11.3 Giant Planet's Atmospheres

1. Introduction to Jovian Atmospheres

• The atmospheres of the jovian planets are the directly observable parts, representing their general compositions more than terrestrial planets due to the lack of solid surfaces.

• These atmospheres exhibit dramatic weather patterns, with storms capable of growing larger than Earth.

2. Atmospheric Composition of the Giant Planets

Learning Objective: Discuss the atmospheric composition of the giant planets.

• Early Spectroscopic Observations:

  • Began in the nineteenth century, but interpretation was difficult.

  • By the 1930s, molecules of methane ($CH<em>4$) and ammonia ($NH</em>3$) were identified in Jupiter and Saturn's atmospheres.

  • Initially thought to be main constituents, but later found to be trace gases.

• Dominant Gases:

  • Hydrogen and helium are actually the dominant gases, but they lack easily detected spectral features in the visible spectrum, leading to initial confusion.

  • Reliable helium abundance was measured by the Voyager spacecraft using far-infrared spectra.

• Jupiter and Saturn's Similarities and Differences:

  • Compositions are generally similar.

  • Saturn has less helium due to its precipitation, which contributes to its internal energy source.

  • The Galileo entry probe in 1995 provided the most precise measurements of Jupiter's atmospheric composition, even surpassing the known elemental abundances of the Sun.

• Voyagers in Astronomy: James Van Allen

  • Physicist James Van Allen significantly contributed to the space age, designing the 'rockoon' technique for high-altitude exploration.

  • Instrumental in the International Geophysical Year (IGY) initiative.

  • Led the design of instruments for Explorer 1, the first U.S. satellite in space, launched January 31, 1958.

  • Discovered the Van Allen belts, a region of highly charged particles surrounding Earth, with Explorer 1, marking the first scientific discovery of the space program.

  • Pioneered measurements of magnetic and particle environments around Jupiter and Saturn with Pioneers 10 and 11, with similar charged-particle zones sometimes also referred to as Van Allen belts.

3. Cloud Formation and Atmospheric Structure of the Gas Giants

Learning Objective: Describe the cloud formation and atmospheric structure of the gas giants.

• Jupiter's Colorful Clouds:

  • Among the most spectacular in the solar system, with white, orange, red, and brown hues.

  • The vibrant colors are puzzling given the atmosphere is nearly 90% hydrogen and cool temperatures; one hypothesis suggests colorful hydrogen compounds rise from warm areas.

• Saturn's Subdued Clouds:

  • Similar activity to Jupiter but much more subdued, displaying a nearly uniform butterscotch hue.

• Cloud Composition and Atmospheric Layers (Jupiter & Saturn):

  • Different gases condense and freeze at distinct temperatures.

  • The primary visible clouds are composed of frozen ammonia crystals.

  • These ammonia clouds mark the upper edge of the troposphere; above this is the colder stratosphere.

  • Cloud top temperatures are approximately $140 K$.

  • On Jupiter, this level is at about $0.1$ bar pressure.

  • On Saturn, it is deeper, at about $1$ bar pressure, making its appearance blander.

  • Deeper Atmospheric Regions (Jupiter):

  • The Galileo probe on Jupiter was expected to pass through frozen and liquid water clouds at deeper levels (e.g., $5$ bars).

  • However, the probe descended through an unusually dry, cloud-free region, dubbed the "desert" of Jupiter, and found a surprisingly low abundance of water vapor.

  • It detected lightning storms at great distances but not locally, supporting the observation of clear weather at its entry point.

  • Upper Atmosphere (Jupiter):

  • Above visible ammonia clouds, a clear stratosphere reaches a minimum temperature near $120 K$.

  • At still higher altitudes, temperatures rise due to molecules absorbing ultraviolet light from the Sun.

  • Cloud Colors and Smog:

  • Cloud colors are due to impurities formed by photochemical reactions among atmospheric gases.

  • In Jupiter's upper atmosphere, these reactions create a thin layer of hydrogen and carbon compounds (smog) far above the visible clouds, which does not block the view of the clouds beneath.

• Saturn’s Cloud Structure:

  • Visible atmosphere: ~75% hydrogen and ~25% helium, with trace hydrocarbons.

  • Overall structure similar to Jupiter, but temperatures are colder, and the atmosphere is more extended due to lower surface gravity.

  • Features a mysterious hexagonal wave pattern around its north pole, with each side longer than Earth's diameter.

  • Winds are extremely high, reaching up to $1800$ kilometers per hour near the equator.

• Uranus's Featureless Atmosphere:

  • Appears almost entirely featureless from ultraviolet to infrared wavelengths.

  • Basic atmospheric structure resembles Jupiter and Saturn, but upper clouds (at 1-bar pressure) are methane-based, not ammonia.

  • Lacks an internal heat source, which suppresses vertical movement and creates a very stable atmosphere with minimal visible structure.

