18 AUSTRALIAN SKY & TELESCOPE APRIL 2016
Length of Great Red Spot (longitude)1880 1900 1920 1940 1960 1980 2000 202015º10º20º25º30º 0.14º
per year0.19º
per year35º40º Visible length (telescopic)
Visible length (spacecraft)LESS AND LESSThe GRS’s length has been shrinking for at least a century, but
the ratehasacceleratedduringthepastthreedecades.
TRACKING
THE WINDS
By using NASA’s
Galileo orbiter to
record the GRS
at near-infrared
wavelengths,
researchers
concluded that the
upwelling gas in
the spot’s interior
is some 3 0 km
higher than the
peripheral collar.
As the gas moves
from areasofhigh
pressure outward
(black arrows),
the Coriolis effect
causes it to spin
counterclockwise
rapidly along its
perimeter (orange
arrows). Some
of that rotational
energy probably
transferstothe
latitudinally
confinedwindjets
flowing past to
the storm’s north
and south (white
arrows).
PERIODIC SCRUTINY
Hubble Space Telescope
enhanced-colour views
from 1995, 2009 and 2014
show both the GRS’s
shrinking size and the
varying intensity of its hue
over two decades.The present
We have more than20 years of Hubble Space
Telescope observations of the GRS. Throughout HST’s
mission, the storm has gotten noticeably smaller and
rounder. Composite images also reveal that the spot
does actually change colour quite dramatically. This is
one key to understanding why the storm is changing.
Jupiter’s cloudtops are dominated by bands
of alternating winds that flow roughly along the
boundaries between dark belts and bright zones.
Storms typically ‘roll’ between these counterflowing
jets like ball bearings in their races. In fact, we
suspect that large vortices such as the GRS help to
maintain the adjacent wind jets by adding convective
energy. Conversely, vortices might draw some energy
from the wind jets by ingesting small eddies.
This is an active area of study, as we try to
understand how waves and vortices are related to
Jupiter’s very stable wind pattern. The GRS is nestled
between a westward-moving jet on the northern edge
of the South Tropical Zone (STrZ) and an eastward jet
on its southern edge. Since the GRS slightly overfills
the STrZ region, those wind jets are deflected around
the big oval, which causes turbulence and cloud
mixing as the flows return to their normal latitudes
after going by.
Small eddies on either of those deflected wind jets can
be drawn into the flow of the GRS as they pass by and
subsequently dragged into its interior. While it’s unclear
if these additions help sustain the storm, they do affect its
colour.Aseddiesareingested and sheared apart, fresh,
whiter material often appears within the GRS. Ingesting
multiple eddies can turn the storm a very pale colour, as
occurred in 20 1 2. Conversely, when no eddies accompany
the jets, the spot’s colour can become quite intense —
as it did during a widely observed fadeout of the South
Equatorial Belt (SEB) in 2010.
In 2014, after a rapid decrease in the spot’s length,
observers noticed that its colour had intensified, even
though there were still eddies on the jets nearby.
Perhaps these eddies could no longer enter the flow of
the GRS for some reason. Why might this be?NASA / ESA / ZOLT LEVAYS&T:GREGGDINDERMAN SOURCE:NASA/JPLS&T:L. TISCIONE; SOURCE: A. SIMON ET AL. /ASTROPHYSICAL JOURNALGRS in 1995 GRS in 2009 GRS in 2014internal structure consists of a high-velocity collar
surrounding a somewhat stagnant core. A cloud in
this collar can complete a full circuit around the spot’s
perimeter in about three days. Some have suggested
that using the location of the high-velocity collar is a
better measure of the GRS’s size. Although this radius
is smaller than the coloured region, it too is shrinking
over time.Shrinking Storm