Ian & Stuart's Australian Mac: Not for Sale/Another.not.for.sale (Australia).iso/Dr. Doyle/Jupiter Comet/Education Notes/sl9-jpl.07

Text File | 1994-04-15 | 19KB | 318 lines

7. The Collisions

        Exactly what will happen as the fragments of Shoemaker-
Levy 9 enter the atmosphere of Jupiter is very uncertain, though
there are many predictions. If the process were better understood,
it would be less interesting. Certainly scientists have never
observed anything like this event. There seems to be complete
agreement only that the major fragments will hit Jupiter and that
these collisions will occur on the back side of Jupiter as seen
from Earth.

Any body moving through an atmosphere is slowed by atmospheric
drag, by having to push the molecules of that atmosphere out of
the way. The kinetic energy lost by the body is given to the air
molecules. They move a bit faster (become hotter) and in turn heat
the moving body by conduction. This frictional process turns
energy of mass motion (kinetic energy) into thermal energy
(molecular motion). The drag increases roughly as the square of
the velocity. In any medium a velocity is finally reached at which
the atmospheric molecules can no longer move out of the way fast
enough and they begin to pile up in front of the moving body. This
is the speed of sound (Mach 1 -- 331.7 m/s or 741 mph in air on
Earth at sea level). A discontinuity in velocity and pressure is
created which is called a shock wave. Comet Shoemaker-Levy 9 will
enter Jupiter's atmosphere at about 60 km/s, which would be about
180 times the speed of sound on Earth (Mach 180!) and is about
50 times the speed of sound even in Jupiter's very light, largely
hydrogen atmosphere.

At high supersonic velocities (much greater than Mach 1) enough
energy is transferred to an intruding body that it becomes
incandescent and molecular bonds begin to break. The surface of
the solid body becomes a liquid and then a gas. The gas atoms
begin to lose electrons and become ions. This mixture of ions and
electrons is called a plasma. The plasma absorbs radio waves and
is responsible for the communication blackouts that occur when a
spacecraft such as the Space Shuttle reenters Earth's atmosphere.
The atmospheric molecules are also dissociated and ionized and
contribute to the plasma. At higher temperatures, energy transfer
by radiation becomes more important than conduction. Ultimately
the temperatures of the plasma and the surface of the intruding
body are determined largely by the radiation balance. The
temperature may rise to 50,000 K (90,000 deg F) or more for very
large bodies such as the fragments of Shoemaker-Levy 9 entering
Jupiter's atmosphere at 60 km/s. The loss of material as gas from
the impacting body is called thermal ablation. The early manned
spacecraft (Mercury, Gemini, and Apollo) had "ablative heat
shields" made of a material having low heat conductivity (through
to the spacecraft) and a high vaporization temperature (strong
molecular bonds). As this material was lost, as designed, it
carried away much of the orbital energy of the spacecraft
reentering Earth's atmosphere.

There are other forms of ablation besides thermal ablation, the
most important being loss of solid material in pieces. In a comet,
fragile to begin with and further weakened and/or fractured by
thermal shock and by melting, such spallation of chips or chunks
of material has to be expected. Turbulence in the flow of material
streaming from the front of the shock wave can be expected to
strip anything that is loose away from the comet and send it
streaming back into the wake. The effect of increasing
temperature, pressure, and vibration on an intrinsically weak body
is to crush it and cause it to flatten and spread. Meanwhile the
atmosphere is also increasing in density as the comet penetrates
to lower altitudes. All of these processes occur at an ever
increasing rate (mostly exponentially).

