Astronomy 311: Lecture 5 - Jovian Planets

Astronomy 311: Lecture 5 - Jovian Planets Scale of solar system established in 1760 s when astronomers used the transit of Venus to estimate the size of 1AU. Sizes of Jovian planets obtained from their angular sizes. Kepler s third law applied to Jovian moons enabled the calculation of Jovian planet mass. Jupiter: 5.2AU, 318M E, ρ = 1.33g/cm 3, mostly H,He. Saturn: 9.54AU, 95M E, ρ = 0.71g/cm 3, mostly H,He. Neptune: 19.2AU, 14M E, ρ = 1.24g/cm 3, H compounds, rock, H, He. Neptune: 30.1AU, 17M E, ρ = 1.67g/cm 3, H compounds, rock, H, He. Hydrogen compounds mentioned above include water, methane, ammonia. Terrestrial planets grew through accretion of solid planetessimals containing rock and metal. Jovian planets formed outside of the frost line where hydrogen compounds could condense into ices. Because hydrogen compounds more abundant, some of the planetessimals grew to a very large size. Then their size allowed them to draw in atmospheres of H, He. All Jovians thought to have grown from an ice-rich planetessimal of about 10M E. Differences in composition stem from the amount of H/He they captured. Ice-rich planetessimal is only about 3 and 10% of Jupiter and Saturn s current mass, respectively. SOlid particles much farther apart further from the Sun than closer to it. Thus the further out you go, the longer it takes for an ice-rich planetessimal capable of attracting other material to it, merely through gravity, and also capable of holding on to H/He atmosphere. Because all planets stopped accreting at the same time: when the solar wind blew all the remaining gas into outer space, more distant planets had less time to capture stuff and thus end up smaller in size. 1

Jupiter is only slightly bigger than Saturn despite being much bigger in size. This is because hydrogen/helium is compressed the deeper you go. Much larger planets than Jupiter may even be smaller in size. Jovian planets rotate much more rapidly than any of the terrestrial worlds. Observations of Jovian features suggest that Jovian rotation rate varies with latitude: equatorial regions rotate faster than polar regions. Rapid rotation means Jovian planets are not spherical. On its own, gravity makes perfect spheres. Gravity plus rotation means material nearer the equator is flung outward: size of equatorial bulge depends on balance between strength of gravity (pulls material in) and rotation rate (pushes material out). Saturn has a fast rotation rate and weak gravity because of its fast rotation, and is about 10% wider at equator than at poles. Jupiter Jupiter has no solid surface. Pressure and temperature increase as you descend into the interior. Computer models predict it has a fairly distinct layers though with similar H/He composition except for the core. Near cloud tops, T 125K, ρ 0.0002g/cm 3, with an atmospheric pressure of 1 bar. From 125K 2000K get gaseous hydrogen, then liquid H from 2000K to 5000K. Metallic H from region above the core. Saturn is similar to Jupiter. To achieve a pressure of about 200atm would you have to go deeper inside Saturn or Jupiter - why? Saturn has a thicker layer of gaseous H and a thinner and more deeply buried layer of metallic H. Pressures within Uranus and Neptune not high enough to form liquid or metallic H. These planets only contain a thick layer of gaseous H surrounding a core of hydrogen compounds, rock and metal. Cores of Unranus and Neptune are larger in radius than cores of Jupiter and Saturn because they are less compressed by the wight of the overlying layers. 2

For terrestrial planets, internal heat was the main driver of surface geology. Jupiter reflects almost twice as much energy as it received from the Sun. Jupiter s large size means it looses heat very slowely. Radioactive decays further heats up Jupiter. Jupiter has lost substantial heat though in 4.6 billion years. This puffed up its atmosphere in the past: Jupiter was larger and had a more flective atmopsphere in th epast. Satun emits twice as much energy as it received from Sun - it too has an ongoing source of heat. But, Saturn s smaller mass means that it cannot be doing this by gravitational contraction. Low pressure and temperature allow for liquid He which sinks to lower levels: differentiation. Saturn atmosphere is somewhat depleted of He. Uranus emits virtually no excess energy. Jovian weather Jovian atmospheres have dynamic winds, cloudy skies and enormous storms. Atmospheres made up o fmostly H/He gas mixed with small amounts of methane (CH 4 ), ammonia (NH 4 ) and water (H 2 O). Spectroscopy also reveals more complex hydrogen compounds (acetylene, ethane and propane. Small amounts but responsible for Jovian appearances. Galileo probe in 1995 into Jupiter implied temperature structure of Jovian atmosphere is similar to that of Earth s atmosphere. Jupiter s thermosphere: low-density gas heated to 1000K by solar X rays. Jupiter has a stratosphere, though this occurs not with O 3 but other gases which can absorb solar UV rays. Peak stratosphereic temperature of about 200K. Below this is the troposphere; temp rises with depth because greenhouse gases trap solar heat and Jupiter s own internal heat. Strong convection exists in the Jovian troposphere: responsible for thick cloud cover over Jupiter. 3

