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Transcript
Stardust Kuiper belt) complicate this picture dramatically. The scattered disk feeds comets into both the Jupiter family and the Oort cloud [and thus into the group of Halley-type comets (1, 2)]. So mixing of comet families complicates the picture. Michael F. A’Hearn The nuclei of even the three Jupiter family comets visited by spacecraft are surprisingly Recent advances in cometary science have indicated the importance of mixing of materials in the diverse in their shapes and in their surface disk where the planets of our solar system formed. Now, the results from the Stardust Discovery topography (Fig. 2). Comet Borrelly has a bend, Mission unambiguously show that even more extensive and earlier mixing of the material took like a banana’s shape (3, 4), Wild 2 is more place, raising new challenges for theories of the protoplanetary disk and the formation of comets. ellipsoidal (5), and Tempel 1 has surfaces that are nearly planar (6). All three show signs of wenty years ago, there was tremendous Neptune in the cold Kuiper belt. Over the 4.5- layering, although it is most obvious in the highinterest in the return of comet P/Halley. billion-year age of the solar system, the comets resolution images of Tempel 1 (6, 7), that may Although that return was somewhat dis- gradually spread among the giant planets, with reflect the original formation scenario (8). The appointing compared with its closer previous some of them ending up in orbits that are visible gas coming out of Tempel 1 (9) showed great heterogeneity, consistent with the idea that the return in 1910, it was scientifically a tremendous from Earth. The long-period comets (and some short- nucleus is made of cometesimals (smaller apparition. Primarily this was because the advance knowledge of its return allowed scientists period comets, including Halley), on the other chunks) of different composition. These results to plan both Earth-based observing programs hand, formed from solids that condensed closer can be interpreted as suggesting extensive radial mixing of cometesimals in the and to design the first space missions outer parts of the early solar to a comet. The first spacecraft to fly system, rather than formation of past a comet was actually intended Comparison of cometary nuclei from cometesimals to study the solar wind but was comet orbits condensed at a uniform distance diverted to pass through the tail of Halley from the proto-Sun. This means comet Grigg-Skjellerup in late 1985. that it will be somewhat harder For comet Halley, however, an ento pin down the locations about tire armada of spacecraft—Suisei, which comets can give us clues Sakigake, Vega 1, Vega 2, and to formation. Giotto—flew past the comet in Tempel 1 The Stardust results presented March 1986. The dramatic breakin this issue provide dramatic throughs in our understanding of Wild 2 evidence for much more extensive comets triggered great interest, r te pi radial mixing at an even earlier largely because comets hold unique Ju rn tu stage in the formation. They show, clues to the origin of our own a S perhaps most importantly, that planetary system. Nevertheless, it us nearly all of the crystalline silicate took 15 years before the next an Ur grains, which have been known missions were under way. e un for some time to exist in comets, Deep Space 1 flew past comet pt e N o must have formed in the solar Borrelly in 2001, heralding a suite of t u Pl system very close to the protocomplementary missions to very Hyakutake Sun rather than being circumstellar different comets: Stardust flew past or other presolar grains that were comet 81P/Wild 2 in January 2004, Deep Impact excavated 10,000 tons Fig. 1. Schematic showing the orbits of comets (T. Farnham, University of transported from the interstellar of material from comet Tempel 1 in Maryland). Wild 2 and Tempel 1 are Jupiter family comets (originally from medium and directly incorporated. July 2005, and Stardust returned to the Kuiper belt in the classical picture); Halley is the prototype of the Only one circumstellar grain was Earth the grains that it had collected Halley-class comets (originally from Oort cloud); Hyakutake is a comet found, showing that material is from comet Wild 2 in January 2006. coming from the Oort cloud. The classical Kuiper belt would be circular preserved from the interstellar These missions, coupled with recent orbits just outside the orbits of Neptune and Pluto. The scattered disk medium but that, at least among dynamical studies, have caused a consists of elongated orbits with perihelia in as close as Uranus and the refractory grains, it is only a small fraction of the material near major rethinking of the origin of aphelia at 50 to 100 AU (up to about three times Neptune’s orbit). the surface of a comet. Although comets. There is no doubt that comets did not form in to the Sun and at higher temperatures. Forming this mixing of near-Sun condensates with orbits even remotely similar to the ones in which in among the giant planets, these comets were preserved circumstellar grains has been sugwe now observe them (Fig. 1). Our classical ejected out to the Oort cloud (extending halfway gested before as an important part of cometary picture has been that the majority of short- to the Sun’s nearest neighbor, alpha Centauri) formation, we now have clear evidence that this period comets (the Jupiter family, of which and then much later perturbed into orbits that are mixing must be taken into account in any theory Borrelly, Wild 2, and Tempel 1 are members) also visible from Earth. Recently, dynamicists of our solar system. The Stardust results also show significant formed from