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Comets
Nature and Nomenclature of Comets
• A comet is a small ice-rich body, about 1 km size, in an eccentric orbit
about the Sun. The nuclei have low albedo (0.03) and red color. The mass
density is 1g/cm3 or less.
• The phenomena that may make the comet spectacular are associated with
the progressively rapid evaporation of ices from its solid nucleus during its
rapid fall toward the Sun.
• These gases, and some entrained dust, expand outward around the nucleus
to form a roughly spherical envelope (coma) about the nucleus. At a
distance of 1,000 - 100,000km from the nucleus, the expanding coma
becomes so tenuous that the gas and dust become uncoupled and begin to
stream systematically away from the nucleus in different direction. The
dust roughly follows ballistic trajectories, whereas the gas flows in the
direction radially outward from the Sun (tail).
Size of comet nuclei
Meech et al. (2004)
Hale-Bopp
--> Largest observed comet
1P/Halley
17P/Holmes
Figure 1
5.5km
1.7km
January 8, 2005
v
Anti-solar direction
Figure 2
C/2004 Q2 (courtesy of Tai, NHAO)
Nature and Nomenclature of Comets
• Many comets have orbits that lie well within the Solar System.
Frequently the apherlia of their orbits are near the Jupiter or Saturn.
Their orbital periods are below 200 years (corresponding to the
semimajor axis < 34AU). These are called “Short-period comets”.
• The other comets have orbits that are nearly randomly distributed in
space, have enormous orbital eccentricities and semimajor axes, and
typically pass perihelion only about once every few million years.
These comets are referred to as “Long-period comets”. Recently, some
dynamical families were found among these long-period comets (e.g.
Kreutz sun-grazer).
Nature and Nomenclature of Comets
• Historically, when a comet is discovered it has been named after the discoverer
(e.g. Comet Holmes was discovered by Edwin Holmes).
• The prefix C/ denotes a comet. When a comet discovered, it is given preliminary
designation such as C/2001 Q3 (SOHO). In this example, “2001” is the date of
discovery and “Q” designates the half-month in which the discovery was made
(A for the first half of January, B for the second half, and so on, skipping “I”).
The name of the discoverer is appended in parentheses.
• Periodic comets are assigned the prefix P/ (for periodic). Since there are many
observers who have found more than one comets, it has become convenient to
affix a catalog number to each periodic comet. It is not unusual for a comet to be
discovered by two or more observers on the same night, an event that produces
names such as 29P/Schwassmann-Wachmann 1, 31P/Schwassmann-Wachmann 2,
73P/Schwassmann-Wachmann 3.
• Occasionally a comet goes dormant or is lost due to the inaccurate orbital data.
The comets that have suffered such a fate bear the designation D/.(e.g.
D/Shoemaker-Levy)
1. Orbits
Cometary Orbits 1: Short-Period Comet
• The orbits of the short-period comets are ellipses of moderate eccentricity and
inclination (Figure 3). Most of comets have inclinations of less than 30
relative to the ecliptic plane. Their eccentricities lie mostly 0.2-0.7. The
perihelion distances of most comets are1-2AU.
• Several comets have retrograde orbits (e.g. 1P/Halley, 55P/Tempel-Tuttle). The
shortest orbital period is 3.3 years for 2P/Encke, a body whose orbit fall within
the range of near-Earth asteroid orbits. 96P/Machholtz 1 has the shortest
perihelion distance of 0.12AU. 29P/Schwassmann-Wachamann 1 have
perihelion beyond the Jupiter orbit (5.77AU).
• The longitudes of the node and the longitudes of the perihelia of short-period
cometary orbits are roughly evenly distributed around the ecliptic.
2P/Encke
23P/Brorsen-Metcalf
8P/Tuttle (Ursids)
1P/Halley (Orionids)
55P/Tempel-Tuttle
(Leonid)
17P/Holmes
29P/Schwassmann-Wchamann 1
Figure 3. Eccentricity-inclination plot for short period comets. (updated 2007.11.5)
Figure 4. Eccentricity-Period plot for short period comets. (updated 2007.11.5)
2P/Encke
P/Read (S3)
Figure 5. e-Q (aphelion distance) plot for short period comets.
