The Changing Face of Jupiter
J
upiter is a favourite target for users of small or moderate
telescopes. The main features are the belts and the
bright zones; there are also spots, wisps and festoons, with
the Great Red Spot often very much in evidence.
Jupiter’s rapid spin means that the markings are car-
ried from one side of the disk to the other in less than five
hours, and the shifts are noticeable even after a few min-
utes’ observation. Jupiter has differential rotation – that is
to say, it does not spin in the way that a rigid body would
do. There is a strong equatorial current between the two
main belts, known as System I; the main rotation period
is 9 hours 50 minutes 30 seconds, while over the rest of
the planet (System II) it is 9 hours 55 minutes 41 seconds.
However, various discrete features have rotation periods
of their own, and drift around in longitude, though the
latitudes do not change appreciably.
Generally there are two main belts, one to either side
of the equator. The North Equatorial Belt (NEB) is almost
always very prominent, and shows considerable detail, but
the South Equatorial Belt (SEB) is much more variable,
and has been known to become so obscure that it almost
vanishes, as happened for a while in 1993. The other
belts also show variations in breadth and intensity. Very
obvious colours can often be seen on the disk, due to pecu-
liarities of Jovian chemistry.
The most famous of all the features is the Great Red
Spot, which has been seen, on and off (more on than off)
ever since the first telescopic observations of Jupiter were
made, during the 17th century. It is oval, and at its maxi-
mum extent it may be 40,000 kilometres (25,000 miles)
long by 14,000 kilometres (8700 miles) wide, so that
its surface area is then greater than that of the Earth. At
times it may be almost brick-red, though at other times the
colour fades and the Spot may even disappear completely
for a few months or a few years. It forms a hollow in
the southern edge of the South Equatorial Belt, and this
hollow can sometimes be seen even when the Spot itself
cannot. Though its latitude is to all intents and purposes
constant at 22° south, the longitude drift over the past cen-
tury has amounted to 1200 degrees. Between 1901 and
1940 there was also a feature known as the South Tropical
Disturbance, which lay in the same latitude as the Spot
and took the form of a shaded area between white patches.
The rotation period of the South Tropical Disturbance was
shorter than that of the Red Spot, so that periodically the
Spot was caught up and passed, producing most interest-
ing interactions.
The Disturbance has vanished, and there is no reason
to suppose that it will return, but the Red Spot is still with
us, though it may be rather smaller than it used to be, and
it may not be permanent. For many years it was assumed
to be a solid or semi-solid body floating in Jupiter’s outer
gas, but the space missions have shown that it is a whirling
storm – a phenomenon of Jovian ‘weather’. It rotates anti-
clockwise, with a period of 12 days at its edge and 9 days
nearer its centre; the centre itself is 8 kilometres (5 miles)
above the surrounding clouds, and it is here that material
rises, spiralling outwards towards the edge. The cause of
the colour is not definitely known, but it may be due to
phosphorus, produced by the action of sunlight upon phos-
phine sent up from the planet’s interior. At any rate, the
Spot is decidedly colder than the adjacent regions.
Many other spots are seen, some of which are bright,
white and well-defined, but generally these features do not
last for long. The Great Red Spot itself may be so long-
lived simply because of its exceptional size.
Amateur observers have carried out important studies
of Jupiter. In particular, they make estimates of the rota-ATLAS OF THE UNIVERSE
▲ North Polar Region
Lat. 90° to 55° approx.
Usually dusky in appearance
and variable in extent.
The whole region is often
featureless. The North Polar
Current has a mean period
of 9hrs 55mins 42secs.
North North North
Temperate Belt
Mean Lat. 45°
An ephemeral feature often
indistinguishable from the
NPR.
North North Temperate Zone
Mean Lat. 41°
Often hard to distinguish
from the overall polar
duskiness.
North North Temperate Belt
Mean Lat. 37°
Occasionally prominent,
sometimes fading altogether,
as in 1924.
North Temperate Zone
Mean Lat. 33°
Very variable, both in width
and brightness.
North Temperate Belt
Mean Lat. 31° to 24°
Usually visible, with a
maximum extent of about
8° latitude. Dark spots at
southern edge of the North
Temperate Belt are not
uncommon.North Tropical Zone
Mean Lat. 24° to 20°
At times very bright. The
North Tropical Current,
which overlaps the North
Equatorial Belt, has a period
of 9hrs 55mins 20secs.
North Equatorial Belt
Mean Lat. 20° to 7°
The most prominent of all
the Jovian belts. This region
is extremely active and has
a large amount of detail.
Equatorial Zone
Mean Lat. 7° to 7°
Covering about one-eighth
of the entire surface of
Jupiter, the EZ exhibits
much visible detail.
Equatorial Band
Mean Lat. 0.4°
At times the EZ appears
divided into two components
by a narrow belt, the EB, at
or near to the equator of
Jupiter.
South Equatorial Belt
Mean Lat. 7° to 21°
The most variable belt.
It is often broader than
the NEB and is generally
divided into two components
by an intermediate zone.
The southern component
contains the Red Spot
Hollow (RSH).South Tropical Zone
Mean Lat. 21° to 26°
Contains the famous Great
Red Spot. The STrZ was the
site of the long-lived South
Tropical Disturbance.
Great Red Spot
Mean Lat. 22°
Although there are other
spots visible on Jupiter’s
surface, both red and white,
the Great Red Spot is much
the most prominent. It
rotates in an anticlockwise
direction.
South Temperate Belt
Mean Lat. 26° to 34°
Very variable in width and
intensity; at times it appears
double.
South Temperate Zones
Mean Lat. 38°
Often wide; may be
extremely bright. Spots
are common.
South South Temperate Belt
Mean Lat. 44°
Variable, with occasional
small white spots.
South South South
Temperate Belt
Mean Lat. 56°
South Polar Region
Lat. 58° to 90° approx.
Like the NPR, very variable
in extent.C Atl of Univ Phil'03stp 2/4/03 3:08 pm Page 96