PREFACE This joint IAU and COSPAR Colloquium, held at the campus of The University of Kent at Canterbury from April 10 to ,41 2000 brought together 129 scientists from 81 countries. It was a continuation of the tradition of holding meetings at regular intervals of a few years in order to review the progress in a broad range of disciplines that are relevant to the study of interplanetary dust and to help to unify progress made through observations, both in situ and from the ground, theory and experimentation. The series of meetings started in Honolulu, Hawaii (USA) in 1967, followed by Heidelberg (Germany) in 1975, then Ottowa (Canada) in 1979, Marseilles (France) in 1984, Kyoto (Japan) in 1990 with the last being in Gainesville, Florida (USA) in 1995. Since the Gainesville meeting, there have been dramatic changes in the field resulting from in-situ space experiments, Earth orbiting satellites and ground based observations. The brightest comet since the early years of the twentieth century, comet Hale-Bopp, appeared, giving an invaluable opportunity to see in action one great source of interplanetary dust. Similarly, the Leonid meteor shower has been at its most active since 1966, producing spectacular displays of meteors and allowing for an array of observational techniques, not available in 1966 to be used, while theory has also been refined to a level where very accurate predictions of the timing of meteor storms has become possible. Prior to the meeting we observed a total eclipse of the Sun in SW England and Northern Europe, traditionally a good opportunity to observe the Zodiacal cloud. Our knowledge of the Near-Earth Asteroid population has also increased dramatically, with the increased study arising from the heightened awareness of the danger to Earth from such bodies. Extrasolar planets have been discovered since the last meeting and it is recognised that we can now study interplanetary dust in other Planetary Systems. Since much of the dust observed in such systems is at a distance of order 100 AU from the star, this brings into focus the production of dust in the Edgeworth-Kuiper Belt of our own system. Recent years have seen a recognition of the importance of dust originating outside our own system, that is now present in the near-Earth environment. As is always the case when great strides take place observationally, much theoretical work follows, and the same is true in this instance. While data about the interplanetary medium from Venus to Jupiter was beginning to be available at the last meeting, the data from both Galileo and Ulysses have now been more fully analysed, with a corresponding increase in our knowledge. Since then however information from SOHO and MSX have become available, giving new insight into the dust population close to the Sun. In addition, ISO allowed us to study the radiation emitted from dust (as opposed to its more normal obscuring properties). There are also new space missions in various stages of planning, Particularly STARDUST and ROSETTA, that will produce a whole new dimension to our knowledge of dust production in the Solar system. The scientific Organizing Committee was responsible for defining the scientific content and selecting the invited reviews. These proceedings contain 31 invited reviews and invited contributions, and 46 contributed papers. The papers reflect the thematic approach adopted at the meeting, with a flow outwards (from meteors in the atmosphere, through zodiacal dust observation and interplanetary dust, to extra solar planetary systems) and returning (via the Edgeworth-Kuiper belt and comets) to the Earth, with laboratory studies of physical and chemical processes and the study of extra-terrestrial samples. Simon Green, Iwan Williams, Tony McDonnell, Neil McBride. -V- SCIENTIFIC ORGANISING COMMITTEE I.P. Williams (UK, Chair) J.A.M. McDonnell (UK, Co-chair) W.J. Baggaley (New Zealand) E. Grtin (Germany) M.S. Hanner (USA) .P Lamy (France) A.C. Levasseur Regourd (France) .T Mukai (Japan) .V Porubcan (Slovak Republic) H. Rickman (Sweden) E. Tedesco (USA) N. Thomas (Germany) LOCAL ORGANISING COMMITTEE J.C. Zamecki (Chair) M.J. Burchell B.J. Goldsworthy S.F. Green N. McBride J.A.M. McDonnell