Lunar Eclipse, Jovian Planets, and Outer Solar System | 9/8 2/2 Astronomy

Lunar Eclipse: what you saw and why it looks red

• Lunar eclipse happens when Earth’s shadow covers the Moon; the Moon looks red because the edge of the sunlight reaching the Moon is filtered by Earth’s atmosphere. The atmosphere scatters blue light, so the red component from sunset is what mostly illuminates the Moon during the eclipse.

• The red color is light that has circled around Earth’s atmosphere and then reflected off the Moon. The atmosphere’s scattering is less for red light, so the Moon appears reddish rather than fully lit.

• Edges of the shadow sometimes look bluish. This is due to the ozone in Earth’s stratosphere preferentially scattering red light, which can shift the color near the shadow edge.

• The total phase of this particular eclipse lasted about $83\ \text{minutes}$.

• The shadow’s edges appear curved because the Earth is round; this curvature is one of the early evidences that the white Earth is brown when seen in shadow and that shadows are not perfectly flat.

• The eclipse’s geometry is affected by: (a) Earth’s tilt, (b) the Moon’s imperfect alignment with Earth’s shadow, and (c) the Moon’s passage through the lower portion of the Earth’s shadow rather than the central axis.

• The Moon’s movement appears offset in the shadow because the Moon is not perfectly aligned with Earth; during some eclipses we see the Moon move through the bottom portion of Earth’s shadow.

• The Moon’s path through the shadow demonstrates the 3D geometry of the Sun–Earth–Moon system, including the tilt of Earth’s axis and the relative orbital plane of the Moon.

• Homework reminder for chapters 6 and 7: due Tuesday night; students experiencing Pearson system access issues should contact the instructor for help; emphasize practice to prepare for the upcoming exam (in less than two weeks).

• Today’s topic transition: planning to cover the planets (Chapter 7). We’ll compare the jovian (gas giant) planets to terrestrial planets and discuss key properties: density, composition, mass, rings, atmospheres.

• Key takeaway: for the Jovian planets, the surfaces are not solid (they are gas/ice layers); most of their mass is in gaseous envelopes,
  with interiors including a rocky core and layers with metallic hydrogen in some cases.

What we care about for planetary science (Chapter 7 overview)

• Major categories to compare across planets: density, composition, mass, rings, atmospheres; these items help infer structure and evolution.

• For gas giants (Jovian planets): predominantly hydrogen and helium in their envelopes; no solid surfaces accessible; internal layering includes a rocky core, a metallic hydrogen layer, and a molecular hydrogen outer layer.

• For ice/gas giants Uranus and Neptune: composed largely of water, ammonia, and methane ices with hydrogen/helium; still no solid surface.

• Real probes and missions mentioned: Voyager spacecraft (Jupiter, Saturn, and outer solar system flybys), Galileo (Jupiter orbiter/probe), Cassini (Saturn orbiter) and the newer Juno (not named in the provided notes but often discussed in class). Voyager 2 conducted a Grand Tour of Jupiter, Saturn, Uranus, and Neptune; Voyager 1 visited Jupiter and Saturn; all four were part of a rare planetary alignment roughly every few hundred years.

• Rotation and shape: Jovian planets are oblate (flattened at the poles) because of rapid rotation and gas/ice composition; radii are larger at the equator than at the poles.

• Axial tilts: Uranus has an extreme tilt, nearly on its side; Jupiter’s tilt is relatively small; Saturn and Neptune have noticeable tilts; each tilt affects seasonal patterns, especially for Uranus where seasons are extreme due to the axial tilt and orbital geometry.

• Orbit and orbital periods: planets have long orbital periods around the Sun; the distance from the Sun and orbital period determine how long a given planet experiences seasons and sunlight during its year.

• Atmospheres: largely hydrogen/helium on Jupiter/Saturn; methane-rich atmospheres on Uranus/Neptune contribute to their blue colors; Saturn’s haze layer mutes visible cloud colors; Jupiter shows dramatic bands (zones and belts) and the Great Red Spot (a persistent storm).

• Interiors and energy sources: gas giants radiate more energy than they receive from the Sun; Jupiter ~ twice as much energy emitted as absorbed; Saturn ~ about three times as much energy emitted as absorbed; Uranus and Neptune show different energy budgets (re-evaluations discussed), with methane and other constituents contributing to greenhouse-like effects; Saturn is thought to radiate heat due to helium rain in its interior.