P/Read (S3)
Main-Belt
29P/Schwassmann-Wachamann 1
Figure 6. e-q (perihelion distance) plot for short period comets.
Halley-type comets
Jupiter-family comets
Figure ７. i-Q (aphelion distance) plot for short period comets.
Figure 8.  plot for short period comets.
Sun-grazing comet
http://soho.esac.esa.int
Figure 9.
Orbital Elements of Comets (Tp>200 years)
Kreutz Sungrazers
Kreutz Sungrazers
Figure 8.
•
The origin of the long-period comets is an interesting problem. It would be helpful to
explore the distribution of the long-period comets over their entire range of semimajor
axes a from 100 AU.
Figure 12.
N
The noticeable features are that there is a strong peak in
the distribution of observed comets near 1/a=0,
parabolic heliocentric orbit and a lack of comets with
energies greater than parabolic must be rare.
The right side, the energy constant for an elliptical
orbit, is a function of its semi-major axis alone and is
independent of the eccentricity. Similar quantities can
be defined for parabolic and hyperbolic orbits. It can
be shown that Cpara=0 and Chyper=/2a.
1/a (AU-1)
Dones et al. (2004)
•
The most common stars are not Sunlike stars but are M9 dwarfs of mass 0.07 M. These
stars perturbed the Oort cloud every 11 million years.
A combination of proper motion and parallax data by Hipparcos and radial velocity
data by ground-based telescope tells us stellar encounters with the Oort cloud
(Garcia-Sanchez et al. 1999)
Figure 14.
•
The huge spheroidal cloud of long-period comets with the radius of ≈104 AU is first
suggested by Dutch astrophysicist, Jan Oort in 1950, and it is referred to as “Oort cloud”
•
The orbital velocity of a comet near aphelion can be calculate from
æ2 1ö
v = GM ç - ÷
è r aø
2
whence the orbital velocity of a comet of a= 104 AU is found to be 0.3 km/s.
•
Long-period comets have very small velocities
aphelion. They spend virtually all of their lives at
distance from the Sun, stored at extremely
temperatures. The blackbody temperature by
illumination at r 104 (AU) is
near
great
low
solar
æ r ö-1/ 2
T(r) = 280ç
÷
è AU ø
= 280 ´ (10 4 )-1/ 2 = 2.8 [K]
• Nowadays, the Kuiper belt is believed to be the main source for short-period
comets. It is a region of the Solar System between 30 AU to ~55 AU from the
Sun. The Kuiper belt is similar to the main-belt asteroid that consists of small
bodies.
• The most widely-accepted hypothesis of its formation is that the Oort cloud's
objects initially formed much closer to the Sun as part of the same process that
formed the planets and asteroids, but that gravitational interaction with young
gas giants such as Jupiter ejected them into extremely long elliptical or
parabolic orbits
First Kuiper-belt object, Jewitt and Luu (1993)
Heating by Passing Stars
•
•
•
The prospect that long-period comets may be pristine, unaltered fossils that have escaped
from significant heating since the time of their formation suggests that they may be the
best probes of ancient Solar System processes available to us.
But the passage of other stars through the Oort cloud might give these comets the chance
to be heated to much higher temperatures than could be provided by the Sun.
Most stellar encounter involve M-class red dwarfs. In order to achieve temperatures high
enough for the loss of very volatile species such as CH4 (~30K), the encounter distance
of the star from the comet must be rather small. The black body temperature at distance r
from a star of luminosity L (in units of the solar luminosity) is roughly,
1/ 4
æ r ö-1/ 2 æ L ö
T(r) = 280ç
÷ ç ÷
è AU ø è L ø
é
æ 10-4 L ö1/ 4 ù
280
r =ê
ç
÷ ú = 0.9AU
êë 30 è L ø úû
2
•
Thus, the comet located around 1AU from the star can be altered by the heat of the star.
However, we have to consider the orbital change of the comet during the encounter. A
star of mass 0.1M passing at a distance D=1 AU from a comet nucleus at a speed of 20
km/s can import a velocity change of 10 km/s, which exceeds the escape velocity 0.3m/s
at 104 AU from the Sun.
