Newsgroups: sci.physics.plasma
From: "HILL, DIANNE" <DHILL@pao.hq.nasa.gov>
Organization: NASA Headquarters
Subject: 94-161 Hubble observations of comet impact with Jupiter

[Note from moderator: This release describes some space plasma effects
of the comet impact including auroral and brightenings upon first entry
into Jupiter's magnetosphere]

Donald Savage
Headquarters, Washington, D.C.
September 29, 1994
(Phone: 202/358-1547)
Jim Elliott
Goddard Space Flight Center, Greenbelt, Md.
(Phone: 301/286-6256)
Ray Villard
Space Telescope Science Institute, Baltimore, Md.
(Phone: 410/338-4514)

RELEASE: 94-161

HUBBLE OBSERVATIONS SHED NEW LIGHT ON JUPITER COLLISION

Was it a comet or an asteroid?

Scientists are debating that question as they continue to
pore over Hubble Space Telescope imaging and spectroscopic data
gleaned in the wake of the spectacular July bombardment of Jupiter by
comet P/Shoemaker-Levy 9.

Their initial findings, combined with results from other
space-borne and ground-based telescopes, shed new light on Jupiter's
atmospheric winds, its immense magnetic field, the mysterious dark
debris from the impacts, and the composition of the doomed comet
itself.

These early results are being presented at a press conference
today at NASA Headquarters, Washington D.C., by astronomers John
Clarke, University of Michigan, Ann Arbor; Heidi Hammel, Massachusetts
Institute of Technology, Cambridge; and Harold Weaver and Melissa
McGrath, Space Telescope Science Institute, Baltimore.

THE LAST DAYS OF THE COMET

Before the comet impact, there was a great deal of speculation
and prediction about whether the 21 nuclei would survive before
reaching Jupiter, or were so fragile that gravitational forces would
pull them apart into thousands of smaller fragments. Hubble helped
solve this question by watching the nuclei until about 10 hours
before impact. HST's high resolution images show that the nuclei, the
largest of which were probably a few kilometers across, did not break
up catastrophically before plunging into Jupiter's atmosphere. This
reinforces the notion that the atmospheric explosions were produced by
solid, massive impacting bodies.

HST's resolution also showed that the nuclei were releasing
dust all along the path toward Jupiter, as would be expected from a
comet. This was evident in the persistence of spherical clouds of
dust surrounding each nucleus throughout most of the comet's journey.
About a week before impact, these dust clouds were stretched out along
the path of the comet's motion by Jupiter's increasingly strong
gravity.

WAS P/SHOEMAKER-LEVY 9 A COMET OR AN ASTEROID?

At present, observations seem to slightly favor a cometary
origin, though an asteroidal origin cannot yet be ruled out. The
answer isn't easy because comets and asteroids have so much in common:
they are small bodies; they are primordial, having formed 4.6 billion
years ago along with the planets and their satellites; either type of
object can be expected to be found in Jupiter's vicinity. The key
difference is that comets are largely icy while the asteroids are
virtually devoid of ice because they formed too close to the Sun. The
attached table summarizes the observational results that shed
light on this question.

WHAT IS THAT DARK STUFF MADE OF?

The HST Faint Object Spectrograph (FOS) detected many gaseous
absorptions associated with the impact sites and followed their
evolution over the next month. Most surprising were the strong
signatures from sulfur-bearing compounds like diatomic sulfur (S2),
carbon disulfide (CS2), and hydrogen sulfide (H2S). Ammonia (NH3)
absorption also was detected. The S2 absorptions seemed to fade on
timescales of a few days, while the NH3 absorptions at first got
stronger with time, and finally started fading after about one month.
During observations near the limb of Jupiter, the FOS detected
emissions from silicon, magnesium and iron that could only have
originated from the impacting bodies, since Jupiter itself normally
does not have detectable amounts of these elements.

SWEPT ACROSS JUPITER

Observations made with HST's Wide Field Planetary Camera-2, a
week and a month after impact, have been used to make global maps of
Jupiter for tracking changes in the dark debris caught up in the
high-speed winds at Jupiter's cloudtops. This debris is a natural
tracer of wind patterns and allows astronomers a better understanding
of the physics of the Jovian atmosphere. The high speed easterly and
westerly jets have turned the dark "blobs" originally at the impact
sites into striking "curly-cue" features. Although individual impact
sites were still visible a month later despite the shearing, the
fading of Jupiter's scars has been substantial and it now appears that
Jupiter will not suffer any permanent changes from the
explosions.

Hubble's ultraviolet observations show the motion of very
fine impact debris particles now suspended high in Jupiter's
atmosphere. The debris eventually will diffuse down to lower
altitudes. This provides the first information ever obtained about
Jupiter's high altitude wind patterns. Hubble gives astronomers a
"three dimensional" perspective showing the wind patterns at high
altitudes and how they differ from those at the visible cloudtop
level. At lower altitudes, the impact debris follows east-west winds
driven by sunlight and Jupiter's own internal heat. By contrast,
winds in the high Jovian stratosphere move primarily from the poles
toward the equator because they are driven mainly by auroral heating
from high energy particles.

PIERCING JUPITER'S MAGNETIC FIELD

About four days before impact, at a distance of 2.3 million
miles from Jupiter, nucleus "G" of comet P/Shoemaker-Levy 9 apparently
penetrated Jupiter's powerful magnetic field, the magnetosphere.
(Jupiter's magnetosphere is so vast, if visible from Earth, it would
be about the size of the full Moon.)

Hubble's Faint Object Spectrograph (FOS) recorded dramatic
changes at the magnetosphere crossing that provided a rare opportunity
to gather more clues on the comet's true composition. During a two
minute period on July 14, HST detected strong emissions from ionized
magnesium (Mg II), an important component of both comet dust and
asteroids. However, if the nuclei were ice-laden -- as expected of a
comet nucleus -- astronomers expected to detect the hydroxyl radical
(OH). Hubble did not see OH, casting some doubt on the cometary
nature of comet P/Shoemaker-Levy 9. Eighteen minutes after comet P/
Shoemaker- Levy 9 displayed the flare-up in Mg II emissions, there was
also a dramatic change in the light reflected from the dust particles
in the comet.

NEW AURORAL ACTIVITY

HST detected unusual auroral activity in Jupiter's northern
hemisphere just after the impact of the comet's "K" fragment. This
impact completely disrupted the radiation belts which have been stable
over the last 20 years of radio observations.

Aurorae, glowing gases that create the northern and southern
lights, are common on Jupiter because energetic charged particles
needed to excite the gases are always trapped in Jupiter's
magnetosphere. However, this new feature seen by Hubble was unusual
because it was temporarily as bright or brighter than the normal
aurora, short-lived, and outside the area where Jovian aurorae are
normally found. Astronomers believe the K impact created an
electromagnetic disturbance that traveled along magnetic field lines
into the radiation belts. This scattered charged particles, which
normally exist in the radiation belts, into Jupiter's upper
atmosphere.

X-ray images taken with the ROSAT satellite further bolster
the link to the K impact. They reveal unexpectedly bright X-ray
emissions that were brightest near the time of the K impact, and then
faded.

The Space Telescope Science Institute is operated by the
Association of Universities for Research in Astronomy, Inc. (AURA) for
NASA, under contract with the Goddard Space Flight Center, Greenbelt,
Md. The Hubble Space Telescope is a project of international
cooperation between NASA and the European Space Agency (ESA).

- end -


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