The Jovian Planets

Jupiter

• Jupiter’s the 3rd brightest object in the night sky (after the moon and Venus).

• Jupiter contains atmospheric bands visible from Earth.

• Jupiter has many moons that vary in properties. The four largest are the Galilean moons and are visible from Earth with even a small telescope.

Saturn

The View from Earth

• Saturn was the most distant planet known to ancient Greek astronomers.

• Saturn orbits at nearly twice Jupiter’s distance from the sun (which makes it considerably fainter than Jupiter or Mars as viewed from Earth).

• Saturn also contains atmospheric bands, but they are less distinct than Jupiter’s.

• Saturn has an overall butterscotch hue.

• Saturn also has many moons orbiting it.

• Saturn’s best known feature - its ring system.

Uranus & Neptune

Spacecraft Exploration

• Voyager 1 (1977) & 2 (1979): remain primary source of data on Uranus and Neptune. Now headed out of solar system and still return data (sparsely).

• Galileo (1989): Reached Jupiter in 1995 and sent probe into Jupiter’s atmosphere. Discovered the likely existence of an extensive saltwater ocean beneath the icy crust of Jupiter’s moon Europa. Mission was ended when the craft was purposefully steered into Jupiter’s atmosphere in 2003.

• Cassini (1997): Reached Saturn in 2004 and sent probe into the atmosphere of Titan, Saturn’s largest moon. Now orbiting Saturn’s moons and still returning data.

Uranus

• Discovered by British astronomer William Herschel in 1781.

• Under the best possible conditions, Uranus is just barely visible to the naked eye.

• Through a large telescope, it appears as a tiny, pale greenish disk.

• From observing Uranus’s orbital motion, it was determined that another planet must be out beyond it as they had trouble fitting an elliptical orbit to the path of Uranus that could accurately predict its position. Introducing another planet into the mix fixed this discrepancy.

Neptune

• • • • • • In 1846, German astronomer Johann Galle found Neptune nearly where it was predicted to be.

Its mass and orbit were both determined by John Adams (English mathematician) and Urbain Leverrier (French mathematician) independently using Uranus’s orbital data.

Neptune cannot be seen with the naked eye.

Through a large telescope, appears as a small bluish disk.

A few markings (cloud bands) can be seen only under the best conditions.

From Voyager 2, Neptune appears to have atmospheric bands and spots, like a blue-tinted Jupiter.

Physical Characteristics

• • • • • • Large masses, large radii, low average densities.

Saturn is less dense than water - it would float!

Jovian planets are large enough that their gravity can hold on even to the light gases - predominantly composed of hydrogen and helium.

Each has a dense, compact core at the center more massive than Earth.

Jupiter, Saturn, and Neptune have significant internal heating.

None of the jovian planets has a solid surface of any kind - the gases just get denser and hotter the further down you go (eventually becoming a liquid, which defines the “surface”).

Rotation Rates

• Lack of solid surface leads to differential rotation - the rotation rate is not constant from one location to the next.

• On Jupiter, the poles rotate slower by about 6 minutes as compared to the equatorial regions. On Saturn, the difference is 26 minutes, again slower at the poles. Difference is even more marked on Neptune.

• On Uranus, the difference is more than 2 hours, with the poles moving faster.

• Differential rotation observed in the surface layers reflects large scale wind flows in the planets’ atmospheres.

• There is no clear relationship between the interior and atmospheric rotation rates - one can move faster than the other and it varies between the 4 planets.

• The rotation axis of most planets is nearly perpendicular to the ecliptic plane (the plane in which the solar system mainly lies). Uranus however has its rotation axis nearly parallel to the plane.

Rotation Rates

Because of the odd orientation of Uranus’s rotation axis, the poles alternately undergo 42 years of darkness at a time. (e.g. For 42 years, the north pole of Uranus will be pointed nearly directly at the sun.)

Overall Appearance and Composition of Jupiter’s Atmosphere

• Appearance dominated by colorful atmospheric cloud bands arranged parallel to the equator and an oval atmospheric blob called the “Great Red Spot.” • The Great Red Spot in the long direction is nearly twice Earth’s diameter and seems to be a hurricane that has persisted for hundreds of years.

• Atmospheric composition: – 86% hydrogen – 14% helium – Small amounts of atmospheric methane, ammonia, and water vapor.

Overall Appearance and Composition of Jupiter’s Atmosphere

• Complex chemical processes occurring in Jupiter’s atmosphere are responsible for the colors that we see.

