By
Jove: Comet Crash Puzzles
It has been
exactly half a year since the fragments of Comet Shoemaker-Levy 9 plowed
into Jupiter. There is no better time to re-visit and re-live the excitement
of the once-in-a-lifetime event. As of today, six months after the crash,
scientists are still continuing their struggle to understand exactly what
the impacts have revealed about the inside of the gaseous planet. But even
if much of the Jovian interior remains a mystery, the collisions have shed
some new insights about the giant planet's upper atmosphere.
Just weeks
after Eugene and Carolyn Shoemaker and David H. Levy discovered the comet
at California's Mount Palomar Observatory on March 24, 1993, further observations
revealed it to consist of 20 or so fragments, eventually labeled A through
W, spread out in space like a strand of diamonds. The scientists calculated
that the comet had broken up during a previous approach near Jupiter and
that it would plunge into the planet for good in July 1994.
On July 16,
1994, for over six days, fragments of Comet Shoemaker-Levy 9 plowed into
Jupiter, leaving behind a necklace of dark bruises. The Jovian show was
big and dazzling. The Jovian fireworks kept electronic mail buzzing for
weeks. How big were the fragments? How large a punch did they pack? How
deeply did they penetrate into Jupiter's atmosphere? Shoving these related
riddles will help reveal how much of the planet's hidden interior the collisions
exposed.
The upper
reaches of Jupiter's clouds appear to consist of three distinct layers.
Visible clouds of ammonia lie at the top. Beneath this may be a layer of
ammonium hydrosulfide. The deepest layer is thought to contain water. How
deep did the fragments go? Scientists have not agreed on the subject. To
observers marvelling over the first fireballs and dark bruises in Jupiter's
atmosphere, it seemed that the comet's fragments must have been big, solid
projectiles, plunging deep into the atmosphere. But as the week wore on
and astronomers analyzed the impact sites, doubts set in. By the time the
curtain had come down, some researchers were arguing that the impact debris
and scars didn't look as if they came from deeply penetrating wounds. Now,
half a year later, what are the conclusions of the collisions?
The intensity
and duration of light from each event offer important clues. Rather than
producing a single burst, the larger fragments staged an extended light
show: an initial flash as a chunk entered Jupiter's upper atmosphere, a
glowing plume of debris shooting thousands of kilometers above the clouds,
and the radiation generated when the plume crashed back down with a high-velocity
splash.
Telescopes
recorded these emissions at a variety of wavelengths, producing a characteristic
set of light curves. The light curves traced the progress of chunks of
ice and rocks. Of all the instruments staring at Jupiter in July, only
those on the Galileo spacecraft had a full view of the fireworks. Heading
toward a 1995 rendezvous with Jupiter, the craft was in the right position
to directly observe the collisions, which occurred on the back side of
the planet just out of direct sight of ground-based and Earth-orbiting
telescopes. Unfortunately, a programming error led to the loss of some
data coinciding with the collisions and a crippled main antenna limits
the spacecraft's ability to transmit information. It hasn't been able to
finish transmitting its data yet. In the meantime, scientists have begun
to make sense of the information already received.
When Galileo
first sighted the fireball from fragment G, believed to be the largest,
the fireball appeared to have a diameter of about 10 km and a temperature
of 7,500 kelvins. Five seconds later, the craft's near-infrared mapping
spectrometer first saw the explosion, recording the rising fireball's expansion
and cooling for 90 seconds, until it was thousands of km across and only
400 kelvins. For the K impact, one of the larger fragments, the craft's
solid-state imaging camera saw a bright flash lasting about 5 seconds.
Over the next 10 seconds, the light dimmed and then brightened again, fading
away after another 30 seconds. For N, one of the smaller impacts, the camera
recorded a similar pattern.
It is thought
that the initial flashes probably reveal the fragments as they streaked
through Jupiter's upper atmosphere and started to glow as meteors. As they
tunneled through thicker atmosphere, the fragments heated material, which
exploded in a a fireball. This produced the second, longer-lasting glow
in visible light as well as the extended emission in the near infrared.
