Overview
This lecture covers the rings of Uranus and Neptune, explaining their composition, why they are hard to see, and current observations about Neptune's disappearing rings.
Rings of Outer Planets
- Uranus and Neptune both have rings, but they are difficult to observe in most images.
- Jupiter’s rings are also hard to see because they are made of rocky material, lacking reflective ice.
- Unlike Jupiter, Uranus and Neptune are cold enough for ice to form in their rings.
Composition of Uranus and Neptune’s Rings
- The rings of Uranus and Neptune are made of methane ice, not water ice like Saturn’s rings.
- Methane ice is subject to solar radiation, breaking down over time.
- Sunlight breaks water ice into hydrogen and oxygen gases, which escape into space.
- Sunlight breaks methane ice (CH₄) into carbon and hydrogen.
- Oxygen and hydrogen produced escape as gases, but carbon remains as a solid, covering the ice.
Visibility Issues
- The carbon residue left on methane ice makes the rings appear black and not reflective.
- If the rings were cleaned of carbon, they would be bright and reflective.
- Atmospheric clouds on these planets avoid this issue because they circulate and do not remain exposed in sunlight.
Neptune’s Disappearing Rings
- Neptune’s rings have been observed to fade over the past 40 years.
- The cause of the disappearing rings is unknown and not seen on other planets.
- One possible explanation is a seasonal effect, as Neptune’s seasons last about 40 Earth years.
- Neptune’s orbital period is 165 Earth years, so one full year on Neptune is much longer than on Earth.
- Humanity has not yet observed Neptune complete a full orbit since its discovery.
Key Terms & Definitions
- Methane Ice — frozen methane (CH₄) found in the rings of Uranus and Neptune.
- Carbonized Methane Ice — methane ice darkened by residual carbon after solar radiation breaks down the methane.
- Seasonal Effect — a change that occurs in cycles based on a planet's long-duration seasons.
Action Items / Next Steps
- Review the composition and visibility of planetary rings for the upcoming exam.
- No assigned readings or homework from this lecture.