The Starbase One Astronomy & Space Collection/STARBASE_ONE.ISO/hst/neptune.txt

Text File | 1996-01-12 | 8KB | 150 lines

PRESS RELEASE:

CONTACT:  Dr. David Crisp
          Jet Propulsion Laboratory, MS 169-237
          4800 Oak Grove Drive
          Pasadena, CA 91109
          [phone]     [e-mail]
or
          Dr. Heidi B. Hammel
          Earth Atmosphere and Planetary Science Dept.
          Massachusetts Institute of Technology, Rm 54416
          Cambridge, MA
          [phone]     [e-mail]


HUBBLE SPACE TELESCOPE WIDE FIELD PLANETARY CAMERA 2 OBSERVATIONS OF
NEPTUNE

Two groups have recently used the Hubble Space Telescope (HST) Wide
Field Planetary Camera 2 (WFPC 2) to acquire new high-resolution images
of the planet Neptune.  Members of the WFPC-2 Science Team, lead by
John Trauger, acquired the first series of images on 27 through 29 June
1994.  These were the highest resolution images of Neptune taken since
the Voyager-2 flyby in August of 1989.  A more comprehensive program is
currently being conducted by Heidi Hammel and Wes Lockwood.  These two
sets of observations are providing a wealth of new information about
the structure, composition, and meteorology of this distant planet's
atmosphere.

Neptune is currently the most distant planet from the sun, with an
orbital radius of 4.5 billion kilometers (2.8 billion miles, or 30
Astronomical Units).  Even though its diameter is about four times that
of the Earth (49,420 vs. 12,742 km), ground-based telescopes reveal a
tiny blue disk that subtends less than 1/1200 of a degree (2.3
arc-seconds).  Neptune has therefore been a particularly challenging
object to study from the ground because its disk is badly blurred by
the Earth's atmosphere.  In spite of this, ground-based astronomers had
learned a great deal about this planet since its position was first
predicted by John C. Adams and Urbain Leverrier in 1845.  For example,
they had determined that Neptune was composed primarily of hydrogen and
helium gas, and that its blue color caused by the presence of trace
amounts of the gas methane, which absorbs red light.  They had also
detected bright cloud features whose brightness changed with time, and
tracked these clouds to infer a rotation period between 17 and 22
hours.

When the Voyager-2 spacecraft flew past the Neptune in 1989, its
instruments revealed a surprising array of meteorological phenomena,
including strong winds, bright, high-altitude clouds, and two large
dark spots attributed to long-lived giant storm systems.  These bright
clouds and dark spots were tracked as they moved across the planet's
disk, revealing wind speeds as large as 325 meters per second (730
miles per hour).  The largest of the giant, dark storm systems, called
the "Great Dark Spot", received special attention because it resembled
Jupiter's Great Red Spot, a storm that has persisted for more than
three centuries.  The lifetime of Neptune's Great Dark Spot could not
be determined from the Voyager data alone, however, because the
encounter was too brief.  Its evolution was impossible to monitor with
ground-based telescopes, because it could not be resolved on Neptune's
tiny disk, and its contribution to the disk-integrated brightness of
Neptune confused by the presence of a rapidly-varying bright cloud
feature, called the "Bright Companion" that usually accompanied the
Great Dark spot.

The repaired Hubble Space Telescope provides new opportunities to
monitor these and other phenomena in the atmosphere of the most distant
planet.  Images taken with WFPC-2's Planetary Camera (PC) can resolve
Neptune's disk as well as most ground-based telescopes can resolve the
disk of Jupiter.  The spatial resolution of the HST WFPC-2 images is
not as high as that obtained by the Voyager-2 Narrow-Angle Camera
during that spacecraft's closest approach to Neptune, but they have a
number of other assets that enhance their scientific value, including
improved ultra-violet and infrared sensitivity, better signal-to-noise,
and, and greater photometric accuracy.

The images of Neptune acquired by the WFPC-2 Science team in late June
clearly demonstrate these capabilities.  The side of the planet facing
the Earth at the start of the program (11:36 Universal Time on July 27)
was imaged in color filters spanning the ultraviolet (255 and 300-nm),
visible (467, 588, 620, and 673- nm), and near-infrared (890-nm) parts
of the spectrum.  The planet then rotated 180 degrees in longitude, and
the opposite hemisphere was imaged in a subset of these colors (300,
467, 588, 620, and 673-nm).  The HST/WFPC-2 program more recently
conducted by Hammel and Lockwood provides better longitude coverage,
and a wider range of observing times, but uses a more restricted set of
colors.

The ultraviolet pictures show an almost featureless disk that is
slightly darker near the edge.  The observed contrast increases in the
blue, green, red, and near-infrared images, which reveal many of the
features seen by Voyager 2, including the dark band near 60 S latitude
and several distinct bright cloud features.  The bright cloud features
are most obvious in the red and infrared parts of the spectrum where
methane gas absorbs most strongly (619 and 890 nm).  These bright
clouds thought to be high above the main cloud deck, and above much of
the absorbing methane gas.  The edge of the planet's disk also appears
somewhat bright in these colors, indicating the presence of a
ubiquitous, high-altitude haze layer.

The northern hemisphere is occupied by a single prominent cloud band
centered near 30 N latitude.  This planet-encircling feature may be the
same bright cloud discovered last fall by ground-based observers.
Northern hemisphere clouds were much less obvious at the time of the
Voyager-2 encounter.  The tropics are about 20 % darker than the disk
average in the 890-nm images, and one of these images reveals a
discrete bright cloud on the equator, near the edge of the disk.  The
southern hemisphere includes two broken bright bands.  The largest and
brightest is centered at 30 S latitude, and extends for least 40
degrees of longitude, like the Bright Companion to the Great Dark
Spot.  There is also a thin cloud band at 45 S latitude, which almost
encircles the planet.

One feature that is conspicuous by its absence is the storm system
known as the Great Dark Spot.  The second smaller dark spot, DS2, that
was seen during the Voyager-2 encounter was also missing.  The absence
of these dark spots was one of the biggest surprises of this program.
The WFPC-2 Science team initially assumed that the two storm systems
might be near the edge of the planet's disk, where they would not be
particularly obvious.  An analysis of their longitude coverage revealed
that less than 20 degrees of longitude had been missed in the colors
where these spots had their greatest contrast (467 and 588 nm).  The
Great Dark Spot covered almost 40 degrees of longitude at the time of
the Voyager-2 fly-by.  Even if it were on the edge of the disk, it
would appear as a "bite" out of the limb.  Because no such feature was
detected, we concluded that these features had vanished.  This
conclusion was reinforced by the more recent observations by Hammel and
Lockwood, which also show no evidence of discrete dark spots.

These dramatic changes in the large-scale storm systems and
planet-encircling clouds bands on Neptune are not yet completely
understood, but they emphasize the dynamic nature of this planet's
atmosphere, and the need for further monitoring.  Additional HST WFPC-2
observations are planned for next summer.  These two teams are
continuing their analysis of these data sets to place improved
constraints on these and other phenomena in Neptune's atmosphere.


Figure Captions:

These almost true-color pictures of Neptune were constructed from
HST/WFPC2 images taken in blue (467-nm), green (588-nm), and red
(673-nm) spectral filters.  There is a bright cloud feature at the
south pole, near the bottom right of the image.  Bright cloud bands can
be seen at 30S and 60S latitude.  The northern hemisphere also includes
a bright cloud band centered near 30N latitude.  The second picture was
compiled from images taken after the planet had rotated about 180
degrees of longitude (about 9 hours later) to show the opposite
hemisphere.