Astronomy 217J o v i a n P l a n e t s

Jovian planetsAs with the terrestrial planets, at first glance the jovian planets appear to have little in common with each other.

Here again we see large differences in the atmospheres (and Saturn’s rings) masking similarities in the interior.

Jovian OrbitsThe Jovian planets dominate the solar system from 5-30 AU.

Their combined efforts cleared most of the solar system of planetesimals, leaving the Asteroid and Kuiper belts.

Orbital Facts

All the Gas Giants are fast rotators and rotationally flattened.

Uranus’ pole is 97.86° to the ecliptic, linking Uranus’ solar day to the planetary seasons.

Jupiter Saturn Uranus Neptune

Orbital Semi-Major Axis (AU) 5.203 9.537 19.19 30.07

Orbital eccentricity 0.048 0.054 0.047 0.009

Orbital Period (yrs) 11.86 29.45 84.02 164.79

Rotational period (days) 0.413 0.444 -0.718 0.671

Solar Day (hrs) 9.926 10.66 84 yr 10.57

Observing JupiterOur view of Jupiter is limited to its very dense, complex atmosphere.

From a small telescope on Earth

Hubble Space Telescope

Cassini

Atmosphere has ~dozen bright zones and dark belts.

Zones are upwelling whose tops have risen higher in the atmosphere and are therefore cooler, while the belts are sinking air.

Zones are likely lighter because of ammonia ice.

Zones and belts are bordered by strong (< 360 km hr-1) zonal winds known as jets.

The jets alternate eastward and westward.

Bands of Jupiter

These alternating jets produce considerable wind shear.

Observation reveals alternating eastward and westward zones.

Zonal winds are similar to the zonal flows on Earth that cause the easterly trade wind near the equator (ʋ ~20 km hr-1) and westerlies at mid-latitudes.

Effect is magnified by Jupiter’s rapid rotation.

Winds of Jupiter

Solid surface of Jupiter is invisible beneath the multiple cloud layers; treat top of troposphere as “surface”.

Lowest cloud layer is obscured to optical telescopes.

Measurements by Galileo probe show high wind speeds persist to great depth.

Winds likely powered by heating from planet, not Sun.

Atmosphere of Jupiter

Jupiter’s atmosphere is composed mostly of hydrogen (90%) and helium (10%); small amounts of methane, ammonia, and water vapor.

The colors are probably due to complex chemical interactions in each of the three main cloud layers, each with quite different colors and chemistry.

The white regions are the tops of the upper layer of ammonia clouds.

The yellows, reds, and browns are associated with the second cloud layer, composed of ammonium hydrosulfide ice.

The lowest cloud layer is bluish water ice; however this level is generally obscured.

Colors of Jupiter

The wind shear and upwellings in the atmosphere power considerable “weather” on Jupiter.

Most notable is the Great Red Spot, an Earth-sized, high pressure, and thus anticyclonic, storm.

The GRS has been seen at 22° south of Jupiter's equator for hundreds of years.

The GRS, or a storm like it, was observed by Giovanni Cassini in 1665 and possibly by Robert Hooke in 1664.

Weather of Jupiter

Great Red Spot is bordered by westerly winds to the north and easterly winds to the south, making its counter-clockwise rotation (period = 6 days) appear to be rolling between there winds.

The cloudtops of the GRS are among the highest and coldest on Jupiter, ~8 km above its surrounding clouds.

The Great Red Spot

+ 4 hours

Storms of JupiterContinued observations of Jupiter show the interactions of storms, providing clues to the dynamics of Jupiter’s atmosphere.

For example, in 2008, the Hubble Space telescope recorded a small storm, the South Tropical Little Red Spot, being disrupted by the Great Red Spot.

+ 1 month + 2 months

In 1939, South Temperate Zone was torn into three long sections by dark features or gaps in the clouds. These sections contracted to form 3 white oval storms, named FA, BC, and DE

Between 1998 and 2000 these three white storms were observed to merge into a single storm, Oval BA.

