We have included gravity in our fluid dynamic model with chemical kinetics of dusty comet comae and applied it with two dust sizes to 2060 Chiron. A progress report on the model and preliminary results concerning gas/dust dynamics and chemistry is given

Boice, Daniel C.

Huebner, Walter F.

Konno, I.

Stern, S. Alan

English

NASA Technical Reports Server

Asteroids, Comets, Meteors 1991, pp. 85-88
Lunar and 1992 _ - "--" 85Planetary Institute, Houston, / _ _, _,,'._ _
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A PRELIMINARY MODEL OF THE COMA OF 2060 CHIRON
D.C. BOICE, I. KONNO, S. ALAN STERN, and W.F. HUEBNER
Southwest Research Institute, San Antonio, TX 78228-0510 U.S.A.
Abstract. We have included gravity in our fluid dynamic model with chemical kinetics of dusty comet
comae and applied it with two dust sizes to 2060 Chiron. A progress report on the model and preliminary
results concerning gas/dust dynamics and chemistry is given.
1. INTRODUCTION
There has been much interest in 2060 Chiron since observations of comet-like activity and a
resolved coma established it is a comet. Determinations of its radius range from 65 to 200 km,
making Chiron unique among bodies with atmospheres in the Solar System. This unusually large
size for a comet suggests that the atmosphere of Chiron is intermediate to the tightly bound, thin
atmospheres typical of planets and satellites and the greatly extended atmospheres in free expansion
typical of cometary comae. Under certain conditions (depending on molecular weight, temperature,
heliocentric distance, and the size and mass of Chiron) it may gravitationally bind an atmosphere
which is thick compared to its size while a significant amount of gas escapes to an extensive exo-
sphere. These attributes coupled with reports of sporadic outbursts at large heliocentric distances
(_ 12 AU) (1,2) and the identification of CN in the coma (3) make Chiron a challenging object to
model. Simple models of gas production and a dusty coma have been recently presented by several
investigators (see, e.g., 4-6) but a general concensus on many basic features has not emerged. We
have begun development of a more complete coma model of Chiton (7). The objectives of this
paper are to report progress on this model and give preliminary results for understanding Chiron.
2. GAS PRODUCTION
Throughout its orbit, Chiron remains too far from the Sun for direct sublimation of water to be
important, but CO, N2, and CH4 ices could be the cause of activity throughout its complete orbit
(4,7). C02 production "turns on" only about 10 years prior to perihelion. At perihelion, on the
order of a Mg/s of CO can be released even if only a small amount (a few %) of Chiron's surface is
active. At these activity levels, the mean free path of a CO molecule at the surface is on the order of
100 m, much smaller than the flow scale length, so the sublimating gas is collisionally coupled and
the fluid dynamic approach is required. The extent of the collision region can be determined only
by detailed modeling or observations but an upper limit can be estimated assuming free expansion
(R -2 density distribution) and isotropic emission. In this limit, its size is roughly 3.103 km, about
the same as the collision zone of P/Halley at perihelion. Under the influence of Chiron's gravity (a
few to 10 cm s-2), fluid dynamic conditions near the surface are even more favorable.
The sublimation of a volatile, like CO, leads to a gas temperature close to 30 K throughout
Chiron's orbit (5), far below that of a blackbody in radiative equilibrium. This results in a thermal
velocity that is comparable to the escape velocity (7), given uncertainities in the size and mass
of Chiron. This situation may lead to a bound atmosphere with extensive exosphere as slower
molecules in the Maxwellian distribution cannot escape while those traveling faster than the escape