solids that condensed beyond have shown that the trans-Neptunian objects in what is now called the scattered disk (dis- differences when compared with the results covered in the 1990s and containing objects in from Tempel 1. At a basic level, the size disDepartment of Astronomy, University of Maryland, College eccentric orbits extending beyond the classical tribution of impacted Wild 2 grains is different Park, MD 20742–2421, USA. E-mail: [email protected] PERSPECTIVE Whence Comets? T 1708 15 DECEMBER 2006 VOL 314 SCIENCE www.sciencemag.org SPECIALSECTION Fig. 2. Cometary nuclei visited in the last decade: (left) Tempel 1 (NASA, University of Maryland, and Deep Impact Team), (middle) Borrelly [NASA, Jet Propulsion Laboratory (JPL), and Deep Space 1 Team], and (right) Wild 2 (NASA, JPL, and Stardust Team). The longest dimensions are 8 km for Borrelly, 6 km for Tempel 1, and 5.5 km for Wild 2. Note the differences in from that inferred for the ejecta from the impact onto Tempel 1 (10), different from that of the particles seen at Halley, and even different from that deduced from the Dust Flux Monitor Instrument on Stardust itself as it flew through the coma of Wild 2. Perhaps more significantly, there are differences in the types of particles inferred for Wild 2 and Tempel 1. What does all this mean for the origin of comets? Are these differences between two comets, both the compositional and grain-size differences reported here and the large-scale morphological differences, related to their somewhat different orbital histories—Wild 2 having only recently been perturbed into the inner solar system, whereas overall shape, even though all are comparable in size. The smooth areas on Tempel 1 are low, whereas the only smooth area on Borrelly is topographically high. On Wild 2, the smooth areas are at the bottoms of circular depressions. The circular features on Wild 2 have a morphology that is very different from those on Tempel 1. Tempel 1 has been in the inner solar system far longer? Or are these differences due to mixing of comets from different reservoirs into the population of Jupiter family comets? Or do they merely represent different mixing ratios for the cometesimals that made up the cometary nuclei? Stardust has certainly brought us plenty of food for thought. Combining the Stardust results with those from other recent comet missions will keep the theoreticians working for some time, while we hope for visits to other comets in the future. References 1. M. Duncan, H. Levison, L. Dones, in Comets II, M. C. Festou, H. U. Keller, H. A. Weaver, Eds. (Univ. of Arizona Press, Tucson, AZ, 2004), pp. 193–204. PERSPECTIVE NASA Returns Rocks from a Comet Don S. Burnett Cometary particles returned by the Stardust Discovery Mission are primarily silicate materials of solar system origin. Some of the grains were formed at high temperatures close to the Sun, but then transported far out to the Kuiper belt region of the solar system before being incorporated in the comet. ntil now, extraterrestrial materials available for study have come from the inner solar system, including meteorites that have fallen from the sky and the returned lunar samples. Meteorites include unequivocal samples of Mars and the Moon as well as an impressive variety of asteroidal materials. U Department of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA. E-mail: [email protected] The Stardust Discovery Mission has, for the first time, returned cometary materials for analyses in terrestrial laboratories. Initial results are reported in this issue of Science. Launched in 1999, the Stardust spacecraft encountered comet P81/Wild 2 in January 2004, passing through the dust cloud surrounding the cometary nucleus and capturing an estimated 1000 particles in the size range of 5 to 300 micrometers (1). These were successfully returned to Earth in January 2006. www.sciencemag.org SCIENCE VOL 314 2. J. A. Fernández, T. Gallardo, A. Brunini, Icarus 172, 372 (2004). 3. R. L. Kirk, E. Howington-Kraus, L. A. Soderblom, B. Giese, J. Oberst, Icarus 167, 54 (2004). 4. J. Oberst et al., Icarus 167, 70 (2004). 5. R. L. Kirk et al., Lunar Planet. Sci. XXXVI, 2244 (2005). 6. P. C. Thomas et al., in preparation. 7. M. F. A’Hearn et al., Science 310, 258 (2005); published online 8 September 2005 (10.1126/science.1118923). 8. M. J. S. Belton et al., Icarus, in press; published online 7 November 2006 (10.1016/j.icarus.2006.09.05). 9. L. M. Feaga, M. F. A’Hearn, J. M. Sunshine, O. Groussin, T. L. Farnham, in preparation. 10. C. M. Lisse et al., Science 313, 635 (2006); published online 12 July 2006 (10.1126/science.1124694). 10.1126/science.1137083 Wild 2 is a Jupiter family comet; evidence suggests that it formed in the Kuiper belt of objects beyond the orbit of Neptune and was then diverted into the inner solar system by orbital perturbations from Neptune and Jupiter. Thanks to Stardust, we now have material to study from a body that unequivocally originated in the outer regions of the solar system. In the inner solar system, volatile constituents, primarily H2O, sublimate from a comet nucleus. Dust grains imbedded in the ices are swept out with the outflowing gas, becoming part of a large cloud of gas and dust, or coma. The cometary dust is there for the taking—the challenge is to capture the particles without destroying both the dust and the spacecraft in the process. Two clever technological achievements led to the success of Stardust: (i) design of a spacecraft trajectory by Chen-Wan Yen (Jet Propulsion Laboratory) that produced a relatively low encounter velocity of 6.1 km/s and (ii) development of an aerogel capture medium by Peter Tsou (Jet Propulsion Laboratory). 15 DECEMBER 2006 1709