Exploring the Trans-Neptunian Solar System (1998) / Chapter Skim
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2 Current Knowledge and Outstanding Issues
2 Current Knowledge and Outstanding Issues
Pages 18-29
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From page 18... ...
Within the region observed in detail by Voyager 2's cameras, Triton exhibits a wide array of features produced by tectonic, volcanic, and atmospheric processes (see Plate 1~. As with other icy satellites, the energy sources for the volcanic and tectonic activity on Triton are not well understood, but they may involve catastrophic tidal heating associated with Triton's capture from solar orbit in the distant past.
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From page 19... ...
The geology of the imaged portion is among the most complex and varied of any of the solar system' s icy satellites.4 There is no evidence for preserved ancient heavily cratered terrain, which implies that Triton was internally active for at least several hundred million years following its formation. The total crater population is low and indicates an average age for Triton's surface on the order of several hundred million years.
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From page 20... ...
Earth-based spectroscopic observations reveal that Triton's surface is covered by a variety of volatile ices.8 The spectra imply that N2 is the dominant ice species with trace amounts (<1% by mass) of CO, CH4, and CO2, if the surface grains are mixed at the granular level.
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From page 21... ...
Each pass through the magnetic equator subjects Triton to an energetic electron energy flux that is about 20 times larger than the solar extreme-ultraviolet energy flux. Some of these electrons enter the upper atmosphere and deposit power estimated to be as large as 108 watts.~4 Significant magnetospheric energetic electron power input is suggested by the high thermospheric temperature (~100 Kelvin)
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From page 22... ...
However, most of the surface features unveiled by HST, including the prominent northern polar cap, are likely produced by frosts that migrate across Pluto' s surface in response to its orbital and seasonal cycles and chemical by-products deposited by Pluto's atmosphere. Earth-based spectroscopic observations show that Pluto's surface, like Triton's, is covered with ices and relatively volatile compounds.~9 Current models of the reflectance spectra suggest that N2 is the dominant species on Pluto, with trace amounts (<2% by mass)
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From page 23... ...
The spectroscopic N2 ice temperature is 40 + 2 Kelvin on Pluto,23 which implies surface N2 atmospheric pressures in the range from 18 to 157 microbars, assuming vapor pressure equilibrium. From the observed abundance of CO ice, the atmospheric CO mixing ratio is inferred to be about 5 x 10-4.24 Plasma Interactions on Pluto The interaction of the tenuous solar-wind plasma with Pluto critically depends on the flux of material escaping from the planet's atmosphere.25 If the escape rate is greater than about 1027 molecules see-i, then Pluto acts like a comet with the solar wind ionizing the outflowing material upstream, slowing down the solar wind, and pulling the mass-loaded solar wind into a downstream ion tail.
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From page 24... ...
Their existence poses the question of whether the Kuiper Belt extends as far as the Oort Cloud. Dynamical studies of the trans-neptunian region show that orbits with a < 35 AU and with 40 < a < 42 AU are very unstable to gravitational perturbations by Neptune and Uranus.39 40 These studies show that a small fraction of KBOs continue to stray into these unstable zones where they are likely to suffer major perturbations,4i 42 confirming an earlier suggestion that the disklike Kuiper Belt is the more probable source of low-inclination, short-period, Jupiter-family comets than is the isotropically distributed Oort Cloud.43 Although the absence of KBOs within 35 AU can be explained by Neptune's perturbations, the lack of objects in the dynamically stable region of low-eccentricity orbits between 36 and 39 AU remains an important mystery.44 Malhotra has proposed that Neptune's orbit has evolved outward, sweeping up objects into the stable 3:2 resonance and clearing the inner Kuiper Belt.45 Others have proposed the presence of as-yet-undetected massive perturbers that have cleared the 36- to 39-AU gap.46 47 Jewitt and colleagues have argued that the inclination distribution of the trans-neptunian objects is important because it controls the velocity dispersion among these objects and hence determines whether the collisional regime is erosive or agglomerative.48 The upper part of Figure 2.1 suggests that objects located in resonant orbits have higher inclinations, consistent with the dynamical studies.49 50 Malhotra's work also shows that the fraction of KBOs whose orbits are pumped up into higher inclinations as they are swept into resonances depends on the time scale for outward migration of the giant planets.
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From page 25... ...
so that searches need to cover a broad band of latitudes in order to avoid a selection bias in sampling the KBO population.5i The size distribution of a population of objects has been a useful diagnostic for understanding the processes that lead to the erosion and/or accretion of planetary bodies. From recent observations and theoretical studies, it is emerging that objects in the trans-neptunian region probably follow a complex size distribution (Figure 2.2~.
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From page 26... ...
If additional observations establish a common origin for Centaurs and KBOs, then the Centaurs can provide compositional information on the more distant Kuiper Belt objects and information about their subsequent processing.
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From page 27... ...
. The organic materials suggested to date include light hydrocarbons or methanol ice, Titan tholin, polymeric HCN, and carbon black.
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From page 28... ...
A.L. Cochran et al., "The Discovery of Halley-Sized Kuiper Belt Objects Using Hubble Space Telescope," Astrophysical Journal 455:342, 1995.
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From page 29... ...
Duncan, "The Discovery of Halley-sized Kuiper Belt Objects Using Hubble Space Telescope," Astrophysical Journal 455:342, 1995.
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