In 2006, Oval BA turned red, earning the nickname Red Spot Jr.

It continues to grow.

Oval BA

The white ovals and red spots are anti-cyclonic storms, rotating clockwise in the Northern Hemisphere, counterclockwise in the Southern Hemisphere.

This is likely an indication that they are high-pressure storms.

Jupiter also exhibits cyclonic (low pressure) storms, called brown ovals (or badges). The color is due to large gap in clouds revealing deeper layers.

Cyclones & ANTICyclones

Saturn’s BandsSaturn’s atmosphere does show zone and band structure like Jupiter, but with coloration that is much more subdued than Jupiter’s.

This true-color image from Cassini shows the very delicate coloration of the cloud patterns on Saturn.

Saturn’s atmosphere is generally similar to Jupiter’s, except the pressure is lower, due to Saturn’s smaller mass.

The three cloud layers are also present, however the cloud layers are thicker than Jupiter’s.

Thus only the top layer is visible, making Saturn’s zones and bands less distinguishable.

Saturn’s atmosphere is notably helium-deficient (96% H and 3% He).

Saturn’s Atmosphere

Wind patterns on Saturn are similar to those on Jupiter, although the zonal flow is less complex.

The wind velocities are larger on Saturn, up to 1800 km/hr, with eastward winds dominating.

These are among the fastest winds in the Solar System.

Saturn’s Winds

False ColorWith false color enhancement, the structure in Saturn’s clouds can be seen more clearly. Storms can also be seen in these images from Voyager.

Storms are much rarer on Saturn, forming less often and quickly dissipating, perhaps due to the wind’s high speed or the lack of westerly flow.

The orange in this image corresponds to farther IR, indicating that storms are cooler and at higher altitudes.

Saturn’s Storms

Great White SpotSaturn does occasionally exhibit the appearance of large storms, termed Great White Spots.

GWS have been observed (in 1933 and 1990) to expand until they encircle the planet.

There is a quasi-periodic component to the GWS, following the 29.5 yr Saturnian year. These storms coincide with the northern summer solstice.

The possibility of similar storms coinciding with the northern winter solstice is obscured by Saturn’s rings.

Saturn’s PolesSaturn’s rapid rotation results in significant flattening, with the polar radius 10% smaller than the equatorial.

The clouds at the north pole exhibit a persisting hexagonal wave pattern around the north polar vortex.

In 2006, Cassini spacecraft observed a 'hurricane-like' storm locked to the south pole with a clearly defined eyewall, the first observation of an eyewall outside of cyclones on Earth.

While Jupiter and Saturn were known to the ancients, Uranus is invisible to the naked eye.

Uranus was discovered in 1781 by Herschel; first planet to be discovered in more than 2000 years.

Little detail can be seen from Earth, even with large telescopes, though Uranus’s moons can be seen.

Most of our knowledge of Uranus comes from spacecraft.

Discovering Uranus

Seeing UranusEven from the Voyager 2 spacecraft at a distance of 1 million km, little detail is visible on the face of Uranus.

The face is uniformly blue, except for a few wispy whitish clouds.

Neptune was discovered in 1846, after analysis of perturbations of Uranus’s orbit allowed predictions of its location by Le Verrier and John Couch Adams.

Details of Neptune also cannot be perceived from Earth, even with large telescopes, though some of Neptune’s moons can be seen.

Discovering Neptune

Seeing NeptuneNeptune revealed more detail to Voyager 2 with more wispy clouds and some storms visible from a distance of 1 million km.

100 km

Atmospheres of Uranus and Neptune are nearly identical to each other.

They are similar to those of Jupiter and Saturn, in that H and He dominate, but the concentration of H is down (~80%) while He (~ 18%) and methane (~2%) are increased.

Uranus and Neptune are cold enough that methane clouds can form in the upper layers, providing the characteristic blue color.