If this interpretation proves correct, it could help indicate how deeply
the fragments plumbed Jupiter. The events viewed by Galileo's camera suggest
the fragments didn't penetrate far. This, in turn, indicates that the chunks
were not larger than 1 km in diameter. Had the fragments plowed deeper,
into thicker layers of atmosphere, Galileo's camera would have recorded
several blank frames until the fireball re-emerged above these light-absorbing
regions. However, some other scientists are skeptical of this interpretation.
Also, did Galileo really see the meteor flashes? The meteor flash provides
a key point in time -- signaling when a fragment first entered the Jovian
atmosphere. By knowing the elapsed time between a fragment's entry and
the time when ground-based telescopes first glimpsed a plume of material
above Jupiter's limb, researchers hope to calculate the energy delivered
by the impact. If the craft did record some flashes, it may not have captured
the most spectacular part of this light show. Even if Galileo did miss
the meteor flashes, the craft still captured much of the light emissions,
since the exploding fragments radiated most of their energy during the
fireball phase.
Several telescopes
on Earth recorded intriguing near-infrared light curves. The light curve
of the R impact shows three separate rises and dips. First came an abrupt
flash of about 10 seconds. After a 1-minute gap, a signal three times as
bright appeared and lasted for about 30 seconds. Finally, 5 minutes after
the initial flash, the telescopes recorded another rise in light intensity
that took 5 minutes to peak and didn't fade completely for half an hour.
It is speculated that the initial flash seen from ground-based telescopes,
like the flashes detected by Galileo, represent the R fragment streaking
into Jupiter's upper atmosphere. But how could a telescope on Earth detect
an event on the back side of Jupiter? Note that it took about 10 minutes
for the impact sites to rotate into view from Earth. It was suggested that
the telescopes may have detected the streak in reflection from light scattering
off dust deposited in Jupiter's atmosphere by the cometary fragments. Alternatively,
the meteor flash may have begun high enough in Jupiter's atmosphere for
it to be visible above Jupiter's darkened limb.
The second
rise in intensity occurred when a plume of hot material generated in the
explosion shot up through the Jovian atmosphere. Data from the Hubble Space
Telescope indicate that this plume rose about 3,500 km above the cloud
tops. The third and longest rise represents the violent shock generated
when the giant plume crashed back into the atmosphere. Forming a dark spot
the diameter of Earth, the falling plume apparently heated the atmosphere
around it to some 500 kelvins, creating an infrared glow.
This scenario
agrees with a model in which fragments no larger than 1 km in diameter
explode relatively high in Jupiter's atmosphere. This model gains support
from the presence and absence of specific molecules in the Jovian atmosphere
soon after the fragments struck.
The answer
to the question how deep did the fragments penetrate the Jovian atmosphere
can be obtained by analyzing an entirely different phenomenon. Rings, like
ripples on a pond, were detected moving outward from several crash sites.
The rings were measured to travel at a speed of about 450 m/s which probably
represent gravity waves. Some researchers argue that gravity wave at this
speed would have originated from the water cloud layer, indicating that
some fragments penetrated to this depth. In contrast, some other scientists
maintain that the gravity wave at the speed measured would lie higher up,
in the stratosphere.
So what are
the answers to all the questions relating to the crash? At the moment,
there is no confirmed answer to any of those question. Researchers aim
to resolve many questions by next May, when they will gather at a special
meeting of the International Astronomical Union in Baltimore. Even if some
puzzles still remain by then, scientists may not have to wait long to solve
them. Next December, a Galileo probe will parachute into Jupiter's atmosphere.
The crash of '94 may be just a prelude for the Jovian exploration of '95.
References:
"After the Crash...", R. Cowen, Science News, Vol. 146, pp. 412-414
(December 17, 1993). "By Jove! ...", J. Horgan, Scientific American,
Oct 1994, pp. 16-20. "Comet collision", D. Graham, Popular Science,
July 1994, pp. 43-45, p.71. And TOO many other references.
