupiter, the largest) or the ice giants - The definition of a gas giant planet is: A large, low-density planet composed primarily of hydrogen, helium, methane, and ammonia in either gaseous or liquid state - The four gas giants in the solar system are: Jupiter, Saturn, Uranus, and Neptune → A few of their common features are:  All have atmospheres of hydrogen and helium, with more hydrogen than helium  On Jupiter and Saturn, the boundary between atmosphere and body of the planet (what we'd call the "surface") is gradual: from gas to liquid, and on Uranus and Neptune, there may actually be a sharp bounding surface between gas and liquid  All have very hot cores  All have cores that are sometimes called "rocky" but are a mix of heavy elements in a solid state  All are surrounded by systems of rings and natural satellites  All rotate rapidly, resulting in strong atmospheric winds that produce cloud bands that parallel their equators — Jupiter rotates fastest, and thus, has the most prominent bands - The outermost atmospheres of all four planets consist of molecular hydrogen and (less) helium (i.e., actual gas state) - Further down, for both Jupiter and Saturn, the "state" changes from gas to liquid and then to something we'd have to call "metal" → The pressure on the interiors of these two massive planets is just so immense that hydrogen starts responding to stimuli exac tly as metal would - For Uranus and Neptune, the molecular hydrogen and (lesser) helium of the outer atmosphere gives way to an atmosphere primarily of methane (also in the gas state), then further down to a semi-solid mix (called "ice") of water, methane, and ammonia → None of these elements transform into a so-called "metal" state prior to the heavy element ("rocky") core — we do not have much data to work with, so there is a lot of guess work involving these two outer gas giants Origin of the Gas Giants - Astronomers figure that all gas giants were formed within the first 10 million years of the development of a star-planet system or they wouldn't have formed → The idea is that all those volatile and liquid elements that would be given off by an early star would gather quickly into fr ozen blobs in the cold outer reaches of the system; there, they would grow and their gravitational attraction would increase rapidly — and they'd start pulling greater and greater quantities of volatiles and liquids into themselves → Inevitably, the larger masses would start interfering with the orbits of the slightly smaller masses, and the combined gravit ational/rotational interactions would have the effect of flinging the smaller ones either out to the margins of the planetary system or in toward the central star Comet Impact on a Gas Giant Comet Impact on Jupiter - The first witnessed occurrence of a comet hitting Jupiter was in 1994 when the comet (named Shoemaker-Levy 9) passed very close to the planet and was pulled into at least 21 pieces that looped out away from Jupiter in long elliptical orbits (which were photographed) - The pieces fell back and slammed into Jupiter over a period of six days - The fragments were no bigger than a kilometre in diameter and contained a fluffy mixture of rock and ices - The impacts occurred just over Jupiter's horizon and the rapid rotation of Jupiter brought the impact points within sight ofEarth only 15 minutes later - We can learn from and use the event in two different ways: 1) Astronomers used the impacts as probes of Jupiter's atmosphere: By combining the observed impacts with the computer models, a stronomers were able to fine- tune the models to better represent Jupiter's atmosphere 2) The spectacle helps us move in a more general way: It reminds us that planets are hit by large objects such as asteroids and the heads of comets Learning Objectives - In general, we need to learn how totally different the four gas/ice giants are from the four terrestrial planets - While we have lots of information about both Jupiter and Saturn, we know very little about Uranus and Neptune, so we extrapolate a lot  Apply some skepticism to the interpretation of "facts" for the latter two  It's important that you remember the "scientific method": we have enough from data for Jupiter and Saturn to progress from observation and hypothesis to theory, but that's not so for Uranus and Neptune - Some of the satellites of Jupiter and Saturn appear to have the potential for primitive life development— learn which satellites make the best candidates and why Unit 5 - The Gas Giant Planets Page 1 Chapter 14: Jupiter April-17-14 12:36 PM - Jupiter is the senior member of Sun's family in size; Jupiter is three times larger than the next largest planet - We know that Jupiter is massive because its satellites race