In what way does the composition of Jupiter's clouds differ from those in the Earth's atmosphere?
Clouds on Earth are made of water; usually in the form of tiny liquid droplets, but sometimes as ice crystals, especially for the highest clouds.. Jupiter's topmost clouds have a different composition (ammonia), and are tiny crystals rather than liquid droplets. The same goes for Jupiter's next layer of cloud below the visible one, which is of ammonium hydrosulfide, but there is believed to be a (hitherto unproven) third layer of cloud made of water ice crystals below this (end of Section 1.5).
Jupiter's topmost cloud layer does not mark the top of the atmosphere, which continues for some hundreds of kilometres above the cloud deck, becoming ever more tenuous with altitude. The traditional interpretation of Jupiter's Hadley circulation (illustrated in Figure 5 and described in Section 1.5) is that the atmosphere rises in the zones and sinks in the intervening belts (which appear darker because the cloud tops are seen through a greater depth of atmosphere). However, thorough analysis of Jupiter images recorded between October 2000 and March 2001 by the Saturn-bound Cassini mission (Table 9.1) suggests that, on the contrary, the belts are where the atmosphere rises.
See Box 1 for the story as released by NASA to the press in March 2003 (too late to be incorporated in Teach Yourself Planets).
Rising Storms Revise Story of Jupiter's Stripes - March 6, 2003
Pictures of Jupiter, taken by a NASA spacecraft on its way to Saturn, are flipping at least one long-standing notion about Jupiter upside down.
Stripes dominate Jupiter's appearance. Darker 'belts' alternate with lighter 'zones'. Scientists have long considered the zones, with their pale clouds, to be areas of upwelling atmosphere, partly because many clouds on Earth form where air is rising. On the principle of what goes up must come down, the dark belts have been viewed as areas where air generally descends.
However, pictures from the Cassini spacecraft show that individual storm cells of upwelling bright-white clouds, too small to see from Earth, pop up almost without exception in the dark belts. Earlier spacecraft had hinted so, but not with the overwhelming evidence provided by the new images of 43 different storms.
'We have a clear picture emerging that the belts must be the areas of net-rising atmospheric motion on Jupiter, with the implication that the net motion in the zones has to be sinking,' said Dr Tony Del Genio, an atmospheric scientist at NASA's Goddard Institute for Space Studies, New York. 'It's the opposite of expectations for the past 50 years,' he said.
Del Genio is one of 24 co-authors from America and Europe reporting diverse results from the Cassini imaging of Jupiter in Friday's edition of the journal Science. Cassini's camera took about 26,000 images of Jupiter, its moons and its faint rings over a six-month period as the spacecraft passed nearby two years ago.
In fact, no one has actually measured Jupiter's atmosphere rising or sinking. The new theory announced in 2003 merely infers that the atmosphere in the belts is rising because the 'small' bright clouds within the belts that are referred to in the press release seem to indicate storm turbulence of a kind that ought to occur only during uprise, but not during sinking. Only time will tell whether this radically new interpretation of Jupiter's atmosphere becomes accepted. This is a timely reminder that little in planetary science is known with absolute certainty. Many mysteries remain, ranging from the apparently trivial, such as the scarcity of very small craters on Eros to the fundamental, such as this example of how the atmosphere of the biggest planet circulates.
The successive collisions of fragments of comet Shoemaker Levy 9 onto Jupiter captivated the world's media in July 1994, and you may remember these events yourself. A view of the tidally disrupted comet six months before the collision is shown in Figure 8 and you can watch a movie of the collision below.
Fragment A collision movie
This is a beta release of a movie of the Fragment A collision from Saturday 16 July 1994. It consists of 55 frames taken on Calar Alto (Spain).
The bright object to the right is the closest Galilean satellite Io, moving slowly towards Jupiter. The fainter oval structure in the southern hemisphere is the well-known Great Red Spot. The impact appears above the southeast (lower left) limb of the planet.
The bright spots near the Great Red Spot appearing in some frames are not real, but bad pixels, they will be corrected in the final release of the movie.
Try the following questions to help to test your understanding of Jupiter's atmosphere and interior before continuing with the rest of the course.
How does Jupiter's Hadley circulation differ from that in the atmospheres of the terrestrial planets, and what might be the main reasons for the differences? (For the purposes of this answer, you can assume that the 'traditional' interpretation of Jupiter's atmosphere is correct.)
What other factors distinguish Jupiter's atmosphere from these others?
(a) Jupiter has four Hadley cells in each hemisphere between the equator and the pole. The Earth has only three., Venus has only one per hemisphere, and Mars has an even simpler pattern with a single dominant cell rising at the sub-solar latitude and flowing across the equator towards the pole in the opposite hemisphere. The differences among the terrestrial planets are caused by Venus's very slow rotation, and the lack of oceans to even out solar heating on Mars. Jupiter's extra cell relative to the Earth is primarily because Jupiter rotates much faster than the Earth, which restricts how far a parcel of atmosphere can travel in a north-south direction before being deflected sideways into zonal (east-west) winds.
(b) Other differences are that Jupiter's atmosphere receives more energy from the planet's interior than from the Sun, and that it is much deeper, with no clearly defined base. The chemistry is different too: Jupiter's atmosphere is almost entirely hydrogen and helium (similar to the Sun) coloured with traces of hydrogen-bearing compounds, whereas the carbon dioxide and nitrogen common to all three main terrestrial planets occur in insignificant proportions. [Comment: This difference is because the atmospheres of the terrestrial planets originated by gas escaping from the rocky interior , whereas the atmospheres of the giant planets were captured directly from the solar nebula, thanks to these planets' much stronger gravity.]
What are the composition and physical state of Jupiter's interior thought to be at a depth of about 12000 km?
According to Section 1.4, at depths greater than about 10 000 km, Jupiter consists mostly of hydrogen having the properties of a molten metal, described as 'metallic hydrogen'. This zone continues much deeper than 12 000 km and at 12 000 km we can infer from Section 1.4 that the pressure must be greater than a million times the Earth's sea-level atmospheric pressure, and the temperature must be greater than 6000°C (which are the values quoted for 10000 km depth).
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