M.L. Watts ACKNOWLEDGEMENTS The Colloquium was sponsored by IAU Commission 22 (Meteors, Meteorites and Interplanetary Dust) and supported by Commission 51 (Physical Study of Comets and Minor Planets), Commission 20 (Positions and motions of Minor Planets, Comets and Satellites), Commission 12 (Light of the Night Sky) and Commission 15 (Bioastronomy: search for Extraterrestrial Life) and also by COSPAR. We are indebted to several organisations for financial support: The Intemational Astronomical Union, COSPAR, The Royal Astronomical Society, The University of Kent at Canterbury and Unispace Kent. This support allowed us to provide travel grants for students and key speakers who would otherwise have been unable to attend. It is a pleasure to thank lla the members of the Local Organising Committee, sa well sa many individuals who worked os hard behind the scenes to make the meeting a success: Esther Aguti, Margaret Fowler, James Galloway, Nadeem Ghafoor, Jon Hillier, Michael MUller, Jo Mann, Naveed Moeed, Manish Patel, Tim Ringrose, and especially Jane Goldsworthy and Mary Watts; Andrew Thompson and his team for flawless organisation of the local tours and Sir Harry Kroto for entertaining us sa guest of honour at the conference dinner. Finally, we thank Louise Hobbs, Michael Mucklow, James Garry, Mary Watts and Michael Willis for assistance with preparation of these proceedings. - vi- SCIENTIFIC ORGANISING COMMITTEE I.P. Williams (UK, Chair) J.A.M. McDonnell (UK, Co-chair) W.J. Baggaley (New Zealand) E. Grtin (Germany) M.S. Hanner (USA) .P Lamy (France) A.C. Levasseur Regourd (France) .T Mukai (Japan) .V Porubcan (Slovak Republic) H. Rickman (Sweden) E. Tedesco (USA) N. Thomas (Germany) LOCAL ORGANISING COMMITTEE J.C. Zamecki (Chair) M.J. Burchell B.J. Goldsworthy S.F. Green N. McBride J.A.M. McDonnell M.L. Watts ACKNOWLEDGEMENTS The Colloquium was sponsored by IAU Commission 22 (Meteors, Meteorites and Interplanetary Dust) and supported by Commission 51 (Physical Study of Comets and Minor Planets), Commission 20 (Positions and motions of Minor Planets, Comets and Satellites), Commission 12 (Light of the Night Sky) and Commission 15 (Bioastronomy: search for Extraterrestrial Life) and also by COSPAR. We are indebted to several organisations for financial support: The Intemational Astronomical Union, COSPAR, The Royal Astronomical Society, The University of Kent at Canterbury and Unispace Kent. This support allowed us to provide travel grants for students and key speakers who would otherwise have been unable to attend. It is a pleasure to thank lla the members of the Local Organising Committee, sa well sa many individuals who worked os hard behind the scenes to make the meeting a success: Esther Aguti, Margaret Fowler, James Galloway, Nadeem Ghafoor, Jon Hillier, Michael MUller, Jo Mann, Naveed Moeed, Manish Patel, Tim Ringrose, and especially Jane Goldsworthy and Mary Watts; Andrew Thompson and his team for flawless organisation of the local tours and Sir Harry Kroto for entertaining us sa guest of honour at the conference dinner. Finally, we thank Louise Hobbs, Michael Mucklow, James Garry, Mary Watts and Michael Willis for assistance with preparation of these proceedings. - vi- SCIENTIFIC ORGANISING COMMITTEE I.P. Williams (UK, Chair) J.A.M. McDonnell (UK, Co-chair) W.J. Baggaley (New Zealand) E. Grtin (Germany) M.S. Hanner (USA) .P Lamy (France) A.C. Levasseur Regourd (France) .T Mukai (Japan) .V Porubcan (Slovak Republic) H. Rickman (Sweden) E. Tedesco (USA) N. Thomas (Germany) LOCAL ORGANISING COMMITTEE J.C. Zamecki (Chair) M.J. Burchell B.J. Goldsworthy S.F. Green N. McBride J.A.M. McDonnell M.L. Watts ACKNOWLEDGEMENTS The Colloquium was sponsored by IAU Commission 22 (Meteors, Meteorites and Interplanetary Dust) and supported by Commission 51 (Physical Study of Comets and Minor Planets), Commission 20 (Positions and motions of Minor Planets, Comets and Satellites), Commission 12 (Light of the Night Sky) and Commission 15 (Bioastronomy: search for Extraterrestrial Life) and also by COSPAR. We are indebted to several organisations for financial support: The Intemational Astronomical Union, COSPAR, The Royal Astronomical Society, The University of Kent at