“Kepler's laws, although not rigidly true, are sufficiently near to the truth to have led to the discovery of the law of attraction of the bodies of the solar system. The deviation from complete accuracy is due to the facts, that the planets are not of inappreciable mass, that, in consequence, they disturb each other's orbits about the Sun, and, by their action on the Sun itself, cause the periodic time of each to be shorter than if the Sun were a fixed body, in the subduplicate ratio of the mass of the Sun to the sum of the masses of the Sun and Planet; these errors are appreciable although very small, since the mass of the largest of the planets, Jupiter, is less than 1/1000th of the Sun's mass.”
— AUTHOR, SOURCE
We've looked in some detail at Earth's atmosphere. Now we revist the topic in a more general context, trying to find the common factors that allow us to explain similarities and differences in the atmospheres of Venus, Earth, Mars, Titan, and the gas giants.
Planetary atmospheres, whether they are superficial (above a solid surface) as on Venus, Earth, Mars, and the moon Titan, or the structure of the planet, as on the gas giants, are driven by energy in the form of heat. Hot gasses expand against their environment, become less dense, and float upwards through any denser medium. As they expand, they cool down, become more dense, and sink into a less dense medium. This constant heating-cooling cycle creates currents away from and toward the center of the planet. If the planet is rotating (which all planets and moons in our solar system do), the rotation introduces a sideways or horizontal component to the circulation of the cell.
To see how this works on Earth, take a look at these diagrams on Atmospheric Circulation. While the solar energy available can be predicted as a function of sunlight spread over a given area as a function of latitude, heat sources from "inside" create differences, and surface variations such as mountains can redirect air flow. On Earth, continents and oceans absorb and reflect heat at different rates, which results in the formation of "thermals" or local upward flows of hotter, less dense air, which create low pressure areas that suck in air (and moisture) from the surrounding atmosphere, generating hurricanes and tornadoes. On other planets, local "hot spots" due to variations in heat convection within the planet create similar local thermal currents, resulting in similar types of cyclonic storms, such as the Giant Red Spot on Jupiter.
We can create models of atmospheric circulation by looking at the heat sources driving the flow of gases. Planetary interiors generate heat outwards in all directions. Solar heat, however, comes in at an angle to the planetary surface, so the influx of heat from the sun is concentrated on a small area when it hits a planetary surface at the ecliptic plane, but spread out over a larger area when it hits surfaces north and south of this plane. The inclination of the planet's rotational axis to the ecliptic plane can also introduce interesting issues: for Mars and Earth, inclinations near 25° create seasonal variations in the northern and southern hemispheres. For Uranus, with its inclination of 98°, the result is that one half of the planet is in constant sunlight while the other is in constant darkness for about 1/4 of the planet's revolution around the sun; then all parts of the planet receive equal amounts of sunlight and dark daily, and finally the sitaution is reversed, with the formerly sunlight side of the planet in darkness for about 21 years.
|All atmospheres have differential rotation above their rocky cores and liquid oceans (whether water or metallic hydrogen). This means that the upper layers of the atmosphere do not move as rapidly as lower layers, and that bands near the equator tend to move with greater angular rotation than bands near the poles. That is, a "cloud" near the equator will complete a rotation in less time than a band at a higher or lower latitude.
These variations in heat source locations, rotation rates, composition of layers result in complex atmospheric flows and persistent structure. A good example is the atmosphere of Jupiter, with its many bands which can flow at different rates, creating interferance and turbulance patterns. We have observed similar storms (although with less intensity) on Saturn and Neptune.
