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Outer or Jovian Planets

• All the Jovian planets are larger than the Terrestrial planets.• All have similar compositions and are similar to the Sun.• Solar composition is mostly Hydrogen, some Helium, etc.• All have low average densities,• All have rings and many satellites.• None have surfaces but only increasingly dense atmospheres and rock and

metal cores.

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Voyager 1 & 2 GalileoPioneer 10 & 11

Ulysses

Some of the unmanned spacecraft sent to the Outer Planets

Cassini

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Jupiter • Largest planet in the Solar System

• Rotates more than twice as fast as Earth this causes it to appear slightly flattened at the poles.

• Jupiter emits more heat than it receives from the Sun.

• The Great Red Spot is a cloud feature that has been observed from Earth for several hundred years!

• Jupiter is a strong source of radio waves due to its powerful magnetic field and its interaction with its moon, Io.

Unmanned missions to Jupiter:• Cassini - Mission to Saturn via Jupiter (2000) • Galileo Orbiter & Probe (1995-2003)• Voyager 1 & 2- Mission to Jupiter (1979), Saturn, Uranus, Neptune, and beyond • Ulysses - (1992- present) Mission to study the Sun via Jupiter • Pioneer 10 & 11(flyby 1973, 1974)

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Jupiter’s Moons• The four largest moons of Jupiter (Io,

Europa, Ganymede and Callisto) are known as the Galilean satellites after Galileo.

• They are roughly the same size as our Moon although the largest one, Ganymede, is larger than the planet Mercury.

• Io, the closest moon to Jupiter, is the most volcanically active body in the Solar System!

• Europa, the second closest moon from Jupiter, is completely covered in ice. There might be an ocean beneath the ice and conditions suitable for life!

Ganymede, Callisto, Io, Europa, the 4 largest of the more than 60 moons

orbiting Jupiter

Io is volcanic due to tides raised by the gravitational pull of Jupiter and the other moons.

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2047 feet long, 66 antennas

courtesy DTM/CIW archives

The Discovery of Radio Emissions from Jupiter

• This discovery was made here in Maryland in Montgomery County, about 20 miles west of the Washington Beltway.• The discovery was made using a radio telescope known as the Mills Cross Array. • The Mills Cross Array was composed of many antennas connected together that worked as one huge antenna.

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The Mills Cross Array from the ground

The receivers were housed in an army surplus truck visible in the distance. Undated photo about 1954. Courtesy of the Archives of the Carnegie Institution of Washington

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• The Mills Cross Array pointed nearly at the zenith, the Earth’s spin allowed objects to move in and out of view of the antennas.

• In early 1955 near the time of transit of the Crab Nebula, Burke and Franklin, two scientists from the Carnegie Institution of Washington noted intermittent bursts of strong radio noise in about 1/3 of their records.

Crab Nebula

from Burke and Franklin [1955]

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• They noticed that the timing of the bursts changed at a sidereal rate, that is at the same rate that the stars appeared to move across the sky so it had to be coming from something in space and not something on the ground.

• Further observations showed that the bursts were actually drifting slightly, (could it be a planet?).

• Only Jupiter was overhead at the same time as these bursts were detected and drifted at the same rate.

• Burke and Franklin announced their discovery on April 6, 1955 at the American Astronomical Society meeting in Princeton, NJ.

How did they determine that the radio waves were coming from Jupiter?

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Jupiter’s “Decametric” Emission• These radio waves have wavelengths

that are a few 10s of meters long, so they are called “decametric”.

• The Jovian emission discovered by Burke and Franklin in 1955 is thought to be emitted in a hollow cone pattern near Jupiter’s north and south magnetic poles.

• Only when Earth lies in the direction of one of these cones can we receive radio waves.

• In 1964, it was found that Jupiter’s moon Io strongly influences the radio emission source.

• The Io-related decametric sources are near the field lines passing through Io.

RadioEmission

RadioEmission

Aurora

Io

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Jupiter’s Van Allen Belts - the “Decimetric” Emission

• Both Jupiter and Earth are surrounded by zones or belts of high energy particles. These are known as the Van Allen Belts.

• In 1959 scientists proposed that the microwave emissions observed from Jupiter could be due to Jupiter’s Van Allen Belt.

• Microwaves have wavelengths of about 1/10 of a meter so they are called decimetric waves.

• In 1960 scientists confirmed that the microwaves are coming from a region along Jupiter’s equator and a few Jupiter radii away from the planet.

A recent image of Jupiter made by observing microwaves of 13 cm wavelength or 2.3 GHz frequency.