Canterbury and Unispace Kent. This support allowed us to provide travel grants for students and key speakers who would otherwise have been unable to attend. It is a pleasure to thank lla the members of the Local Organising Committee, sa well sa many individuals who worked os hard behind the scenes to make the meeting a success: Esther Aguti, Margaret Fowler, James Galloway, Nadeem Ghafoor, Jon Hillier, Michael MUller, Jo Mann, Naveed Moeed, Manish Patel, Tim Ringrose, and especially Jane Goldsworthy and Mary Watts; Andrew Thompson and his team for flawless organisation of the local tours and Sir Harry Kroto for entertaining us sa guest of honour at the conference dinner. Finally, we thank Louise Hobbs, Michael Mucklow, James Garry, Mary Watts and Michael Willis for assistance with preparation of these proceedings. - vi- 33 YEARS OF COSMIC DUST RESEARCH "Welcome to Canterbury 2000", extended to the Interplanetary Dust community, was phased to mark progress in research over 33 years at Kent. The group, founded by Roger Jennison and myself in 1967, commenced research with space dust experiments involving collaboration with Otto Berg of NASA GSFC, later taking a big stride forward with the NASA and USSR Lunar Sample analyses. Deep space experiments on Pioneers 8 and 9, developed by Merle Alexander and Otto Berg showed the potential, and high reliability, needed for measurements in sparsely populated interplanetary space. With dust accelerators then at Kent and at Heidelberg, experiments such as those on Ulysses and Galileo were able to be proposed and, vitally, calibrated; impact detectors for the Giotto Halley Mission, for Cassini and now for Stardust followed. Results, which will be flowing for many years, provide that vital in-situ link between distant regions and observations at planet Earth. Equally vital to this "ground truth", albeit in space, are the fields of modelling, laboratory measurements, radar studies and extended astronomical measurements such as those of the Zodiacal Light. Without these different approaches and the different data acquired, each would be the weaker. These proceedings underscore the breadth and strength which has developed since that first coherence was created in "Cosmic Dust" (1978). The Canterbury welcome coincided with farewells from the majority of space academics who, with their equipment, expertise and experience, joined the well established lines of success developed by Colin Pillinger at the Open University, Milton Keynes. Success for a research group is very much due to the efforts and response of each individual; the essential contributions are not confined to academics. I thank therefore all of the group members throughout my time at Kent and all of the UK and International colleagues who have been both a stimulus and pleasure in sharing a career at Canterbury. From The Open University ..... where even greener pastures may unfold! ~176 - VII - LIST OF ATTENDEES S. Abe V. Haudebourg I.S. Murray P. Abraham R.L. Hawkes H. Ntibold E. Aguti S. Helfert H. Ohashi D.J. Asher M.K. Herbert R. Ohgaito P.B. Babadzhanov J.K. Hillier E. Palomba D.E. Backman T.-M. Ho C. Park W.J. Baggaley E.K. Holmes M.R. Patel L.R. Bellot Rubio S.S. Hong A. Pellinen-Wannberg S. Benzvi J.E. Howard S.B. Peschke D.E. Brownlee S.I. Ipatov T. Poppe M.J. Burchell M. Ishiguro H. Rickman A. Bursey D. Janches F.J.M. Rietmeijer M. Burton S. Jayaraman T.J. Ringrose B.C. Clark P. Jenniskens S. Sasaki L. Colangeli E.K. Jessberger G. Schwehm M.J. Cole T.J.J. Kehoe H. Sdunnus J. Crovisier H.U. Keller Z. Sekanina S.F. Dermott S. Kempf H. Shibata V. Dikarev K.V. Kholshevnikov N.R.G. Shrine C. Dominik H. Kimura A.A. Sickafoose J.R. Donnison D. Koschny M.B. Simakov G. Drolshagen A.V. Krivov R. Srama E. Epifani N.A. Krivova D.I. Steel F. Esposito H. Krtiger M. Sttibig G.J. Flynn J. Kuitunen H. Svedhem S. Fonti S.M. Kwon S. Takahashi M. Fulle P.L. Lamy H. Tanabe D.P. Galligan M. Landgraf E.A. Taylor J. Galloway M.R. Leese S.P, Thompson M.J. Genge A.-C. Levasseur-Regourd K. Torkar N.A.L. Ghafoor G. Linkert P. Tsou F. Giovane J-C. Liou R. Vasundhara B.J. Goldsworthy C.M. Lisse R. VickramSingh M.M. Grady K. Lumme K.W.T. Waldermarsson G.A. Graham J.C. Lyra M.K. Wallis A.L. Graps Y. Ma I.P. Williams S.F. Green J. Mann M.J. Willis I.D.S. Grey M. Matney J.