METEOROID STREAM SOURCES FROM DYNAMIC AND PROBABILISTIC TRAJECTORY ANALYSIS By LARS GORAN ADOLFSSON A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 1996 To Madeleine, Christopher and Danniella ACKNOWLEDGMENTS It is a pleasure to thank the members of my examining committee, Humberto Campins, Stanley Dermott, Bo Gustafson, Carl Murray, and Yngve Ohm. I owe special thanks to Bo Gustafson for serving as chairman on my committee. Bo has shown me that there is another side to science than just being able to "put up" equations. He always want to think physically about the problem at hand, something I wish to master one day. I also value very much the many long talks we have had on how to survive in science. I firmly belive that it should be part of the curriculum. Lastly I wish to thank Bo for his friendship! During my first couple of days in graduate school, far from home, I realized how different this country was from my own. Keith Grogan, the "Liverpuddlian", helped me stay (almost) sane throughout graduate school. I wish to thank Keith for conversations, beers, soccer-updates, taking care of first submissions, and many more things. I thank all faculty and graduate students for very interesting years. Special thanks goes to Seppo Laine, Sumita Jayaraman, Dave Osip, Tim Spahr, Joanna Thomas-Osip, and Steve Kortenkamp. I wish to express deep gratitude to my parents, Karin and Hugo Adolfsson, for their support and almost endless interest in my work. Finally, I thank Madeleine for all the love and emotional support she has given me through many difficult times, and all the happiness she has shared with me through good times. iii TABLE OF CONTENTS ACKNOWLEDGMENTS iii ABSTRACT vi CHAPTER 1: INTRODUCTION 1 Meteoroids and Streams 1 Identification of Meteoroid Stream Sources 4 CHAPTER 2: PRODUCTION OF METEOROIDS 8 Cometary Activity 10 Collisions 16 CHAPTER 3: METEOROID DYNAMICS 19 Cometary Coma 19 Collisions 23 Interplanetary Space 24 Atmospheric Flight 31 CHAPTER 4: PARENT-DAUGHTER ASSOCIATION METHOD 36 Sampling of Error Distributions and Orbital Integrations 38 Meteoroids 39 Parent Bodies 44 Intersection Condition 45 Ejection Condition 47 Probability Computations 48 CHAPTER 5: OBSERVATIONAL MATERIAL AND DATA REDUCTION 52 Meteoroids 52 Harvard Small Camera Program 54 Harvard Super-Schmidt Program 60 Prairie Network 66 Parent Bodies 73 iv Geminids: 3200 Phaethon, 1566 Icarus, 5786 Talos 73 Taurids: P/Encke 74 Computations and Hardware 75 CHAPTER 6: CROSS-SECTION TO MASS RATIO 77 CHAPTER 7: RESULTS 82 Geminids 82 Taurids 92 CHAPTER 8: DISCUSSION 96 CHAPTER 9: CONCLUSIONS 112 REFERENCES 114 BIOGRAPHICAL SKETCH 120 v Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy METEOROID STREAM SOURCES FROM DYNAMIC AND PROBABILISTIC TRAJECTORY ANALYSIS By Lars Goran Adolfsson May 1996 Chairperson: Bo A. S. Gustafson Major Department: Astronomy We develop a method to estimate the probability of a genetic relation between a specific meteoroid and a proposed parent body. From meteor data, using single-body meteor theory, we estimate the errors in the velocity vector and the cross-section to mass ratio of the meteoroid. We sample these errors and probe the orbit probability distribution by numerically integrating the equations of motion back in time. The integrations account for planetary perturbations, radiation pressure, Poynting-Robertson light drag, and solar wind corpuscular drag. Two conditions are used to ensure the opportunity of a genetic relation. First, we require that the orbits of the meteoroid and the parent body intersect; the intersection condition. Identification of an ejection point allows estimates of ejection conditions; relative velocity, direction of ejection, heliocentric distance, and epoch of ejection. Secondly, an intersection point is rejected as a potential ejection site unless the relative velocity is deemed vi to be a physically realistic ejection velocity; the ejection condition. The probability of a genetic relation is estimated as the fraction of the sampled orbits that satisfy both criteria. The method is applied to Geminid and Taurid meteors. Only the most precise meteors are considered. A total of 55 Geminids from the Harvard Small Camera Program, the Harvard Super-Schmidt Program, and the Prairie Network, are used. Twenty-six Taurids from the Harvard Super-Schmidt Program and the Prairie Network are also chosen. The error in velocity is typically 0.01-0.1%. A large error in the velocity vector lead to a situation where we are less likely to find the ejection point and establish a genetic relation. A genetic relation for 28 of the Geminids and their proposed parent body Phaethon is established, indicating cometary activity as the formation process. Phaethon was probably active over at least 5,000 years, terminating its activity as recently as = AD 1,800. None of the Taurids