around it at high speed and don't fly off into space → Io is the innermost of the four Galilean satellites, and its orbits around Jupiter in less than two days — Jupiter has to be a very massive world to hold on to such a rapidly moving satellite - Jupiter is not exactly a "nice" place:  Hurricane-force winds whip up storms that could swallow the continent of Asia  Lightning bolts capable of vaporizing a small city rip open the sky  A swirling vortex of clouds (called the Great Red Spot) larger than Earth has been raging for at least 300 years  Above all, lethal radiation permeates the surrounding environment - Jupiter emits about 1.7 times as much energy as it receives from Sun — it must be very hot inside to have so much heat flowing outward into space - Some 67 satellites (at last count) orbit the planet, but 4 are particularly interesting: 1) Io is the most volcanically active body in the solar system 2) Europa apparently has a significant liquid water ocean beneath its ice surfaces 3) Ganymede seems to have a water "slush" ocean beneath a solid ice cover 4) Callisto is a body that apparently never chemically differentiated → Together these are known as the Galilean satellites, in honor of the man who first described them - In 1979, the Voyager 1 spacecraft discovered ghostly rings around Jupiter — they are very tenuous and composed of dust particles that are kicked up as interplanetary meteoroids smash into Jupiter's four small inner satellites Missions to Jupiter - There was no race involved to explore Jupiter because it's a lot further away and landing would be quite unthinkable for any existing technology we currently have (because Jupiter does not have a solid surface) - The following automated flights to Jupiter are worth noting: Pioneer 10 - Returned over 500 images of Jupiter and satellite family during a fly-by - Pioneer10 has now left the Solar System - Scientists mounted a plaque on the spacecraft that displayed diagrams capable of being translated by any scientifically educated civilization that might encounter the spacecraft in the future Pioneer 11 - Took better pictures than Pioneer 10 - Measured Jupiter's intense charged-particle and magnetic field environment - It, too, has now left the Solar System Voyager 1 and Voyager 2 - Voyager 1 made its closest approach while Voyager 2 followed during the same year - Images from both crafts showed the complicated, swirling turbulence of Jupiter's atmosphere in exquisite detail - Voyager 1 found nine active volcanoes erupting on Io — four months later, Voyager 2 found that eight of the nine volcanoes were still erupting - A thin, dusty ring was also discovered around Jupiter, forcing scientists to revise their thinking on the origins and mechanics of planetary ring systems Galileo - Galileo is the only craft to orbit Jupiter; it was designed to study Jupiter's atmosphere, satellites, and surrounding magnet osphere for two years - After 14 years of flight time and eight years collecting data around Jupiter, Galileo was deliberately destroyed by sending i t crashing into Jupiter's crushing atmosphere → If this had not been done, there was a possibility that it could contaminate one of its own discoveries - Because of data Galileo collected, scientists now suspect Europa has a salt water ocean that may contain microbial life - Galileo has changed the way we think of the Solar System New Horizons - Launched to reach Pluto, New Horizons took pictures of Jupiter on its journey Juno - Juno is a solar powered craft that roams far into the outer reaches of the Solar System and "returns" to Earth for a "gravity assist" - It should arrive at Jupiter in 2016 where it will collect data to help scientists figure out the earliest history of the Solar System (since Jupiter was the first planet to form), after which it will end with a dive straight into the planet Planetary Facts  Distance from Sun - 5.3 AU  Diameter - 142 984 km (about 11.2 Earths)  Density - 1.326 g/cc  Rotational period - 9.925 hours  Equatorial rotational speed - 12.6 km/s (45 300 km/h)  Orbital period - 4331.6 days (11.86 years)  Orbital speed - 13.07 km/s  Orbital eccentricity - 0.0488  Orbital inclination - 1.305°  Axial tilt - 3.13°  Satellites - 67 (4 big ones)  Rings - one main ring and one minor ring Orbit and Rotation - Jupiter does not share similarities with Earth - Jupiter maintains a very "upright stance" of a mere 3° of obliquity - Jupiter has a rotational period that's really fast as it orbits Sun - Because most of the planet is liquid, and it doesn't have a hard surface, and it rotates so fast, it has a detectable oblateness (i.e., it bulges slightly at the equator) Unit 5 - The Gas Giant Planets Page 2 Interior Heat Engine - From a true gaseous