-C. Worms K. Grogan N. McBride H. Yano E. Grtin J.A.M. McDonnell S. Yokogawa B./k.S. Gustafson N.S. Moeed J.C. Zarnecki E. Hadamcik M. Mtiller Y. Hamabe K. Muinonen M.S. Hanner T. Mukai - viii - Meteoroid streams and meteor showers I.P.Williams ~ gAstronomy Unit, Queen Mary and Westfield College, Mile End Rd, London E1 4NS, UK The generally accepted evolution of meteoroids following ejection from a comet is first spreading about the orbit due to the cumulative effects of a slightly different orbital period, second a spread in the orbital parameters due to gravitational perturbations, third a decrease in size due to collisions and sputtering, all in due course leading to a loss of identity as a meteor stream and thus becoming part of the general sporadic background. Finally Poynting-Robertson drag causes reduction in both semi-major axis and eccentricity producing particles of the interplanetary dust complex. The aim of this presentation si to review the stages involved in this evolution. 1. HISTORICAL BACKGROUND This meeting si about dust in our Solar System and Other Planetary Systems. Planets have been discovered in about 03 nearby systems, but in these we have not as yet observed dust. On the other hand, a number of young stars are known to have a dust disk about them, but in these direct detection of planets is absent. At present, our system si the only one where dust and planets, as well as comets and asteroids to provide a source for the dust is present. Many phenomenon show the presence of the interplanetary dust complex, the zodiacal light, grains captured in the near-Earth environment as well as a number of in-situ measurements from spacecraft both in Earth orbit and in transit to other regions of the Solar System. We start the discussion with proof that must have been visible to humans since pre-history, namely the streaks of light crossing the sky from time to time, popularly called shooting stars, but more correctly known as meteors. Indeed, many of the ancient Chinese, Japanese and Korean records, talk of stars falling like rain, or many falling stars. A detailed account of these early reports can be found in the work of Hasegawa 1. The same general thought probably gave rise to the English colloquial name for meteors, namely Shooting Stars. In paintings of other events, meteors were often shown in the background (see for example 2). These historical recordings are very valuable, for they show that the Perseids for example have been appearing for at least two millenia. Recording and understanding are however two different things so that the interpretation of these streaks of light as interplanetary dust particles burning in the upper atmosphere is somewhat more recent. The reason probably lies in the belief that the Solar System was perfect with each planet moving on its own well determined orbit. Such beliefs left no room for random particles colliding with the planets, especially the Earth. Meteors were thus regarded as some effect in the atmosphere akin to lightning, -3- .P.I smailliW hence the name. About two centuries ago the situation changed. First, there were a number of well observed meteorite falls where fragments were actually recovered. This at least proved that rocks could fall out of the sky though it did not by itself prove that they had originated from interplanetary space, however, as more observations of meteors took place, so thoughts changed. The measurement of the height of meteors as about 90kin by Benzenberg & Brandes in 1800 3 in essence spelt the end of the lightning hypothesis. When Herrick (1837, 1838) 4,5 demonstrated that showers were periodic on a sidereal rather than a tropical year, the inter-planetary rather than terrestrial in origin was proved. 