satisfy all criteria within the method, and could not be linked to their proposed parent body comet P/Encke. vii CHAPTER 1 INTRODUCTION Meteoroids and Streams Meteoroids are small interplanetary bodies orbiting the Sun. Meteoroids come in a wide range of sizes; considerably larger than atoms and up to a size comparable to the smallest asteroids. However, no hard-and-fast boundaries exist. A size range below 100 um is usually implied by the word dust, a subset of meteoroids. The word meteor is a general term describing the luminous track and the ionization associated with the entry of a meteoroid into the atmosphere (see Bronshten, 1983, for a thorough review of meteor phenomena). The apparent maximum brightness of a meteor is, to first approximation, proportional to the mass of the meteoroid. Since the mass (proportional to the size to the third power) range of meteoroids is wide, this implies that the same holds true for the range of the maximum brightness of meteors. The faintest meteors (not observable to the naked eye) are observed through the use of radar, and they are subsequently described as radar meteors. Most people have at some time or other seen a "shooting star" or a "falling star." This corresponds to a visual or photographic meteor. The brightest, and most rare, meteors, brighter than Venus and occasionally rivaling the Moon, are denoted fireballs. 1 2 The meteoroid material that survives atmospheric entry, and reaches the surface of the Earth, is called a meteorite. It has to be noted, though, that every meteoroid impacting the Earth's atmosphere does not produce a meteorite. A meteoroid made of a tough material and impacting the atmosphere at low speed, is much more likely to produce a meteorite, than a meteoroid made of a friable material which is prone to fragment, and impacting the atmosphere at high speed. While it holds true that a large enough meteoroid will produce a meteorite, it is also true for a small enough meteoroid (although it is usually close to or within the size range of dust). The kinetic energy (proportional to the square of the impact speed) of the meteoroid is, to first approximation, converted into heating, sublimation, and thermal radiation of the meteoroid. The sublimation is extremely dependent on the surface temperature. A small meteoroid, with a large cross-section to mass ratio, can effectively control the temperature through thermal radiation, and at the same time it is decelerating at a high rate (Whipple, 1950a). This has the implication that the meteoroid can escape sublimation and slowly sediment through the atmosphere. These meteoroids, called micro-meteoroids or micro- meteorites, may then be collected in the Earth's stratosphere (Brownlee, 1978), and are then called interplanetary dust particles (IDPs). On certain calendar dates, the number of observed meteors is greater than the average. In addition, it is found that the meteor trails, when projected back, are emanating from a common point on the celestial sphere - the radiant. This is an effect of perspective; the meteoroids all enter the atmosphere along roughly parallel tracks. These attributes constitute the annual meteor showers. In general, a meteoroid stream is a large number of meteoroids travelling around the Sun in similar orbits. If it is such that the meteoroid stream intersects the orbit of the Earth, it will produce an annual meteor shower. This implies that on the Earth, we are only able to observe a tiny fraction of the meteoroid streams in our Solar System. A meteoroid stream is not a static phenomenon. Due to planetary perturbations and differences in forces acting on meteoroids of different sizes, the orbits of the individual meteoroids will become less similar. Eventually it is not possible to distinguish the meteoroid stream from the sporadic background. From this it is also seen that the annual meteor showers observed at the present epoch were not observed in the past, and they will not be observed at some future time. The annual meteor showers usually show the same level of activity, with minor fluctuations, year after year; implying that the meteoroids are equally distributed along the stream orbit. However, an intense outburst of activity, significantly above the regular value, is sometimes observed (cf. Jenniskens, 1995). This is called a meteor outburst or meteor storm, and is usually associated with the recent ejecta of comets such that the meteoroids have not yet spread out along the orbit. The most famous meteor storm is that of the Leonids. There are several conceivable sources for meteoroids. For example, the decay of comets through cometary activity, collisions among asteroids and the subsequent collisions between fragments, impact ejecta from planetary or satellite surfaces, production through volcanism (Jupiter's moon Io), and interstellar origin.