Space Mission to Comets
Target
Comet type
Mission type
Year
19P/Borrelley
JF
Flyby
2001
Stardust
81P/Wild 2
JF
Sample return
2004
Deep Impact
9P/Tempel 1
JF
Impact
2005
67P/Churyumov-Gerasimenko
JF
Rendezvous
2014
1P/Halley
H
Deep Space 1
Rosetta
Vega, Giotto,
Suisei, Sakigake
1986
v [1] Soderblom et al. et al. “Observations of Comet 19P/Borrelly by the Miniature Integrated Camera
and Spectrometer Aboard Deep Space 1”, Science 296, 1087 (2002)
v [2] Boice et al. “The Deep Space 1 Encounter with Comet 19P/Borrelly”, Earth, Moon and Planets 89,
301 (2002)
v [3] A’Hearn et al. “Deep Impact: Excavating Comet Tempel 1”, Science 310, 258 (2005)
v [4] Brownlee et al. “Surface of Young Jupiter Family Comet 81P/Wild 2: View from the Stardust
Spacecraft”, Science 304, 1764 (2004)
[5] Sunshine et al. “Exposure Water Ice Deposits on the Surface of Comet 9P/Tempel 1”, Science
311, 1453, (2006)
Deep Space 1 Mission and 19P/Borrelly
•
Deep Space 1 (DS1) is a part of NASA’s program to test technologies to
lower the cost and risk. It succeeded in the flyby of its target, 19P/Borrelly, on
September 22, 2001.
•
At encounter, 25 visible-wavelength images and 45 short-wave infrared
spectra (1.3-2.6 m) were obtained. The images cover solar phase angles from
88o to 52o.
•
There are two classes of dust feature: “jets” and “fans (or very wide jets)”.
By comparing the seven images, two sets of collimated dust jets were
distinguished. The  jet is aligned at the core of the main jet, and the  jet are
offset by about 15o from the direction of the  jet. The fan-shaped dust feature
is centered on the Sun line. It is found that the main jet is nearly aligned with
the rotational axis of the nucleus.
Main Jet
Main Jet
 jet
 jet

Boice et al. (2002)
•
There are two classes of dust feature: “jets” and “fans (or very wide jets)”.
By comparing the seven images, two sets of collimated dust jets were
distinguished. The  jet is aligned at the core of the main jet, and the  jet are
offset by about 15o from the direction of the  jet. The fan-shaped dust feature
is centered on the Sun line. It is found that the main jet is nearly aligned with
the rotational axis of the nucleus.
19P/Borrelly nucleus topography
•
The nucleus can be divided into two fundamental units: Smooth Terrain and
Mottled Terrain.
•
The smooth terrain occupies the broad basin that dominates the central part of
the comet. It shows higher albedo. Areas of smooth terrain appear to be the
general source region of both the main jets and the fans.
Several mesa-like features are found within the smooth terrain, and appear to
be associated with the active jets.
*Mesa: an elevated area of land with a flat top and sides that are usually steep cliffs.
Soderblom et al. (2002)
Smooth terrain on 19P/Borrelly
•
A possible scenario for mesa formation could be that
– Thick, dusty lag deposit insulate the comet’s volatile-rich substrate.
– Mesa slopes are area of thinner deposits and the source of volatile loss.
– Volatile loss erode the mesa tops and collapses the lag material to the
lower floor of the smooth terrain.
Boice et al. (2002)
Dust and Gas
Mesa
scarp
Smooth terrain
Heavy rocks
1.55 1.75 1.95
2.15 2.35 2.55
Wavelength (m)
Reflectance (normalized at 2.359m; offsets +0.02)
Reflectance (normalized at 1.7m)
1.35
1.3
1.5
1.7
1.9
2.1
2.3
2.5
Wavelength (m)
•No water ice
•High temperature
•Unknown absorption at 2.39 m
•
No evidence of impact crater with diameter larger than 200 m, are present. It
indicate a young and active surface compared to typical asteroids and most
small planetary satellites.
•
The entire nucleus is extremely dark with geometric albedo of 0.007 - 0.035.
•
Short-wavelength infrared spectra (1.3-2.6 m) show a red-ward slope and
reveal that the surface is hot (345K) and dry.