On Earth a sizable iron meteoroid or even some relatively low
velocity stony meteoroids can survive all of this and impact the
surface, where we collect them for study and exhibition. (Small
bodies traveling in space are called meteoroids. The visible
phenomena which occur as a meteoroid enters the atmosphere is
called a meteor. Surviving solid fragments are called meteorites.
There is no sharp size distinction between meteoroids and
asteroids. Normally, if the body has been detected telescopically
before entering the atmosphere, it has been called an asteroid.)
Many meteoroids suffer what is called a "terminal explosion" when
crushed while still many kilometers above the ground. This is what
happened in Tunguska, Siberia, in 1908. There a body with a mass
of some 109 kg (2.2 billion lb.) and probably 90 to 190 m in
diameter entered Earth's atmosphere at a low angle with a velocity
of less than 15 km/s. It exploded at an altitude of perhaps 5-
10 km. This explosion, equivalent to 10-20 megatons of TNT,
combined with the shock wave generated by the body's passage
through the atmosphere immediately before disruption, leveled some
2,200 km^2 of Siberian forest. The Tunguska body had a tensile
strength of some 2x10^8 dynes/cm^2, more than 100,000 times the
strength of Shoemaker-Levy 9, but no surviving solid fragments of
it (meteorites) have ever been found. The fragile Shoemaker-Levy 9
fragments entering an atmosphere of virtually infinite depth at a
much higher velocity will suffer almost immediate destruction. The
only real question is whether each fragment may break into several
pieces immediately after entry, and therefore exhibit multiple
smaller explosions, or whether it will survive long enough to be
crushed, flattened, and obliterated in one grand explosion and
terminal fireball.

Scientists have differed in their computations of the depths to
which fragments of given mass will penetrate Jupiter's atmosphere
before being completely destroyed. If a "terminal explosion"
occurs above the clouds, which are thought to lie at a pressure
level of about 0.5 bar or roughly 0.5 Earth atmosphere (see
Section 5), then the explosion will be very bright and easily
observable by means of light reflected from Jupiter's satellites.
Using ablation coefficients derived from observation of many
terrestrial fireballs, Sekanina predicts that the explosions
indeed will occur above the clouds. Mordecai-Mark Mac Low and
Kevin Zahnle have made calculations using an astrophysical
hydrodynamic code (ZEUS) on a supercomputer. They assume a fluid
body as a reasonable approximation to a comet, since comets have
so little strength, and they predict that the terminal explosions
will occur near the 10-bar level, well below the clouds. Others
have suggested still deeper penetration, but most calculations
indicate that survival to extreme depths is most unlikely. The
central questions then appear to be whether terrestrial experience
with lesser events can be extrapolated to events of such magnitude
and whether all the essential physics has been included in the
supercomputer calculations. We can only wait and observe what
really happens, letting nature teach us which predictions were
correct.

O.K. So an explosion occurs at some depth. What does that do? What
happens next? Sekanina calculates that about 93% of the mass of a
10^13-kg fragment remains one second before the terminal explosion
and the velocity is still almost 60 km/s. During that last second
the energy of perhaps 10,000 100-megaton bombs is released. Much
of the cometary material will be heated to many tens of thousands
of degrees, vaporized, and ionized along with a substantial amount
of Jupiter's surrounding atmosphere. The resulting fireball should
balloon upward, even fountaining clear out of the atmosphere,
before falling back and spreading out into Jupiter's atmosphere,
imitating in a non-nuclear fashion some of the atmospheric
hydrogen bomb tests of the 1950s. Once again, the total energy
release here will be many thousands of times that of any hydrogen
bomb ever tested, but the energy will be deposited initially into
a much greater volume of Jupiter's atmosphere, so the energy
density will not be so high as in a bomb, and, of course, there
will be no gamma rays or neutrons (nuclear radiation or particles)
flying about. The energy of these impacts will be beyond any prior
experience. The details of what actually occurs will be determined
by the observations in July 1994, if the observations are
successful.

