Download 12 Comets, Kuiper Belt Objects, and Pluto

Chapter
12
Comets, Kuiper Belt Objects, and Pluto
Every few decades a new comet lights up the night sky in a spectacular fashion. A
spacecraft rendezvous with such bright, young comet would return fundamental data on
the chemistry and early history of the outer solar system, but the lack of warning makes
such an encounter a practical impossibility. There is plenty of time to plan and launch a
mission to one of the nearly 100 periodic comets with well known orbits, but these tend
to be dim, old comets that have lost most of their volatiles. There exists one, and only
one, periodic comet that is consistently spectacular, namely Halley’s Comet. The last
thirty appearances of P/Halley (the “P/” denotes “periodic”) have been recorded since
the year 240 BC. The comet is named in honor of the English astronomer Edmond Halley,
who predicted its 1758 apparition, based on similarities in the orbits of comets that had
appeared in 1531, 1607, and 1682. The 1985-1986 apparition of P/Halley was the first to
occur in the space age, and the spacecraft and telescopic observations of its passage have
provided planetary scientists with an enormous amount of new information about comets.
12.1 Structure and Composition
12.1.1 Parts of a Comet
12–1
Comets travel in highly elliptical orbits inclined in random angles to the ecliptic (apparent annual path of Sun on celestial sphere – i.e. the plane of the solar system). As a
comet approaches the sun, solar heating begins to vaporize the ices. The liberated gases
begin to glow, producing a fuzzy, luminous ball called a coma. The coma can be 1 million
km in diameter. The solar wind and solar photons blow these luminous gases outward into
a long flowing tail, which can stretch to 100 million km in length (roughly the distance of
Venus from the Sun). The solid, inner part of the comet is called the nucleus. The nucleus, we think, consists of approximately equal parts of ice and dust, typically measuring
only a few km to a couple of tens of km across.
12.1.2 Composition
To explain the composition of comets, Harvard astronomer Fred Whipple coined the
term dirty snowball, a simple description which has proved to be very accurate after many
years of observations. However, it is important to note that the nucleus of a comet is never
visible to Earth-based observers because it is obscured by the coma. Observations of the
nucleus of Halley’s Comet by 6 flyby spacecraft in 1986, which we discuss in detail later,
showed that its nucleus was potato-shaped and darker than coal, reflecting only about 4% of
the light falling on it. Water and water-derived molecules (H2 O+ , OH, OH+ , H3 O) appear
to be the principal constituents of the nucleus. It appears that water ice makes up 50%
or more of many comets and is the dominant constituent. Basic photochemistry provides
insight on the chemical species that may form from water ice as the comet approaches the
sun. When water vapor is exposed to ultrviolet solar flux the following reactions can occur
by photodissociation:
H2 O + hν → OH + H ,
H2 O + hν → O + H + H
H2 O + hν → O + H2 .
(12.1a)
(12.1b)
(12.1c)
Of these, reaction (12.1a) is the most common because it occurs in response to photons
with energies that commonly occur in the solar flux. The others are rarer because they
require more energetic photons. At even higher energies there can be additional reactions
due to photoionization.
Other common molecules are HCN, CH3 CN, and (H2 CO)n . The dark color of cometary
nuclei is mostly due to carbon-rich compounds and dust that remain as the comet’s ice sublimates. Spectroscopic observations from Earth-based sensors and spacecraft indicate that
gases within cometary comas are generally neutral molecules rich in CHON (Carbon, Hydrogen, Oxygen, Nitrogen) elements, with elemental abundances that are consistent with
solar composition. Lyman-alpha observations in the ultraviolet (120.6 nm) indicate that
some comets are surrounded by a vast (107 km) halo of hydrogen atoms. Calculations
have shown that the production rate of hydrogen required to explain the atomic density
of the cloud is too great by an order of magnitude or so to be explained by sublimation of
the nucleus. This hydrogen is probably a consequence of dissociation of hydroxyl (OH) by
sunlight.