Stardust Mission and 81P/Wild 2
•
Stardust was launched in 1999 by NASA and returned to the Earth on January
2006 to release a sample material capsule. It is the first sample return mission
to collect cosmic dust and return the sample to the Earth.
•
The mission target of Stardust, 81P/Wild 2 passed within only about one
million kilometers of Jupiter, whose strong gravitational pull altered the
comet's orbit and brought it into the inner solar system. Its orbital period
changed from 40 years to about 6 years, and its perihelion is now about 1.59
AU.
•
On July 3, 2007, a second mission was
approved to re-visit the 9P/Tempel 1, under
the designation of New Exploration of
Tempel 1 (NExT). NExT will take images
of crater formed by Deep Impact mission.
NExT is scheduled to fly by Tempel 1 on
February 14, 2011.
Brownlee et al. (2004)
Pit-halo
Flat-floor
•
There are two distinguish morphologies for circular depression: pit halo and
flat floor.
Pit-halo feature have a rounded central pit surrounded by an irregular and
rough region of excavated material, whereas flat-floor feature do not have halo
region and bounded by steep cliffs.
•
The authors argued that most of depression results
from the compression of the weakly cohesive, porous
target. They experimentally reproduced these two
depression observed on 81P/Wild 2 by hypervelocity
impacts. Pit-halo structures formed when a loose sand
layer covered a harder underlying layer, and flat-floor
crater resulted when the unconsolidated surface layer
was modestly baked to give it some cohesion.
Brownlee et al. (2004)
Hemenway
Mayo
Sekanina et al. (2004)
Triangulation of 20 jets shows that 2 spread out from the dark side and 7
coincide with relatively bright surface spots.
Deep Impact Mission and 9P/Tempel 1
•
Deep Impact is NASA space probe launched on January 12, 2005 that was
designed to study the composition of the interior of the comet by colliding a
section of the spacecraft into the comet.
•
The mission target is 9P/Tempel 1. At 5:52 UTC on July 4, 2005, the impactor
of the Deep Impact probe successfully impacted the comet's nucleus,
excavating debris from the interior of the nucleus.
•
9P/Tempel 1 is a member of Jupiterfamily comets. The perihelion distance is
1.5AU and the orbital period is 5.5 years.
It was discovered in 1867.
9P/Tempel 1
1 km
Composite of ITS images. Arrows a and b
point to the large, smooth areas. The impact
site is indicated by the other large arrow. The
small arrows highlight the scarp, bright due to
the illumination angle. Celestial north is near
the rotational pole; ecliptic north is more
nearly upward. (A’Hearn et al. 2005)
•
The mean radius is estimated to have been 3.0±0.1 km.
•
There are several dozen circular features, ranging from 40 to 400 m. The
cumulative size-frequency distribution of these features is consistent with
impact crater population.
•
Two region of smooth surface exist. One smooth region is bounded to the
north by a scarp (급사면) ~20 m in height. The Rougher terrain (east), smooth
terrain (west), and the scarp strongly suggests removal by backwasting of a 20m layer, leaving an exhumed surface containing the circular features.
•
Albedo variations are within 50% of an average of 0.04 in the visible
wavelength. Color ratios between 300 nm and 1000 nm are uniform to <2%.
No exposures of clean ice or frost have been identified on the basis of albedo
or color.
-A’Hearn et al. (2005)
scarp
Rough terrain
Smooth terrain
9P/Tempel 1: Temperature Map
R=1.5AU
With the near infrared spectrometer, the two dimensional spectral data were obtained.
The temperature is derived by fitting the thermal component. The maximum
temperature is around the sub-solar point. There are no areas on the sunlit surface colder
than 260 K. The obtained temperature is above the temperature expected for sublimation
of volatile on the surface. Therefore, volatiles probably sublimate below the surface.
Exposed Water Ice Deposits
on the Surface of Comet 9P/Tempel 1
Sunshine et al. (2006)
81P/Wild 2
19P/Borrelly
9P/Tempel 1
The appearances of three comets
explored by the spacecrafts are so
different from one another.
Both 9P/Tempel 1 and 19P/Borrelly
have had long times in the inner
solar system, whereas 81P/Wild 2
has not.