If differential gravitation (tidal forces) should further fragment
a piece of the comet, say an hour or two before impact, the pieces
can be expected to hit within a second of each other. In one
second a point at 44 deg. latitude on Jupiter will rotate 9 km
(5.6 mi.), however, so the pieces would enter the atmosphere some
distance apart. Smaller pieces will explode at higher altitudes
but not so spectacularly. If smaller pieces do explode above the
clouds, they may be more "visible" than larger pieces exploding
below the clouds. It is also possible that implanting somewhat
less energy density over a wider volume of atmosphere might create
a more visible change in JupiterUs atmosphere. Sekanina notes that
pieces smaller than about 1.3-km mean radius should not be further
fragmented by tidal forces unless they were already weakened by
earlier events.

One of the more difficult questions to answer is just how bright
these events will be. Terrestrial fireballs have typically
exhibited perhaps 1% luminous efficiency. In other words about 1%
of the total kinetic energy has been converted to visible light.
The greater magnitude of the Jupiter impacts may result in more
energy appearing as light, but let's assume the 1% efficiency.
Then Sekanina calculates that a 10^13-kg fragment, a reasonable
value for the largest piece, will reach an apparent visual
magnitude of -10 during the terminal explosion. This is
1,000 times Jupiter's normal
brilliance and only 10 times fainter than the full moon! Sekanina,
of course, calculates that the explosions will occur above the
clouds. And, remember that, unfortunately, these impacts will
occur on Jupiter's back side as seen from Earth. There will be no
immediate visible effect on the appearance of Jupiter. The light
of the explosion may be seen reflected from the Galilean
satellites of Jupiter, if they are properly placed at the times of
impacts. Ganymede, for example, might brighten as much as six
times, while Io could brighten to 35 times its normal brilliance
for a second before fading slowly, if the explosions occur above
the clouds. This would certainly be visible in an amateur
telescope and could conceivably be visible to the naked eye at a
dark mountain site as a tiny flash next to Jupiter at the location
of the normally invisible satellite. Emphasis on "tiny"! The
brightness of explosions occurring below the clouds will be
attenuated by a factor of at least 10,000, making them most
difficult to observe. In the best of cases, these events will be
spectacles for the mind to imagine and big telescopes to observe,
not a free fireworks display.

The most recent predictions are that at least some of the impacts
will occur very close to the planetary limb, the edge of the
planetUs disk as seen from Earth. That edge still has 11 degrees to
rotate before it comes into sunlight. This means that the tops of
some of the plumes associated with the rising fireballs may be
just visible, although with a maximum predicted height of 3,000 km
(0.8 arc second as projected on the sky) they will be just
"peeking" over the limb. The newly repaired Hubble Space Telescope
(HST), with its high resolution and low scattered light, may offer
the best chance to see such plumes. By the time they reach their
maximum altitude the plumes will be transparent (optically thin)
and not nearly so bright as they were near the clouds. Some means
of blocking out the bright light from Jupiter itself may be
required in order to observe anything. A number of observers plan
to look for evidence of plumes and to attempt to measure their
size and brightness.

It also is difficult to predict the effects of the impacts on
Jupiter's atmosphere. Robert West points out that a substantial
amount of material will be deposited even in the stratosphere of
Jupiter, the part of the atmosphere above the visible clouds where
solar heating stabilizes the atmosphere against convection
(vertical motion). Part of this material will come directly from
small cometary grains, which vaporize during entry and recondense
just as do meteoritic grains in the terrestrial atmosphere. Part
will come from volatiles (ammonia, water, hydrogen sulfide, etc.)
welling up from the deeper atmosphere as a part of the hot buoyant
fireballs created at the time of the large impact events. Many
millimeter-sized or larger pieces from the original breakup will
also impact at various times for months and over the entire globe
of Jupiter. There is relatively little mass in these smaller
pieces, but it might be sufficient to create a haze in the
stratosphere.

James Friedson notes that the fireball created by the terminal
explosion will expand and balloon upward and perhaps spew
vaporized comet material and Jupiter's entrained atmospheric gas
to very high altitudes. The fireball may carry with it atmospheric
gases that are normally to be found only far below Jupiter's
visible clouds. Hence the impacts may give astronomers an
opportunity to detect gases which have been hitherto hidden from
view. As the gaseous fireball rises and expands it will cool, with
some of the gases it contains condensing into liquid droplets or
small solid particles. If a sufficiently large number of particles
form, then the clouds they produce may be visible from Earth-based
telescopes after the impact regions rotate onto the visible side
of the planet. These clouds may provide the clearest indication of
the impact locations after each event.