12.1.3 The Tails
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A comet’s tail always points away from the sun, regardless of the direction of the
comet’s motion. The implication is that something from the sun is ”blowing” at the
comet. In fact, we now know that the sun usually produces two comet tails – an ion or
plasma tail and a dust tail. Ionized atoms are swept directly from the sun by the solar
wind to form the ion tail, which is generally blue in color due to fluorescing ions of carbon
monoxide (CO+ ).
The dust tail, which is yellowish in appearance as a consequence of reflected sunlight,
forms when solar photons strike micron-sized dust particles that dislodge from the sublimating nucleus. Light exerts a pressure on any object that absorbs or reflects it. This
radiation pressure is very weak, but fined-grained dust particles in a comet’s coma offer
little resistance are are blown away from the comet, producing the dust tail. Dust tails
generally form sweeping arcs have lengths that range from 1 to 10 million km. They are
more prominent than ion tails and usually form opposite the comet’s direction of motion.
Occasionally a comet, such as P/Kohoutek in 1979, is observed to exhibit an anti-tail
that is oriented sunward. This is just an extension of the dust tail projected in the line
between the comet and Earth.
Ion tails, which may reach lengths of up to 100 million km, are usually narrow and
linear in appearance, and display more fine structure than dust tails. The ion tail, which
forms in the line of the solar wind, is composed of electrons and ionized molecules. The
structure and ionization properties of the cometary plasma indicate that comets have
associated magnetic fields. Ion concentrations in the tail, referred to as streamers or
rays, provide insight about the nature of magnetization. Spacecraft measurements have
detected the presence of a bow shock that marks the interaction of the magnetic field
and solar wind. In the plasma tail fields of opposite polarity meet, forming a current
sheet. Occasionally, the ion tail is observed to detach from the visible coma in an event
referred to as a disconnection event. The reason for disconnection events is a matter of
debate, with suggestions ranging from reversals in the magnetic field of the solar wind to
turbulence resulting in pinching of field lines behind the coma.
In one case observations of P/Hyakutake by the ROSAT satellite detected the presence
of x-rays, which have yet to be satisfactorily explained. In targeting Hyakutake, there was
an expectation of faint x-ray return at best. Instead, the signal was 100 times stronger
than even the most optimistic predictions. subsequent analyses of many other comets
extablished them as strong x-ray sources. the mechanism of x-ray emission appears to
be charge exchange between highly-charged heavy ions in the solar wind and cometary
neutrals. This discovery dictates that cometary x-ray emissions can be used as probes of
the heavy-ion content of the solar wind.
12.1.4 Sublimation
To address the devolatilization of a comet as it approaches the sun, we begin with
the concept of temperature developed in the chapter on Asteroids. Assuming steady-state
conditions, the balance between the absorbed and emitted energy on a body is
(1 − A)
F0
= 4σTe4 = 4²σTs4 ,
R2
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(12.2)
where A is albedo, F0 is the solar constant, R is radius, σ is the Stefan-Boltzmann constant,
² is the thermal emissivity (≈ 1), Te is the effective temperature, and Ts is the surface
temperature of the body averaged over its entire surface. Another factor that must be
taken into account is the effect of vaporization (sublimation), because under certain P/T
conditions (high P and T) this process can absorb a significant amount of the heat flux.
It is possible to show that the sublimation rate (dm/dt) is related to the vapor pressure
(Pv ap) by
µ
¶1/2
m
dm
= Pvap
,
(12.3)
dt
2πkTs
where k is Boltzmann’s constant. The heat flux Φ due to sublimation is just
Φ=L
dm
,
dt
(12.4)
where L is the latent heat of evaporation for ice.
Once a comet gets close to the sun, a significant amount of the incident solar heat
flux will be carried away by evaporation. It is then necessary to modify the steady-state
flux balance to account for this effect:
(1 − A)
F0
dm
.