• Magnetic fields: all four gas giants have magnetic fields; Jupiter’s field is very strong (roughly 20,000 times stronger than Earth’s field) and extends well beyond Saturn’s orbit; Saturn’s field is aligned closely with its rotation axis; Uranus and Neptune have magnetic fields that are both offset from their centers and misaligned with their rotation axes, suggesting unusual internal flows.

• Rings: Saturn’s rings are prominent; ring shadows and ring dynamics are used to study Saturn’s system.

• Clouds and colors in Jupiter’s atmosphere: color differences arise from different clouds at various altitudes and temperatures; ammonia ice sits higher in the atmosphere, ammonium hydrosulfide (AHS) ice at mid-levels, and water ice deeper still; altitude and temperature affect color and observed bands.

• Vertical atmospheric structure and clouds: the five-layers framework is used to model cloud structure and temperatures; temperature minima and pressure relations help locate cloud layers and understand radiative transfer.

• Persistent cloud features and storms: the Great Red Spot and numerous smaller storms show long-lived and dynamic atmospheric activity; storms can be torn apart or stretched by the planet’s strong zonal winds.

• Methane’s role in color: methane absorbs red light, deepening blue hues especially on Uranus and Neptune; methane abundance correlates with observed color shifts.

• Internal heat and energy transport: a variety of mechanisms transport internal heat toward the surface; helium rain on Saturn is a specific mechanism that releases gravitational energy as helium droplets settle inward.

• Question-and-answer highlights from class discussion:

  • Why do gas giants have more moons than terrestrial planets? More mass allows capture and formation of multiple moons around gas giants.

  • How do we determine rotation in gas giants? Magnetic field rotation is used because atmospheric winds can be differential and less reliable as a single rotation marker.

  • Are all Saturn/Jupiter moons in the same phase in a given image? Yes, because they are illuminated by the same Sun angle at that time; observed phase depends on Sun-planet-moon geometry and observer’s vantage point.

Jupiter: a detailed look

• Size and shape: Jupiter is significantly larger than Earth; radius is about $R<em>J \approx 11\,R</em>\oplus$; oblate due to rapid rotation.

• Mass: Jupiter is about $M<em>J \approx 320\,M</em>\oplus$ (roughly 318 Earth masses).

• Rotation: very fast; a day on Jupiter is about $P_J \approx 10\ \text{hours}$; rapid rotation drives strong zonal winds and banded structure.

• Density and composition: overall low density compared to terrestrial planets; internal structure includes a rocky core with a massive envelope of hydrogen/helium; outer layers dominated by molecular hydrogen; a metallic hydrogen layer exists beneath the molecular envelope due to extreme pressures.

• Cloud layers and colors: ammonia ice (high altitude, brighter), ammonium hydrosulfide ice (mid-altitude, orange-ish), and water ice deeper (blueish tints in some reduced light); temperature vs altitude is plotted to show cloud layers and chemical composition effects on color.

• Galileo atmospheric probe: direct measurements of composition and temperature through the cloud layers down to the lower atmosphere, confirming the layered cloud model and thermal structure.

• Great Red Spot and storms: a persistent hurricane-like storm larger than Earth; multiple other storms present; winds show differential rotation and complex flow patterns; storms can be sheared by zonal winds and interact with each other.

• Cloud bands and chemistry: zones (lighter bands) tend to be higher and associated with rising hotter gas; belts (darker bands) are lower with differing cloud cover; ongoing debate over exact convection direction (zones up, belts down vs. the reverse) based on Voyager and Cassini data; current consensus emphasizes a complex, transitional behavior rather than a simple up/down picture.

• Atmospheric circulation and color: color variations reflect altitude, temperature, and chemical species; persistent storms and moving vortices create a dynamic, Gogh-like appearance in images.

• Internal heat and energy emission: Jupiter radiates about twice as much energy as it receives from the Sun, indicating a residual heat source from formation and slow cooling; energy transport mechanisms connect core heat to the outer atmosphere.

• Magnetic field and radiation environment: Jupiter’s magnetic field is extremely strong and creates an intense radiation environment, especially near the poles; auroras are observed in ultraviolet as well as visible light, shaped by the planet’s rapid rotation and strong magnetic field.

