Jupiter Clouds in Depth
December 29, 2000
Images from NASA's Cassini spacecraft using three different filters reveal
cloud structures and movements at different depths in the atmosphere
around Jupiter's south pole.
Cassini's cameras come equipped with filters that sample three wavelengths
where methane gas absorbs light. These are in the red at 619 nanometer
(nm) wavelength and in the near-infrared at 727 nm and 890 nm. Absorption
in the 619 nm filter is weak. It is stronger in the 727 nm band and very
strong in the 890 nm band where 90 percent of the light is absorbed by
methane gas. Light in the weakest band can penetrate the deepest into
Jupiter's atmosphere. It is sensitive to the amount of cloud and haze down
to the pressure of the water cloud, which lies at a depth where pressure
is about 6 times the atmospheric pressure at sea level on the Earth).
Light in the strongest methane band is absorbed at high altitude and is
sensitive only to the ammonia cloud level and higher (pressures less than
about one-half of Earth's atmospheric pressure) and the middle methane
band is sensitive to the ammonia and ammonium hydrosulfide cloud layers as
deep as two times Earth's atmospheric pressure.
The images shown here demonstrate the power of these filters in studies of
cloud stratigraphy. The images cover latitudes from about 15 degrees north
at the top down to the southern polar region at the bottom. The top two
images are ratios, the image in the methane filter divided by the image at
a nearby wavelength outside the methane band. Using ratios emphasizes
where contrast is due to methane absorption and not to other factors, such
as the absorptive properties of the cloud particles, which influence
contrast at all wavelengths.
The most prominent feature seen in all three filters is the polar
stratospheric haze that makes Jupiter bright near the pole. The equatorial
band is also very bright in the strong 890-nm image and to a lesser extent
in the 727 band (middle image) but is subdued in the weak 619-nm image at
the top. These are high, thin, haze layers that are nearly transparent at
wavelengths outside the methane absorption bands. Another prominent
feature is the Great Red Spot. About a third of it appears at the
right-hand edge of the frame. It is a bright feature in methane absorption
because it has extensive cloud cover reaching to high altitude. A wisp of
high thin cloud can be seen trailing off its western rim in the middle and
Features mentioned above have been seen from ground-based telescopes, from
NASA's Hubble Space Telescope and from NASA's Galileo spacecraft. This is
the first high-resolution image in all three methane bands, and a
comparison of all three reveals some interesting features. Chief among
these is the very dark patch seen in the top (weak methane) image near the
top-middle of the frame. It is almost invisible in the bottom image and it
appears to be composed of strands of bright clouds in the middle image.
This is a region similar to the hot spot where the Galileo Probe entered
Jupiter's atmosphere in 1995. These images indicate that cloud cover is
present at the higher altitudes but absent from the lower altitudes. This
is also what the Galileo Probe found when it entered Jupiter's atmosphere.
To the northwest (above and to the left) of the dark feature is a small
cloud that is bright in the 619-nm (top) image but has no contrast at the
other wavelengths. This is the signature expected for a thick water cloud.
Another feature seen only in the weak-methane (top image) ratio is a dark
ring near the center of the image. This feature is probably a
counter-clockwise rotating, upwelling core surrounded by a sinking
perimeter with diminished cloudiness. The fact that it is seen only in the
weak methane ratio indicates the effects of a lower-level circulation that
does not penetrated to the upper ammonia cloud level and may be confined
to the deeper water cloud.
The opposite behavior is evident in an oval storm that appears dark in the
middle and bottom images but is absent in the weak, 619-nm image. It is
located to the southwest of the Great Red Spot. Further to the west at
slightly more northerly latitudes are a series of small spots that are
dark at all wavelengths. These and a myriad of other contrast features at
many latitudes reveal much about Jupiter's complicated cloud structure and
Additional information about Cassini is available online at:
The Cassini spacecraft is scheduled to arrive at Saturn in July 2004 to
begin a four-year exploration of the ringed planet and its moons. The
Cassini mission is managed by NASA's Jet Propulsion Laboratory in Pasadena,
Calif., for NASA's Office of Space Science, Washington, D.C. JPL is a
division of the California Institute of Technology in Pasadena.
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