= 4²σTs4 + L
2
R
dt
(12.5)
So evaporation carries heat away at a rate that depends on the vapor pressure but the vapor
pressure is dependent on the temperature. As a comet approaches the sun it undergoes a
transition from a radiatively-controlled regime to a sublimation-controlled regime. In the
latter the sublimation of ice controls the temperatue of the comet surface. But comets
approaching close to the sun have a surface temperature of about 180 K independent
of distance, because closer approach results in an enormous increase in sublimation rate
with only a small increase in temperature. Note that the eutectic temperature for the
ammonia-water ice system is 173 K. This suggests that for a water ice-ammonia mixture
it is marginally possible to have a liquid phase.
A typical comet is estimated to lose 0.1% to 1% of its ice every time it passes near
the sun. So its ice should be gone after about 100 to 1000 perihelion passages, leaving
only the dust and rock component. This material forms meteor showers. The process
is accelerated if the comet passes very close to the sun, producing a sun-grazing comet.
Occasionally, a comet’s nucleus fragments due to gravitational perturbations or other processes that induce stress in the weakly bound nucleus. As discussed later, this happened
to P/Shoemaker-Levy 9.
12.2 Comet P/Halley: The 1985-1986 Apparition
P/Halley has an average period of 76 years, a perihelion of 0.59 AU, and an aphelion of 35 AU. An armada of spacecraft, containing some fifty different experiments, was
sent out in a coordinated effort to encounter the comet as it crossed the ecliptic plane
in March, 1986. Because P/Halley is in a retrograde orbit, the closing speeds for the
spacecraft encounters were about 70 km per second at closest approach. All the spacecraft
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performed well, and in particular, the two Soviet Vega spacecraft and the Giotto spacecraft
of the European Space Agency (ESA) successfully returned the first-ever images of a comet
nucleus.
The only records we have of P/Halley prior to modern observations are artists’ depictions. The 1301 apparition seems to have been particularly spectacular. The Florentine
artist Giotto di Bondone painted an Adoration of the Magi scene in the Scrovegni Chapel
in Padua, around 1303-1305, in which he used a comet for the Star of Bethlehem. Good
indirect evidence suggests that what Giotto painted was an accurate rendition of P/Halley
as he had witnessed it only two years beforehand. In recognition, ESA named its spacecraft to P/Halley Giotto. We will now take a brief look at each of the spacecraft that met
P/Halley in 1986, in reverse order of closest approach.
12.2.1 ICE and IUE
Much to the chagrin of American planetary scientists, none of the proposed US missions to P/Halley were acted upon. However, a few existing spacecraft already in orbit
were reprogrammed for the 1986 apparition. A 1978 International Sun-Earth Explorer
satellite, parked in a near-Earth libration point, was renamed the International Cometary
Explorer (ICE) and redirected to fly through P/Giacobini-Zinner in September, 1985, and
then to fly past P/Halley in March, 1986, in order to monitor the solar wind before it
reached P/Halley. The closest approach by ICE to P/Halley came on March 25, at a
distance of 28,100,000 km.
The ever-dependable Pioneer Venus orbiter was used to observe UV emissions from
P/Halley’s hydrogen coma as seen from Venus, and even the twenty-year-old Pioneer 7
spacecraft, with its plasma detectors, was reactivated because it just happened to be
in the right place at the right time. The Solar Maximum satellite was used to image
P/Halley during the six weeks when it was too close to the Sun to be seen from Earth.
The Dynamics Explorer 1 satellite was used to obtain UV images after perihelion. The
International Ultraviolet Explorer (IUE) in Earth orbit provided extensive UV coverage of
P/Halley all through the 1986 apparition. It is remarkable how many useful sentries we
have in space.
12.2.2 Suisei and Sakigate
Japan’s Institute for Space and Astronautical Science (ISAS) made its debut into
interplanetary exploration by sending the twin satellites Suisei (“comet”) and Sakigate
(“pioneer”) to P/Halley. Suisei carried a UV camera and a solar wind detector, and
Sakigate carried a plasma-wave probe and a magnetometer. Suisei observed a 2.2 day
period in the UV emission from P/Halley. This was very close to the rotation period for
the nucleus derived in 1985 by Z. Sekanina and S. Larson based on a reanalysis of 1910
photographs of P/Halley. Suisei made its closest approach to P/Halley on March 8 at
a distance of 151,000, and the more distant Sakigake passed P/Halley on March 11 at a
distance of 6,990,000 km.
