comets
Nucleus
Coma or the Head
Types of Tails
Type I Tail (Ionic or Plasma Tail)
Type II Tail (Dust Tail)
Sodium Tail
What is a nucleus?
The nucleus is a compact body of irregular
form with a diameter between 1 and 100 km (from 40 to 50 km for
Hale-Bopp).
However it is not visible from the Earth. The American astronomer Fred
Whipple proposed the " dirty snow ball " model: the nucleus is a solid
amalgam of ices and dust which sublimates when it approaches the Sun.
Sublimation begins at 15 AU from the
Sun. The spectroscopic analysis shows that the ensuing gases appear in
the order of their temperature of sublimation. The model also explains
the so-called non-gravitational forces as being due to the ejection of
gas jets (cfr infra). A final confirmation was provided by the probe
Giotto
which took a close up (18 000 km) photograph showing the nucleus of
Halley's
comet in March 1986.
How do we know the size of the nucleus?
Although in most cases one cannot observe
the nucleus directly, its diameter can be determined either by radar
echo
or by the albedo method.
The first method is based on the precise determination of a return trip of a radar pulse. The returning echo from a sphere will be spread out over a time of the order of radius divided by the speed of light. Such observations also provide the period of revolution and the orientation of the axis of rotation from the Doppler shift of the return signal. Unfortunately, the comet Hale-Bopp passed too far from the Earth so that its nucleus cannot be studied in this way.
What is the coma?
Comets are small, fragile, irregularly
shaped bodies composed of a mixture of non-volatile grains and frozen
gases.
They usually follow highly elongated paths around the Sun. Most become
visible, even in telescopes, only when they get near enough to the Sun
for the Sun's radiation to start subliming the volatile gases, which in
turn blow away small bits of the solid material. These materials expand
into an enormous escaping atmosphere called the coma, which becomes far
bigger than a planet, and they are forced back into long tails of dust
and gas by radiation and charged particles flowing from the Sun. Comets
are cold bodies, and we see them only because the gases in their comae
and tails fluoresce in sunlight (somewhat akin to a fluorescent light)
and because of sunlight reflected from the solids. Comets are regular
members
of the solar system family, gravitationally bound to the Sun. They are
generally believed to be made of material, originally in the outer part
of the solar system, that didn't get incorporated into the planets --
leftover
debris, if you will. It is the very fact that they are thought to be
composed
of such unchanged "primitive" material that makes them extremely
interesting
to scientists who wish to learn about conditions during the earliest
period
of the solar system.
It is a very luminous halo which one
observes all around the nucleus.
The nucleus of a comet, which is its
solid, persisting part, has been called an icy conglomerate, a dirty
snowball,
and other colorful but even less accurate descriptions. Certainly a
comet
nucleus contains silicates akin to some ordinary Earth rocks in
composition,
probably mostly in very small grains and pieces. Perhaps the grains are
"glued" together into larger pieces by the frozen gases. A nucleus
appears
to include complex carbon compounds and perhaps some free carbon, which
make it very black in color. Most notably, at least when young, it
contains
many frozen gases, the most common being ordinary water. In the low
pressure
conditions of space, water sublimes, that is, it goes directly from
solid
to gas -- just like dry ice does on Earth. Water probably makes up
75-80%
of the volatile material in most comets. Other common ices are carbon
monoxide
(CO), carbon dioxide (CO2), methane (CH4), ammonia (NH3), and
formaldehyde
(H2CO). Volatiles and solids appear to be fairly well mixed throughout
the nucleus of a new comet approaching the Sun for the first time. As a
comet ages from many trips close to the Sun, there is evidence that it
loses most of its ices, or at least those ices anywhere near the
nucleus
surface, and becomes just a very fragile old "rock" in appearance,
indistinguishable
at a distance from an asteroid.
A comet nucleus is small, so its gravitational
pull is very weak. You could run and jump completely off of it (if you
could get traction). The escape velocity is only about 1 m/s (compared
to 11 km/s on Earth). As a result, the escaping gases and the small
solid
particles (dust) that they drag with them never fall back to the
nucleus
surface. Radiation pressure, the pressure of sunlight, forces the dust
particles back into a dust tail in the direction opposite to the Sun. A
comet's tail can be tens of millions of kilometers in length when seen
in the reflected sunlight. The gas molecules are torn apart by solar
ultraviolet
light, often losing electrons and becoming electrically charged
fragments
or ions. The ions interact with the wind of charged particles flowing
out
from the Sun and are forced back into an ion tail, which again can
extend
for millions of kilometers in the direction opposite to the Sun. These
ions can be seen as they fluoresce in sunlight.