hydrogen-rich atmosphere it turns into liquid hydrogen then to "metallic" hydrogen and finally a heavy metal core - Jupiter's weird interior is responsible for a very high heat flow - Jupiteractually emits about 1.7 times as much energy as it receives from Sun - Astronomers theorize that the source of Jupiter's excess energy is the slow compaction of the planet — the atoms of hydrogen and helium move as gravity very slowly pulls them into a more compact configuration, and that motion generates heat - This planet compaction mechanism is thought to explain the internal heat generated by all the gas giants, with the exception of Uranus (which doesn't appear to generate any internal heat at all) The Magnetosphere - Like most planets, Jupiter has a magnetic field; we're not sure how it's generated but it likely is to do with differential m otion by the different physical states of hydrogen (particularly the metallic state surrounded by the liquid state) - The field is the strongest of any body within the solar system except for spots on the surface of Sun (it's 14 times stronger than Earth's) - The strong magnetic field efficiently traps electrically charged particles (from the solar wind) and shapes them into a teard rop form of magnetosphere of radiation around the planet — the trapped charged particles are so abundant, that they would do damage to instruments of any spacecraft flown through the magnetosphere - Jupiter's rings and satellites occur within this radiation, although certainly the values drop considerably with distance out ward (there's speculation about life in the oceans of two of the satellites — Europa and Ganymede; for that to be the case, the strength of the radiation would have to decrease considerably by the time it reached them) - Charged particles in the magnetosphere leak downward along the magnetic field, and, where they enter the atmosphere, they produce auroras 1000 times more powerful than those on Earth → The auroras on Jupiter, like auroras on Earth, occur in rings around the magnetic poles Unit 5 - The Gas Giant Planets Page 3 Atmosphere - Shortly after the Galileo probe "arrived on station," it sent the Atmosphere Structure Instrument (ASI) it carried into the atmosphere of Jupiter - The probe made the most difficult planetary atmospheric entry ever attempted - It relayed data obtained during its 61 minute descent mission that has revolutionized our thoughts on Jovian/gas giant planets - The probe detected extremely strong winds and very intense turbulence during its descent through Jupiter's thick atmosphere — this confirmed theories that the energy source driving much of Jupiter's distinctive circulation phenomena is probably heat escaping from the deep interior of the planet - Jupiter's atmosphere is composed of significantly lower-than-expected levels of helium, neon, and certain heavy elements, such as carbon, oxygen, and sulfur - Some indication of a high-level ammonia ice cloud was detected - The vertical temperature gradient obtained by the ASI was characteristic of a dry atmosphere, free of condensation - The clouds of Jupiter are organized into dark belts and bright zones — all a result of the high rate of rotation of the planet → The zones tend to be white or yellow, while the belts are often some shade of reddish brown → The temperature of the dark belts is higher than that of the light zones, implying that the former are lower in the atmosphere - The belts appear to be regions of descending gas and the zones are regions of rising gas → The zones appear to be high-pressure regions of rising gas that cools as it rises and forms clouds higher in the atmosphere where they receive more sunlight and look brighter → The belts are low-pressure regions with sinking gas and lower clouds that are not as brightly lit - Each hemisphere has around six bands with winds blowing at very high velocities in opposite directions — this explains the extensive shear and turbulence at the boundaries between these regions Unit 5 - The Gas Giant Planets Page 4 boundaries between these regions - On Earth, high- and low-pressure regions are bounded by high-speed winds, and the same is true on Jupiter — winds blowing hundreds of kilometers per hour separate the belts and zones - The clouds on Jupiter are twisted and swirled by the winds, but the belts and zones are highly stable → Although their colors and brightness change sometimes, they have not changed their position since humans have been mapping them - Astronomers think that that colors are caused by traces of compounds containing sulfur or phosphorus modified by sunlight - Mixed within the belt-zone circulation are light and dark spots: Dark spots appear to be openings through which we see deeper, darker