• Moons and orbital geometry: observed moons appear in similar phases in a given image due to Sun angle and observer geometry; many moon discoveries and orbital dynamics relate to Jupiter’s strong gravity.

• Rings: visible as faint lines in some images; not as prominent as Saturn’s rings but present and detectable with sufficient resolution.

Saturn: atmosphere, rings, and unique features

• Atmosphere: more muted color bands than Jupiter due to a thicker haze layer that scatters light; less gravity leads to less compression of atmospheric layers, making cloud structures less dramatic in visible light.

• Ring system: prominent rings; rings are thin, and the associated shadows appear as dark bands; rings contribute to the planet’s overall appearance and are essential to understanding Saturn’s dynamics.

• Hexagonal polar feature: Saturn’s north pole hosts a six-sided hexagonal jet stream (polar hexagon) that is a persistent atmospheric pattern; the exact cause is not fully known, but simulations show that certain wind patterns can produce such geometric shapes.

• Methane and haze effects: methane in Saturn’s atmosphere also influences color but is modulated by the haze layer that reduces visible contrast; infrared imaging can reveal deeper cloud layers not visible in the optical.

• Interior and energy: Saturn radiates energy—likely due to helium rain in its interior releasing gravitational energy as helium droplets descend; this energy transport contributes to Saturn’s overall luminosity beyond solar input.

• Magnetic field: Saturn’s magnetic field is strong but not as intense as Jupiter’s and is notably aligned with its rotation axis; the alignment and field strength have implications for magnetospheric dynamics and auroral processes.

• Comparisons to Jupiter: while Jupiter’s bands are pronounced, Saturn’s are subtler; the differences arise from mass, gravity, haze, and internal heat generation styles.

Uranus and Neptune: extreme tilts, methane atmospheres, and distant climates

• Uranus: rotational axis is tilted dramatically, almost on its side; extreme seasons occur because the tilt causes one pole to be in darkness for decades and the other to receive continuous sunlight for decades.

• Seasonal dynamics on Uranus: during certain orbital positions, the northern hemisphere may be completely dark for years, while the southern hemisphere experiences extended daylight, and vice versa. The equator experiences extended twilight for long periods.

• Atmospheres: both planets are methane-rich; methane absorbs red wavelengths, giving them blue hues; Uranus is the coldest planet; cloud features are less distinct than Jupiter due to the colder temperatures and deeper cloud layers that form under conditions that reduce visible banding.

• Infrared vs visible: infrared observations reveal deeper cloud layers; visible images show fewer obvious bands; methane-rich atmospheres contribute to the characteristic blue tones.

• Internal heat and energy budgets: both Uranus and Neptune radiate energy, but the exact balance between absorbed solar energy and emitted internal energy remains uncertain and subject to revisions.

  • Uranus: earlier data suggested it radiates roughly as much energy as it absorbs; recent reanalyses indicate it may radiate slightly more energy than it absorbs, implying it absorbs somewhat less from the Sun than previously thought.

  • Neptune: radiates about three times as much energy as it absorbs, indicating a stronger internal heat source or slower cooling relative to Uranus.

• Interiors: Uranus and Neptune share a similar general structure (rocky/ice core with a substantial water/ammonia/methane-rich outer shell and an H/He envelope); exact layering deeper inside remains uncertain due to limited direct measurements.

• Magnetic fields: Uranus and Neptune have magnetic fields that are significantly offset from their rotation axes and misaligned with the rotation axes; the fields are centered near the planet’s interior but not aligned with the rotational symmetry, suggesting unusual internal flow patterns.

• Atmosphere vs interior: the color differences and atmospheric dynamics reflect both composition (e.g., methane) and the altitude at which certain clouds form; deeper layers and higher pressures influence the observed spectra.

Interior structure and energy transport across Jovian planets (summary)

• General architecture for Jupiter and Saturn:

  • Rocky core (dense, deep interior) surrounded by layers of metallic hydrogen (conducting) and molecular hydrogen (outer envelope).

  • A hydrogen/helium outer atmosphere with progressively higher pressure as you move inward.