It is about spherical with an average
diameter of 200 000 km at 1 AU of the Sun (100 000 km for Hale-Bopp).
Its
size varies inversely with the square of the heliocentric distance.
Why is there a luminous halo?
Sublimation creates a gaseous environment
that become excited under the influence of sunlight and of the protons
from the solar wind. The ensuing fluorescence results from a subsequent
transition of an electron between two energy levels.
Why does the coma vary in size?
Comets are very small in size relative
to planets. Their average diameters usually range from 750 m or less to
about 20 km. Recently, evidence has been found for much larger distant
comets, perhaps having diameters of 300 km or more, but these sizes are
still small compared to planets. Planets are usually more or less
spherical
in shape, usually bulging slightly at the equator. Comets are irregular
in shape, with their longest dimension often twice the shortest. The
best
evidence suggests that comets are very fragile. Their tensile strength
(the stress they can take without being pulled apart) appears to be
only
about 1,000 dynes/cm^2 (about 2 lb./ft.^2). You could take a big piece
of cometary material and simply pull it in two with your bare hands,
something
like a poorly compacted snowball.
When the nucleus approaches the Sun,
the condensed gases evaporate. As the temperature increases, so does
the
rate of sublimation and new species (which have a higher temperature of
sublimation) are set free. So, the size of the coma increases when it
approaches
the Sun. The size of the coma can also vary from one comet to another
depending
upon the materials it is made of (because they have different
temperatures
of sublimation).
Types of
Tails
Every comet then really has two tails,
a dust tail and an ion tail. If the comet is faint, only one or neither
tail may be detectable, and the comet may appear just as a fuzzy blob
of
light, even in a big telescope. The density of material in the coma and
tails is very low, lower than the best vacuum that can be produced in
most
laboratories. In 1986 the Giotto spacecraft flew right through Comet
Halley
only a few hundred kilometers from the nucleus. Though the coma and
tails
of a comet may extend for tens of millions of kilometers and become
easily
visible to the naked eye in Earth's night sky, as Comet West's were in
1976, the entire phenomenon is the product of a tiny nucleus only a few
kilometers across.
The ionic tail, known as type
I or plasma, is made up of ions. This tail is straight and is always
opposed
to the direction of the Sun. It is up to 100 000 km wide and of a few
tens
of million km long. Blue is the colour which dominates its spectrum.
Why are there ions?
In order to understand that, it is
important to know another phenomenon: solar wind. This is a wind of
charged
particles wound in an helix around the solar magnetic field. This wind
moves at a speed of 400 km per s. The gases are ionised either by
photoionisation
of the neutral molecules under the action of the solar ultraviolet
radiation,
or under the action of the solar wind by a phenomenon where a proton
removes
an electron from an atom.
Why is it straight and opposed to the
direction of the Sun?
The initial velocity of the ions is
provided by the reaction that produced them. The ions are carried by
the
solar magnetic field at 10 to 100 km per s. The tail is thus straight
on
account of the speed of the ions, and forms an angle of a few degrees
with
direction of the Sun, in the direction opposed to the motion.
Where does the light come from?
It is emitted by fluorescence: a particle
of solar wind excites an electron of the atom or molecule concerned.
This
electron reaches a metastable level, goes down again and releases its
surplus
energy in the form of a photon (particle of light) of a well determined
energy and thus, of a specific colour.
Why is it blue?
The blue colour is explained by the
fact that the most abundant ion, CO+ emits around 400 to 460 nm, in the
blue part of the spectrum.
The Type II Tail is the most spectacular part: it extends over several millions km and is made up of particles of dust whose size varies between 0.1 and 10 m m. It is very different from the ion tail. Although remaining in the plane of the orbit, the dust trails behind the motion of the comet giving this tail a curved appearance. The dominant colour of the dust tail is yellowish.
Why is dust accelerated in the direction
opposite to the Sun?
The motion of the particles leaving
the coma is influenced by forces external to the comet: the
gravitational
attraction of the Sun, Fg = (where M is the mass of the Sun and m, that
of dust), and also a repelling force which will project them towards
the
tail; this force comes from the radiation pressure of the Sun (the
photons
communicate to the particles an impulse directed opposite to the Sun;
it
is the pressure of radiation, Fr = (where a = the radius of the
particle;
s r(a) = coefficient measuring the effectiveness of the radiation
pressure;
F = Solar constant; c = the speed of the light and r = the heliocentric
distance).
