Chapter 13 lecture notes Uranus, Neptune, and the Voyager Explorations

Historical Observations and Imaging of Uranus

• Voyager 2 Spacecraft (1986): Our understanding of Uranus increased substantially after this mission. The spacecraft captured natural color images revealing a beautiful hue and wispy clouds in the Northern Hemisphere.

• Keck Telescope Observations (2004): These images of the two hemispheres showed significant activity within the clouds, including bands reminiscent of Jupiter and Saturn. Other features included:

  • Circulating clouds and hurricanes.

  • Strange convective features located near the North Pole (positioned at approximately the $04:00$ position in these specific images).

• Keck Telescope Observations (2012): These represent the most detailed images of Uranus ever obtained from Earth. They were composite images taken in the near-infrared spectrum. Findings included:

  • An astounding level of detail in the clouds.

  • Confirmation of the same small convective spots near the North Pole seen in $2004$.

  • A broadband just left of the disc center covering latitudes from the Equator to approximately $10$ degrees North.

  • A scalloped wave pattern just South of the Equator, never before seen on Uranus, which is similar to instabilities caused by horizontal wind shear.

  • A small dark spot with bright companion clouds located near the bottom of the left image.

Discovery and Observation of Neptune

• Mathematical Discovery (1846): Neptune was the first planet discovered through mathematical predictions rather than visual search. Astronomers noticed Uranus did not follow Newton’s laws of motion, suggesting a gravitational pull from an unknown body.

  • Calculations: Two researchers independently calculated the location. French astronomer Le Verrier sent his calculations to Gala at the Berlin Observatory.

  • Observation: Gala found the planet on the same night he received the letter, precisely where calculated.

• Earth-based Observation: Neptune is challenging to see because it is farther than Uranus. It is never visible to the naked eye but appears as a faint, blue-tinted, star-like object through a telescope.

• Voyager 2 (1989): This was the first and only spacecraft to observe Neptune up close. It passed within 4,800\,
  ext{kilometers} of Neptune's North Pole. Findings included:

  • A dynamic blue world.

  • Wind speeds cited at 1,100\,
    ext{km/hr}.

  • A large storm called the Great Dark Spot.

  • Faint rings and six new moons.

• James Webb Space Telescope (JWST): In its first images, the Webb Observatory provided the sharpest look at Neptune's rings in over $30$ years (using infrared).

  • Captured faint dust bands and $7$ of Neptune's $14$ known moons.

  • In $2023$, JWST captured bright auroral activity on Neptune.

  • Aurora Explanation: High-energy particles, often from the sun, become trapped in the magnetic field and strike the upper atmosphere; the resulting energy release creates a signature glow.

  • Past hints of aurorae were detected by Voyager 2, but JWST confirmed them using an infrared spectrograph.

Physical Comparison of Ice Giants

• General Characteristics: Uranus and Neptune are ice giants with similar sizes, compositions, and chaotic magnetic fields. They are the smallest Jovian planets but are still $4$ times the size of Earth.

• Composition: Primarily composed of water, ammonia, and methane ices over a small rocky core.

  • Cores: Each core is roughly $10$ times the mass of Earth.

  • Neptune vs. Uranus: Neptune is more massive than Uranus despite having a smaller radius. Neptune is significantly denser, indicating a higher proportion of rocky material and compressed ices, while Uranus has more hydrogen and helium.

• Uranus' Axial Tilt: Among all planets, Uranus has a unique, extreme axial tilt of $97.77$ degrees. It rotates on its side like a rolling ball rather than a top. This was likely caused by a massive collision. This orientation means poles point toward the sun, creating $21$-year-long extreme seasons.

Atmospheres and Internal Processes

• Atmospheric Composition: Primarily hydrogen, helium, and methane.

  • Coloration: Blue-green colors result from methane. Neptune appears a deeper blue due to a thinner haze layer and an active, warmer atmosphere. Uranus is lighter because of a thicker, stagnant haze.

• Temperatures and Clouds: Uranus is the coldest planet in the solar system, with temperatures reaching $-224^{\circ}C$. This is due to a lack of significant internal heat. Clouds exist in warmer, deeper layers:

  • Bottom layer: Water ice.

  • Middle layers: Ammonium hydrosulfide.

  • Upper layers: Ammonia hydrogen sulfide.

• Storm Systems: Both planets feature dark spots, which are massive, short-lived anticyclonic storms.

  • Uranus: Hubble first captured its dark spot in $2006$. Clouds appear as bright spots and can move significantly over a four-hour period.

  • Neptune: First discovered by Voyager 2. These are high-altitude methane ice clouds with intense winds, often dissipating within a few years.