velocity leave. The loss of these more energetic molecules results in substantial cooling of the
remaining gas, making it more tightly bound. However, slightly higher thermal velocities lead to
hydrodynamic escape or "blow-off" of the atmosphere (8). This situation is complex and sensitive
to the actual bulk parameters.
Other processes can heat and cool the gas (7), including photo-reactions (heating), radiative
"cooling, collisions with grains that are hotter than the gas (heating), sublimation from icy grains,
https://ntrs.nasa.gov/search.jsp?R=19930009943 2020-03-17T07:34:49+00:00Z
86 Asteroids, Comets, Meteors 1991
and expansion cooling of the gas. In the case of photo-destruction of CO, typical rates (9) at 10 AU
yield a lifetime of 4.5 years, making this a minor source of energy and ions on smaller timescales.
Charge exchange with the solar wind may be an important ion source at Chiron also. Each of these
effects requires investigation with detailed modeling to assess its significance, as we are undertaking.
3. THE DUST COMA
The sublimating gas entrains dust particles as it leaves Chiron's surface. The dust dynamics is
also influenced by the gravity and rotation of Chiron within a sphere of influence given by (10),
RGS = r( Mchiron/2Msun) 1/3.
At r = 10 AU, adopting a radius of 120 km and a density of 1 gcm -3 for Chiron, Ras -_
1500 Rchiron. Outside of this region, both solar gravity and radiation pressure must be taken into
account. The maximum particle size that can be lifted by gas drag has been estimated to be on
the order of 100/*m for CO sublimation (5). In a typical comet, the gas density decreases as R -2,
and for large particles (with velocities much smaller than the gas) the ratio of the gas drag to
gravity is constant (11). Once lifted off the nucleus, these particles will leave the comet. However,
considering gas production from restricted active areas and the effects of gravity on the gas, the
gas density will decrease more rapidly than R -2 and these larger particles may decouple from the
gas drag before escape, traveling in bound orbits and may eventually fall back to the surface. The
extent of the gas-dust interaction region depends on particle size and bulk density. Based on our
preliminary model, we estimate the size of the acceleration region to be on the order of ten Chiron
radii for 1 to 10 #m particles.
Beyond the gas-dust interaction region, the dust trajectories are increasingly influenced by ra-
diation pressure and can be approximated by the fountain model (12). Micrometer-sized particles
follow parabolic orbits confined within an envelope which is a paraboloid with focus in the nucleus
and apex in the sunward direction. The standoff distance can be roughly estimated for these parti-
cles, using typical parameters for Chiron, to be on the order Of 100 Reh_ro,_. At further distances,
the solar gravity and the Poynting-Robertson effect influence the dust dynamics. Other effects that
may need to be considered in a realistic model of the dust coma include grain-grain collisions and
charging of particles by solar wind, secondary electron emission, or coma plasma.
4. PRELIMINARY MODEL OF 2060 CHIRON
One-dimensional, multi-fluid simulations of the coma of Chiton at perihelion have been per-
formed. These simulations are based on our model that treats the physics and chemistry of the
inner coma in great detail (9). Recent progress of the model includes incorporation of dust en-
trainment by the gas, dust size distributions, dust fragmentation, and distributed coma sources of
gas-phase species related to the dust described in (13).
In this preliminary model of Chiron, CO is assumed to be the only volatile. Other model
parameters include r = 8.51 AU, Rchiron --= 120 km, A = 0.03, PChiron ---- 1. The dust-to-gas mass
production ratio (X) is assumed to be 1. Two sizes of dust (a = 1 and 10/_m) are considered and the
dust mass distribution is approximated by a power law (exponent = 0.5). The model capabilities