Atmospheres of Uranus and Neptune

Band structure can clearly be seen on Neptune in high resolution images.

Weather on Neptune

Wind speeds are typically < 400 km hr-1, though nearly supersonic speeds (~ 600 km hr-1) are seen in storms.

Higher abundances of methane, ethane and ethyne at Neptune's equator are interpreted as signs of upwelling at the equator and subsidence near the poles.

Storms of NeptuneDuring Voyager 2’s flyby in 1989, an Earth-sized Great Dark Spot, due to anti-cyclonic storm like Jupiter’s GRS, was seen.

GDS has since vanished but other smaller dark spots have been observed.

The dark spot are lower in the atmosphere than the bright clouds and appear to be holes in the cloud deck.

Uranus exhibits similar band and cloud structures as Neptune, but much less intensely. Only a single dark spot has been observed, in 2006.

Clouds on Uranus

The clouds motions also reveal that Uranus’s rotational orbit lies nearly in the Ecliptic plane.

Uranus’s weaker weather is thought to result from lesser amounts of internal energy coming to the surface.

There is some evidence that weather in strengthening as Uranus approaches its equinox.

Because Uranus axis of rotation lies close to its orbital plane, its solar day is tied to seasonal cycle, rather than its rotational period. As a result it’s seasonal variations are extreme.

Extreme Inclination

The reason for the axial tilt is unknown, but a collision with an Earth sized protoplanet has been proposed.

Planetary FactsJupiter Saturn Uranus Neptune Earth

Radius (km) 71,493 60,267 25,557 24,766 6,370

Mass (1026 kg) 18.99 5.685 0.868 1.024 0.06

Density (kg m-3)

1330 700 1270 1638 5515

Mean Surface Temperature

152 134 76 72 287

Escape Velocity (km s-1)

59.5 35.5 21.3 23.7 11.2

Magnetic Field (μT)

428 22 23 14 30

Central Pressure

Applying the mean density ρ̅ ⇒ M(r) = 4πρ̅r3/3,

or as an integral

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For planets or moons where seismology is possible, we can accurately map the density, pressure and other quantities as a function of radius.

In other cases, we can estimate the central pressure, Pc, using the equation of hydrostatic equilibrium and the mean density.

For Jupiter, Pc ≈ 1 × 1012 N m-2

For Saturn, Pc ≈ 2 × 1011 N m-2

For such pressures (106-7 atmospheres), hydrogen exists as a metal.

Dense HydrogenThe outer layers of the Jovian planets are 98% or more hydrogen and helium, but what phase do these gases take in the planet’s interior?

The central pressure is an important guide.

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Little direct information is available about Jupiter’s interior, but computer modeling allows us to deduce its structure.

The H & He rich atmosphere transitions to a molecular liquid at a depth of ~1000 km.

At a depth of ~20,000 km, the pressure favors metallic H.

Because metallic hydrogen is less dense that rock, differentiation would result in a rocky core, ~10,000 km in radius.

Jupiter’s Interior

The interior structure of Saturn is similar to Jupiter’s, though the evidence is just as scant in Saturn’s case.

Modeling suggests Saturn’s metallic H layer is thinner than Jupiter’s, so much so that Saturn’s molecular H layer is likely thicker than Jupiter’s.

The rocky/icy core is estimated to have a mass 9-22 M⨁, possibly larger than Jupiter’s, estimated to be 14-18 M⨁.

Saturn’s Interior

Contraction of matter due to the force of gravity results in energy release (Kelvin–Helmholtz contraction).

As a mass element moves inward, its potential energy becomes more negative, providing either kinetic or thermal energy to maintain energy conservation.

Gravitational Heating

for a spherical shell

Assuming a constant density, �̅� ⇒ M(r) = 4π�̅�r3/3,

Total gravitational potential energy is

Heating Rate

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Unlike the terrestrial planets, the jovian planets are radiating significantly more energy than they capture from sunlight.