• General architecture for Uranus and Neptune:

  • Likely rocky/icy core with thick mantles of water, ammonia, and methane ices, surrounded by an atmosphere rich in hydrogen and helium.

  • Metallic hydrogen layer is not as prominent or extensive as in Jupiter/Saturn due to lower mass.

• Why these internal structures matter:

  • They explain magnetic fields, heat budgets, and gravitational moments;

  • They influence atmospheric dynamics and observed cloud features.

• Density considerations and what they tell us:

  • Jupiter is much less dense than terrestrial planets, reflecting its gas-dominated composition; Saturn’s density is even lower (~$<br>ho_S \approx 710\ \text{kg m}^{-3} = 0.710\ \text{g cm}^{-3}$), which is lower than water’s density (~$1\ \text{g cm}^{-3}$). If Saturn could be placed in a bathtub, it would float.

  • The density contrast across these planets helps infer internal layering and the presence of heavy-element cores.

Magnetic fields and radiation environments

• Jupiter: extremely strong magnetic field; the magnetosphere extends far, influencing the radiation environment; Pioneer 10/11 demonstrated that Jupiter’s magnetic field can extend beyond Saturn’s orbit; the field is many thousands of times stronger than Earth’s field. Observations show intense auroras in ultraviolet.

• Saturn: magnetic field aligned closely with rotation axis; less intense than Jupiter but still significant for magnetospheric dynamics and auroras.

• Uranus and Neptune: magnetic fields are offset from centers and misaligned with rotation axes; the origin of this misalignment is not fully understood and indicates unusual internal flow patterns.

• Interactions with the solar wind and other planets: Jupiter’s large magnetosphere can interact with Saturn’s magnetosphere when the latter passes through or near Jupiter’s field — complex interactions with solar wind and other planetary fields can occur.

Rocks, rings, and moons: formation and evolution themes

• Moons around gas giants: gas giants host many moons because they have enough mass to form satellites in their circumplanetary disks; the outer solar system’s abundance of mass made moon formation around the giants more common than around terrestrial planets.

• Rings: visible prominently around Saturn; rings can be shadowed and observed as dark regions against the planet’s body in some images; next class will discuss rings in more detail.

• Moon phases and illumination: in images, moons appear in similar phases when observed from a fixed vantage point, because the Sun’s angle relative to the planet remains roughly constant during the observation window. The changing view comes from the planet’s rotation bringing different longitudes into view.

Quick notes on the exam and study guidance

• The exam will cover material from seven chapters; approximately forty questions on the paper, distributed across the material covered.

• The section on stellar positioning was part of early chapters and will be included as a small portion of questions; not a large fraction of the exam.

• The professor plans to distribute questions approximately evenly among chapters; the exam structure, timing, and format will be discussed in class prior to the test.

• Practical tips: work through the Pearson system issues ahead of time, practice energy budgets, internal structure, and cloud-layer models for Jupiter/Saturn; ensure you understand the implications of axial tilts, magnetic field geometries, and energy transport mechanisms for the gas giants.

Key concepts and equations to remember (LaTeX)

• Jovian planet masses and radii: $M<em>J \approx 320\ M</em>\oplus, \quad R<em>J \approx 11\ R</em>\oplus$

• Saturn density: $\rho_S \approx 710\ \mathrm{kg\,m^{-3}} = 0.710\ \mathrm{g\,cm^{-3}}$

• Jupiter rotation period: $P_J \approx 10\ \mathrm{hours}$

• Effects of atmosphere on eclipse color: red sunset light transmitted around Earth’s atmosphere; blue scattering from atmosphere reduces red color near edges; ozone effects can contribute slight blue tints in shadow boundaries.

• Energy budgets (approximate):

  • Jupiter radiates about twice the energy it receives from the Sun: $L<em>J \approx 2\, L</em>{\odot,\text{incident}}$

  • Saturn radiates about three times as much as it receives: $L<em>S \approx 3\, L</em>{\odot,\text{incident}}$

• Interior structure keywords: rocky core, metallic hydrogen layer, molecular hydrogen envelope (outer atmosphere for Jupiter/Saturn); Uranus/Neptune have icy/rocky interiors with H/He envelopes.

• Methane effects on color: methane absorbs red light, enhancing blue coloration in Uranus and Neptune; observed colors also depend on atmospheric hazes and cloud chemistry.