• Neptune's Active Atmosphere:

  • Differs from Uranus in appearance, despite similar basic atmospheric temperatures.

  • Upper clouds are methane, forming a thin layer near the tropopause ($70 K$, $1.5$ bars).

  • The atmosphere above this layer is clear and transparent, with less haze than Uranus, giving Neptune a pale blue color from sunlight scattering.

  • An internal heat source drives convection currents, forming high-altitude clouds (about $75$ kilometers higher than the main clouds) composed of methane ice crystals, which appear as bright white patterns and cast shadows.

4. Giant Planets’ Wind and Weather Patterns

Learning Objective: Characterize the giant planets’ wind and weather patterns.

• Fundamental Differences from Terrestrial Planets:

  • Rapid rotation smears circulation into horizontal (east-west) patterns parallel to the equator.

  • Absence of a solid surface below the atmosphere means circulation patterns do not lose energy through friction.

  • Internal heat contribution (except Uranus) drives deep convection currents of rising hot air and falling cooler air.

• Jupiter's Wind Patterns:

  • Dominated by alternating dark (belts) and light (zones) bands that are semi-permanent but shift in intensity and position.

  • The pattern is stable across seasons due to Jupiter's small axial tilt.

  • Underlying east-west wind patterns are stable over decades.

  • An equatorial jet stream flows eastward at about $90$ meters per second ($300$ kilometers per hour).

  • Higher latitudes show alternating east- and west-moving streams, mirroring each other across the equator.

  • Light zones are regions of upwelling air and white ammonia cirrus clouds, indicative of upward convection.

  • Dark belts are regions where cooler atmosphere moves downward, completing the convection cycle, appearing darker because fewer ammonia clouds allow deeper visibility (possibly to ammonium hydrosulfide clouds).

• Saturn's Wind Patterns:

  • Exhibits a similar pattern to Jupiter but with a significantly stronger equatorial jet stream.

• Uranus's Wind Patterns:

  • Basic circulation is parallel to its equator, despite its $98^ ext{o}$ axial tilt.

  • The massive atmosphere's heat storage capacity minimizes the effect of 42-year periods of sunlight and darkness.

  • Atmospheric temperature can be slightly higher on the dark winter side, presenting a complex problem not fully understood.

• Neptune's Wind Patterns:

  • Characterized by strong east-west winds, comparable to Jupiter and Saturn.

  • Equatorial jet stream reaches speeds up to $2100$ kilometers per hour, exceeding Saturn's peak winds and approaching supersonic speeds.

5. Scale and Longevity of Storms on the Giant Planets

Learning Objective: Understand the scale and longevity of storms on the giant planets.

• Local Disturbances/Storms:

  • Superimposed on regular atmospheric circulation patterns.

  • Typically large, oval-shaped, high-pressure regions.

• Jupiter's Great Red Spot (GRS):

  • The largest and most famous storm, a reddish oval in the southern hemisphere.

  • Size: Measured $25,000$ kilometers long in 1979, shrinking to $20,000$ kilometers by 2000; its eventual disappearance is speculated.

  • Has persisted for over 300 years since telescopic observation began.

  • Juno spacecraft data suggests its depth is only a few hundred kilometers, challenging existing models.

  • It is a high-pressure region, unlike terrestrial storms which are low-pressure.

  • Rotates counterclockwise with a period of six days.

  • Three similar but smaller white ovals (Earth-sized) formed in the 1930s, with observed mergers (e.g., two merging in 1998).

• Longevity and Stability Factors:

  • Unlike Earth, Jupiter has no solid surface to slow down atmospheric disturbances, allowing them to persist.

  • The immense size of these disturbances inherently lends them stability.

  • Calculated lifetimes are centuries for the GRS and decades for white ovals, consistent with observations.

• Neptune's Great Dark Spot (GDS):

  • Exhibited features surprisingly similar to Jupiter's GRS despite Neptune's smaller size and different cloud composition.

  • Nearly $10,000$ kilometers long, formed at latitude $20^ ext{o} S$.

  • Rotated with a period of 17 days.

  • Was no longer detectable by the Hubble Space Telescope in the mid-1990s.

• Conclusion:

  • Jovian planets offer dramatic and spectacular weather phenomena.

  • Studying these atmospheric features provides insights into conditions on gas giants and helps improve understanding of Earth's weather.

• Example: Storms and Winds Calculation

  • To calculate wind speed, use the formula Speed = Distance / Time.

  • Earth Hurricane: Diameter = $400$ km, Rotation = $24$ hours. Circumference $( ext{distance}) = ext{approx. } 1250$ km. Speed $ext{at edge} ext{approx. } 52$ km/h.

  • Jupiter's Great Red Spot: Radius = $10,000$ km (Circumference $ext{approx. } 63,000$ km), Rotation = $6$ days ($144$ hours). Speed $ext{at outer} ext{edge} ext{approx. } 436$ km/h, much faster than Earth's winds.