ScienceWeek

PLANETARY SCIENCE: ON THE KUIPER BELT

The following points are made by Rodney Gomes (Nature 2003 426:393):

1) Since the first member of the Kuiper belt was discovered(1) in 1992, many unexpected features of their orbits and physical properties have been uncovered. One surprise was the very low total mass observed in the belt -- approximately one-tenth of the mass of the Earth, when it was predicted to be a hundred times larger than that. To explain the missing mass, it has been proposed that collisions between Kuiper-belt objects over the lifetime of the Solar System have gradually transformed most of their mass into dust; bombarded by solar radiation, these dust grains are eventually expelled from the Solar System. But such theories have never been able to satisfactorily explain the extent of the depletion of the original Kuiper-belt mass.

2) According to current theory, the planets of the Solar System formed from a primordial disk of gas and dust, as the dust accumulated into gradually larger objects. Of course, in going from dust to planets there must have been intermediate stages, such as a disk of fledgling planets, or planetesimals, of roughly asteroid size. In regions where the total mass in the disk was not large enough for the accumulation process to produce planets, this planetesimal disk would have been preserved to the present day. This would certainly be the case for the Kuiper belt: because of its less dense mass distribution and large distance from the Sun, the accretion process in the belt would have ended when object sizes were no bigger than Pluto. However, in contrast to the present estimate for the Kuiper-belt mass, this accretion theory would require there to be around 10 Earth masses in the Kuiper-belt region, to allow the formation of objects as big as those presently seen in the belt.

3) The paucity of mass in the Kuiper belt is not the only enigma. Lying at such great distances from the rest of the Solar System, and experiencing no great perturbations from other large bodies, Kuiper-belt objects were expected to have orbits that were nearly circular and located close to the average plane of the Solar System. In fact, the orbits are quite eccentric, and are inclined out of the Solar System plane. Planetary scientists have wondered how these orbits could have been so dynamically excited. One idea(3) is that these objects formed much closer to the Sun and were then propelled outwards by a mechanism involving close gravitational encounters with the outward-migrating, primordial Neptune. Other work(4) shows that if there had originally been a large mass beyond Neptune's present position, this planet would have moved much further out than it is today (effectively, into the Kuiper-belt region).

4) Thus, the puzzle seemed to be nearly solved. On the one hand, the original planetesimal disk would be truncated near Neptune's present position, and was massive enough to form the large bodies now observed in the Kuiper belt; on the other hand, some objects would have been transported to the belt from this dense inner planetesimal disk by a mechanism that induced high-inclination orbits through close encounters with Neptune(3). However, there was one piece that did not fit. In addition to the Kuiper-belt objects in high-inclination orbits, there is a roughly equal number of objects at low inclination. These could not have been pushed out by the same mechanism.

5) Levison and Morbidelli(2) have proposed an alternative view. They demonstrate that some objects that now exist in the Kuiper belt might have been pushed there from original positions near Neptune's present orbit: the original Kuiper-belt region could, in fact, have been virtually empty, and only a small amount of mass was subsequently deposited there.

References (abridged):

1. Luu, J. & Jewitt, D. Nature 362, 730-732 (1993)

2. Levison, H. F. & Morbidelli, A. Nature 426, 419-421 (2003)

3. Gomes, R. S. Icarus 161, 404-418 (2003)

4. Gomes, R. S., Morbidelli, A. & Levison, H. F. Icarus (in the press).

5. Malhotra, R. Nature 365, 819-821 (1993)

Nature http://www.nature.com/nature

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ON PLUTO AND THE KUIPER BELT

In 1951 the astronomer Gerard P. Kuiper (1905-1973) postulated the existence of a belt of objects beyond the orbit of Pluto. Both the existence and nature of the objects were matters of speculation for decades, until finally in 1992 Jewitt and Luu identified the first Kuiper object. The current estimate is that as many as 10^(8) objects larger than 10 kilometers in diameter may exist in what is called the "Kuiper belt", a disc that hugs the plane of the planetary system and lies between 35 and 1000 AU from the Sun. Observations to date have yielded some 55 trans-Neptune bodies with radii on the order of 100 km or larger, and Pluto is considered by some astronomers to be a member of this population.

The moon Charon, discovered by Walter Christy in 1978, is the only satellite of Pluto, approximately one-tenth Pluto's mass, so that the pair are considered an instance of a double-planet. Charon is 1186 km in diameter, and in the Solar System, is the largest satellite compared to its primary. Its axial rotation period is the same as the rotation period of Pluto itself, so that Charon keeps one face permanently turned towards Pluto and hangs stationary over one point on Pluto's surface.

The following points are made by William B. McKinnon (Nature 2002 418:135):

1) The discovery of the Kuiper Belt in the far regions of the Solar System is one of the great achievements of the space age. In addition to the small planet Pluto and its large moon Charon, the belt contains approximately 100,000 worlds greater than 100 km in diameter, as well as a vast number of smaller, cometary bodies(1). Unlike the domains of the terrestrial planets (Mercury to Mars) or the gas giants (Jupiter to Neptune), the Kuiper Belt has never been explored by spacecraft -- although one mission, "New Horizons", has been competitively selected and is in its final design phase(2).

2) In astronomy, there is no substitute for resolution and Pluto and Charon are, to put it mildly, poorly resolved from Earth. One clear signature, however, is Pluto's rotational lightcurve --the variation of the planet's apparent brightness with time. Pluto's lightcurve is quite pronounced, both in terms of brightness and spectral features, and implies at least three separate types of surface terrain: a bright, nitrogen-ice-rich terrain containing dissolved methane and carbon monoxide; another bright, reddish terrain dominated by methane ice; and a third dark, volatile-depleted terrain betraying only the slightest hint of the broad infrared absorptions of water ice(4,5). Such a complex, variegated surface goes a long way towards explaining the peculiarities of Pluto's heat signature: the planet simultaneously exhibits a nitrogen-dominated atmosphere in vapor-pressure equilibrium with nitrogen-frosted terrain at a temperature of 40 K, and warmer regions where volatile ices have burned off. In this regard, Pluto is Mars-like in its surface-atmosphere interaction.

3) The lightcurve data only hint at the complexity of Pluto's surface. A higher-resolution map -- derived from the mutual eclipses and transits of Pluto and its moon in the 1980s --shows that even within the dark, volatile-depleted regions there exist significant visual color differences, although these differences are not as extreme as those seen in the Kuiper Belt population as a whole.

4) The fate of the New Horizons mission presently rests with the US Congress, but if it goes ahead it will provide the best answers by far to our fundamental questions about Pluto-Charon and the Kuiper Belt.

References (abridged):

1. Committee on Planetary and Lunar Exploration Exploring the Trans-Neptunian Solar System (National Academy Press, Washington DC, 1998).

2. Stern, S. A. Sci. Am. 286, 56-63 (2002).

3. Stern, S. A. & Tholen, D. J. (eds) Pluto and Charon (Univ. Arizona Press, Tucson, 1997).

4. Doute, S. et al. Icarus 142, 421-444 (1999).

5. Grundy, W. M. & Buie, M. W. Icarus 157, 128-138 (2002)

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