12.2.3 Vega 1 and Vega 2
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The Soviets used their powerful Proton rockets to advantage to launch two Vega
spacecraft towards P/Halley, by way of Venus. The term “Vega” is a contraction of “Venera” (“Venus”) and “Gallei” (“Halley”). The Venera part of the mission was quite novel
because it contained two French-made balloons that were successfully floated in Venus’
atmosphere.
Vega 1 was the first spacecraft to encounter P/Halley, and made its closest approach
on March 6 at a distance of 8,890 km. Two days later Suisei made its closest approach,
and then the next day Vega 2 made its closet approach at a distance of 8,030 km. Vega 1
was the first spacecraft to image the nucleus of P/Halley. One of the best images of the
nucleus from the Vega missions was made by Vega 2 at its closest approach. It appeared at
first that P/Halley had a double nucleus, but by combining several Vega images together
it was finally determined that the nucleus is in fact a single, irregularly shaped object. The
nucleus is approximately 16×8×7.5 km3 in size, which is larger than had been anticipated.
12.2.4 Giotto
The boldest mission to P/Halley was ESA’s Giotto mission, because it was sent closest
to the nucleus. It was targeted for a closest approach on March 14, with a planned
miss distance of only 540 ± 40 km. The actual distance turned out to be 596 km, at
68.4 kms−1 . The position of the nucleus using Earth-based observations was known to
no better than ±400 km, and so instead of relying on Earth-based observations for lastminute corrections to Giotto’s trajectory, a very successful collaboration between the Vega
and Giotto missions, called the “Pathfinder” concept, was employed. The close-range
navigational data from the Vega encounters was used to reduce the uncertainty of Giotto’s
miss distance by an order of magnitude, and the collaboration worked extremely well.
Giotto passed so close to Halley’s nucleus that the magnetometer team recorded a
sudden drop to zero of the interplanetary magnetic field (IMF), at about 1 minute before
closest approach. The spacecraft experienced the expected large amount of dust impacts
on its way past the nucleus. At 33 seconds before closest approach, the first instrument
failed. At 21 seconds, another two instruments stopped working, and at 18 seconds a
fourth went dead. The camera failed at 9 seconds. Signals from the spacecraft stopped
altogether 7 seconds before closest approach, which was alarming because it was calculated
that the spacecraft had a 90% chance of survival. In fact, the spacecraft was not dead, but
just wobbling from the off-center impact of a 0.1 g dust particle, such that its telemetry
connection to Earth was temporarily broken. The possibility of just such a disruption had
led mission planners to equip Giotto with nutation dampers, which successfully brought
the spacecraft under control within half an hour. The mission turned out to be a great
success.
12.2.5 The Nucleus
The most exciting outcome of the 1986 apparition of P/Halley was the first images of
a comet nucleus, returned by both the Vega and Giotto spacecraft. A detailed image of
the P/Halley nucleus has been constructed by adding together 60 Giotto images, revealing
two prominent jets on the sunward side and what appear to be craters and significant
topography on the surface.
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The volume of the nucleus of P/Halley was larger than expected. Given the apparent
magnitude of the comet before it reaches the sun, the Bond and geometric albedos work
out to ≈ 0.05 and ≈ 0.03, respectively, which makes P/Halley one of the darkest known
objects in the solar system. The implication is that all periodic comet nuclei are probably
twice as large as previously thought.
The total gas and dust lost by P/Halley during its 1986 apparition was just slightly
larger than expected. The estimate for the mean density of P/Halley is a low 100200 kg m−3 , with an upper limit not exceeding 500 kg m−3 . Earlier ideas that collisions
would tend to increase the density may have been biased by terrestrial experience with
snow near its melting point. The material in P/Halley’s nucleus is so cold that it probably shatters on impact without significant compression. The low densities mean that
radiogenic heating from the silicate component of the nucleus is not a significant factor,
and therefore differentiation of the nucleus is not expected. The 1986 Halley encounters
upheld Whipple’s classical dirty snowball hypothesis. However, even Whipple was surprised at how low the density turned out to be. The other surprise was a significantly
large amount of dust particles with masses less than 10−17 kg. The dust contained CHON
elements.
