Sky & Telescope - July 2023 - 53

SKETCH: WILL HAY, COURTESY OF THE ROYAL
ASTRONOMICAL SOCIETY / MARTIN MOBBERLEY
feeble compared to the planet's internal
store of heat, the effect of its 27° axial
tilt is greatly augmented by the shadow
cast by the rings, which blocks sunlight
from reaching the tropical and temperate
latitudes for long periods of time
(S&T: Sept. 2021, p. 52). Saturn ponderously
circles the Sun once every 29.4
years, so each season lasts more than
seven Earth years.
The first indication of a Great White
Spot is the appearance of a tiny, brilliant
" pearl. " This is typically followed
by multiple, closely spaced eruptive
nuclei, which are violent convective
upwellings reminiscent of the cumulonimbus
towers of terrestrial thunderstorms.
When instruments aboard
NASA's Cassini spacecraft monitored
the 2010 storm from Saturnian orbit,
they detected intense radio static emitted
by lightning generated in its fierce
turbulence.
The warm plume cools as it expands,
causing billowing clouds of white
ammonia crystals to freeze out high
above the main cloud canopy. The visible
wake of the intense storm expands
very rapidly in an eastward direction
from its trailing end and within only
a week can extend to a length of more
than 100,000 kilometers (62,100 miles)
to become one of the most striking
planetary phenomena visible through
amateur telescopes. The equatorial
Great White Spots were visible even
through apertures as small as 3 inches.
As they continue to expand, the
clouds slowly lose contrast with their
surroundings. Zonal winds shear them
Apr 22, 2011
into increasingly dissipated streaks that
eventually blend into the surrounding
zones. Although expansion in latitude
is much less pronounced, these storms
can displace or even obliterate adjoining
belts. Activity usually subsides after
several months, although outbreaks in
the Equatorial Zone persisted for 4 years
following the Great White Spot of 1990.
In 2015, Caltech planetary scientist
Andrew Ingersoll proposed a detailed
mechanism for the seasonal genesis of
the Great White Spots. He suggested
that when Saturn's upper atmosphere
undergoes seasonal cooling, it initially
becomes less dense as water vapor condenses
and rains out. After achieving a
density minimum, there is a reversal as
the remaining desiccated air (composed
almost entirely of hydrogen and helium,
the lightest of all gases) continues to
cool. Convection is suppressed when the
density of the upper atmosphere is low
but resumes when its density increases
sufficiently. It's this interaction of
descending cold, dry air with rising
warm, moist air that triggers titanic
storms. The decades-long lag before the
onset of storms arises from the very
slow rate at which Saturn's massive
atmosphere radiates heat into space.
Comparable planet-encircling
storms don't occur on Jupiter because
Jupiter's axial tilt is only 3°, so seasonal
effects are practically nonexistent.
In addition, Jupiter's upper atmosphere
is drier than Saturn's, which
is replenished by the water supplied
by a steady rain of ice particles from
the ring system. A lesser though still
May 18, 2011
British comedian and amateur astronomer
Will Hay discovered a Great White Spot
outbreak using his 6-inch Cooke refractor on
August 3, 1933.
significant source of water comes from
geysers on the icy moon Enceladus.
No other satellite in the solar system
is known to influence the chemical
composition of its parent planet.
Summer solstice in Saturn's northern
hemisphere occurred in May 2017,
and the autumnal equinox is coming
up in May 2025. It's high summer right
now in Saturn's northern hemisphere,
and it's been over three decades since a
Great White Spot appeared in the planet's
Equatorial Zone. Observers should
monitor this region closely during the
current Saturn observing season. If you
need any encouragement, bear in mind
that of the six Great White Spots we
have witnessed, four were first detected
by vigilant amateur astronomers.
¢Despite anticipating storms on Saturn,
Contributing Editors TOM DOBBINS and
BILL SHEEHAN don't consider themselves
meteorologists.
Aug 12, 2011
sk yand tele scope .o r g * JULY 2023 53
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