81P/Wild 2 have lost only a meter
or so of surface after 1974
apparition.
Deep Impact: Cratering Experiment
•
Impactor struck the nucleus near its southern limb. At about 20 s and 10s
before impact, large dust particles hit the impactor, causing the pointing to turn
away.
•
The impact time is 05:44:35.4 ~ 05:44:36.2. The final crater has not been
observed because of a ejecta.
Blue dotted line is the position of
the spectrograph slit.
1st flash
700-ms2nd flash
First flash lasted <200 ms. This is
probably
associated
with
vaporization of the impactor and
part of the comet.
Second flash may be associated
with the first eruption of material
at the surface. A hot material
moved at a projected velocity of
5km/s.
Ejecta
Impactor (364kg, 10km/s)
2nd flash
shadow
A’Hearn et al. 2005
285K850K
Brightness of the peak pixel in the plume versus
time in MRI. The origin of time is based on
extrapolating the motion of the plume back to its
origin at constant velocity.
The dashed curve is what is expected for
uniformly expanding particles reflecting sunlight.
1.7 km/s
Total mass = 4000kg
Initial temperature=3500K
Liquid silicate droplet
Spectra 3km beyond the limb of the nucleus
Pre-impact
Radiance (W m-2 sr-1 m-1)
0.0025
Post-impact
0.015
0.0020
0.010
0.0015
0.0010
0.005
0.0005
0.000
0.0000
-0.0005
-0.005
2.5
4.5
3.0
3.5
4.0
Wavelength (m)
2.5
3.0
3.5
4.0
Wavelength (m)
4.5
The most dramatic difference was the large increase in organics.
The feature at 4.4 m is CN stretch band. Methyl cyanide could be present in the ejecta.
Chemical Composition (1)
•
The information about the chemical composition of the nucleus comes from
the observed chemical composition of the gaseous coma.
•
The elements H, C, N and O present in the spectra of comets by the telescopic
observations.
•
The in-situ measurements of Comet 1P/Halley with ionization mass
spectrometer on board the Vega spacecraft have given the elemental
composition of dust.
-Solar
-Halley’s dust
-Comet gas
-Chondrite
Anders & Grevesse (1989), Geochim. Cosmochim. Acta. 53, 197
Jessberger et al. (1988), Nature 332, 21, 691
Swamy, in Physics of Comets
Andrers & Ebihara, Geochim. Cosmochim. Acta. 46, 2363 (1982)
Chemical Composition (2)
•
There was a standing problem on the depletion of carbon by a factor of 4. The
observations of 1P/Halley helped in resolving this carbon depletion problem as
carbon is mostly tied up in organics, called the CHON particles.
•
A direct comparison of the abundances of 1P/Halley and in the Sun shows that
the elemental abundances in comet material are very similar to the solar values
except for hydrogen.
•
The detailed results derived from 1P/Halley make it possible to build a
consistent model for the volatiles escaping from the nucleus. A heuristic
model is shown below.
92.0% with O
5.6% with N
2.2% Hydrocarbon
0.2% with S
78.5
H2O
2.6
N2
1.5
C2H2
0.1
H2S
4.5
HCO OH
1.0
HCN
0.5
CH4
0.05
CS2
4.0
H2CO
0.8
NH3
0.2
C3H2
0.05
S2
3.5
CO2
0.8
N2H4
1.5
CO
0.4
C4H4N2
Delsemme 1991, In Comets in the Post-Halley Era
Evolution of Comets into Asteroids
In Asteroids III, page 669-685
Paul R. Weissman
Willam F. Bottke Jr.
Harold F. Levison
Introduction
•
Solar system astronomers have long speculated on the possible existence of extinct or
dormant comets, objects that appear asteroidal but in truth are icy objects that had their
origins as comets.
•
The chaotic nature of the dynamical evolution of objects in planet-crossing orbits, as
well as nongravitational accelerations on comets caused by outgassing, make it
impossible to track orbits accurately backward (or forward) in time more than a few
decades or centuries.
•
In the past decade, several lines of research have provided new data (e.g. NEO findings
and physical observations of comets and asteroids) and new tools (numerical simulation)
with which to pursue these questions.