While the P/Halley observations of cometary nucleus have been extremely important
in aiding our understanding of comets they must be interpreted with some caution. Because P/Halley has experienced many solar passages it is quite possible that the molecular
abundances measured are not characteristic of pristine cometary material.
12.3 Cometary Dust
12.3.1 Zodiacal Light
Some dust particles blown off comets are too large to become entrained in the dust
tail. These particles are not affected significantly by solar radiation pressure and fall into
heliocentric orbits. Sunlight reflected by these particles is sometimes visible as a subtle
glow in the night sky called zodiacal light.
12.3.2 Poynting-Robertson Drag and the Yarkovsky Effect
Interplanetary dust undergoes forcing due to radiation from the sun. Unlike gravity,
a force with scales with the mass (i.e., volume, or radius r3 ) of the dust particle, the force
of solar radiation scales with surface area (r2 ). Thus, the smaller the particle, the larger
the relative importance of radiative efects.
Solar radiation effects can take the form of radiation pressure from solar photons
or particle and energy flux from the solar wind.
12.3 Cometary Sources
12.3.1 The Oort Cloud
Cometary orbits provide important clues as to the origin of these bodies. Astronomers
typically discover about a dozen or so new comets in a given year. Some are short-period
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comets, which have inclinations with respect to the ecliptic of 30◦ or less and orbit the
sun in less than 200 years. Like Halley’s comet, they return again and again in predictable
intervals. The majority of comets discovered each year are long-period comets, which
typically take 1 million to 30 million years to complete one orbit of the sun. These comets
travel along in extremely eccentric orbits, inclined at random angles to the plane of the
ecliptic plane, and spend most of their time at distances of 104 to 105 years from the
sun, which is about one-fifth of the distance to the nearest star. Long period comets are
discovered at roughly the rate of one per month, so it is reasonable to suppose that there is
a huge population of cometary objects out at about 50,000 AU. This reservoir of cometary
material surrounding the sun is called the Oort Cloud, after the Dutch astronomer Jan
Oort, who proposed its existence in the 1950’s. Oort talked of ”a garden, gently raked
by stellar perturbations”. Occasionally, random stars pass right through the Oort Cloud,
significantly perturbing comets in this reservoir. (Stars are expected to pass within 10,000
AU of the sun every 35 Ma and within 3,000 AU every 400 Ma). Though it has not
been observed directly, the Oort Cloud is a widely accepted paradigm because of the longperiod comet orbital properties. Comets do not as a rule display hyperbolic trajectories
that would indicate their origin in interstellar space.
Estimates of the number of comets in the Oort Cloud range as high as 1 trillion. Only
such a large population would explain the fact that we see one comet a month even though
each one takes several million years to complete an orbit. The Oort Cloud was probably
formed 4.5 BY from the numerous icy planetesimals that orbited the sun in the vicinity
of the newly formed Jovian planets. During near-collisions with the giant planets, many
of these chunks of ice and dust were catapulted by gravity into the highly elliptical orbits
that they now occupy, much in the way that the Voyager spacecraft were flung far from
the sun after their encounters with the giant planets. The gravitational attraction from
the sun streches much farther than Pluto, in fact about half way to the nearest star, so
cometary bodies perturbed by Jupiter are not lost from the solar system. Gravitational
perturbations by passing stars or interstellar gas clouds may force a long period comet into
a short period orbit. There the comet will eventually be destroyed by frequent passages
near the sun.
