Scientists assume Jupiter's clouds are composed primarily of ammonia, but
only about 1% of the cloud area displays the characteristic spectral
fingerprint of ammonia. This composite of infrared images taken by the New
Horizons Linear Etalon Infrared Spectral Imager (LEISA) captures several
eruptions of this relatively rare breed of ammonia cloud and follows the
evolution of the clouds over two Jovian days. (One day on Jupiter is
approximately 10 hours, which is how long it takes Jupiter to make one
complete rotation about its axis.)
The New Horizons spacecraft was still closing in on the giant planet when
it made these observations: Jupiter was 3.4 million kilometers (2.1
million miles) from the New Horizons spacecraft for the LEISA image taken
at 19:35 Universal Time on February 26, 2007, and the distance decreased
to 2.5 million kilometers (1.6 million miles) for the last image shown.
LEISA's spatial resolution scale varied from approximately 210 kilometers
(130 miles) for the first image to 160 kilometers (100 miles) for the last
one.
New Horizons scientists originally targeted the region slightly northwest
(up and to the left) of the Great Red Spot to search for these special
ammonia clouds because that's where they were most easily seen during
infrared spectral observations made by the Galileo spacecraft. But unlike
the churning, turbulent cloud structures seen near the Great Red Spot
during the Galileo era, this region has been quieting down during the past
several months and was unusually tranquil when New Horizons passed by.
Nevertheless, LEISA managed to find other regions of fresh, upwelling
ammonia clouds, and the temporal evolution of one such region is displayed
in this figure. In the first image, a fresh ammonia cloud (the blue
region) sprouts from between white clouds and a dark elongated region.
This blue cloud subsequently stretches along the white-dark border in the
next two images.
These fresh ammonia clouds trace the strong upwelling of gases from the
largely hidden depths of Jupiter to higher altitudes. Presumably, water is
also being dragged up from below, and the subsequent condensation of that
water, which is far more abundant than ammonia in Jupiter's atmosphere,
into cloud droplets energizes the lower troposphere.
LEISA produces images at infrared wavelengths, which is heat radiation
that cannot be sensed by the human eye. These "false color" images were
produced by putting images of Jupiter at wavelengths of 1.99 micrometers,
1.94 micrometers and 2.04 micrometers into the red, green and blue
channels, respectively, of the image display. Ammonia has an absorption
feature at 1.99 microns, and when the colors are combined in this way the
fresh ammonia clouds take on a bluish hue.
