Overview
This lecture compares the planets Uranus and Neptune, focusing on their similarities, internal structures, atmospheres, magnetic fields, weather, and unique features.
Similarities Between Uranus and Neptune
- Uranus and Neptune are similar in size and both appear bluish.
- Their atmospheres are primarily hydrogen and helium, with about 5% methane giving them their blue color.
- Both planets have three main layers: a solid rocky core, a water-ammonia ocean, and a gaseous atmosphere.
- Both planets have magnetic fields generated by their conductive water-ammonia oceans.
Internal Structure
- The innermost layer is a solid, rocky core composed mainly of iron and silicates under high pressure.
- The middle layer is an ocean of water with ammonia (and some methane), similar to industrial cleaning fluid.
- The outer layer is an atmosphere mostly made of hydrogen, helium, and some methane.
Magnetic Fields
- Magnetic fields on Uranus and Neptune are generated by the motion of their water-ammonia oceans.
- Their magnetic fields are significantly tilted, almost 90 degrees off from the planetary rotation axis.
- Possible explanations for this tilt include magnetic field flipping or the presence of a theorized “liquid diamond” layer.
Atmospheric Appearance and Weather
- Uranus and Neptune show little atmospheric detail compared to Jupiter and Saturn due to lower gravity spreading out cloud layers.
- Despite plain appearances, both planets have active storms and turbulence deep in their atmospheres.
- Infrared imaging reveals hidden atmospheric bands and storms, especially on Uranus.
Axial Tilt and Rotation
- Uranus is highly tilted, rotating almost completely on its side (most tilted planet).
- Neptune is not significantly tilted compared to Uranus.
Clouds and Storms
- Visible clouds on Uranus and Neptune are made of methane ice due to extremely cold temperatures.
- Methane ice clouds appear white because frozen particles reflect sunlight.
- Neptune formerly had a large storm called the "Great Dark Spot," similar to Jupiter’s Great Red Spot, which has since disappeared.
- Uranus has its own storms, mostly visible in infrared wavelengths.
Key Terms & Definitions
- Methane — a colorless, odorless gas; present in the atmospheres of Uranus and Neptune, causing their blue color.
- Water-ammonia ocean — liquid layer under the atmosphere, composed of water and ammonia.
- Rocky core — dense inner center made of rock and metal.
- Magnetic field — invisible field generated by moving, electrically conductive materials inside a planet.
- Axial tilt — the angle between a planet's rotational axis and its orbital axis.
- Methane ice clouds — frozen methane particles forming cloud layers in the upper atmospheres.
Action Items / Next Steps
- Review planetary layers and magnetic field concepts for Uranus and Neptune.
- Read assigned textbook section on outer planets’ atmospheres and internal structures.