123.2 The Kuiper Belt
Beyond the orbit of Neptune lies another source region of comets. The existence of the
Kuiper Belt was first speculated upon by Edgeworth, and is named after Gerard Kuiper,
a well-known planetary astronomer, who among other discoveries, first identified water ice
in Saturn’s rings. He proposed that no large planets could have accreted beyond Pluto
because the long orbital periods at those distances led to very long formation times. He
had proposed that there would be a belt of icy planetary remnants there that were never
perturbed into the Oort Cloud by Jupiter or one of the other gas giants and therefore
represent the icy remnants of solar system accretion. The Kuiper Belt was first detected
by David Jewitt and colleagues, on the basis of identification of bodies with slow, retrograde
motions indicative of large heliocentric distances. For an angular velocity dθ/dt (in arcsec
hr−1 ) determined by parallax,
148
dθ
=
,
(12.6)
dt
r + r1/2
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where r is heliocentric distance and ∆ = r -1 is geocentric distance in AU. The apparent
red magnitude mR of an object can be written
pR R2 φ(α) = 2.25 × 1016 r2 ∆2 100.4(msun −mR ) ,
(12.7)
where pR is geometric albedo, R is the radius of the object, φ(α) is the phase function, and
msun is the apparent magnitude of the sun. These expresssions indicate that 100-km-sized
objects at distances beyond Neptune (≈30 AU) should exhibit apparent red magnitudes
greater than 22, in other word, they are extremely dim.
Kuiper Belt Objects (KBOs) fall into three dynamical classes:
(1) Classical KBOs occur in low eccentricity orbits and are mostly observed from
42-47 AU. These bodies are not associated with resonances of any planet and this class
appears to be the most stable in a dynamic sense. Classical KBOs have small eccentricities
(e=0.07) and maintain a large separation (≈10 AU) from Neptune. They have a wide range
of inclinations, i=0◦ -32◦ .
(2) In contrast, resonant KBOs, of which Pluto is the largest known member, have
orbital periods that are commensurate with that of Neptune and thus avoid close encounters with that planet. These objects are sometimes referred to as Plutinos, to reflect
their dynamical similarity to the ninth planet; these resonant KBOs reside in the 3:2 mean
motion resonance, as does Pluto. KBOs in this class possess relatively high orbital inclinations and eccentricities that may have been excited by Neptune during an early, transient
period of orbital migration. Corrected for their smaller mean heliocentric distance, this
class of KBOs probably constitute about 10%-50% of the population inside 50 AU. The
eccentriciities (0.1 ≤ e ≤ 0.34) and inclinations (0◦ ≤ i ≤ 20◦ ) bracket the values for Pluto
(e = 0.25, i = 17◦ ). The resonance provides immunity to the destabilizing influence of
Neptune.
(3) Finally, scattered KBO’s occupy highly inclined and eccentric orbits, which
likely resulted from close encounters with Neptune. Scattered KBOs likely represent a
source of short-period comets.
Occasionally a Kuiper Belt object becomes gravitationally perturbed by an encounter
with another similar object or by the accumulated gravitational influence of one of the
outer planets. The object becomes diverted into the inner solar system. By this scenario it
is believed that the Kuiper Belt is the source of most short period comets. While scientists
had long assumed that short period comets were long period comets that were perturbed
into smaller orbits, there had always been a problem that the short period comets were
restricted to low inclinations. Having the Kuiper Belt as an alternative source of short
period comets solves that problem. The unusual Saturn-crossing body Chiron may have
had its origin in the Kuiper Belt. Long thought to have been an asteroid, Chiron has on
occasion been observed to display a coma and have comet-like outbursts.
Because the Kuiper Belt is much more accessible by remote Earth-based and even
spacecraft exploration, it has been an area of significant recent interest. Kuiper Belt objects
are now beginning to be resolved by astronomical surveys and about 100 such objects are
have so far been identified. There are preliminary plans for spacecraft missions to observe
one or more Kuiper Belt objects up close but this will not occur until some time in the
future. Up-close measurements of a Kuiper Belt Object would provide extremey useful
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information on the chemistry of early volatile solar system bodies before solar encounters
that modify surface structure and chemical abundances.
12.4 The Pluto-Charon System
The best-known inhabitant of the Kuiper Belt is Pluto. Although it is clear that Pluto
shares dynamical and chemical properties with other KBO’s, there is an opinion on the
part of many that it retain its place as the ninth planet.
