The Imaging Science System (ISS) cameras come equipped with filters that sample three
wavelengths where methane gas absorbs light. These are in the red at
619 nano-meter (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% of the light is
absorbed by methane gas. Light in the weakest band can penetrate the
deepest into Jupiter’s atmosphere and is sensitive to the amount of
cloud and haze down to the pressure of the water cloud (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 1/2 of an atmosphere) and the
middle methane band is sensitive the ammonia and ammonium hydrosulfide
cloud layers as deep as 2 times atmospheric pressure. The images
shown here demonstrate the power of these filters in studies of cloud
stratigraphy. The images cover latitudes from about 15 N 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. By taking ratios we are
able to isolate contrast 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 which 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 lowest images.
Features mentioned above have been seen from ground-based telescopes
and from the
Hubble Space Telescope and from the Galileo spacecraft. This is the
first time we have had high-resolution images 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 which is
similar to the hot spot where the Galileo Probe entered Jupiter’s
atmosphere. 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 which 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 an anticyclonic (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 that we are seeing the effects of a lower-level circulation
which 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 which is 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
which 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 meteorology.
Credit: NASA/JPL/University of Arizona
Released: December 28, 2000