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Saturn • Spectacular rings surround the planet. Saturn has at least 33 moons.

• The rings are composed of chunks of ice and rock of various sizes most just a few inches across.

• One of Saturn’s moons, Titan, has a thick atmosphere and may be covered in ices composed of nitrogen or ethane.

• The Cassini mission arrived at Saturn in 2004 and will spend 4 years orbiting the planet. In 2005 the Huygens probe landed on the surface of Titan.

Saturn as seen from one of the Voyager spacecraft. A crescent Saturn is impossible to see from Earth.

Unmanned missions to Saturn:Cassini/Huygens - (2004-present)Voyager 1&2 - NASA Missions to Jupiter, Saturn (1980,1981), Uranus, Neptune, and beyond Pioneer 11 - (flyby 1979)

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Saturn’s Moon Titan • Saturn’s moon Titan is larger than the planet Mercury.

• Titan’s surface is very hard to observe due to Titan’s opaque atmosphere.

• Results about Titan from the Huygens Lander:– Titan’s surface temperature is -179°

C (-290° F)– Instead of liquid water, Titan has

liquid methane. Instead of silicate rocks, Titan has frozen water ice. Instead of dirt, Titan has hydrocarbon particles settling out of the atmosphere, and instead of lava, Titanian volcanoes spew very cold ice.

Images like this one from Voyager were

among the best we had until the

Cassini/Huygens mission

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Uranus• The blue color of Uranus and

Neptune is mainly due to methane.• Its rings are narrow and dark.• The spin axis of Uranus is tilted so

that it lies nearly in its orbital plane.

• Uranus has more than 20 moons.• The rings and moons all orbit the

planet so that they are also tilted the same way as the planet.

• This tilt causes one pole to be in perpetual day for part of its orbit.

Unmanned mission to Uranus:Voyager 2 (1986)

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Neptune• Sometimes the farthest planet

from the Sun.• Has dark, narrow rings.• Neptune has more than 13

moons.• Triton, one of Neptune’s moons

has strange geysers erupting from its surface and has a thin atmosphere.

• Triton orbits backwards and at high inclination. This might mean that it was “captured” by Neptune. It might be a Kuiper belt object. May also be similar to Pluto.

Unmanned missions to Neptune:Voyager 2 (1989)

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Pluto• Is not really a Terrestrial nor a Jovian

planet, its composition is mostly rock and ice.

• Has the most elliptical and inclined orbit of all the planets

• Sometimes the farthest known planet from the Sun.

• Has one moon, Charon.• Pluto and Charon are locked in a spin-

orbit resonance so they always have the same side facing one another.

• Like Uranus, is tipped on its side. • Is the smallest of all the planets, even

smaller than our Moon.Neptune’s moon, Triton.Pluto might look like this.

Current best images of Pluto (small figures) and computer enhanced versions (large figures).

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Comets• Comets are composed of a solid

nucleus, surrounded by a coma and two tails.

• Comets have a dust tail and an ion tail.

• The tails form when gas and dust from the nucleus is pushed back due to the pressure of sun light and the fast-moving solar wind.

• Comet tails always point away from the Sun.

• Tails generally don’t form until the comet is about 1 AU from the Sun. Comas generally don’t form until the comet is less than 5 AU from the Sun

Comet Hale-Bopp (notice yellow dust tail and blue ion tail).

The nuclei of Halley’s Comet and Wild2

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Comets• Some comets come from the Oort

cloud others are from the Kuiper Belt both are beyond the orbit of Pluto.

• Comets have very elliptical orbits. Some have orbital periods of decades others have periods of thousands of years.

• Comet nuclei are basically “dirty snowballs” since they contain both ice and dust.

• The dust that comets leave behind in their orbital path sometimes hits the Earth and we see them as meteor showers.

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Asteroids• Most asteroids orbit between

Mars and Jupiter but there are some that cross the orbit of the Earth and other planets.

• Most asteroids are not large enough to form into spheres.

• Ceres, one of the largest asteroids, is about 1000 km in diameter (less than half the size of Pluto) and is spherical.Mathilde(left) and Eros(right),

two asteroids observed by theNEAR spacecraft. NEAR orbited and eventually landed on Eros.Eros is about 22 x 8 x 8 miles.

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Asteroids• Some asteroids

may have been fragments from larger bodies that had differentiated into metallic cores and stony crusts.

• Other asteroids seem to have compositions that might be very similar to the original solar nebula. Eros again now shown to scale

with Vesta(artist conception) and Ceresthe two largest asteroids.