12.4.1 Planet X
Subsequent to the discovery of Neptune, the search was on for a ”Planet X” that might
explain unexplained aspects of the motion of Neptune that were hypothesized to be due to
a more distant perturbing body. After an exhaustive sky search, Pluto was discovered by
Clyde Tombaugh on February 18, 1930, as a dim (15th magnitude) object. After discovery
its orbit was refined, and analyses of its light curve showed its preiod of rotation to be
about 6.4 days. But even after repeated observations with the largest telescopes little more
was learned about Pluto until the 1970’s. In 1976, spectral analysis showed the existence
of frozen methane (CH4 ) on Pluto’s surface. This discovery was exciting because until
then only water ice and frozen carbon dioxide had been detected spectrographically. The
detection of an ice surface was also important because it indicated that Pluto must have a
high albedo, which put a constraint on its size. Unlike the other planets of the outer solar
system Pluto was small – smaller in fact than the Earth’s moon.
12.4.2 Pluto-Charon
A second major discovery was made shortly thereafter. On June 22, 1978 James
Christy performed a systematic study of photographic plates of Pluto and noticed a consistent distortion that turned out to be a satellite, which was named Charon. The presence
of a satellite allowed the first estimate of Pluto’s mass, and it was found to be about 0.2
that of the Moon. Further observations show the radius of Pluto to be 2302 ± 14 km,
while Charon is 1186 ± 20 km. Charon has a mass of about 0.12MP luto , which makes
Charon larger than any other satellite relative to its primary body, including the EarthMoon system. The average density of the combined system is about 2000 kg m−3 , which
is consistent with a composition that is about 0.6-0.7 rock and 0.3-0.4 ice, predominantly
water ice. The spectrum of Pluto shows that in addition to methane and water ice, the
surface shows the signature of frozen nitrogen and carbon monoxide. The presence of ices
on the surface combined with the suspected gravity field led to the expectation that Pluto
has an atmosphere.
Pluto and Charon are locked in a synchronous rotation and revolution. This means
that one side of Pluto always faces Charon and vice versa. In this configuration, no tidal
evolution of Charon’s orbit can occur as in the Earth-Moon system. Interestingly, the orbit
of Charon around Pluto is slightly eccentric. The deviation from circularity is small (e =
0.0076), but any deviation is unusual for a system that is so tidally locked. This has led
people to speculate that Charon has been the target of a relatively recent collision. This
orbital eccentricity opens the possibility of internal energy dissipation due to solar tides,
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that could conceivably result in heating and perhaps the loss of volatiles from the interior
or surface.
12.4.3 Orbital Properties
At certain times, the plane of Charon’s orbit around Pluto can be seen in an ”edge-on”
configuration as observed from Earth. This allows the possibility of analysis by occultation.
The first definitive observation of a grazing occultation was made by Richar Binzel. As
these events continued it became possible to refine the diameter, masses, and mean desnity
of the system. They also allowed surface spectra of Pluto and Charon alone and it was
determined that Charon’s surface is covered mostly by water ice. High resolution imaging
by the Hubble Space telescope showed areas of dark albedo on the surfaces of the bodies
that could either be outcroppings of rock or (more likely) radiation darkening (by UV) of
surface ices.
12.4.4 Seasonal Changes?
With respect to the sun, the Pluto-Charon system has a significant orbital eccentricity
(e=0.249). The orbital configuration is such that Pluto is closer to the sun than Neptune
for about 20 years out of each 248-year orbit. At perihelion, the last of which was in
1990, Pluto was 29.7 AU, but at aphelion in 2114 it will be at 49.5 AU. The orbit is tilted
significantly with respect to the plane of the solar system (approx 17◦ ), and in addition,
the inclination of Charon’s motion to the plane of Pluto’s orbit is 99◦ . A ”pole-on” configuration can sometimes occur at perihelion. As a consequence of the orbit orientation and
tilts, temperatures on the surface of Pluto and Charon can vary greatly. Spectral data of
bright areas have yielded an approximate surface temperature of 35-60 K. The atmosphere
is likely regulated by vapor pressure equilibrium with the surface of the dominant species,
likely to be methane. It is possible that during ”winter” (when Pluto is beyond Neptune’s
orbit), that much of all of the atmosphere of Pluto freezes out, possibly falling as methane
snow to the surface. Because of a desire to study the atmosphere, proponents of a Pluto
mission argue that it should occur in the next decade or so, before the possible deep freeze.
