RISC DISC 2/RISC_DISC_2.iso/resources/nasa/shoemaker/info/hstobservationsonsl9_collision

Text File | 1995-05-03 | 8.3 KB | 170 lines

9/29/94:  HUBBLE OBSERVATIONS SHED NEW LIGHT ON JUPITER COLLISION

94-161 HUBBLE DISCOVERIES

Donald Savage
Headquarters, Washington, D.C.
September 29, 1994
(Phone:  [phone])

Jim Elliott
Goddard Space Flight Center, Greenbelt, Md.
(Phone:  [phone])

Ray Villard
Space Telescope Science Institute, Baltimore, Md.
(Phone:  [phone])

RELEASE:  94-161


        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).

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