Mankind’s Explanation: Jupiter's Red Spot
“THE
CLOUDS AND THE GREAT RED SPOT. Even a modest telescope can show much detail on
Jupiter. The region of the planet’s atmosphere seen from the Earth contains
several different types of clouds that are separated both vertically and
horizontally. Changes in these cloud systems can occur in a few hours, but an
underlying pattern of latitudinal currents has maintained its stability for
decades. It has become traditional to describe the appearance of the planet in
terms of a standard nomenclature for alternating dark belts and bright zones.
The currents, however, seen to have a greater persistence than this pattern.
The close-up views of Jupiter from the Voyager spacecraft revealed a variety of cloud forms, with a predominance of elliptical features reminiscent of cyclonic and anticyclonic storm systems on the Earth. All these systems are in motion, appearing and disappearing on time scales dependent on their sizes and locations. Also observed to vary are the pastel shades of various colours present in the cloud layers--from the tawny yellow that seems to characterize the main layer, through browns and blue-grays to the well-known salmon-coloured Great Red Spot, Jupiter’s largest, most prominent, and longest-lived feature (see Figure 44). This vertical and horizontal segregation of the cloud systems is evidently accompanied by chemical differences as well.
Jovian
meteorology can be compared with the global circulation of the Earth’s
atmosphere. On the Earth huge spiral cloud systems often stretch over many
degrees of latitude and are associated with motion around high-and low-pressure
regions. These cloud systems are much less zonally confined than the cloud
systems on Jupiter and move in latitude as well as longitude. Local weather on the Earth is often closely tied to the local
environment, which in turn is determined by the varied nature of the planet’s
surface.
Jupiter
has no solid surface, hence no topographic features, and the planet’s
large-scale circulation is dominated by latitudinal currents. The lack of a
solid surface with physical boundaries and regions with different heat
capacities makes the persistence of these currents and their associated cloud
patterns all the more remarkable. The Great Red Spot, for example, moves in
longitude with respect to all three of the rotation systems, yet it does not
move in latitude. The three white
ovals found at a latitude just south of the Great Red Spot exhibit similar
behaviour; white ovals of this size are found nowhere else on the planet. The
dark brown clouds, evidently holes in the tawny cloud layer, are found almost
exclusively at latitudes near +18 degrees.
The blue-gray or purple areas, from which the strongest thermal emission
is detected, only occur in the equatorial region of the planet.
THE
NATURE OF THE GREAT RED SPOT. The true nature of Jupiter’s unique Great Red
Spot was still unknown by the early 1990’s, despite extensive observations
from the Voyager spacecraft. On a planet whose cloud patterns have lifetimes
often counted in days, the Great Red Spot has survived as long as detailed
observations of Jupiter have been made--at least 100, and perhaps 300, years.
There is some evidence that the spot may be slowly shrinking, but a longer
series of observations is needed to confirm this suggestion. Its present
dimensions are about 26,000 by 14,000 kilometers, making it large enough to
accommodate, side by side, two planets the size of the Earth. These huge
dimensions are probably responsible for the feature’s longevity and possibly
for its distinct colour.
The
rotation period of the Great Red Spot with respect to the rotation of Jupiter
itself shows a variability that has not been successfully correlated with other
Jovian phenomena. Earth-based observations in 1966 and 1967 revealed the
counterclockwise circulation of the material within the spot itself to have a
period of 12 days. This period was confirmed by the Voyager observations, which
recorded a large number of interactions between the Great Red Spot and much
smaller disturbances moving in the current at the same latitude (Figure 43). The
Voyager pictures showed the interior of the spot to be remarkably tranquil, with
no clear evidence for the expected upwelling (or divergence) of material from
lower depths.
The
Great Red Spot, therefore, appears to be a huge anticyclone, a vortex, or eddy,
whose lateral dimensions are greater that the Earth’s diameter. This lateral
size is presumably accompanied by a huge vertical extent that allows the feature
to reach well below and well above the main cloud layers. The extension above
the main clouds can be observed directly, and it also is manifested by lower
temperatures and by less gas absorption above the Great Red Spot than at
neighboring regions on the planet.”
Information acquired within the quotes is from:
(Encyclopedia Britannica copyright 1997 15th edition macropaedia volume 27 pages
502-503)