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12 Comets, Kuiper Belt Objects, and Pluto

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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 rst 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 (ap- parent 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 owing 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 nu- cleus, 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 yby spacecraft in 1986, which we discuss in detail later, showed that its nucleus was potato-shaped and darker than coal, reecting only about 4% of + + the light falling on it. Water and water-derived molecules (H2O , OH, OH ,H3O) 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 ux the following reactions can occur by photodissociation: H O + h OH + H, (12.1a) 2 → H O + h O + H + H (12.1b) 2 → H O + h O + H . (12.1c) 2 → 2 Of these, reaction (12.1a) is the most common because it occurs in response to photons with energies that commonly occur in the solar ux. 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, CH3CN, and (H2CO)n. The dark color of cometary nuclei is mostly due to carbon-rich compounds and dust that remain as the comet’s ice sub- limates. Spectroscopic observations from Earth-based sensors and spacecraft indicate that gases within cometary comas are generally neutral molecules rich in CHON (Carbon, Hy- drogen, 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 12–2 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 uorescing ions of carbon monoxide (CO+). The dust tail, which is yellowish in appearance as a consequence of reected sunlight, forms when solar photons strike micron-sized dust particles that dislodge from the sub- limating nucleus. Light exerts a pressure on any object that absorbs or reects it. This radiation pressure is very weak, but ned-grained dust particles in a comet’s coma oer 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 ne 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 elds. 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 eld and solar wind. In the plasma tail elds 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 eld of the solar wind to turbulence resulting in pinching of eld 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 F (1 A) 0 =4T 4 =4T 4, (12.2) R2 e s 12–3 where A is albedo, F0 is the solar constant, R is radius, is the Stefan-Boltzmann constant, is the thermal emissivity ( 1), Te is the eective 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 eect of vaporization (sublimation), because under certain P/T conditions (high P and T) this process can absorb a signicant amount of the heat ux. It is possible to show that the sublimation rate (dm/dt) is related to the vapor pressure (Pvap)by dm m 1/2 = P , (12.3) dt vap 2kT s where k is Boltzmann’s constant. The heat ux due to sublimation is just dm =L , (12.4) dt where L is the latent heat of evaporation for ice. Once a comet gets close to the sun, a signicant amount of the incident solar heat ux will be carried away by evaporation. It is then necessary to modify the steady-state ux balance to account for this eect: F dm (1 A) 0 =4T 4 + L . (12.5) R2 s dt 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.
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