• Banded cloud structures: – Zones - lighter colored – Belts - darker colored • The zones and belts vary in position and intensity with time, and are the result of convective motion in the atmosphere.

– Zones - high-pressure areas moving upward.

– Belts - low-pressure areas moving downward.

– However, Cassini observations conflict with the above stated standard view.

Overall Appearance and Composition of Jupiter’s Atmosphere

• Zones and belts are equivalent to low and high pressure systems we experience on Earth. Due to Jupiter’s rapid differential rotation, these systems wrap completely around the planet.

• Because of the pressure difference, belts and zones lie at slightly different heights in the atmosphere, and thus have slightly different temperatures. This temperature difference is responsible for the difference in colors between the two regions.

• Zonal flow - underlying the bands is a very stable pattern of eastward and westward wind flow.

• The equatorial regions of the atmosphere rotate the fastest, at a rate of 500 km/h, similar to the jet stream on Earth.

• At higher latitudes, there are alternating regions of eastward and westward flow, corresponding to the pattern of zones and belts.

• Near the poles, where the zonal flow disappears, the band structure vanishes also.

• • • • • • • • • • • •

Jupiter’s Atmospheric Structure

White ammonia clouds generally lay over the colored layers.

Above the ammonia clouds lies a thin, faint layer of haze created by chemical reactions similar to those on Earth that cause smog.

Since it lacks a solid surface, the zero point is taken to be the top of the troposphere (the turbulent region where the clouds lie that we see). As on other planets, weather on Jupiter is the result of convection in the troposphere.

Above the troposphere, the temperature rises as the atmosphere absorbs solar UV light.

Below the haze layer, at -30 km, lie white, wispy clouds of ammonia ice at a temperature of 125 to 150 K.

A few tens of km below the ice, the temperature is above 200 K and we have clouds of ammonium hydrosulfide.

Even deeper, we have clouds of water ice or water vapor.

The top of the deepest cloud layer, which is not visible to us, lies 80 km below the troposphere.

In 1995, Galileo probe arrived at Jupiter.

Galileo survived in the atmosphere for about an hour before being crushed by atmospheric pressure at an altitude of 150 km below the troposphere.

Galileo’s findings are consistent with the picture we just described.

Weather on Jupiter

• In addition to the large-scale zonal flow, Jupiter experiences many smaller-scale weather patterns.

• The Great Red Spot is one example.

– The center remains tranquil in appearance, like the eye of a hurricane.

– This storm resembles a hurricane in that it rotates.

– The storm has persisted for at least 300 years.

• Also many smaller circulating storms that appear as white ovals.

• Brown ovals are holes in the overlying clouds that allow us to see down into the lower atmosphere.

• Storms are localized anomalies in the zonal flow.

• Storms survive on Jupiter if the they are large enough to not be affected by other storms. On Earth, storms (hurricanes) die out once they reach land as this interrupts their flow of energy. Jupiter has no land, thus no interruptions to the storms.

• • • • • • • • • •

Saturn’s Atmospheric Composition

Not as much atmospheric structure as Jupiter.

Yellowish and tan cloud bands that parallel the equator.

Storms do exist, but there’s largely no difference in color between them and the cloud bands.

Composition: – 92.4% Hydrogen – 7.4% Helium – Traces of methane and ammonia Much less helium than on Jupiter believe Saturn’s helium liquefied at some point and sank toward the center of the planet.

Very similar to Jupiter’s, just lower temperatures due to increased distance from the sun.

Troposphere contains clouds arranged in 3 distinct layers composed of (in order of increasing depth) ammonia ice, ammonium hydrosulfide ice, and water ice.

Above the clouds, there is a layer of haze.

Total thickness of the 3 cloud layers is about 3 times the thickness on Jupiter due to Saturn’s weaker gravity (Jupiter pulls much harder on its cloud layer).

Saturn’s yellow color is likely due to the same photochemistry as on Jupiter, but on Saturn we only see the less colorful top layer (remember holes in Jupiter’s top layer lets us see below).

Weather on Saturn

• Computer enhanced images of Saturn show the presence of bands, oval storm systems, and turbulent flow patterns.

• Stable east-west zonal flow, greater wind speed (1500 km/h) than on Jupiter and fewer east-west alternations than on Jupiter.

• “Dragon Storm” thought to be similar to a thunderstorm on Earth. Millions of times stronger than we’ve experienced here on Earth, but does contain lightning and water and ammonia rain. Thought to be a long-lived phenomena that occasionally flares up.