After the particles condense, they will grow in size by colliding
and sticking together to form larger particles, eventually
becoming sufficiently large to "rain" out of the visible part of
the atmosphere. The length of time spent by the cloud particles at
altitudes where they can be seen will depend principally on their
average size; relatively large particles would be visible only for
a few hours after an impact, while small particles could remain
visible for several months. Unfortunately, it is very difficult to
predict what the number and average size of the particles will be.
A cloud of particles suspended in the atmosphere for many days may
significantly affect the temperature in its vicinity by changing
the amount of sunlight that is absorbed in the area. Such a
temperature change could be observed from Earth by searching for
changes in the level of Jupiter's emitted infrared light.

Glenn Orton notes that large regular fluctuations of atmospheric
temperature and pressure will be created by the shock front of
each entering fragment, somewhat analogous to the ripples created
when a pebble is tossed into a pond, and will travel outward from
the impact sites. These may be observable near layers of condensed
clouds in the same way that regular cloud patterns are seen on the
leeward side of mountains. Jupiter's atmosphere will be
sequentially raised and lowered, creating a pattern of alternating
cloudy areas where ammonia gas freezes into particles (the same
way that water condenses into cloud droplets in our own
atmosphere) and clear areas where the ice particles warm up and
evaporate back into the gas phase. If such waves are detected,
measurement of their wavelength and speed will
allow scientists to determine certain important physical
properties of Jupiter's deep atmospheric structure that are very
difficult to measure in any other way.

Whether or not "wave" clouds appear, the ripples spreading from
the impact sites will produce a wave structure in the temperature
at a given level that may be observable in infrared images. In
addition there should be compression waves, alternate compression
and rarefaction in the atmospheric pressure, which could reflect
from and refract within the deeper atmosphere, much as seismic
waves reflect and refract due to density changes inside Earth.
Orton suggests that these waves might be detected "breaking up" in
the shallow atmosphere on the opposite side of the planet from the
impacts. Others suggest the possibility of measuring the small
temperature fluctuations wherever the waves surface, but this
requires the ability to map fluctuations in Jupiter's visible
atmosphere of a few millikelvin (a few thousandths of a degree).
Detection of any of these waves will require a very fine infrared
array detector (a thermal infrared camera).

Between the water and other condensable gases (volatiles) brought
with the comet fragments and those exhumed by the rising
fireballs, it is fairly certain that a cloud of condensed material
will form at the location of the impacts themselves, at high
altitudes where such gases seldom, if ever, exist in the usual
course of things. It may be difficult to differentiate between the
color or brightness of these condensates and any bright material
below them in spectra at most visible wavelengths. However, at
wavelengths where gaseous methane and hydrogen absorb sunlight, a
distinction can easily be made between particles higher and lower
in the atmosphere, because the higher particles will reflect
sunlight better. Much of the light is absorbed before reaching the
lower particles. Observing these clouds in gaseous absorption
bands will then tell us how high they lie in the atmosphere, and
observations over a period of time will indicate how fast high-
altitude winds are pushing them. The speed with which these clouds
disappear will be a measure of particle sizes in the clouds, since
large particles settle out much faster than small ones, hours as
compared to days or months.

Orton also notes that in the presence of a natural wind shear (a
region with winds having different speeds and/or directions) such
as exists commonly across the face of Jupiter, a long-lived
cyclonic feature can be created which is actually quite stable. It
may gain stability by being fed energy from the wind shear, in
much the same way that the Great Red Spot and other Jovian
vortices are thought to be stabilized. Such creation of new,
large, fixed "storm" systems is somewhat controversial, but this
is a most intriguing possibility!