•
In this chapter, the authors discuss how these new tools have made it possible to argue
for, at least statistically, the existence of extinct or dormant comets among the observed
asteroid population, in particular among the NEOs.
Cometary Definitions
•
Active comet: a comet nucleus losing volatiles and dust in a detectable coma.
•
Inactive comet: a comet nucleus that is active during part of its orbit, but presently is
in a part of the orbit where volatile loss is negligible and there is no detectable coma.
•
Dormant comet: a comet nucleus that, although once active, has lost the ability to
generate a detectable coma in any part of its present orbit. A dormant comet perturbed
to a smaller perihelion distance might be reactivated. Or an impact might remove an
overlying nonvolatile crust and expose fresh icy materials to sublimation, reactivating
the comet.
•
Extinct comet: a comet nucleus that has lost its ices or has its ices so permanently
buried under a nonvolatile crust that it is incapable of generating a coma.
•
Asteroid: an interplanetary body formed without significant ice content, and thus
incapable of displaying cometary activity.
•
Exceptions: Kuiper belt objects, Trojan asteroids, some outer main-belt asteroids
Physical End States of Cometary Nuclei (1)
•
Many cases of comets that disappeared can likely be attributed to poorly determined orbits.
•
But in other cases, well-observed comets seem to have simply disappeared (e.g. 3D/Biela). Note that
the designation “D” refers to “defunct” comets. Several of these comets displayed unusual brightness
changes on their last apparition (outbursts and/or fading).
•
Three physical mechanisms have been proposed to explain the disappearance of comets: (1) random
disruption, (2) loss of all volatiles, and (3) formation of a nonvolatile crust or mantle on the
nucleus surface. Unfortunately, none of these mechanisms are well understood or quantified.
•
An example of (1) is comet LINEAR, D/1999 S4. Recently, this comet was observed to completely
disrupt as it passed through perihelion in July 2000.
•
Weissman (1980) compiled records of observations of disrupted or split comets and showed that 10%
of dynamically new comets from the Oort cloud split, 4% for returning long-period comets, and only
1% for short-period comets (per orbit). The splitting events did not show any correlation with
perihelion distance, distance from the ecliptic plane, or time of perihelion passage.
1999 S4 (LINEAR)
i = 149º
q = 0.765 AU
e = 0.9999970
Weaver et al. (2001)
Physical End States of Cometary Nuclei (2)
•
Formation of a nonvolatile crust or mantle on the nucleus surface is the one process that would
presumably lead to asteroid-like objects. This was first proposed by Whipple (1950). There are two
ways that such crusts may form.
•
First, irradiation of comets by galactic cosmic rays and solar protons will sputter away volatiles and
transform organic molecules to more refractory forms during the comets’ long storage in the Oort
cloud or Kuiper belt (Johnson et al., 1987; Moore et al., 1983; see also Weissman and Stern, 1997).
This irradiated crust would extend ~1/ρ m below the nucleus surface, where ρ is the density of the
cometary materials in g cm–3. An interesting and unsolved problem is how such a crust is removed
when the comet re-enters the planetary region on it first perihelion passage, allowing it to become
active.
•
A second method for forming a cometary surface crust is through the lag deposit of nonvolatile
grains, left behind or launched on suborbital trajectories that do not achieve escape velocity, as water
and more volatile ices sublimate from the nucleus surface during perihelion passage. It is not clear
how such large non-volatile grains, which presumably are too heavy to be lifted off the nucleus by
evolving gases, might form. However, if they did, thermal models have shown that a layer only a few
to perhaps 10 cm thick would be sufficient to insulate the underlying ices from sublimation (Brin and
Mendis, 1979; Fanale and Salvail, 1984; Prialnik and Bar-Nun, 1988).
Luu (1994)
Physical End States of Cometary Nuclei (3)
•
1P/Halley and 19P/Borrelly images appear to support the idea that comets can develop
crusts and thus evolve to dormant or extinct objects.
•
Further support comes from estimates of the active fraction of other cometary nuclei.