Atmospheric Composition of Uranus and Neptune

• Very similar to that of Jupiter.

– Molecular hydrogen (84%) – Helium (14%) – Methane • 3% on Neptune • 2% on Uranus • Ammonia not observed in any significant amount (probably exists in the form of ice crystals due to low atmospheric temperatures).

• Blue color of outer jovian planets is due to the relatively high presence of methane. Methane is quite efficient at absorbing red light, so mostly blue light is reflected by the planets (deeper blue means more methane).

Weather on Uranus

• Few clouds exist in Uranus’s cold upper atmosphere. This means we must look deeper into the atmosphere to see structures (bands and spots) which characterize the flow patterns like on the other jovian planets. • Stratospheric haze also washes out any of the structure that would be visible below it.

• Uranus’s atmospheric clouds and flow patterns move around the planet in the same direction as its rotation, with wind speeds from 200 to 500 km/h.

Weather on Neptune

• Neptune’s upper atmosphere is hotter than Uranus’s due to internal heating.

• Extra warmth makes the haze thinner and clouds higher, so its easier to see.

• Among the main cloud tops, there are white methane clouds.

• Equatorial winds blow east to west at speeds over 2000 km/h. Interior rotates west to east. No explanation for the winds behavior.

• Largest storm - “Great Dark Spot” - akin to Jupiter’s Great Red Spot. Comparable in size to Earth. However, it has since disappeared and new storms similar to it have formed.

Internal Structure - Jupiter

• Based on models that best fit the observed data (have never explored the interiors of the jovian planets!).

• Temperature and pressure increase with depth.

• Gas gradually turns to liquid, and even deeper the liquid hydrogen enters a liquid metallic state (excellent conductor of electricity like other metals).

• Jupiter is flattened - bulges at the equator. This implies a dense core having a mass 10 times the mass of Earth.

• Believe the core is molten or semi-solid rock.

• Enormous central pressure (50 million times that on Earth’s surface) compresses the core to no more than 20,000 km in diameter.

Internal Structure - Saturn

• Same basic internal parts as Jupiter, but in different proportions.

• Metallic hydrogen layer is thinner, central dense core is larger (about 15 Earth masses).

• Due to lower mass, core temperature is less extreme, as are core density and pressure. Central pressure is 1/10 that of Jupiter (not too different from that of Earth).

Internal Structure Uranus and Neptune

• Low enough interior pressures that hydrogen stays in molecular form all the way to the cores.

• May have high-density “slushy” interiors below the clouds composed of highly compressed water clouds.

• Also possible that much of the ammonia is dissolved in the water clouds, producing a thick electrically conducting layer.

• From their relatively large densities, it is assumed that they have Earth-sized cores that are similar in composition to those of Saturn and Jupiter.

Magnetospheres

• Jupiter has strongest magnetic field in the solar system.

• All four Galilean moons lie within Jupiter’s magnetosphere.

• Jupiter experiences aurorae that are larger and more energetic than those on Earth.

• Saturn also has a strong magnetic field, however it is only 1/20 that of Jupiter’s due to the lower level of metallic hydrogen.

• Saturn’s magnetosphere contains the planet’s ring system and most of its moons.

• Uranus and Neptune also have strong magnetic fields (a few % of Saturn’s, 30 to 40 times that of Earth).

• Uranus and Neptune’s magnetic fields are not aligned with the planets’ rotation axes and are offset from the planets’ centers no idea why.

• • • • • • • •

Internal Heating

Jupiter emits about twice as much energy as reaches the planet in the form of sunlight.

Excess heat emission from Jupiter is likely here from its formation which is still leaving the planet today - it is still cooling.

Saturn emits about three times as much energy as reaches the planet in the form of sunlight.

Since Saturn is smaller than Jupiter, the heat from its formation must have left it long ago.

On Jupiter, liquid helium dissolves in liquid hydrogen due to the high temperatures. This doesn’t happen on Saturn.

On Saturn, helium condensed in the clouds and it has been raining helium ever since. As the helium sinks toward the center of the planet, gravity compresses it and heats it up. The energy released in this process is the source of Saturn’s internal heating. Eventually, the process will cease and Saturn will cool.

Uranus has no internal heating - it emits exactly what it absorbs from the sun.

Neptune does have internal heating, but we don’t know its source. Possible that the high concentration of methane insulates the planet, helping it maintain its initially high internal temperature (didn’t allow the planet to cool as much).