12.4.5 Origin
Given the unusual orbits and the similarity of Pluto to Neptune’s satellite Charon,
that Pluto is an escaped satellite of Neptune. This is an interesting idea but does not
explain several aspects related to orbit geome try. Neptune and Pluto are in a 3:2 orbital
resonance. This resonance is ”librating” such that Neptune passes Pluto in its orbit only
when Pluto is near aphelion. So a collision between Neptune and Pluto can’t occur in the
current orbital configration.
The tilt of the rotational axis of the system and the fact that Pluto has a moon could
potentially both be explained by a massive impact.
12.5 Comets and Planetary Evolution
12.5.1 Sources of Water and Organics
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Comets may have been important sources of volatile material in the terrestrial planets.
They may have supplied material to planetary atmospheres and existing and/or extinct
oceans. An important question in planetary science is how much water planets accreted
and how much was deposited subsequently from comets. To address this question, it is important to understand the isotopic signature of cometary water. This is done by measuring
the ratio of deuterium/hydrogen (D/H). When water is lost from a system, the ligher hydrogen atom is lost preferentially compared to the heavier deuterium. Mass spectrometer
measurements of Halley from the Giotto spacecraft indicate that the deuterium/hydrogen
(D/H) ratio of the Halley nucleus is very similar to that of the Earth’s oceans. Howevever,
re-analysis of that data in concert with Earth-based spectral data indicate that the ratio
is more correctly about twice Earth’s value.
A single measurement of D/H in the Venus atmosphere has been interpreted to represent evidence that that planet may also once have had an ocean. Further, the source of
the ocean was hypothesized to be early cometary impacts.
Recent detections of radar-bright deposits at the poles of Mercury from Earth-based
observations and the detection of hydrogen at the lunar poles by the Lunar Prospector
mission have both been interpreted to be water ice in perenially shaded craters. If these
interpretations are correct, then the source of the water must be cometary.
If impacts delivered water to the Earth and other planets, then they also could have
delivered complex organic molecules, which would have possible implications for the origin
of life. Comets could also have had negative implications for life, as their impacts would
have catastrophic consequences on the biosphere, as discussed in Chapter 5.
12.5.3 Comet P/Shoemaker-Levy 9
Jupiter perturbed a previously undiscovered comet out of its orbit, causing it to insert
into Jovian orbit. When the comet passed near perihelion, the gravitational attraction of
Jupiter broke it apart into at least 18 pieces. The comet, which was discovered by Eugene
and Carolyn Shoemaker and David Levy, was described as a string of pearls. The orbit
degraded and the pieces impacted in August, 1994. The collisions were more energetic
than the impact at the Earth’s K-T boundary, which has been linked to a major biological
extinction event. The comet pieces entered on night side of Jupiter, but just before dawn
so observations were possible soon after the impacts. Upwelling plumes were observed in
some cases. The pieces ripped holes in Jupiter’s atmosphere and it was possible for the first
time to look down through the upper layers. The holes lasted for weeks, which provided
information on upper atmospheric dynamics and mixing on Jupiter. These impacts are
also providing much information on the composition of the comet, and of the mechanics
of the impact process.
Problems
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References
Comet P/Halley Special Issue, 1987, Astronomy and Astrophysics, 187, 1-936.
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Jewitt, D., J. Luu, and C. Trujillo, AJ, 115. 225, 1998.
Newburn, R.L., M. Neugebauer, and J. Rahe, Editors, 1991, Comets in the Post-Halley
Era, Kluwer Academic Publishers.
Wilkening, L., 1982, Comets, University of Arizona Press.
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