A’Hearn (1988) showed that many Jupiter-family comets have active fractions of only a
few percent. For comets 49P/Arend-Rigaux and 28P/Neujmin 1, the active fractions are
estimated as only 0.08% and 0.1% respectively.
•
This suggests that comets may slowly age and evolve toward total inactivity, either by
developing non-volatile crusts on their surfaces or by some other as-yet-unrecognized
mechanism. Thus, crust formation does appear to provide a mechanism for evolving
cometary nuclei to dormant or extinct states.
Cometary Dynamics:
Evolution to Asteroidal Orbits (1)
•
Comets have traditionally been divided into two major dynamical groups: long-period (LP) comets
with orbital periods >200 yr, and short-period (SP) comets with periods <200 yr. Long-period comets
typically have random orbital inclinations while short-period comets typically have inclinations
relatively close (within ~35°) to the ecliptic plane.
•
In addition it has become common to divide the SP comets into two subgroups: Jupiter-family comets
(JFC), with orbital periods <20 yr and a median inclination of ~11°, and Halley-type comets (HTC),
with periods of 20–200 yr and a median inclination of ~45°.
•
A more formal dynamical definition of the difference between the JFC and HTC comets was
proposed based on the Tisserand parameter. The Tisserand parameter is an approximation to the
Jacobi constant, which is an integral of the motion in the circular restricted three-body problem. It
was originally devised to recognize returning periodic comets that may have been perturbed by
Jupiter and is given by
T=
aJ
a
2
+2
1e
cos(i)
(
)
a
aJ
where aJ is Jupiter’s semimajor axis, and a, e, and i are the comet’s semi-major axis, eccentricity,
and inclination, respectively.
An example of such an encounter for Jupiter family comet 81P/Wild 2 is shown below. A
close approach to Jupiter alters the orbital elements of the comet with the semi-major axis
from 4.95 AU to 1.49 AU.
81P/Wild 2 in early 1970’s

Cometary Dynamics:
Evolution to Asteroidal Orbits (2)
•
Jupiter-family comets have T > 2 while
long-period and Halley-type comets
have T < 2. Levison (1996) proposes
that comets with T > 2 be known as
ecliptic comets, while comets with T <
2 be known as nearly isotropic comets.
HTC
HTC
JFC
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Comets on planet-crossing orbits are transient members of the solar system. Close and/or
distant encounters with the giant planets, in particular Jupiter, limit their mean
dynamical lifetimes to ~0.4–0.6 m.y. (Weissman, 1979; Levison and Duncan, 1997).
Thus, they must be continually re-supplied from long-lived dynamical reservoirs.
•
The different inclination distributions of the ecliptic and nearly isotropic comets reflect
their different source reservoirs. Nearly isotropic comets (Long-Period and Halley-Type
Comets) are believed to originate from the nearly spherical Oort cloud. Ecliptic comets
(Jupiter-family comets) are fed into the planetary system from the highly flattened
Kuiper belt.
•
Fernández et al. (2001) compared measured albedos for 14 cometary nuclei and 10
NEOs with Tisserand parameters T<3, and 34 NEOs with T > 3. They showed that all of
the comets and 9 out of 10 of the NEOs with T < 3 had low albedos, ≤0.07, while most
of the NEOs with T >3 had albedos >0.15. This result suggests that the T <3 objects
have a cometary origin.
From Interstellar Material to Cometary
Particles and Molecules
P. Ehrenfreund
S.B. Charnley and D.H. Wooden
In Comet II, Univ. of Arizon Press (2004)
Introduction of molecules
Is There Interstellar Cloud Material in Comets?
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The gases observed in cometary comae originate from the nuclear ices and
offer insight into the nucleus composition.
•
Coma molecules can either have
– a “native” source, which have been sublimed directly from the nucleus
itself,
– An “extended” source, due to the decomposition of large organic particles
or molecules.
•
A comparison with the list of interstellar ices and gases found in dark
molecular clouds and in regions of massive and low-mass star formation,
suggests a direct link.
•
The abundances of the spices in star forming-regions are similar to those in
comets (see Table).
