Observed Atmospheres Of The Giant Planets

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The observable atmospheres of the giant planets are dominated by molecular hydrogen and helium, in proportions roughly similar to that found in the Sun. The abundance of "heavy" elements (which in this context refers to elements heavier than helium) is found, or estimated, to be approximately 3-5 times the solar value for Jupiter, ~10 times the solar value for Saturn, increasing to 30-50 times the solar value

Figure 1.2. Total thermal-infrared radiation flux (Wm~2) emitted by the giant planets as a function of latitude (Ingersoll, 1990). While some belt/zone variations are visible, the emitted flux is to a first approximation independent of latitude. The radiation is emitted predominantly from the 0.3 bar to 0.5 bar pressure levels. On the right-hand axis the radiative flux has been converted to brightness temperature (the temperature of a black body that would emit the same flux). Reprinted with permission from Ingersoll (1990). Copyright 1990 American Association for the Advancement of Science.

for Uranus and Neptune. As we shall see in Chapter 2, the generally favored interpretation of this and the mean size and density measurements is that the outer planets accreted originally from icy planetesimals and became so massive that they were able to attract gravitationally hydrogen and helium from the solar nebula. It would appear that Jupiter and Saturn grew large enough and rapidly enough to capture a huge mass of hydrogen and helium, while Uranus and Neptune were not able to attract so much. Hence, the abundance of icy materials is higher in Uranus and Neptune than in Jupiter and Saturn. In the upper, cooler parts of the giant planet atmospheres that are actually observable, these heavy elements are mainly present in their fully reduced form and thus after H2 and He the next most abundant molecules inferred or measured (prior to any condensation) are, in decreasing order, H20, CH4, NH3, and H2S. In fact, the upper atmospheres of the giant planets are so cold that H20, H2S, and NH3 condense at various levels forming the cloud decks observed on these giant planets. The upper atmospheres of Uranus and Neptune are so cold that even CH4 condenses.

The observed atmospheres of the giant planets reveal many very interesting properties which will be briefly described here, and expanded upon in Chapters 2 to 5.

1.2.1 Jupiter

Through a telescope, Jupiter appears as a dusky ochre-colored oblate planet with dark horizontal stripes aligned parallel to the equator. Two of these dark "belts" are especially noticeable on either side of the brighter equatorial "zone", with other thinner "belts" seen closer to the poles. In fact, the atmosphere of Jupiter has the

Figure 1.4. Jovian zonal nomenclature (Irwin, 1999). Reprinted with kind permission of Kluwer Academic.

most color contrast of any atmosphere in the solar system, including that of the Earth's (Dowling, 1997); an image of Jupiter recorded by the Cassini spacecraft in 2000 is shown in Figure 1.3 (see color section). The general belt/zone structure appears to be very stable and a universally accepted naming scheme is shown in Figure 1.4. Although the general structure is long-lived, the contrast of the different features varies with time. These changes are usually gradual, although the South Equatorial Belt (SEB) often displays dramatic outbursts of cloud activity (Dowling, 1997; Rogers, 1995), the most recent upheaval occurring in 2007. The belt/zone structure is generally thought to be formed by a global circulation system that upwells moist air in the "zones", forming bright cloudy regions and subsides in the belts, forming relatively cloud-free regions that appear dark in the visible, although it has been suggested that this circulation reverses at deeper levels (as we shall see in Chapter 5). The upper observable cloud deck is almost certainly predominantly composed of ammonia crystals, although we shall see in Chapter 4 that these appear to be modified in some way such that their pure spectral absorption features are usually masked. Above the main cloud decks, various processes such as photochemistry act to create hydrocarbon haze particles, of uncertain composition, which gradually settle down through the atmosphere and are eventually pyrolyzed and destroyed at deeper levels.

In addition to the general zonal structure, Jupiter is found to have a number of large oval structures or vortices. Unlike three-dimensional turbulence where one expects a large eddy to split up into smaller ones (a good example is a smoke ring, which can be observed to rapidly dissipate), weather systems are governed predominantly by two-dimensional turbulence, which has the counter-intuitive property that smaller eddies merge into larger ones by a process known as the backwards energy cascade. The most famous of these ovals is the Great Red Spot

(GRS). The GRS is a vast anticyclonic weather system that is currently ^20,000 km wide in the east-west direction and ^ 12,000 km in the north-south, making it large enough that the Earth would easily fit in the middle! Winds in the center of the spot are light, but increase rapidly towards the edge of the spot, reaching speeds of 100 m/ s. The GRS appears to be extremely long-lived. Robert Hooke (1635-1703) first reported a large spot in 1665 and "Hooke's spot" was subsequently observed intermittently from 1664 to 1701. Although this may have been the GRS itself, continuous observations of the current GRS can be traced back only to 1831 (Dowling, 1997; Simon-Miller et al., 2002). Indeed it has been argued that Hooke's spot became unstable and dissipated, only for the current GRS to form later (e.g., Simon-Miller et al., 2002). The current GRS has undergone numerous changes since observations began. For example, it became nearly invisible in the 1860s, but within 10 years was very prominent again. It was particularly prominent during the Pioneer and Voyager flybys. The GRS is currently shrinking in the longitudinal direction. A hundred years ago the east-west diameter of the spot was ^46,000 km, almost twice as wide as it is today. If the GRS continues to shrink in the east-west direction at the current rate then by 2040 the spot will be circular. We shall see in Chapter 5 that such a configuration is thought unlikely to be stable and thus the GRS may actually break up and disappear in our lifetimes, perhaps to spawn the generation of a new great oval!

Other well-known ovals include the South Tropical Belt-South (STrBs) white ovals and the North Equatorial Belt-North (NEBn) brown barges. The current STBs white ovals initiated as a disturbance in the South Temperate Zone (STZ) in 1939 and coalesced into three white-colored ovals. Two of these ovals merged together in 1998, and in March 2000 the two remaining ovals merged to form a single white oval, which took on the same red color as the GRS in 2005. While 90% of the Jovian vortices are anticyclonic, only 10% are cyclonic and the most well-known of these are the NEBn brown barges, which appear at the boundary between the NEB and the North Tropical Zone (NTrZ) and were particularly prominent during the Voyager encounters.

We shall see in Chapter 5 that the winds on the giant planets blow almost entirely in the zonal direction (i.e., east to west, or west to east), and the winds alternate in direction in association with the belts and zones. The zonal wind speed on Jupiter varies particularly rapidly with latitude and is puzzlingly strong at the equator, reaching speeds of 100 m/s in the eastward direction. The equatorial region of Jupiter is thus super-rotating (i.e., rotates faster than the bulk of the planet), a state that is difficult to simulate with numerical models pointing to considerable underlying complexity (as we shall see in Chapter 5). Observations of ovals and other atmospheric features by early astronomers such as Jean-Dominique (a.k.a. Gian-Domenico) Cassini (1625-1712) had to be referred to a longitude system and as the equator rotates at a noticeably faster rate than the rest of the planet due to the high wind speeds there, two conventions arose. The System I frame referred to features at equatorial latitudes within 10° of the equator, while the System II frame referred to all other latitudes. Both systems have since been superseded by System III, which is referenced to the bulk rotation of the interior as inferred from radio observations of the rotation of the magnetosphere.

Satellite

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Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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Responses

  • monika
    Why is the 1664 grs so long lived?
    8 years ago

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