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ScienceWeek
PLANETARY SCIENCE: ON THE KUIPER-BELT BODY QUAOAR
The following points are made by David J. Stevenson (Nature 2004 432:681):
1) Our planetary system does not end at Pluto. Hundreds of bodies exist in the Kuiper belt, which extends outwards from Pluto, sharing the same plane as the planetary orbits. The largest known bodies in the Kuiper belt are not much smaller than Pluto, and some have similar dynamics to that planet. Although the existence of the Kuiper belt had long been hypothesized, the first Kuiper-belt body was discovered only in 1992, by Jewitt and Luu[1]. These same authors have proposed[2] that Quaoar, the largest known of these bodies, has crystalline water ice on its surface and possibly also ammonia. The presence of crystalline ice is surprising, because it is widely believed that its formation requires a temperature of approximately 100 K or more --substantially higher than the surface temperature of these bodies. The precise temperature required, however, is not known and may not be the same in laboratory experiments as it is in space. Yet it might be that we are seeing evidence for "planetary" processes such as volcanism within these bodies.
2) The discovery and characterization of the Kuiper belt is among the most important developments in planetary science in the past decade[3]. As is usual with the discovery of new bodies (inside or outside the Solar System), the initial excitement focused on the dynamical implications: why do they occupy these orbits and how did they form? Some orbital migration may occur, but it is likely that these bodies never experienced much higher surface temperatures than the ambient conditions provided by the Sun (temperatures of about 50 K or less). Quaoar, discovered in 2002 by Trujillo and Brown, is the largest body to be found in our system since the discovery of Pluto in 1930. It has a radius of approximately 650 km, roughly half that of Pluto. The composition and nature of the surfaces of these bodies are difficult to determine, yet such characteristics may be important for understanding their history. Color and brightness (albedo) can be informative, but spectroscopy at near-infrared wavelengths is the preferred technique of investigation.
3) Water is the most abundant condensed material in the Universe, and it should form the "bedrock" for solid bodies in the outer Solar System. This does not necessarily mean that the water would be readily observed; it could be hidden beneath a mantle of other material. Still, it is not surprising that Jewitt and Luu[2] observed the distinctive spectroscopic feature of water ice. But water ice that forms and remains at very low temperatures would be expected to be amorphous -- that is, lacking the periodic structure of crystalline water ice. This is because the highly coordinated architecture of a crystal lattice is difficult to establish when molecules are added to a substrate at very low energy (temperature). At higher temperature, amorphous ice rearranges to crystallize into the ordered, thermodynamic ground state (and releases latent heat as it does so).
4) More controversially, Jewitt and Luu[2] claim evidence for ammonia ice. The slight dip they see in the spectrum of reflected light around a wavelength of 2.22 micrometers is a subtle characteristic, and seems to be part of a broader spectral feature that is imperfectly understood. Brown and Trujillo have observed the crystalline water-ice feature independently (personal communication), and they also report a putative feature at 2.22 micrometers -- but they favour methane or some other explanation for the shape of the spectrum in this region. Irrespective of whether one believes the evidence for ammonia --and it would indeed survive for only a limited time on the surface -- it is likely that this molecule is present to some extent in the internal make-up of Quaoar: ammonia is present in interstellar space and is a natural (although possibly minor) carrier of nitrogen in the Universe. This raises the intriguing possibility of water-ammonia volcanism.[3,4]
References:
1. Jewitt, D. C. & Luu, J. Nature 362, 730-732 (1993)
2. Jewitt, D. C. & Luu, J. Nature 432, 731-733 (2004)
3. Brown, M. E. Phys. Today 57, 49-55 (May 2004)
4. Van Dishoeck, E. F. Annu. Rev. Astron. Astrophys. 42, 119-167 (2004)
Nature http://www.nature.com/nature
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ASTRONOMY: ON THE KUIPER BELT
The following points are made by Michael E. Brown (Physics Today 2004 April):
1) Astronomers studying the formation and evolution of the outer Solar System have had, until recently, few data points around which to spin their theories. The outer Solar System's four giant planets were each created and moved around by a complex suite of interactions. Trying to piece together all of those interactions to discern a history would be like trying to use just four large shipwrecks washed ashore to understand the flows of all of the oceans' currents. The breakthrough for astronomers came in November 1992. After a long search, Jewitt and Luu (1) found a single faint, slowly moving object in the sky. After they followed it for a few days, they realized that they had found the first object beyond Neptune since Clyde Tombaugh (1906-1997) discovered Pluto in 1930.
2) Today, more than 800 additional members of what is now known as the "Kuiper belt" have been discovered in the outer Solar System.(2) Their existence is forcing a change in the picture that astronomers held just a decade ago of the outer Solar System's dynamical evolution. In many ways, Kuiper belt objects behave like test particles that trace the gravitational effects of the giant planets' rearrangements and perturbations. The hundreds of such objects strewn throughout the outer Solar System give concrete data that allow astronomers to trace the history of that system.
3) The prediction of a belt of small bodies, or planetesimals, beyond the orbit of Neptune was made in 1950 by Gerard Kuiper (1905-1973), who used a seemingly weak but ultimately correct line of argument. Kuiper proposed a method to conceptually reconstruct the initial disk of gas and dust from which the entire Solar System formed. He began by taking Jupiter, smashing it flat, and spreading its entire mass into an annulus centered around the orbit of the giant planet. That annulus represented the region of the nebula that went into making Jupiter. Kuiper knew, however, that some material initially in that part of the nebula must have been lost, because Jupiter has a higher abundance of elements heavier than hydrogen and helium than the Sun does. So he added a little more mass to the annulus to make up for the lost hydrogen and helium and bring that region of the theoretical nebula to solar composition. Kuiper applied the same procedure to the remaining giant planets and to the terrestrial planets. (The terrestrial planets have lost almost all of their hydrogen and helium so a large amount of extra material had to be added in.) His method yields an approximate reconstruction of the mass distribution of the initial nebula.