clouds, and w hite spots appear to be higher cloud circulations → The largest is the Great Red Spot (GRS) which is twice the diameter of Earth, and was formed by rising gas carrying heat upward from deep below the clouds and creating a vast, rotating storm - Rising currents of hot gas can produce thunderheads that tower and spawn powerful winds and tremendous lightning - Weather patterns on Jupiter appear to be dominated by the interior Geology of Jupiter - With no hard surface to look at, there is not much geology to recount Jupiter's Family - Galileo discovered The Galilean Moons orbiting Jupiter through a telescope → We must learn to call them satellites to avoid confusion → There is no clear definition of a satellite - Jupiter is host to a very large family of 67 natural satellites - The satellites fall into three categories (these classifications are general — not specific to Jupiter): 1) Regular 2) Irregular 3) Trojan - Jupiter is the first planet to have all three categories of satellites Regular Satellites - These satellites define the outer orbital reaches of a planet's domain in space - Examples include Jupiter's Galilean satellites (Io, Europa, Ganymede,and Callisto) - Regular satellites:  Are large and round  Tend toward entirely stable, simple, nearly circular orbits  All move through a plane in space that is roughly equal to the planet's equatorial plane  There are two hypotheses of formation of regular satellites: 1) They formed out of the same nebula gas and dust that built the planet, and likely at the same time (this is what virtually everyone now believes) 2) They formed by collision (our Moon is a great example — so there is always an exception to the rule) Irregular Satellites - These satellites are of unknown origin - Irregular satellites:  Are almost certainly captured objects, and they were captured early in the history of the solar system  Many irregulars travel in packs that indicate they were once related objects  The irregulars are typically small  They orbit at great distances and often on odd trajectories → More often than not their orbits are retrograde → About the only thing they don't do is orbit in a plane perpendicular to that of the planet's orbit  A recently discovered fact is that the irregular satellites all look to have similar compositions (identified by their albedo) Trojan Satellites - These are weird - Outside the traditional categories of satellites are two sorts of objects that really stretch definitions  Ahead of and behind Jupiter are two packs of asteroids that orbit Sun but are also gravitationally bound to Jupiter → They occupy positions called Lagrangian Points — 60° ahead of and 60° behind Jupiter on its Sun orbital path → They are technically satellites of Jupiter, some astronomers say, but others consider them mere "companions" to Jupiter Unit 5 - The Gas Giant Planets Page 5  The major problem is that they do not orbit Jupiter (Note: Mars and Neptune are each known to "control" one or more Trojan objects in a similar fashion) → Earth has one true Trojan (2010 TH7) that shares Earth's orbit → Earth has acquired a sort of enhancedTrojan, too — a rock called 2002 AA29 orbits Sun while also carving a horseshoe-shaped path around Earth  Every few centuries it gets close enough to Earth to be considered more than a Trojan, becoming what astronomers now call a "quasi-satellite" The Galilean Satellites - The Galilean Moons are large and have interesting geology - They are all tidally locked to Jupiter, thus keeping the same face forever facing the planet - Our study of the satellites of Jupiter will illustrate three important principles in comparative planetology: 1) First, a body's composition depends on the temperature of the material from which it formed (this is illustrated by the prevalence of ice as a building material in the outer Solar System where sunlight is weak) 2) The second principle is that cratering can tell us the age of a hard surface 3) Finally, we will see that internal heat has a powerful influence over the geology of these larger satellites Callisto - The outermost of Jupiter's four Galilean satellites, Callisto is a bit larger than Earth's Moon - Its density is 1.79 g/cm (calculated by its gravitational influence on passing spacecraft) - By knowing the density of ice and rock, it can be determined that Callisto must be a mixture of rock and ice - Photographs show that the surface of Callisto is dark, dirty ice (with some rock) heavily pocked with craters → Old, icy surfaces in the Solar System become dark because of dust and impacts of small meteoroids - Slow radioactive decay in its interior may provide enough heat to melt some of the interior; when the water approaches the surface, it freezes - Callisto has never fully differentiated to form a dense core and a lower-density mantle → It's interior is a mixture of rock and ice — this is consistent with the observation that it has only a weak magnetic field Ganymede - The next Galilean satellite inward is Ganymede — the largest of the satellites of Jupiter (it's also larger than Moon, Mercury, and over three-quarters the diameter of Mars) - Its density is 1.936 g/cm2 - It is composed of silicate rock and water ice - Ganymede is completely differentiated, and even has a molten iron-rich core - Below a 200 km thick surface layer of water ice appears to be a liquid water salty ocean - Below that (between the hot metal core and the ocean) is a layer of solid silicate rock material - Ganymede is one of the only planetary satellites (along with other Galilean moons) in the Solar System known to have a magnet ic field, and the only one that is known to generate its own field — clearly a result of it having a molten metal core inside a solid "sheath" - Because the interior is hot, there must be enough radioactive material in the mantle to generate that heat — thus allowing both the differentiation of the satellite plus the convection that makes the magnetic field - Ganymede has its own atmosphere, composed primarily of oxygen and very minor hydrogen, that is very thin - On Ganymede, the oxygen is a result of radiation breaking down the water molecules of ice into oxygen and hydrogen - Ganymede formed shortly (and over a short time span) after Jupiter from the disk of gas and dust surrounding Jupiter - After formation, the heat of both the gravitational energy of the accretion itself plus the heat of radioactive decay process es allowed the body to differentiate, to begin to generate a magnetic field, and to produce a liquid water sub-ocean with a thick ice cover - Today, Ganymede will be in a slow cooling stage, but it will likely take millions of years before the temperature drops below the point of stability of liquid water Europa Unit 5 - The Gas Giant Planets Page 6 Europa - The next Galilean satellite inward is Europa (which is a bit smaller than Moon) 2 - Europa has a density of 3 g/cm — making it mostly rock and metal with a surface of ice that is almost free of craters → *Note: The density is increasing as we work our way in toward Jupiter - Since the satellite is older than can be determined through crater-counting, the surface must therefore be "resurfaced" periodically → The easiest hypothesis is that liquid water (from an underlying sub-ocean) rises through cracks in the ice (perhaps in response to some convecting motion in the water) to flood over the surface (like resurfacing a skating rink) — this is what has given rise to the idea that there is a significant sub-surface liquid water ocean on Europa - Europa is much too small to have retained much heat from its formation or from radioactive decay, it has no magnetic field of its own, and it cannot have a molten conducting core — so how has it undergone complete differentiation? → The heat came from tidal heating which provides enough heat to keep the little satellite active - Orbiting deep inside Jupiter's radiation belts, Europa is bombarded by high-energy particles that damage the icy surface → Water molecules are freed by the abrasion and broken up → The oxygen and hydrogen are sort of "sputtered" into space where they form a very tenuous and temporary atmosphere — actually forming a doughnut-shaped cloud spread around Jupiter and enclosing Europa's orbit Io - Io orbits closest to parent Jupiter - It has a high density of 3.53 g/cc — which roughly mid-way between that of Moon and Mars, and the highest density of any satellite in the Solar System - Io is just a bit bigger than Europe, but smaller than both Ganymede and Callisto - The interior looks to be a core of iron plus or minus sulfur (not sure if it's solid or molten), a mantle of about the same c omposition as that of Earth (i.e., heavy silicate minerals) that's at least 20% molten, and a brittle outer crust of silicate rock - Geological activity is driven by heat flowing out of a planet's interior, and nothing could illustrate this principle better than Io - Photographs show no impact craters at all (surprising considering Jupiter's power to focus meteoroids and asteroids inward) - Over 150 active volcanoes are visible on the satellite's surface, blasting sulfurous gas and ash and lava out over the surface to bury any newly formed craters - Io is bursting with energy, and Io is the most volcanically active body in the Solar System; volcanism is virtually non-stop - The colors of Io have been compared to those of a badly made pizza → The reds, oranges, and browns of Io are caused by sulfur and sulfur