Sky & Telescope - July 2023 - 52

JULY 2023 OBSERVING
Exploring the Solar System
by Thomas A. Dobbins and William Sheehan
Suspense at Saturn
Will observers be treated to an atmospheric
display this apparition?
W
e're overdue for a recurrence of
one of the ringed planet's most
arresting spectacles. Known as " Great
White Spots " by analogy to Jupiter's
iconic Great Red Spot, Saturn's colossal
storms appear suddenly at intervals of
about three decades and can expand to
encircle the entire planet.
Shorn of its magnificent rings, Saturn
would appear as a paler version of
Jupiter. While Saturn's dusky belts and
bright zones resemble Jupiter's richly
detailed cloudscape, they are comparatively
muted in contrast and less sharply
defined. The blandness of features on
Saturn is largely attributed to the planet's
lower temperatures, arising from
its greater distance from the Sun. As a
result, Saturnian clouds form deeper in
its atmosphere.
Saturn's weaker gravity greatly reinforces
this thermal effect. Although the
planet is just 15% smaller in diameter
This series of images recorded by NASA's
Cassini orbiter shows the evolution of the 2010
Great White Spot over an 8-month period.
Dec 5, 2010
than Jupiter, it's only 30%
as massive. The Jovian
atmosphere is pulled
much more powerfully
toward the planet's center,
so the pressure gradient
is almost three times
steeper than in Saturn's
atmosphere.
Nevertheless, the
to Jupiter, but its wind speeds are up to
four times higher and its jet streams are
much broader as well. Vertical convection
in Saturn's atmosphere is also far
more pronounced than within Jupiter
thanks to the lower gravity and greater
buoyancy. These factors play a key role
in generating the massive atmospheric
upwellings of its Great White Spots.
Throughout observational history,
ringed planet's distended
atmosphere is similar
in both structure and
composition to Jupiter's, with a deck of
water clouds at the bottom, a layer of
ammonium hydrosulfide clouds in the
middle, and clouds of frozen ammonia
tinted by traces of sulfur and phosphorous
compounds at the top. Above
the ammonia clouds, Saturn's upper
troposphere and stratosphere contain a
murky haze of photochemical smog that
washes out details and subdues colors.
These overlying aerosols obscure
very dynamic meteorological activity.
Saturn may appear quiescent compared
Jan 2, 2011
The intense 1990 equatorial
storm imaged by the
Hubble Space Telescope on
November 9th of that year
we've witnessed just six of these spots,
occurring in 1876, 1903, 1933, 1960,
1990, and 2010. The 1903, 1960, and
2010 Great White Spots
appeared at northern
latitudes of 36°, 58°, and
38°, respectively. The far
more spectacular events
of 1876, 1933, and 1990
formed in Saturn's Equatorial
Zone. This pattern
suggests a cycle of roughly
30 years for outbreaks
occurring alternately in
the Equatorial Zone and
at higher temperate latitudes,
though the seemingly
premature 2010 event broke the
pattern. Curiously, each of the Equatorial
Zone outbreaks first appeared at a
hotspot located at a longitude extending
from 290° to 315°. This calls to mind
the vents that serve as the sources of
Jupiter's South Equatorial Belt revivals
(S&T: Sept. 2010, p. 50).
All of these Great White Spots
occurred during summer in Saturn's
northern hemisphere, strongly suggesting
a seasonal correlation. Although
heating by the distant Sun is rather
Feb 25, 2011
52 JULY 2023 * SKY & TELESCOPE
1990 EQUATORIAL STORM: NASA / JPL / STSCI; SERIES OF SIX
SATURNS: NASA / JPL-CALTECH / SPACE SCIENCE INSTITUTE
Sky & Telescope - July 2023

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