Sky and Telescope - March 2016 - 20

Visible length (telescopic)
Visible length (spacecraft)

Length of Great Red Spot (longitude)

40º
35º
0.14º
per year

30º
25º
20º

0.19º
per year

15º
10º
1880

1900

1920

1940

1960

1980

2000

S&T: L. TISCIONE; SOURCE: A. SIMON ET AL. / ASTROPHYSICAL JOURNAL

Jupiter's Shrinking Storm

2020

S&T: GREGG DINDERMAN, SOURCE: NASA / JPL

LESS AND LESS The GRS's length has been shrinking for
at least a century, but the rate has accelerated during the past
three decades.

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
30 km higher than the peripheral collar. As the gas moves from
areas of high pressure outward (black arrows), the Coriolis force
causes it to spin counterclockwise rapidly along its perimeter
(orange arrows). Some of that rotational energy probably transfers to the latitudinally confined wind jets flowing past to the
storm's north and south (white arrows).

NASA / ESA / ZOLT LEVAY

GRS in 1995

20

March 2016 sky & telescope

GRS in 2009

the high-velocity collar is a better measure of the GRS's
size. Although this radius is smaller than the colored
region, it too is shrinking over time.

The Present
We have more than 20 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 color 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 westwardmoving 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 color. As eddies are ingested and sheared apart,
fresh, whiter material often appears within the GRS.
Ingesting multiple eddies can turn the storm a very pale
color, as occurred in 2012. Conversely, when no eddies
accompany the jets, the spot's color 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 color 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?

GRS in 2014

PERIODIC SCRUTINY
Hubble Space Telescope
enhanced-color views
from 1995, 2009, and 2014
show both the GRS's
shrinking size and the
varying intensity of its
hue over two decades.



Sky and Telescope - March 2016

Table of Contents for the Digital Edition of Sky and Telescope - March 2016

Contents
Sky and Telescope - March 2016 - Cover1
Sky and Telescope - March 2016 - Cover2
Sky and Telescope - March 2016 - 1
Sky and Telescope - March 2016 - Contents
Sky and Telescope - March 2016 - 3
Sky and Telescope - March 2016 - A
Sky and Telescope - March 2016 - B
Sky and Telescope - March 2016 - 4
Sky and Telescope - March 2016 - 5
Sky and Telescope - March 2016 - 6
Sky and Telescope - March 2016 - 7
Sky and Telescope - March 2016 - 8
Sky and Telescope - March 2016 - 9
Sky and Telescope - March 2016 - 10
Sky and Telescope - March 2016 - 11
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Sky and Telescope - March 2016 - Cover3
Sky and Telescope - March 2016 - Cover4
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