2. OVERVIEW OF METEOR SHOWERS Meteors can be seen at any time of the year, appearing on any part of the sky and moving in any direction. Such meteors are called sporadic and the mean sporadic rate is very low, no more than about ten per hour. Nevertheless, the flux of sporadics, averaged over a reasonable time span, is greater than the flux from any major stream averaged over the same time span. The major streams appear at well-determined times each year with the meteor rate climbing by two or three orders of magnitude. For example around 21 August meteors are seen at a rate of one or two per minute all apparently radiating from a fixed well determined point on the sky, called the radiant. This is the Perseid meteor shower, so named because the radiant of this shower lies in the Constellation of Persius. This behaviour is generally interpreted in terms of the Earth passing through a stream of meteoroids at the same siderial time each year. Olmstead 6 and Twining 7 are credited with first recognizing the existence of a radiant. Many of the well-known showers are rather consistent from year to year, but other are not. The best-known of these latter is the Leonids, where truly awesome displays are sometimes seen. For example, in 1966, tens of meteors per second were seen. Records show that such displays may be seen at intervals of about 33 years, with the displays of 1799, 1833 and 1966 being truly awesome, but good displays were also seen for example in 1866 and 1999. These early spectacular displays helped Adams 8, LeVerrier 9 and Schiaparelli 10, all in 1867, to conclude that the mean orbit of the Leonid stream was very similar to that of comet 55P/Tempel- Tuttle and that 33 years were very close to the orbital period of this comet. Since then comet-meteor stream pairs have been identified for virtually all recognizable significant stream. These simple facts allow a model of meteor showers and associated meteoroid streams to be constructed. Solid particles (meteoroids) are lost from a comet as part of the normal dust ejection process. Small particles are driven outwards by radiation pressure but the larger grains have small relative speed, much less than the orbital speed. Hence these meteoroids will move on orbits that are only slightly perturbed from the cometary orbit, hence in effect generating a meteoroid cloud about the comet which is very close to co-moving with the comet. As the semi-major axis of each meteoroid will be slightly different, each will have a slightly different orbital period, resulting in a drift in the epoch of return to perihelion. After many orbits this results in meteoroids effectively being located at all points around the orbit. With each perihelion passage a new family of meteoroids is generated, but, unless the parent comet is heavily perturbed, the new set -4- Meteoroid streams and meteor showers of meteoroids will be moving on orbits that are almost indistinguishable from the pre- existing set. Various effects, drag, collisions, sputtering, will remove meteoroids from the stream, changing them to be part of the general interplanetery dust complex and seen on Earth as Sporadic meteors. An annual stream is thus middle-ages, with meteoroids having spread all around the orbit so that a shower is seen every year. In a very old stream where the parent comet may not still be very active, the stream is never very noticeable, but again constant each year. A very young stream on the other hand will only show high activity in certain years only since the cloud of meteoroids has had insufficient time to spread about the orbit. 3. THE LIFE OF A METEOROID STREAM The basic physics behind the process of ejecting meteoroids from a cometary nucleus became straightforward as soon as a reasonably correct model for the cometary nucleus became available. Such a model for the nucleus was proposed in 1950 by Whipple 11, the so called dirty snowball model, in which dust grains were embedded in an icy matrix. As the comet approaches the Sun, the nucleus heats up until some of the ices sublime and become gaseous. The heliocentric distance at which this occurs will depend on a number of parameters, the composition, the albedo and the rotation rate for example, but the process which follows this is independent of these details. When sublimation occurs, the gaseous material flows outwards away from the nucleus at a speed which is comparable to the mean thermal velocity of the gas molecules. Any grains, or meteoroids not still embedded in the matrix will experience drag by the outflowing gas. The outward motion of the meteoroid will be