4) Kuiper noted that the surface density of the nebula smoothly dropped from the inside to the outside until, beyond Pluto, the density plummeted. He reasoned that the nebula should not have an abrupt edge and that beyond Pluto was a realm where densities were never high enough to form large bodies, but where small icy objects existed instead. Furthermore, he suggested that the outer region could be the source for the comets that periodically come blazing through the inner Solar System.
5) In 1987, almost four decades after Kuiper's analysis, Duncan et al (3) took advantage of the growing power of computers to simulate the long-term gravitational influence of planets and showed that one class of comets -- the Jupiter-family comets --were best explained by the existence of a band of small bodies just beyond Neptune's orbit. They named that group of bodies the "Kuiper belt". Jewitt and Luu found the first object in the hypothesized Kuiper belt just five years later, in 1992.(1,4,5)
References (abridged):
1. D. Jewitt, J. Luu, Nature 362, 730 (1993)
2. See the list updated daily at http://cfa-www.harvard.edu/iau/Ephemerides/Distant/index.html
3. M. Duncan, T. Quinn, S. Tremaine, Astron. J. 94, 1330 (1987)
4. S. A. Stern, Astron. J. 110, 856 (1995)
5. R. L. Allen, G. M. Bernstein, R. Malhotra, Astrophys. J. 549, L241 (2001); C. A. Trujillo, M. E. Brown, Astrophys. J. 554, L95 (2001)
Physics Today http://www.physicstoday.org
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PLANETARY SCIENCE: ON BINARIES IN THE KUIPER BELT
The following points are made by Joseph A. Burns (Nature 2004 427:494):
1) In the frigid outskirts of the Solar System, beyond the orbit of Neptune, lies the Kuiper belt. This doughnut-shaped region contains 100,000 frozen bodies, each more than 200 kilometers across. It is scarcely a decade since the first Kuiper-belt object (KBO) was sighted, but now the positions and orbits of nearly 800 are known. Sizes have been measured for a handful and, for scores more, crude colors have been detected. A few KBOs have been found to be binary -- that is, two objects looping around each other while, as a pair, they circle the Sun.
2) When Pluto's satellite Charon was discovered in 1978, these two bodies became the first identified binary in the Kuiper belt -- although this was well before the belt itself was recognized as a distinct province of our Solar System. The next pair was not found until 2001, but now a dozen binary KBOs have been pinpointed, roughly half of them by ground-based observatories and the others by the Hubble Space Telescope.
3) Kuiper-belt couples(1,2) seem strikingly different from other binaries(3) in the Solar System. Among the known KBO pairs, the components are roughly equal in size and move on highly elongated orbits around each other, typically separated by a distance hundreds to thousands of times greater than their radii. In contrast, binary systems in the main asteroid belt, between Mars and Jupiter, are usually made up of tiny companions that are tightly bound in circular orbits around their much larger partner. The abundance and orbital configurations of the known KBO binaries, as well as the relative sizes of the individuals themselves, provide potentially pivotal clues to how KBO binaries originated and, more generally, to how bodies accumulate and evolve in the outer Solar System.
4) Like the solid planets themselves, individual KBOs are believed to have formed in the early Solar System through the agglomeration of smaller bodies. So how do any KBOs end up travelling in tandem? Cataclysmic collisions probably led to the creation of Earth's Moon, the Pluto-Charon system and many asteroid companions(4), but the progeny of such events are born too close together and are too different in size to account for the Kuiper-belt binaries. Furthermore, each of the Kuiper-belt systems contains more angular momentum than could have been transferred in any collision that did not totally destroy and disperse the colliding bodies. The formation rate through collisions alone is, anyway, much too low to account for the total binary population of KBOs(5).
5) Debris ejected in a two-body collision could, however, have been captured to yield a broad binary system had there been a third body nearby. Alternatively, two passing bodies can undergo permanent mutual capture, as long as their relative energies are lowered sufficiently through the dynamical drag of small background objects or by the gravitational scattering of another large passing KBO. This scheme(4) should also generate many small, distant companions. But to produce a reasonable yield of binaries, these mechanisms, require either many more primordial building-blocks or relatively greater numbers of small bodies than expected.
6) The theory of Funato et al(1) combines aspects of both of these mechanisms. They suggest that objects of disparate masses initially form a close binary system through gravitational instability or through collisions. Then the pair encounters another KBO that typically has a mass similar to that of the larger of the companions. Following an elaborate gravitational dance, the two massive objects pair up and the small object is ejected from the system. This idea produces binaries whose attributes match those of the observed binary population of the Kuiper belt.
References (abridged):
1. Funato, Y., Makino, J., Hut, P., Kokubo, E. & Kinoshita, D. Nature 427, 518-520 (2004)
2. Noll, K. Earth Moon Planets (in the press)
3. Merline, W. J. et al. in Asteroids III (eds Bottke, W. F., Cellino, A., Paolicchi, P. & Binzel, R. P.) 289-312 (Univ. Arizona Press, Tucson, 2003)
4. Michel, P., Benz, W., Tanga, P. & Richardson, D. C. Science 294, 1696-1699 (2001)
5. Stern, S. A. Astron. J. 124, 2300-2304 (2002)
Nature http://www.nature.com/nature
ScienceWeek http://scienceweek.com
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