Infrared Views of the Solar System from Space 689
FIGURE 11 The most prominent dust trail in 1983 was
associated with the short period comet Tempel 2. The dust coma
and tail appear as the fish to the dust trail’s stream. Trails are
characteristically narrow (as a consequence of the small relative
velocities of the constituent dust relative to the nucleus of the
parent comet) and trace out a portion of the comet’s orbit. The
particles ahead of the comet (to the left) are preferentially larger
than those following the comet.
Viewed from space in the thermal infrared, many short-
period comets were found to have very extensive, narrow
trails consisting of millimeter- to centimeter-sized particles
extending degrees to tens of degrees across the sky (Fig. 11).
The narrowness of the trails is due to the low velocities with
which their constituent particles are ejected. They retain
a record of comet emission history over a period of years
to centuries. For comets having perihelia interior to the
Earth’s orbit, trails represent the birth of a meteor stream.
The number density of particles within them are such that
were the Earth to pass through one, there would be a “me-
teor storm” equal to or exceeding the famous Leonid storms
of 1833 and 1966. First discovered byIRAS, it was inferred
thatdust trailswere common to short-period comets. Con-
tinuing surveys bySpitzersuggest this is the case (Fig. 12).
Space-based infrared observations revealed that comets
possessed far more dust than had been thought. Classical
“gassy” comets such as P/Encke were found to possess both
a significant large particle dust coma and trail (Fig. 13).
Encke’s trail was found to extend over 80◦of its orbit. It
was determined that the ejection of large particles into trails
was the principal mechanism by which comets lose mass.
These particles quickly devolitilize after leaving the comet
nucleus; this means that most of the comet’s mass loss is in
refractory particles.
The discovery of cometary dust trails is changing the
picture of comet nuclei from being primarily icy bodies to
objects more akin to “frozen mudballs” because of their
much higher than expected fraction of refractory dust. The
fraction of dust-to-gas in comet nuclei provides important
information about where the comets formed and how they
evolve, once captured into short-period orbits.
FIGURE 12 Spitzer has detected the first new dust trails in the
infrared since IRAS. Shown are P/Johnson (top) and
P/Shoemaker-Levy 3 (bottom). Spitzer is confirming the
commonality of such large particle emissions across the
short-period comet population. (Figure courtesy of W. Reach.)Dust-to-gas mass ratios corresponding to the canonical
dirty snowball model range between 0.1 and 1. If we were to
compress comet nucleus material so that refractories have
a density of 3 g/cm^3 and volatiles had a density of 1 g/cm^3 ,
this would give us a nucleus in which 3–33% of the volume
consisted of refractory material.FIGURE 13 Comet Encke (left) is considered a classical ‘gassy’
comet based on visible wavelength observations showing only a
gas coma and no dust tail. (Image courtesy of J. Scotti.) An ISO
map (right) of P/Encke and its trail at 11.5μm, evidencing
anisotropic emission and requiring the spin axis of the nucleus to
lie nearly in the orbital plane. The inferred dust-to-gas mass ratio
of 10–30 is even higher than that inferred from IRAS
observations. (Figure courtesy of W. Reach.)