The speed of the dust which makes it up is about 0.1 to a few km per s (much smaller than the orbital velocity of the comet around the Sun). Each grain of dust follows a slightly different ellipse leaving a curved trail.
Why is it of yellowish colour?
The dust is mainly made up of silicates
(principal component of sand). This dust does not emit any light (like
the ion tail does it by fluorescence) but only reflects that of the Sun
(which is of a yellowish white).
The anomalous tail or anti-tail, is observed when the Earth is about in the plane of the cometary orbit; it is thus rather rare and of short duration. One then sees a tail pointing towards the Sun, hence its name. Such anti-tails were observed in the comets Arend-Roland (1957 III), Kohoutek (1973 XII) and Hale-Bopp (C/1995 O1).
Why does this tail point towards the
Sun?
To explain the appearance of this tail,
one can put forward two hypothesis:
1. It could be due to the accumulation of grains of large size (approximately 50 mm) which escape very slowly from the nucleus and would be emitted at a much earlier time. They need to be seen " on edge " so that the light that they diffuse is sufficient to simulate a tail.
2. It could be due to a visual effect
where the curve of the tail of dust would
reveal a small point opposed to the
principal tail. We think that the second hypothesis is more likely
because
the first one does not explain why the anti-tail is so narrow.
Physics
Comets, of course, must obey the same
universal laws of motion as do all other bodies. Where the orbits of
planets
around the Sun are nearly circular, however, the orbits of comets are
quite
elongated. Nearly 100 known comets have periods (the time it takes them
to make one complete trip around the Sun) five to seven Earth years in
length. Their farthest point from the Sun (their aphelion) is near
Jupiter's
orbit, with the closest point (perihelion) being much nearer to Earth.
A few comets like Halley have their aphelions beyond Neptune (which is
six times as far from the Sun as Jupiter). Other comets come from much
farther out yet, and it may take them thousands or even hundreds of
thousands
of years to make one complete orbit around the Sun. In all cases, if a
comet approaches near to Jupiter, it is strongly attracted by the
gravitational
pull of that giant among planets, and its orbit is perturbed (changed),
sometimes radically. This is part of what happened to Shoemaker-Levy 9.
Sodium Tail
As the photograph of its discovery
shows, its appearance is markedly different from the other tails. It
would
be about 800 000 km broad and at least 12 000 000 km long. It is
observable
only with a narrow band yellow filter because the concentration of
neutral
sodium is very small.
Is this the first time this phenomenon
is observed?
No, it has already been observed during
a few hours on certain comets with a perihelion lower than 0.05 AU. At
that point, temperature is so high that metals like sodium can be
ionised.
Hale-Bopp however did not reach that close to the Sun, and its sodium
tail
is thus one of a kind.
Where does this tail come from?
We know that the neutral sodium is
not directly emitted from the nucleus but is ejected from grains of
sand
from the tail of dust.
How is this tail formed?
That remains a mystery because the
magnetic field cannot carry molecules or atoms which are not ionised
and
the pressure of radiation does not act on grains of diameter lower than
0.1 m m.
How was it discovered?
From spectroscopy carried out by satellites
in 1970 hydrogen was discovered in the comets Tago-Sato-Kosaka and
Bennett
from the Lyman a line at 121.6 nm. Ionised hydrogen forms in fact an
enormous
halo (150 millions km for Hale-Bopp) which largely exceeds the limits
of
the coma.
Why isn't it not visible since the
Earth?
Atomic hydrogen emits in the ultraviolet
(l < 300 nm) but the ozone layer (O3) stop these wavelengths. The
hydrogen
cloud can only be observed from space, with satellites, for example. It
is also for this reason that the hydrogen cloud was discovered so late.
Because comet nuclei are so small, they are quite difficult to study from Earth. They always appear at most as a point of light in even the largest telescope, if not lost completely in the glare of the coma. A great deal was learned when the European Space Agency, the Soviet Union, and the Japanese sent spacecraft to fly by Comet Halley in 1986. For the first time, actual images of an active nucleus were obtained and the composition of the dust and gases flowing from it was directly measured. The opportunity to reconstruct the events that occurred when Shoemaker-Levy 9 split and to study those that will occur when the fragments are destroyed in Jupiter's atmosphere is uniquely important.