Uranus and Neptune Flashcards

Uranus and Neptune Overview

• Images: Neptune image taken from Voyager 2 at a distance of 4.4 million miles (about 18 times Earth-Moon distance).
  • Features of Neptune:
  • Great Dark Spot: Dark feature with a bright companion smudge.
  • Scooter: Fast-moving bright feature.
  • Little Dark Spot: Observed on the lower left of the image.

Discovery of Uranus

• Date of Discovery: 1781 by William Herschel.
• Initial Name: Georgium Sidum ("George’s Star"), in honor of King George III.
  • Reason for Name Change: Astronomical community preferred names from Greek and Roman mythology, leading to the name Uranus, father of Cronus (Saturn).
• About William Herschel:
  • Originally a trained musician, he emigrated to England.
  • Developed larger telescopes to detect stellar parallax.
  • Discovered Uranus unexpectedly while searching for stars.

Reaction to Discovery

• Public Reception: Herschel became famous, but some continental astronomers viewed him as just an amateur.
• Herschel's Defense:
  1. He built one of the best telescopes of the time.
  2. Conducted systematic searches for stars before discovering Uranus.
  3. Had a keen eye for details to differentiate planet and star properties.

Discovery of Neptune

• Orbital Discrepancy: Uranus’s orbit was inconsistent with Newton’s laws, suggesting another gravitational influence.
• Predictions by Le Verrier and Adams:
  • Used mathematical predictions to find an unknown planet, Neptune.
• Date of Discovery: September 23, 1846, by Johann Galle, only 1° from predicted location.

Orbital Characteristics

• Orbital Distance:
  • $d_{Uranus} = 19.2 ext{ au}$
  • $d_{Neptune} = 30.1 ext{ au}$
  • Neptune receives 40% less solar radiation than Uranus.
• Orbital Period:
  • Uranus: 84 years.
  • Neptune: 165 years.
• Day Length:
  • Uranus: 17.3 hours.
  • Neptune: 16.1 hours.
• Axis Tilt:
  • Uranus: 97.9° (retrograde rotation).
  • Neptune: 29.6° (larger than Earth’s 23.5°).

Physical Properties

• Size and Mass: Similar sizes.
  • $R{Uranus} hickapprox 4.0 R{E}$
  • $R{Neptune} hickapprox 3.9 R{E}$
  • $M{Uranus} hickapprox 15 M{E}$
  • $M{Neptune} hickapprox 17 M{E}$
• Density:
  • $SG{Uranus} = 1.3$ and $SG{Neptune} = 1.8$ (indicate significant ice and rock).

Interior Composition

• Core: Molten rocks and metals, liquid hydrogen compounds (mostly water).
  • Comparable to Earth’s mass, but denser due to surrounding material.
• Core Temperatures:
  • $T_{Uranus} hickapprox 5300 K$ (9000 °F)
  • $T_{Neptune} hickapprox 7300 K$ (12600 °F)
• Regions Above Core: Slushy ice layers of water ($H{2}O$), ammonia ($NH{3}$), methane ($CH_{4}$), and hydrogen/silicate matter.
• Chemical Processes: High pressures disturb chemical bonds, possibly leading to diamond crystallization in the slushy region.

Magnetospheres

• Composition: Conducting fluid is salt water with ammonia mixed in.
• Orientation: Magnetospheres are off-centered and inclined relative to rotational axes, potentially due to ammonia reservoirs.

Atmospheric Characteristics

• Mass Composition: ~80% Hydrogen, 18% Helium, 2% other gases, predominantly methane.
• Cloud Layers: Aerosolized ammonia in outer cloud layers at lower cloud top temperatures (~60 K).
• Appearance: Both planets appear blue due to frozen methane; Neptune is "bluer" than Uranus.
• Weather Patterns: Less atmospheric structure than gas giants, with observed strong winds, storms, and lightning.
  • Example: The Great Dark Spot observed in the mid-1980s, vanished by 1995.

Studies from Spacecraft

• Voyager Missions:
  • Voyager 2 (1986: Uranus; 1989: Neptune) provided major insights, such as Neptune emitting more than twice the thermal energy it absorbs.
• Hubble Space Telescope: Used for long-term atmospheric monitoring of both planets.

Thermal Energy Comparison

• Neptune vs. Uranus:
  • Neptune is still contracting and converting gravitational energy into thermal energy, whereas Uranus does not.
  • Several hypotheses address differences in energy outputs, including age, formation, and historical impacts on thermal evolution.

Key Questions for Further Study

• Why are Neptune's and Uranus's thermal characteristics so different?
• What mechanisms govern their thermal energy outputs and contractions?