However, on Jupiter there are several gases that could condense as clouds. Gases include water (H 2 O), ammonium hydrosulphide (NH 4 SH) and annomina (NH 3 ). First rising gas encounters temperatures for water vapor to condense: lowest layer of clouds contains water droplets. At the next layer, ammonium hydrosulphide condenses to make second cloud layer. After about 50km, ammonia can condense to form the upper cloud layer. Other three Jovians have similar structure but their different distances from the Sun means the three layers happens at different height for each planet. For example, to find water vapor clouds on Saturn need to look about 200km deeper into Saturn s atmosphere than for Jupiter. Also smaller mass of Saturn means its cloud layers separated by a greater distance than is the case for Jupiter: its weaker gravity causes less atmospheric compression. Cannot see deep enough into Uranus and Neptune to detect these cloud layers. Can see methane clouds high in Uranus and Meptune s atmospheres. Jupiter and Saturn are too warm for methane clouds. Colors of Jovian planets caused by differnt clouds reflecting different colors eg. Neptune and Uranus look blue because methane gas absorbs red light and transmits blue light which is reflected back into space by methane clouds. Neptune is also emitting twice as much energy as it received from the Sun - how? Perhaps it too is still contracting. Jupiter weather Solar heating at equator expands and spills air towards Jupiter s poles where cooler air flows toward equator: similar to air flow on Earth. But Jupiter s greater size, faster rotation makes for a stringer Coriolis force and Jupiter s air circulation is split into many bands of rising and falling air. Note on Earth you have 3 such cells in each hemisphere. These bands are the stripes that you see on the surface of Jupiter and come from cloud reflectivity differences that form the rising and cooling air. Strong Coriolis force due to fast rotation drives winds to 400km/hr on Jupiter. 4

The Great Red SPot is a storm more than twoce as wide as the entire Earth - also very long lived, upto now 300 years. No real seasons due to low axis tilt. Saturn s fast rotation also creates bands of rising and falling air and these winds are faster than those on Jupiter - not clear why. Axis tilt similar to Earth so there do exist some seasonal changes but Saturn s internal heat keeps temperatures almost the same all year round. Neptune s atmosphere is also banded. There is the Great Dark Spot as opposed to the Great Red Spot. Relatively little seasonal change due to internal heat source. Uranus has no clouds and no banded structure. Storms just starting to show because its northern hemisphere is just beginning to see sunlight for a few decades. Jovian planets also have magentic fields and bubble-like magnetospheres that shield them from the solar wind. Jupiter s magnetosphere some 20,000 times stronger than Earth s magnetic field. For a magneic field, need: interio region of electrically conducting fluid, b) raoid rotatio and c) convection in the layer of liquid. For Jupiter, the conducting fluid region is the thick layer of metallic hydrogen. Strong magnetic field because layer of metallic H is so extensive. Juputer s magnetosphere traps more charged particles than Earth s magnetosphere. Not only solar wind but also particles from volcanic Io are trapped in these areas: belts of intense radiation around Jupiter. Charged particles also strike Io leading to continuous escape of gases released by volcanic outgassing. Can even generate thin atmospheres on some of the Jovian moons. Escaping gases create a further charged belt, the Io torus, around Io. Other Jovian planets also have magnetospheres but much weaker than Jupiter s: Saturn s is weaker because it has a thinner layer of electrically conducting metalic hydrogen. Size of magnetosphere depends not only on strength of magnetic field but also on solar wind pressure which decreases as you go further from the Sun. Jupiter and Saturn s magnetic field is aligned with rotation axes but Voyages showed that the magnetic field of Uranus is tipped by 60 5

degrees relative to its rotation axis and is offset from the center. Neptune s is inclined by 46 degrees to its rotation axis. This is unexplained. Jovian satellites More than 100 moons orbit Jovian planets. Small: < 300km in diameter, medium-size:300 1500km in diameter, large: > 1500km in diameter. Useful because size relates to geological acitivity. Large moons show signs of geological activity and most mediumsized ones have had geological activity in the past. Jupiter has more than 60, largest are Callisto, Ganymedem Europa and Io - the Gailiean moons. Saturn s largest is Titan. Titan and Ganymede larger than Mercury. But these moons contain substantial amounts of ice in addition to metal and rock. Most medium-sized moons formed within the disks of gas surrounding individual Jovian planets - these moons lie close to the equatorial plane of their parent planet and have almost circular orbits. Also they orbit in the same direction as their parent planet. Nearly all keep the same face always turned to their parent planet - just like the Moon: synchronous rotation arose because of strong tidal forces. See later. Small moons are captured asteroids, very numerous, not circular - why? Galilean moons Io is the most volcanically active world in the solar system. No impact craters survived - that means its surface is very young. Voyager recorded volcanic eruptions as it passed by. Probably also tectonic activity since this goes hand in hand with vulcanism Volcanic eruptions result in outgassing of S, SO 2. Some goes into space for the Io torus or into Jupiter s magnetosphere but some results in a think atmosphere. Some condenses and falls back on Io giving Io its distinctive red and and orange colors. Volcanos suggest it is quite hot inside. But Io is about the same size as the Moon which is volcanically dead. How? Some other process must be heating its interior: tidal heating due to tidal forces exerted by Jupiter. Size and strength of Jupiter s gravitational pull on it means its orbit is slightly elliptical (unlike our Moons) and it is being continuously flexed in different directions which generates friction inside it which heats the interior. 6

