Physics and Chemistry of Comets 567
Modeling the outflow of hydrogen (the lifetime of the
H atoms is determined primarily by the proton flux in the
solar wind) to produce the observed cloud size shows that
the required outflow speed is 8 km sec−^1. This is much
larger than the outflow speed in the coma,≈1kmsec−^1.
An additional energy source is needed. If H 2 O were pho-
todissociated, a speed of 19 km sec−^1 would result, and this
value is too high. The likely scenario is that OH is produced
by photodissociation and then is further dissociated into H
outside the thermalization region. These H atoms and the
thermalized H atoms from H 2 O photodissociation combine
to give the deduced outflow speed of 8 km sec−^1.
The outflow rate of hydrogen,QH, provides a good sur-
rogate for the total gas production rate from a comet. For
large comets, this rate can approach 10^31 atoms sec−^1 , and
the general range is 10^27 –10^30 atoms sec−^1. The heliocentric
variation is roughlyrh−^1.^3 (rh=the heliocentric distance).
This expression follows the practice of basing variations
on the value at 1 AU (where comets are most easily ob-
served) and using a power law to give the heliocentric varia-
tion.
Early dust measurements were made in the coma of
comet Halley by dust detectors on theVEGAspacecraft
and onGiotto.Three basic types of dust composition were
found. The CHON particles have only the light elements
Carbon,Hydrogen,Oxygen, andNitrogen. The silicate par-
ticles are rich in Silicon, Magnesium, and Iron. The third
type is essentially a mixture of the CHON and silicate types.
The differential size distribution can be represented by a
power law in size,ra, witha∼−3.5, for grain sizes greater
than 20μm. This implies that most of the dust mass is
emitted in large grains. There was also evidence for large
numbers of small dust grains down to sizes of 0.01μm. The
results are compatible with the sizes needed in models of
the dust tail.
Interest in the dust particles from the coma has increased
with the return to Earth of the dust collected byStardust.
Some of the coma dust particles may be similar to the fluffy
particles collected in the Earth’s upper atmosphere, the in-
terplanetary dust particles (IDP) or “Brownlee particles.”
But, having particles collected in the coma and available for
analysis in the laboratory opens a whole new era. The sam-
ple return portion of theStardustmission to comet Wild 2
was accomplished by catching the particles in an ultra low-
density glass-like material called aerogel. The collection ex-
ceeded expectations with thousands of particles embedded
in the aerogel. The mineral structure has been preserved
for many of the grains. Some first results indicate the pres-
ence of high-temperature minerals such as olivine, one of
the most common minerals in the universe. It certainly did
not form inside the comet’s cold body. It probably formed
near the Sun or from hot regions around other stars. In any
event, the discovery that cometary material contains sub-
stances formed in hot and cold environments adds a new
constraint to formation scenarios.
5. Tails
The dust and gas in the coma are the raw materials for the
comet’s tails. The prominent dust and gas (plasma) tails are
the traditional identifying characteristic of comets. Dust
tails are flat, curved structures and, compared to plasma
tails, are relatively featureless. They can reach lengths
∼ 107 km.
Dust particles, once they are decoupled from the coma
gas, are in independent orbits around the Sun. But the so-
lar gravitational attraction is not the full value because the
dust particles generally stream away from the Sun. An extra
force, solar radiation pressure, is acting on the particles. Be-
cause both solar gravity and radiation pressure vary asrh−^2 ,
the orbit is determined by initial conditions and an effec-
tive gravity. The parameterμis the ratio of the net force on
the tail particle to the gravitational force. Or, the parameter
(1−μ) gives the normalized nongravitational force.
For a constant emission rate of dust particles with a single
size or a small range of sizes, thesyndyne(or same force)
from the Bessel–Bredichin theory is a good description.
The tails are tangent to the radius vector (the prolonged
Sun–comet line) at the head, and the curvature of the tail
increases with decreasing (1−μ). An important concept is
the fact that the shape of a particle’s orbit is not the observed
shape of the tail. The observed tail shape is the locations
of dust particles at a specific time of previously emitted
particles.
Another case from the Bessel–Bredichin theory is the
synchrone(or same time). It is produced by particles with
many sizes [or values of (1−μ)] being emitted at the same
time. These features are rectilinear, and the angle with the
radius vector increases with time. This type of feature is
occasionally observed as synchronic bands.
In practice, comets emit dust particles with a range of
sizes and at a rate that varies with time. Several computa-
tional approaches that accurately model observed dust tails
with reasonable assumptions are available. The size distri-
bution generally peaks at a diameter around 1μm.
Besides the synchronic bands (mentioned earlier), fine
structure in the form ofstriaeoccasionally appear in dust
tails. They are a system of parallel, narrow bands found
at large distances from the head. So far, striae appear at
heliocentric distances greater than 1 AU and always after
perihelion. Figure 14 shows a spectacular example in comet
Hale–Bopp. Currently, there is no satisfactory explanation.
Organization by the solar wind’s magnetic field acting on
electrically charged dust particles or dust particle fragmen-
tation has been proposed.
Two other dust features are sometimes observed. Anti-
tails or sunward spikes are produced by large dust particles
in the plane of the comet’s orbit. These particles do not ex-
perience the relatively large force that sends the smaller
dust particles into the dust tail. They remain near the
comet and, when seen in projection, appear to point in the