690 Encyclopedia of the Solar System
FIGURE 14 The canonical “dirty-snowball” model of comets,
inferred from groundbased observations at visible wavelengths,
is compared to the “frozen-mudball” model inferred from
spacebased observations in the thermal infrared. All refractories
are collected at the center in both cases.
This picture is based largely on ground-based observa-
tions of dust at visual wavelengths, sensitive to particles
within a decade or so of 1μm in size. These observations
underestimate the mass fluence of dust from comets.
Most of cometary mass loss appears to be in much larger
(and dark) particles, which are difficult to detect at visual
wavelengths. This conclusion was also reached after the
EuropeanGiottospacecraft was struck by a small number
of large particles as it flew by comets Halley and Grigg–
Skellerup.
Analysis of theIRASobservations of eight trails indi-
cates that short-period comets lose their mass primarily in
refractory particles in the mm to cm diameter size range. An
average dust-to-gas mass ratio of 3 was calculated (Fig. 14).
This was the upper limit inferred for Halley byGiotto(with
a nominal value of 2). Assuming the same densities for re-
fractories and volatiles as previously, this corresponds to a
comet nucleus that is 75% refractive by mass and 50% by
volume (Fig. 15). Mixing equal volumes of dirt and water
in a backyard experiment demonstrates the apt description
of such a mixture as a mudball.
These dust-to-gas ratios also provide insight into the
formation location of short-period comets. Dynamical
considerations have lead investigators to focus on the
proto-Uranus and proto-Neptune regions as that location.
Significant amounts of ice have long suggested the outer
solar system as the source of short-period comets. Consid-
eration has also been given to their formation beyond the
solar system, for instance in molecular clouds. Models of
comets forming in such interstellar locations yield comets
dominated by their volatile components, contradicting in-
ferences drawn from space-based thermal infrared obser-
vations. On the other hand, it is very interesting that both
FIGURE 15 Dust to gas mass ratios are shown for comets
having detected dust trails. For comparison, values are shown for
Halley, Triton, and Pluto. The shaded area spans the “canonical”
ratios between 0.1 and 1.Pluto and Triton have effective dust-to-gas mass ratios that
are identical to the average comet values determined from
IRASandGiotto(Fig. 15). This is not unexpected if Pluto
and Triton accumulated from proto-comets in the vicinity
of Neptune’s orbit.
The existence of dust trails indicates that short-period
comets are losing mass more rapidly than previously
thought. Hence, their lifetime against sublimation may be
shorter. A greater fraction of refractory material, however,
would allow for the rapid formation of a nonvolatile man-
tle that is difficult to blow off, progressively choking off
cometary activity. Such a mantle was apparent in theGiotto
images of the Halley nucleus, which was near perihelion at
the time. When activity is choked off, the comet would look
like an asteroid until such time as sufficient pressure built
up from subsurface ices to break through the crust in a burst
of resumed activity. The discovery in August 1992 that as-
teroid 4015 was actually comet P/Wilson–Harrington (last
seen in outburst in 1949) provided the first hard evidence
of such “dormant” comets in the inner solar system.
In addition to trails associated with known short-period
comets, IRAS also detected trails having no known source
(Fig. 16). Unfortunately, since these were discovered in the
data long after the mission had ended, it was not possible
to follow up theIRASobservations with observations from
the ground in order to determine their orbits. So these ob-
jects are now lost. However, assuming a cometary origin,
the numbers of these “orphan trails” suggest that there
may be twice as many short-period comets as previously
recorded, with the majority of them being less active and
hence more difficult to detect by traditional means. The
serendipitous detection of orphans requires a space-based
thermal infrared survey of the sky having sufficient spa-
tial resolution. Such a survey is planned for the ongoing
JapaneseAkarimission and the future NASAWide-field
Infrared Survey Explorer(WISE).
Space-based thermal infrared telescopes have also
provided direct compositional information about comets
through spectroscopic observations raising questions about