Astro 101 Lecture 12 The Jovian Planets

2-28-2018

Jupiter, Saturn, Uranus and NeptuneASTR-101Section 004

Bulk Properties of Terrestrial and Jovian Planets• All Jovian planets have strong magnetic fields• Jovians are about as dense as water (1000 kg/m3)• Very cold• Way more massive

Discovery of the Jovians

• Jupiter and Saturn have been know since antiquity.

• Jupiter is the third brightest object in our sky (Moon and Venus are brighter)

• Saturn can be seen with binoculers

• Uranus was discovered in 1781 and was the first planet to be “discovered”. It was almost named George after the king of England.

• Neptune was discovered after a careful study of Uranus’s orbit.

• Astronomers found that in order to explain why Uranus’s orbit was deviating from Newtonian mechanics, another planet father out must be pulling on it.

Jupiter

• Jupiter is composed of 71% hydrogen, 24% helium, and 5% all other elements by mass. Has a higher percentage of heavy elements compared to the Sun

• Rotates so rapidly that the planets are noticeably flattened – Jupiter has 98% of the angular momentum of the solar system!

• The visible “surfaces” of Jupiter are actually the tops of clouds

• The rapid rotation of the planets twists the clouds into dark belts and light zones that run parallel to the equator

• The outer layers of the atmospheres shows differential rotation, meaning the equatorial regions rotate slightly faster than the polar regions.

• Strong convection currents cause the bands of color seen on Jupiter’s upper cloud layer. Due to Jupiter’s rapid rotation, these belts and zones east and west.

• None of the gases in the clouds can explain the color of Jupiter’s belts.

• These strong convective currents create a complex cloud chemistry that is very sensitive to pressure and temperature, which leads to the cloud coloration.

• Underlying these bands is a zonal flow region that is a stable east-west wind that flows at 500 km/h (310 mph)

Jupiter’s Weather

Optical – colors dictated

by how molecules

reflect sunlight

Infrared - traces heat in

atmosphere.

So white colors from cooler, higher clouds, brown and red from

warmer, lower clouds.

Storms of Jupiter

• Because of Jupiter’s differential rotation and strong convective currents, massive storms occur on Jupiter’s surface.

• The Great Red Spot has existed for at least 300 years.

• It is sustained by large scale atmospheric motion.

• Jupiter’s storms are similar to hurricanes that we experience on earth.

The internal heat of Jupiter and Saturn has a major effect on the planets’ atmospheres

Saturn

• Saturn’s atmosphere is similar to Jupiter’s, except pressure is lower.

• Cloud layers are wider than Jupiter’s because Saturn’s gravity is 2.5 times weaker.

• Lower pressure implies a lower temperature. This has caused much of the helium to liquefy.

Saturn’s Atmosphere

• Saturn has planet wide storms that are similar to thunder storms

• The white clouds seen are from warm methane gas come up from deep inside the planet.

• Earth-based observations reveal three broad ringsencircling Saturn

• Saturn’s rings of are composed of numerous particles of ice and ice-coated rock ranging in size from 1 um to about 10 m

• Jupiter’s faint rings are composed of a relatively small amount of small, dark, rocky particles that reflect very little light

Uranus

• The blue color of both Uranus and Neptune is due to their high concentration of methane.

• Methane absorbs long wavelengths like red much better than it short wavelengths like blue.

• Uranus has a very thick haze above its atmosphere, thus making it hard to observe any features. This is due to it being the coldest Jovian.

• The rotation was measured by watching storms as they moved across the surface of the planet

Uranus’s Atmosphere

Sun

Neptune is a cold, bluish world with Jupiterlikeatmospheric features

• Much more cloud activity is seen on Neptune than on Uranus.

• This is because Uranus lacks a substantial internal heat source.

• White methane clouds can be seen in stormy areas

• Neptune has equatorial winds that blow at 2000 km/h and are retrograde relative to the rotation of the planet.

• Neptune and Pluto have intersecting orbits.

Uranus and Neptune each have a system of thin,dark rings

• Because of the large pressure and temperature of Jupiter’s and Saturn’s interior, hydrogen and helium compresses into its metallic state.

• No direct information is available about Jupiter’s interior, but we know that it has a solid rocky core.

• Jupiter flattens because of its fast rotation. It is 7% fatter around the middle.

• It would be much more pronounced if the planet lacked a solid core.

• The metallic hydrogen and helium are great conductors and are the source of the planet’s magnetic field

Jupiter’s and Saturn’s Interior

• Both Uranus and Neptune may have a rocky core surrounded by a mantle of water and ammonia

• Water and ammonia have been compressed into a slushy.

• Uranus and Neptune contain a higher proportion of heavy elements than Jupiter and Saturn

• No metallic hydrogen and helium. The pressure is not large enough, ammonia water slushy may be able to explain the existing magnetic field

The magnetic fields of both Uranus and Neptuneare oriented at unusual angles

• The magnetic axes of both Uranus and Neptune are steeply inclined from their axes of rotation

• The magnetic and rotational axes of all the other planets are more nearly parallel

• The magnetic fields of Uranus and Neptune are also offset from the centers of the planets

Jupiter’s Magnetic Field• At the surface of the Jupiter, the

magnetic field is 14 times strong than on earth. Enough to stop a pacemaker from working.

• When Jupiter’s size is taken into account, the magnetic field strength is ~20,000 times stronger than earth.

• It extends well into Saturn’s orbit 4 AU away.

Jovian Internal Heating

• If no internal heating occurred, jovians would radiate back all of the suns energy.

• Jupiter emits twice as much energy, Saturn emits three times as much energy, and Neptune emits 2.7 times as much energy.

• Uranus is a cold gas planet.

• Jupiter and Neptune create their heat because they are still contracting.

• Saturn has a much more complicated heating mechanism that involves liquefied helium raining down into the interior. Not well understood.