• The "Diamond Rain" Hypothesis: Under extreme pressures and temperatures beneath the atmospheres, methane ($CH_4$) molecules are crushed.

  • Carbon clusters are squeezed into a diamond structure, the most stable form of carbon under high pressure.

  • These carbon crystals would sink through the methane, ammonia, and water toward the core and accumulate in layers.

Magnetic Fields and Internal Structure

• Structure: Both planets are ice giants because they possess an icy mantle (compressed water, methane, ammonia) above a rocky core, rather than being mostly gas.

• Magnetic Fields: Unlike Earth's dipole field (generated by a spinning liquid metallic outer core), ice giant magnetic fields are a "jumbled maze."

  • Generation: Research from $2024$ suggests fields are generated in the mantle in a watery layer that becomes an ion-rich, electrically conductive fluid under extreme pressure.

  • This stratification (layers like oil and water) leads to the disordered nature of the fields.

Moons of Uranus

• Total Moons: $28$ known moons (updated figure).

• Major Moons: Miranda, Ariel, Umbriel, Titania, and Oberon. They are roughly half-ice and half-rock.

• Miranda: The smallest and innermost major moon. It features the most varied landscape in the solar system, including:

  • Coronae: Three large racetrack-like oval structures with concentric grooves and ridges. These are 20\,
    ext{km} deep and formed by upwellings of warm ice.

  • Fault Canyons: These can be up to $12$ times deeper than the Grand Canyon.

  • Gravity: On Miranda, dropping a rock from the highest cliff would take $10$ minutes to reach the bottom.

Moons of Neptune

• Total Moons: $16$ known moons (updated figure).

• Visibility: Triton and Myriad are the only moons visible from Earth.

• Triton: Discovered by an amateur in England $17$ days after Neptune's discovery.

  • Retrograde Orbit: Triton orbits in the opposite direction of Neptune's rotation, suggesting it is a captured Kuiper Belt object.

  • Surface: Geologically young (less than $100\,\text{million}$ years old) with few impact craters.

  • Geysers: Erupt nitrogen gas and dark dust up to $8\,\text{km}$ high.

  • Ice Volcanoes (Cryovolcanism): Erupt methane ice driven by internal or solar heat.

• Neriod: Known for one of the most eccentric orbits in the solar system, ranging from $1,380,000\,\text{km}$ to $9,660,000\,\text{km}$ from the planet.

Ring Systems

• Detection: Discovered via stellar occultation (starlight flickering as the planet passes in front).

  • Uranus: $1977$.

  • Neptune: $1984$ (confirmed by Voyager 2 in $1989$).

• Uranus Rings: $13$ distinct rings (inner narrow rings and outer colorful dusty rings). They consist of water ice and organic compounds.

  • Epsilon Ring: The brightest and densest ring, ranging from $20$ to $100\,\text{km}$ wide.

  • Shepherd Moons: Cordelia (inner) and Ophelia (outer) find the particles in place using gravity to keep the ring narrow.

• Neptune Rings: At least $5$ main rings and $4$ prominent ring arcs.

  • Ring Arcs: These clumps of dust are stabilized by the gravitational effect of the moon Galatea, located just inward of the ring.

The Voyager Missions and Orbital Mechanics

• Overview: NASA's Voyager program (Voyager 1 and 2) launched in $1977$. Voyager 2 is the only craft to visit all four Jovian planets.

• The Three-Body Problem: Solved by Michael Minovich (a $25$-year-old PhD student) in $1961$.

  • Gravity Assist: His model showed a craft can steal a planet's orbital speed to accelerate away from the sun without using fuel.

  • Alignment: The mission took advantage of a rare alignment of the outer planets occurring once every $175$ years.

• Voyager 2 Planetary Encounters:

  • Jupiter (1979): Imaged the Great Red Spot and Europa; discovered the moon Adrastea.

  • Saturn (1981): Imaged ring spokes and shepherding moons.

  • Uranus (1986): Discovered $10$ moons and $2$ rings. Found evidence of a boiling water ocean $800\,\text{km}$ below the surface. Captured the magnetosphere during a rare solar windstorm (occurring only $4\%$ of the time).

  • Neptune (1989): Discovered $6$ moons and $4$ rings. Imaged the Great Dark Spot and Triton’s geysers.

• Current Status:

  • Both are in interstellar space (Voyager 1 since $2012$, Voyager 2 since $2018$).

  • They are over $20,000,000,000\,\text{km}$ away and still transmitting data in their $48$th year of operation.

  • They monitor magnetic fields, high-energy particle radiation, and interstellar plasma properties.