compounds, but the crust is true silicate rock - Great lava flows are visible carrying molten material downhill, where it buries the surface under layer after layer - Io is heated by tidal heating → Because Io is so close to Jupiter, tides are powerful and should have forced Io's orbit to become circular long ago → Io, Europa, and Ganymede are locked in an orbital resonance: In the time it takes Ganymede to orbit once, Europa orbits twice, and Io four times → This gravitational interaction keeps the orbits slightly elliptical, and Io, being closest to Jupiter, suffers dramatic tides, with its surface rising and falling by about 100 m — the resulting friction is enough to melt the interior and drive volcanism → The energy flowing outward is continually recycling Io's crust: Deep layers melt, are spewed out through volcanoes, fall back to cover the surface, and are later covered themselves until they are buried so deeply that they are again melted - Spectra reveal that Io has a tenuous atmosphere of gaseous sulfur and oxygen; however, those gases cannot be permanent — the erupting volcanoes pour out at about one ton of gases per second, but because of Io's low escape velocity, the gases leak into space easily Some Irregular Satellites - The fifth satellite to be found, Amalthea, is irregular in shape and has a reddish surface that may have been colored by contamination from Io The "Life Conundrum" - The deliberate crash of the Galileo spacecraft into Jupiter was to prevent a future crash into Europa - Scientists are protective of Europa because there is some possibility of bacteria-like life living in the liquid waters of Europa which we must avoid contaminating with Earth bacteria - The next best bet would be Ganymede (if the saltwater oceans are truly liquid) - It is postulated that such life forms could live without sunlight if they rely on chemical sources of energy in the oceans Unit 5 - The Gas Giant Planets Page 7 Chapter 15: Saturn and Family April-17-14 12:36 PM - Saturn is the most distant of the five planets easily visible to the naked eye from Earth (the other four being Mercury, Venus, Mars, and Jupiter) Saturn Missions Pioneer 11 - Pioneer 11 was the first spacecraft to visit Saturn during a fly-by - Low-resolution images were acquired of the planet and a few of its satellites Voyager 1 and Voyager 2 - The Voyager missions provided the data for the recent Cassini-Huygens mission - The Voyagers:  Determined the hydrogen/helium ratio of the atmosphere  Checked out winds, and atmosphere belt patterns  Found auroras at the poles (thus the planet had a magnetic field)  Got some good pictures of satellites and rings Cassini-Huygens - The Cassini-Huygens was the first spacecraftto orbit Saturn, and the volume of data arriving is phenomenal(most of the details in this chapter are obtained from the Cassini-Huygens spacecraft) - Cassini-Huygens is neither a small project nor a small craft - Cassini was built by NASA and the Huygens probe by the European Space Agency - Designed to use as little fuel as possible and be dependent upon the gravity assist received from other Solar System bodies plus a small plutonium nuclear reactor fuel cell (a matter which did not endear this craft to environmental activists) - After three "gravity assist" boosts from Venus twice and Earth, Cassini-Huygens finally had enough orbital momentum to reach the outer Solar System - One last gravity assist from Jupiter gave Cassini-Huygens the final thrust of energy it needed to project itself all the way to Saturn - The Cassini spacecraft is the first to explore the Saturn system of rings and satellites from orbit New Horizons - The New Horizons space probe has proceeded through the Jupiter system and has cut through the orbit of the Saturn system as it proceeds toward its target: Pluto Planetary Facts  Distance from Sun - 9.5 AU  Diameter - 120 660 km (about 8.85 Earths)  Density - 0.687 g/cc (it would float on water (1.00 g/cc))  Rotational period - 10.5 hours  Equatorial rotationalspeed - 9.87 km/s  Orbital period - 10 832 days (29.66 years)  Orbital speed - 9.69 km/s  Orbital eccentricity- 0.0557  Orbital inclination- 2.485°  Axial tilt - 26.73°  Satellites - at least 60 (some seem to just be blocks of ice)  Rings - individually, there are hundreds, but organized into 8 to 10 groups Saturn: The Planet 3 3 - Saturn has almost the same compositionas Jupiter but is much less dense (Jupiter: 1.33 g/cm , Saturn: 0.69 g/cm ) — this is a matter of relative compaction → The planets generally follow the principle that planet density is greatest in the inner Solar System and decreases markedly outward - Saturn rotates about as fast as Jupiter, but it is
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