In the case of Jupiter, 3.8 × 1017 W of solar energy is absorbed while ≈ 4 × 1017 W is generated by Kelvin-Helmholtz contraction.

Planetary Cooling

This rate of energy generation requires contraction at the rate

This is equivalent to an imperceptible 0.5 mm yr-1 or 500 km Gyr-1 or ~3% RJ over Jupiter’s 4.6 Gyr life.

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For Saturn, the mismatch between absorbed solar radiation and emitted infra-red heat is more unbalanced than Jupiter, with Saturn radiating 2.5 times more energy than it absorbs.

This rate of emission seems too large to be accounted for solely by Kelvin-Helmholtz contraction.

The leading idea to explain this luminosity is the condensation of atomic helium droplets in metallic hydrogen, which fall like rain, heating the hydrogen by friction.

Helium rainout also explains the observed He depletion.

Helium Rain

Interior structures of Uranus & Neptune are quite similar to each other, but distinct from Jupiter & Saturn.

Internal Structure of Neptune & Uranus

Structural DetailsDetailed modeling predict much lower central pressures for U&N, too low for metallic hydrogen.

In its place is a layer of “slush” composed of H20, CH4, NH3; hot (2000-5000 K), but dense and semi-solid.

Rocky cores are relatively larger in U&N than Jupiter & Saturn, with radii ~ 5000 km and masses of 0.6 & 1.2 M⨁.

Intriguingly, while Neptune also emits 250% of its absorbed solar flux, Uranus emits only 110%.

Jupiter’s rapid rotation and conductive liquid metallic hydrogen result in a prodigious magnetic field.

The intrinsic magnetic moment is 18,000 times that of Earth. Even at the radius of Jupiter’s cloud tops (RJ= 11.2 R⨁), the field is 5 × 10-4 T (14 B⨁).

Jupiter is surrounded by belts of charged particles, like the Van Allen belts but ~1000 times stronger.

There is also a torus of plasma fueled by the moon Io.

Jupiter’s Magnetic Field

Jupiter’s remarkable magnetic field generates an impressive magnetosphere ~5 million km across.

Magnetosphere was observed by Pioneer 10 to extend beyond the orbit of Saturn, a distance of 500 million km.

Unlike the teardrop shape of Earth’s magnetosphere, Jupiter’s is flattened by interaction with the plasma originating from Io.

Jupiter’s Magnetosphere

Radio EmissionThe high particle densities and large expanses inside Jupiter’s magnetosphere and plasma torus makes Jupiter a strong source of radio waves. An electron spiraling in a helix with the Larmor radius,

spirals at the cyclotron frequency,

It emits radio waves with

A relativistic electron, with Lorentz Factor spirals at the synchrotron frequency ωs = eB/γc

Synchrotron radiation exhibits a broader peak than cyclotron radiation with

Jupiter’s broad 1 GHz radio emission ⇒ ʋe ~ .998 c

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For the same reasons as Jupiter, Saturn also has a strong magnetic field, but only 5% as strong as Jupiter’s.

The magnetic moment is 580 times that of the Earth, but because of Saturn’s large radius, the field is 2.1 × 10-5 T at the cloudtops, ⅔ B⨁.

Saturn’s magnetosphere is the second largest in the solar system, though ~¼ the size of Jupiter’s.

Enceladus supplies a plasma torus, but < 10% the density that Io supplies for Jupiter’s torus.

Strong polar aurorae are frequently visible on Saturn.

Saturn’s Magnetic Field

Uranus & Neptune have substantial magnetic fields, 2.3 × 10-5 T & 1.4 × 10-5 T, at the respective cloudtops, but at a large angle to their rotation axes and with large quadruple contributions (unlike the largely dipole fields of the other planets).

Visualizing the field as a bar magnet within each planet that would produce a similar field, highlights the oddity of the Uranian and Neptunian fields

One possibility is field generation in the “slush” layer.

Dipole Comparison

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