Titan This generates 200 times as much heat as radioactive decay on Earth. But almost al other large satellites have circular orbits - so why is Io s orbit elliptical? During the time Ganymede completes one orbit of Jupiter (7 days), Europa completes two orbits and Io completes four orbits. So sometimes they line up and the gravitational tugs they exert on each other add up over time. Alignment creates a resonance. Orbital periods falling into a simple mathematical relationship is an orbital resonance. These are quite common in planetary systems, affect planetary rings, asteroid belt, etc. Europa is covered by water ice. But frozen surface hides an interior made hot by tidal heating as for Io. Has a handful of older craters suggesting a surface no more than a few tens of millions of years old. Active geology has covered up the surface. Water could be the agent of change: either liquid water rising up from beneath the icy crust or ice that is sufficently warm to convect. SUrface g measurements from Galileo suggest that Europa has a metallic core and a rocky mantle. Above the rocky mantle, there is enough water to make a layer of ice about 100km thick. Upper 5-10km is solidly forzen but below that could be liquid. Strong possibility of liquid water. Europa has a magnetic field which changes as Jupiter rotates implying it is induced in response to the rotation of Jupiter s strong magnetic field. Thus there needs to be a liquid layer of electrically conducting material which argues for liquid water rather than soft, convecting ice. Europa s ocean must be global in extent. Ganymede is the largest moon in the solar system with a surface of water ice with a young surface. Callisto is the outermost Galilean moon and is a heavily cratered iceball - this its surface is quite old. No volcanic or tectonic activity. 7

Titan is unique among the solar system moons in having a thick atmosphere which hides the surface from view except at a few wavelengths. Titan s reddish color comes from chemicals in its atmosphere. Atmosphere is 90%N 2 with the rest consisting of argon, methane, ethane and other hydrogen compounds. Ccomplex atmospheric chemistry may produce organic compounds - chemical basis for life: rivers of methane and lakes of hydrocarbons - see Cassini-Huygens website, Since Jovian moons contain more icy substances than rocky and since ice can melt or deform at much lower temperatures than rocky ones, small icy moons are more geologically active than small rocky planets - vulcanism probably produced icy lava that is really water mixed with methane and ammonia. Jovian planet rings All four Jovian planets have rings. For Saturn, appear to be continuous concentric sheets of material separated by a large gap (Cassini division). Sheets made up many individual rings, each separated from the next by a narrow gap. Actually made up of countless icy particles ranging in size from dust grains to large boulders. Spectroscopy suggests they are made up of reflective water ice. Rings look bright when they contain enough particles to intercept sunlight and scatter it back to us. Gaps where there are too few particles to intercept sunlight. Each individual ring particle orbits Saturn independently according to Kepler s laws. Frequent collisions, perhaps every few hours. Frequent gentle collisions imply thinness: diameter 270000km but only a few tens of meters thick. Frequent collisions explain thinness and circular orbits. Lots of structure: rings, gaps caused by particles bunching up at some orbital distances and being forced out out at others. Nudging occurs due to gravity. Sometimes, moons loctaed in gaps between rings: gap moons - thich can create ripples in surrounding rings plus creates a gap around the moon. Sometimes two gap moons can force particles between them into a very narrow ring: shepherd moons. 8

Orbital resonances can also effect ring structure. Rings of Jupiter Uranus and Neptune are much fainter, darker, but lie in equatorial plane, orbits circular. Gap moons, Shepherd moons, orbital resonances. Ring origin? Roche tidal zone: within 2-3 planetary radii (of any planet) tidal forces tugging apart an object become comparable to the gravitational forces holding it together. Only relatively small objects held together by non-gravitational forces (such as electromganetic forces that hold spacecraft, rock etc. human beings together) can avoid being ripped apart. So a large moon within 2-3 planetary radii would be ripped apart but its highly unlikely that a large moon wondered close to all of the Jovians. Cannot be leftover from planet building since particles of such size would not survive this long (age of the Solar System): continuosly being ground down by sand-size particles in Solar System. Thus none of the abundant small particles that form the Jovian rings could be there since the Solar System formed more than 4 billion years ago. Thus new particles are continually being supplied to the system to replace ones that are ground down. Possible source is numerous small moons formed when the planet formed. Impacts on these moons release particles, both large and small. Number of particles in rings vary with time. 9