ISO spectrum of high-mass star
Comparison of molecules in star-forming region and comets
Star-forming regions
Comets
High-mass stars
Solar-type stars
1P/Halley
Hyakutake
Hale-Bopp
H2O
100
100
100
100
100
CO
9—20
5—50
15
6—30
20
CO2
12—20
12—37
3
2—4
6—20
CH3OH
0—22
0—25
1—1.7
2
2
CH4
1—2
<1
0.2—1.2
0.7
0.6
H2CO
1.5—7
—
0—5
0.2—1
1
OCS
0—0.3
<0.08
—
0.1
0.5
NH3
0—5
<9.2
0.1—2
0.5
0.7—1.8
0.4—3
—
—
—
0.06
C2H6
<0.4
—
—
0.4
0.3
HCN
<3
—
0.1
0.1
0.25
C2H2
—
—
—
0.5
0.06—0.1
H2S
—
—
0.04
0.8
1.5
HCOOH
Ehrenfreund & Charnley, Annu, Rev. Astron. Astrophys. 2000, 38:427
Ehrenfreund, Charnley & Wooden, in Comet II, Arizon Press
Paucity of CO2
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H2O, CO, CO2 and CH3OH are important ice species that can be used as a tracer for the
interstellar/cometary link.
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ISO identified CO2 ice as one of the major components in interstellar ice mantles with
an average abundance of ~15–20% relative to water ice. CO2 appears to be much less
abundant in comets than in molecular clouds (Feldman et al., 2004).
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Whereas CO2 ice is ubiquitous in the interstellar medium, the measured abundance of
CH3OH is highly variable and in fact is undetected toward many sources. The cometary
abundance of methanol is appreciable but generally much smaller (~2%) when
compared to interstellar ices (5–25%).
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The possible explanation is that these molecules are interstellar but have had their
original populations partially depleted in the nebula. For example, CO2 is very
susceptible by H atoms in warm gas and partially destroyed at the accretion shock or in
nebular shock waves. In the hot gas, the CO2 abundance is quickly diminished through
reactions which proceed efficiently at temperature above ~1000K.
H2S and CS2 in comets
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The problem in comparing the abundances of S-bearing compounds is that the
cometary parents CS2 and H2S are not detected in interstellar ices.
•
On the other hand, the most abundant of the interstellar S-bearing compounds
have all been detected in comets (e.g. OCS). It is very unlikely that these
sulfuretted species represent pristine interstellar material. Probably, these are
photoproducts of other compounds or may have a distributed (polymeric?)
source.
HNC in comets
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The difficulties in connecting observed cometary volatiles with interstellar
molecular cloud material can be illustrated for the case of the HNC molecule.
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Cometary HNC was originally discovered in the coma of Hyakutake (Irvine et
al., 1996); the high HNC/HCN ratio in the comet Hyakutake is similar to that in
the interstellar materials, which suggests that this HNC was preserved
interstellar material.
•
However, subsequent observation showed that the HNC/HCN ratios in HaleBopp varied as a function of the heliocentric distance. Furthermore, Hale-Bopp
showed two HNC components: one correponding to nuclear emission and
another corresponding to extended (or jet) source. The HNC/HCN ratio of the
nuclear component is similar to hose of Hyakutake and comet Lee, whereas the
extended component is almost 2-3 times as large (Blake et al. 1999).
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The most likely source of HNC in the coma is the decomposition of a large
organic, perhaps polymeric, compound (Rodgers and Charnley, 2001a; Irvine et
al., 2003).
C chain radicals in comets
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The C chain radicals, C2 and C3 are widespread in comets.
•
Helbert et al. (2002) have shown that the abundances of these molecules could
be derived from C2H2, CH33CH, and C3H8 in a coma chemistry driven by
electron-impact dissociations. However, methylacetylene (CH33CH) and
propane (C3H8) have not yet been detected in comets.
•
Synthesis of long, unsaturated, C-chain molecules appears to be one signature
of interstellar organic chemistry. There have been tentative detections of C4H
and its possible parent C4H2 in Halley and Ikeya-Zhang, respectively (Geiss et
al., 1999; Magee-Sauer et al., 2002). C4H2 and C6H2 were found in the
protoplanetary nebula CRL 618. These detections of long C-chain molecules in
comets would be strong evidence for an interstellar origin of these organics
H-C º C-C º C-H