opposed by the gravitational field of the comet nucleus and a meteoroid will escape from the cometary nucleus into inter-planetary space only if the drag force exceeds the gravitational force. Now, drag is roughly proportional to surface area while gravity depends on mass, thus smaller grains might experience a greater acceleration while gravity will win for grains over a given size. Hence there is a maximum size of meteoroid that can escape, though this size might vary from comet to comet depending on the size and activity level of the comet. The final speed achieved by any meteoroid that does escape will similarly depend on these factors as well on the grain properties. These considerations were first quantified by Whipple 12. He obtained (1 ) /~2_ 4.3 x 105Rc bcrr2.25 0.013Rc , (1) where rc is the bulk density of the meteoroid of radius b and r the heliocentric distance in astronomical units. Rc is in kilometers and all other quantities in cgs units. A number of authors have suggested modifications to this general formula, for example Gustafson 13 pointed out that the drag formula was incorrect if the meteoroids were non-spherical while Harris and Hughes 14 suggest that the gas outflow down a tube or cone is slightly faster than is suggested by considering the mean thermal velocities. Both these points are undoubtedly correct but the end result leads to only a slight increase in the ejection velocity. Finson and Probstein 15 produced a model for dust outflow that related the observed brightness variations along the cometary tail to the dust flow rate. The dust that causes light scattering in the tail is somewhat smaller than dust -5- LP. Williams that evolves into meteors, but nevertheless, there is no major difference between the dust velocities given by this approach and that given for example by Whipple's formula. The main conclusion, in terms of meteoroid stream formation, is that the ejection velocity is in all cases considerably less than the orbital velocity of the parent comet. As an illustration, consider comet 1P/Halley. Grains of up to a few centimeters will escape, while at 1AU, a one millimeter meteoroid would have an ejection speed of about 70ms -1. The orbital speed at 1AU is of the order of 3Okras -1. The effect of the meteoroid being ejected with a speed given by the mechanism above relative to the comet will be to produce differences between the orbit of the meteoroid and that of the comet. These changes will of course depend on the direction at which the meteoroid is ejected and the point on the cometary orbit at which the ejection takes place. There will always be a change in the specific energy E. Now, standard theory of Keplerian motion tells us that E - -aM| (2) 2a ' and that p2 _ a .3 (3) where a is the semi-major axis of the orbit in Astronomical Units and P the orbital period in years. Hence we can obtain AE -Aa -2AP -_ - . (4) E a 3P a change in semi-major axis and period thus is an inevitable consequence of the ejection process, but since -~ is likely to be small in view of the fact that the ejection velocity is small compared to the orbital velocity, changes in a and P are also likely to be small. Observationally, it will be very difficult to detect such changes in the semi- major axis. However, changes in the orbital period are different in that their effect is cumulative. After n completed orbits, the time difference between a meteoroid and the comet passing perihelion will be nAP . For a typical situation, in about 50 orbits meteoroids will be found at all points of the orbit that is an annual stream is formed. If there is a component of the ejection velocity in the transverse direction, then the specific angular momentum h will also be changed, we have h 2 - GM| (5) where p is the semi-parameter of the orbit, that is p - a(1 - e2). This yields Ah _ _ Ap_ _ Aa eAe (6) h 2p 2a 1( - e2)" This implies that in general there is a change in eccentricity as well. Detecting changes in the eccentricity from observations of meteors will also be very difficult. Unless the ejection took place exactly at perihelion, the changes in a and e, together with the requirement that the ejection point is on both the comet and meteoroid orbit, -6-