Observations made by the imaging science experiment and the long-wavelength
infrared spectrometer aboard the Cassini spacecraft reveal that the large, longlived
cyclonic vortex at Saturn’s south pole has a 4200-km-diamter eye. The
eye has a 4 K warm core extending from the troposphere into the stratosphere,
concentric eyewall clouds extending 70 km above the internal clouds, and numerous
external clouds whose anticyclonic vorticity suggests a convective origin.
The large-scale relative vorticity outside the eye is near zero, and there are
no high clouds within the eye. The Saturn polar vortex has features in common
with terrestrial hurricanes and with the Venus polar vortex, and probably
resembles these two features more than any other large vortices on Venus,
Jupiter, and Earth.
2
Earth-based telescopic observations (1) revealed a hot spot at Saturn’s south pole in 2003.
Cassini imaging observations (2, 3) revealed cyclonic rotation around the spot in 2005.
To
explore this feature further, a series of high-resolution observations by Cassini were
planned for
a three-hour period on October 11, 2006. Figure 1 is a false-color image from those
observations
that shows cloud heights (4–6). The spatial resolution is 20 km/pixel. The central eye
looks
dark and red in Fig. 1. This indicates a nearly cloud-free upper atmosphere with some
low
clouds at the bottom. The blue-green ring outside the eye indicates high clouds and haze,
which
is consistent with air that has been lifted. The eye has two concentric boundaries. The
inner
boundary is oblong; the outer one is circular. They measure 2000 and 4200 km in
diameter,
respectively. Throughout this paper, latitudes are planetocentric and the geometry is that
of an
oblate spheroid.
The eyewall clouds cast shadows on the clouds inside the eye. Figure 2 demonstrates
that the shadows follow the sun in a counterclockwise direction as the planet turns during
the
three-hour period. From the shadow lengths, assuming the clouds inside the eye are flat
and
horizontal, we estimate the height of the outer wall as 40±20 km and the height of the
inner
wall as 70±30 km (7). The latter value is about twice the pressure scale height of Saturn’s
atmosphere. The eyewalls are consistent with rising motion, since clouds form on
updrafts.
From the opacity of Saturn’s atmosphere in the three wavelengths used in Fig. 1, it
appears that
the eyewall clouds extend up to the tropopause, which is at the 100 mbar level (7).
In Fig. 3A, the points represent the zonal velocity (positive eastward) of individual cloud
features. The average of the points is the mean zonal velocity ¯u. The peak value of ¯u is
150±20
m s−1, and it occurs near the outer eyewall between latitudes -86.5◦ and -89◦. The smooth
curves assume absolute vorticity ζ + f = f0 = constant poleward of latitude φ = φ0, where
ζ = 0 and ¯u = 0. Here ζ is the relative vorticity and f is the planetary vorticity - the
vertical
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Abstract
Solar system exploration reveals a variety of large vortices that resemble vortices on
Earth in some, but not all, respects. Here we present observations of Saturn's south polar
vortex (SPV) showing that it shares properties with terrestrial hurricanes - cyclonic
circulation (same direction as the planet's spin), a warm central region (the eye)
surrounded by a ring of high clouds (the eyewall), and convective clouds outside the ring.
It also shares properties with the polar vortices on Venus - polar location, cyclonic
circulation, warm center, and long lifetime. Neither Saturn nor Venus has oceans, so the
energy source is fundamentally different from that of terrestrial hurricanes. The SPV is
different also from mid-latitude vortices on the giant planets and from the polar vortices
on Earth. [125 words]
Start text
Earth-based telescopic observations (1) revealed a compact hot spot within 3 of Saturn's
south pole in 2003. The feature was unexpected, and the authors concluded that
dynamical as well as radiative forcing was required to explain the strong temperature
enhancement. Cassini imaging observations (2,3) revealed a 135 m s-1 eastward jet
surrounding the hot spot in 2005. The Cassini high-resolution observations reported here
were taken over a three-hour period on October 11, 2006. They reveal the similarities and
differences between this feature and atmospheric vortices throughout the solar system.
Figure 1 is a false-color image ....