Naked Eye Observation
|Superior planet, brightest after Venus. Surface features (banding, Red Spot) and four largest moons easily observed with small 8X telescope.||Superior planet. Rings and ring divisions, moon Titan easily observed with 20X telescope.||Visible with the naked eye under optimum conditions, but because of faint magnitude and slow orbital period, not identified as planet until William Herschel plotted its orbit using a telescope in 1781.||Not visible with naked eye. Discovered when planetary gravitational field perturbed Uranus, leading astronomers Johann Galle and Heinrich d'Arrest to locate the "star" based on French astronomer Jean Le Verrier and English mathematician John Couch Adams' predictions.|
|Pioneer 10 (1973), Pioneer 11 (1974), Voyager 1 (1979), Voyager 2 (1979), Galileo (1995-2003), New Horizons (2014 FlyBy)||Pioneer 11 (1980), Cassini (2004-current)||Voyager 2 (1986)||Voyager 2 (1989)|
|Normal revolution, high speed of rotation (differential).||Normal revolution, high speed of rotation (differential)||Normal revolution, high speed of rotation (differential), but retrograde. Tilted 98°||Normal revolution, high speed of rotation (differential).|
|Magnetic field 19000X Earth's total field; 14X stronger at atmosphere top than Earth's surface field. Dynamo due to liquid metallic hydrogen ~ 20K times Earth's dynamo. Resulting magnetosphere drives solar wind and gases from Io's eruptions to form plasma.||Magnetic filed 570X Earth's total field; 0.67X as strong at atmosphere top than Earth's surface field. Dynamo 800 times Earth's. Magnetosphere 20% of Jupiter's. Lower plasma concentration (solar wind particles captured in rings, no erupting moon).||Magnetic filed 50X Earth's total field; 0.7X as strong at atmosphere top than Earth's surface field. Magnetic field tilted 60° to rotational axis and off-center from core.||Magnetic filed 35 Earth's total field; 0.4X as strong at atmosphere top than Earth's surface field. Magnetic field tilted 47° to rotational axis and off-center from core.|
Composition:H2: 86.2%, He: 13.6%, methane (CH4): < 0.2%; trace ammonia, water vapor.
Giant Red Spot, white ovals, brown oval storms.
Composition:H2: 96.3%, He: 3.3%, other: 0.4%.
Theory: He "rains" into Saturn's interior.
Composition:H2: 82.5%, He: 15.2%, CH4: 2.3%. Higher level of heavy elements than Jupiter or Saturn. No NH3 (freezes out).
Relatively featureless in visible light; storms discernable in IR. No internal heat to force convection. Upper atmosphere ~ 55K.
Composition:H2: 79%, He: 18%, CH4: 3%, no ammonia or water vapor (freezes out). Strong internal heat source; upper atmosphere ~ 55K. Banding, Great Dark spot visible in 1989 Viking pictures dissipated by 1994.
|Rarified system 129000km from center of Jupiter; mostly rock ~ 1μm.||System of rings extending from 92000km to 137000km from planetary center; mostly ice 1cm to 5m (with most around 10cm)||System of 13 narrow rings; mostly water and methane ice 0.1 - 10m in diameter darkened by radiation.||Mostly water and methane ice 10μm to 10m in diameter that have undergone radiation darkening.|
|Rocky core coated with ices: 2.6% by mass. Surrounded by liquid metallic hydrogen/liquid helium core shell, liquid hydrogen mantle, hydrogen/helium gas atmospheres.||Rocky core coated with ices: 10% by mass. Surrounded by liquid metallic hyrdogen/liquid helium core shell (but not as much as Jupiter), liquid hydrogen mantle, hydrogen/helium gas atmospheres.||Rocky core surrounded by highly compressed liquid water and ammonia, with outer mantle of liquid hydrogen and helium, hydrogen/helium gas atmospheres.||Larger rocky core surrounded by highly compressed liquid water and ammonia, with outer mantle of liquid hydrogen and helium, hydrogen/helium gas atmospheres.|
|Multiple competing theories to account for variation in size of cores, variation in amount of helium (missing from Saturn), presence of noble gas concentrations on Jupiter.||Varying theories to account for high level of heavier elements: either both planets formed closer to sun and were expelled outward, or both formed in current position but lost H2 and He in UV bombardment from nearby star (now gone).|
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