FOR RELEASE:  November 20, 1997

PHOTO NO.:  STScI-PRC97-36B


HUBBLE SPOTS NORTHERN HEMISPHERIC CLOUDS ON URANUS

Using visible light, astronomers for the first time this 
century have detected clouds in the northern hemisphere of 
Uranus. The newest images, taken July 31 and Aug. 1, 1997 
with NASA Hubble Space Telescope's Wide Field and Planetary 
Camera 2, show banded structure and multiple clouds. Using 
these images, Dr. Heidi Hammel (Massachusetts Institute of 
Technology) and colleagues Wes Lockwood (Lowell Observatory) 
and Kathy Rages (NASA Ames Research Center) plan to measure 
the wind speeds in the northern hemisphere for the first time.

Uranus is sometimes called the "sideways" planet, because 
its rotation axis is tipped more than 90 degrees from the 
planet's orbit around the Sun. The "year" on Uranus lasts 
84 Earth years, which creates extremely long seasons - 
winter in the northern hemisphere has lasted for nearly 20 
years. Uranus has also been called bland and boring, because 
no clouds have been detectable in ground-based images of the 
planet. Even to the cameras of the Voyager spacecraft in 
1986, Uranus presented a nearly uniform blank disk, and 
discrete clouds were detectable only in the southern 
hemisphere. Voyager flew over the planet's cloud tops near 
the dead of northern winter (when the northern hemisphere 
was completely shrouded in darkness).

Spring has finally come to the northern hemisphere of 
Uranus. The newest images, both the visible-wavelength ones 
described here and those taken a few days earlier with the 
Near Infrared and Multi-Object Spectrometer (NICMOS) by 
Erich Karkoschka (University of Arizona), show a planet with 
banded structure and detectable clouds.

Two images are shown here. The "aqua" image (on the left) is 
taken at 5,470 Angstroms, which is near the human eye's peak 
response to wavelength. Color has been added to the image to 
show what a person on a spacecraft near Uranus might see. 
Little structure is evident at this wavelength, though with 
image-processing techniques, a small cloud can be seen near 
the planet's northern limb (rightmost edge). The "red" image 
(on the right) is taken at 6,190 Angstroms, and is sensitive 
to absorption by methane molecules in the planet's 
atmosphere. The banded structure of Uranus is evident, and 
the small cloud near the northern limb is now visible.

Scientists are expecting that the discrete clouds and banded 
structure may become even more pronounced as Uranus 
continues in its slow pace around the Sun. "Some parts of 
Uranus haven't seen the Sun in decades," says Dr. Hammel, 
"and historical records suggest that we may see the 
development of more banded structure and patchy clouds as 
the planet's year progresses."

Some scientists have speculated that the winds of Uranus are 
not symmetric around the planet's equator, but no clouds 
were visible to test those theories. The new data will 
provide the opportunity to measure the northern winds. 
Hammel and colleagues expect to have results soon.

Credits: Heidi Hammel (Massachusetts Institute of 
Technology), and NASA.


EDITOR'S NOTE:

This is an electronic photo release.

A photo and caption are available via the World Wide Web at
http://oposite.stsci.edu/pubinfo/PR/97/36.html and via links in
http://oposite.stsci.edu/pubinfo/Latest.html or
http://oposite.stsci.edu/pubinfo/Pictures.html.  

Images are available via the World Wide Web at http://oposite.stsci.edu/pubinfo/gif/uranhh.gif (GIF),
http://oposite.stsci.edu/pubinfo/jpeg/uranhh.jpg (JPEG).

Image files also may be accessed via anonymous ftp from oposite.stsci.edu
in /pubinfo:  gif/uranhh.gif (GIF) and jpeg/uranhh.jpg (JPEG).  Higher
resolution digital versions (300 dpi JPEG) of the release photograph are
available in /pubinfo/hrtemp: 97-36b.jpg (color) and 97-36bbw.jpg (black &
white).  A full resolution TIFF image is available in
/pubinfo/tiff/1997/36b.tif.