Uranus and Neptune

Module 5: Uranus and Neptune

Discovery of Uranus

• Discovered by William Herschel in 1781 using a telescope.

• Noted that it moved among stars; its motion was too slow for that of a planet.

• Initially wanted to call it "George's Star" in honor of King George.

• Eventually settled on Uranus, the Father of Saturn.

• Barely visible to the naked eye; appeared as a large, faint star with a pale green disk.

Early Uranus Exploration

• Voyager 2 conducted exploration.

• Found that the atmosphere appeared to have no distinct features, no bands or spots observed.

• Discrepancies noted between predicted/observed orbits; did not comply with Kepler's planetary laws, implying external gravitational influence.

Discovery of Neptune

• The orbit of Uranus influenced by the presence of an unseen nearby planet (Neptune).

• John Adams and Urbain Leverrier calculated the mass and orbit of Neptune based on Uranus' irregularities.

• Johann Galle successfully located Neptune in the night sky.

Neptune Exploration

• Neptune cannot be seen with the naked eye; requires telescopes or binoculars.

• Appears as a small bluish disk with a few cloud bands.

• Most information about Neptune derived from Voyager 2 observations.

• Discovered more bands and spots; dubbed the "Blue Jupiter" due to its coloration.

Uranus - Axis of Rotation

• Unique axis of rotation with the highest tilt among planets at 98 degrees.

• Uranus rotates on its side, switching which pole points towards the Sun.

• Each pole experiences a 42-year period of light and dark.

• The cause of the unique tilt is uncertain; one theory suggests a major collision but lacks evidence.

Composition of Atmospheres

• Uranus and Neptune primarily consist of:

  • 84% Hydrogen

  • 14% Helium

  • Methane: 3% in Neptune and 2% in Uranus.

• Contain low ammonia and cold ice crystals, difficult to detect.

Color of Atmospheres

• Blue color results from methane absorption of red light and reflection of blue/blue-green light.

• More methane leads to a more vivid blue color:

  • Uranus: blue-green (less methane)

  • Neptune: blue (more methane)

Weather on Uranus

• Few atmospheric features evident; upper clouds obscured by haze.

• Clouds seen with enhancement, moving in the same rotational direction.

• Wind speeds vary between 200-500 km/h.

Atmosphere of Neptune

• Upper atmosphere is warmer than that of Uranus due to internal heat.

• Atmospheric features observable with a thinner haze; clouds less dense but at higher altitudes.

Weather on Neptune

• First explored by Voyager 2; observed white methane clouds above other cloud layers.

• Equatorial winds blow from east to west at 2000 km/h with occurrences of retrograde motion.

• Weather patterns remain largely unknown.

Great Dark Spot on Neptune

• Similar to Jupiter's Great Red Spot, located south of the equator.

• Shows counterclockwise flow and turbulence; noted to be short-lived.

• A new Dark Spot appeared in the northern hemisphere, the nature of which is not fully understood.

Interiors of Uranus & Neptune

• Both planets have smaller masses and lower gravities when compared to other gas giants.

• Internal structure theory suggests a high-density, slushy composition with compressed water clouds and dissolved ammonia.

• Likely have large cores, approximately 10 times the mass of Earth, resembling those of Jupiter and Saturn.

Uranus and Neptune Magnetic Fields

• Both have strong magnetic fields, though weaker than Jupiter's and stronger than Earth's.

• Uranus' magnetic axis is tilted significantly compared to its rotational axis, as is Neptune's.

  • Uranus: Magnetic field lines are directed towards the Sun.

  • Neptune: Magnetic field lines are also directed towards the Sun, but are arranged differently due to the planet's unique rotation axes.

Internal Heating

• Uranus shows no internal energy source; radiates energy received back to space.

• Considered to have given off extra energy in the past.

• Neptune, in contrast, possesses an internal heat source, emitting 2.7 times more energy than it receives; theorized that methane acts as an insulator, contributing to a warm South Pole.