for dust fragmentation are not used in the present calculations. Being a diatomic molecule, CO
is not an efficient emitter in the infrared so the radiative cooling term has been omitted in this
first-order model. For comparison, a dust-free model (X = 0) and a model with a modest amount
of dust (X = 0.1) were produced also.
In the simulations, gas and dust are rapidly accelerated upon leaving the nucleus as illustrated
in Fig. 1. For standard dust densities, small particles are more efficiently entrained with the
gas flow than large particles, resulting in higher terminal speeds. The acceleration zone for all
particles is approximately within 10 RChiron. The inclusion of dust has two important effects on
Asteroids, Comets, Meteors 1991 87
the gas flow. The first is an initial mass-loading of the gas, reducing the gas velocity to subsonic
values close to the surface of the nucleus. The second effect is a strong coupling of the gas and
dust temperatures near the nucleus as shown in Fig. 2. Upon release, the dust heats rapidly to its
radiative equilibrium value of 95 K. Collisions of molecules with dust particles heat the gas (initially
at 30 K) to 85 K within a Chiton radius. This results in a terminal gas velocity about 80% higher
than that calculated from a pure gas model. Even with a modest amount of dust (X = 0.1), the gas
is significantly heated in the near-nucleus region. A complete description of our model and more
extensive results will be presented in a forthcoming publication.
Acknowledgements. This research was supported by grants from NASA (Nos. NAGW-2205
and NAGW-2370).
5. REFERENCES
(1) Tholen D.J., Hartmann W.K., and Cruikshank D.P. (1988) IAU Circ. No. 4554.
(2) Bus S.J., Bowell E., Harris, A.W., and Hewitt A.V. (1989) 2060 Chiron: CCD and Electrono-
graphic Photometry. Icarus 77, 223-238.
(3) Bus S.J., A'Hearn M.F., Schleicher, D.G., and Bowel], E. (1991) Detection of CN Emission
from (2060) Chiron. Science 251,774-777.
(4) Stern S.A. (1989) Implications of Volatile Release from Object 2060 Chiron. PASP 101,126-
132.
(5) Luu J.X. and Jewitt D.C. (1990) Cometary Activity in 2060 Chiron. Astron. J. 100, 913-932.
(6) Meech K.J. and Belton M.J.S. (1990) The Atmosphere of 2060 Chiron. Astron. J. 100,
1323-1338.
(7) Boice D.C., Stern S.A., and Huebner W.F. (1991) On the Atmosphere of 2060 Chiton. LPSC
XXII, 121-122.
(8) Hunten D.M. and Watson A.J. (1982) Stability of Pluto's Atmosphere. Icarus 51,665-667.
(9) Schmidt tt.U., Wegmann R., Huebner W.F., and Boice D.C. (1988) Cometary Gas and Plasma
Flow with Detailed Chemistry. Comp. Phys. Comm. 49, 17-59.
(10) Opik E. (1963) Survival of Cometary Nuclei and the Asteroids. Adv. Astron. Astrophys. 2,
219-262.
(11) Griin E. and Jessberger E.K. (1990) Dust. In Physics and Chemistry of Comets (W.F.
Huebner, ed.), pp. 122-126. Springer-Verlag, Berlin.
(12) Hazer L. (1965) Calcul de Distribution d'Intensit_ Relative dans une T_te Comhtaire. Congr_s
Colloques l'Universitd de Liege, 37, 233-241.
(13) Konno I., Huebner W.F., and D.C. Boice (1991) Icarus, submitted.
88 "Asteroids, Comets, Meteors 1991
0.5
[
0.4t-
Figure 1.
_-_0.3
o
0.2
0.1
.-" - - vd (l i_)
/
/
j/
I
0 i ........... I
1O0 1000
DISTANCE (KM)
Gas and dust speeds for Chlron at perihelion.
104
The dust accelerationregion is within
about 10 RChiron. For comparison, the dotted line is the gas speed for a dust-free model (X = 0).
_d
100
80
20
0
100 IOOO 104
DISTANCE (KM)
Figure 2. Temperature profiles of the gas and dust for Chiron at perihelion. Upon release from the
nucleus, both sizes of dust quickly reach radiative equilibrium at 95 K. The gas is strongly heated
by the dust in the near-nucleus region for the X = 1 case. Even with a modest dust-to-gas mass
ratio (X = 0.1), the gas is heated significantly above the dust-free model (X = O).


A preliminary model of the coma of 2060 Chiron

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