Dinosaurs exercise a strange fascination over humanity, owing, perhaps, to their disportionate sizes, exotic structures, and their temporal remoteness. Some loomed taller than buildings and extended over a THE SOUTHWORTH PLANETARIUM 207-780-4249 www.usm.maine.edu/planet 70 Falmouth Street Portland, Maine 04103 43.6667° N, 70.2667° W Founded January 1970 hundred feet. They assumed various forms: horned, quadrapedal, duck-billed and even winged. And, the last of the dinosaurs trod on Earth more than sixty-five million years ago. They were the enormous aliens reigning over the world we now inhabit. Dinosaurs are also compelling because they vanished so quickly. Well, "quickly" in the geological sense. After their 135-million year dominion over terra firma, they disappeared and became part of the geological strata that is forever dinosauria. Their precipitous departure remains mysterious, even though the notion that an asteroid impact killed the dinosaurs has insinuated itself into popular science. (Even the "Dinosaurs at Dusk" program concludes with an asteroid strike.) Though geologists and palentologists would concede that the asteroid impact scenario is the most likely explanation, the matter remains unresolved. We know that the clade Dinosauria appeared during the Triassic period about 231 million years ago, but did not become the dominant vertebrae until the Jurassic period commenced nearly 30 million years later. They maintained this dominance throughout the Jurassic and then Cretaceous period (started 145 million years ago), which ended with the Cretaceous-Paleogene extinction event 80 million years after its beginning. This event caused the extinction of more than seventy-five percent of all plant and animal life forms, including the non-avian dinosaurs, those incapable of flight. It is assumed that some avian dinosaurs survived the catastrophe and were the progenitors of modern day birds. What could have caused this event? The prevailing theory is that a 10-mile asteroid struck a region now called the Yucatan Peninsula. Apart from the localized destruction that vaporized thousands of square miles of surface material, the asteroid engendered a swarm of secondary impacts which ignited wildfires around the globe. The resultant eco-system disruption and atmospheric dust obscuration destroyed myriad life forms and impeded photosynthesis. Only twenty five percent of all species then extant perservered through the subsequent years of deep cold and widespread starvation. The discovery of iridium along the boundary between the Cretaceous and later Tertiary layers lent credence to this theory. The heavy element iridium is abundant in asteroids, but rare on Earth's surface. Geologists assume that only an asteroid would have conveyed such iridium stores to our planet around 65 million years ago. Of course, as is true with all theories, this one is unproven. (The unforgiving scientific method allows us only to disprove theories.) Some believe that an impacting comet, or comets, might have also laid siege to Earth around the same time of this asteroid strike. This hypothesis relates to a 1984 paper produced by geologists Raup and Sepkoski indicating that Earth experiences a 26- 30 million year extinction periodicity. Or, mass extinctions seem to occur every 26 to 30 million years. In fact, two comparatively recent extinction events Cretaceous–Paleogene (65 million years ago) and Eocene–Oligocene (33.5 million years ago) have separation periods approximately equal to this periodicity. This purported periodicity suggests a periodic causation: some other periodic event must be causing these waves of mass extinction. Soon after the publication of Raup and Sepkoski's paper the concept of Nemesis arose: the notion that the Sun has a binary, red dwarf companion the theorists named Nemesis. With a mean distance of 1.5 light years from the Sun, Nemesis's orbital period would have been comparable to the 26 - 30 million year extinction periodicity. The Nemesis theory proposed that when Nemesis reached its periastron (closest distance to the Sun), its gravity would propel a swarm of Oort Cloud particles toward the inner solar system. Extending almost 0.75 light years from the Sun -some suggest its outer edge is more than 1 light year away- the Oort Cloud is the vast spherical repository from which all long period comets originate. These cometary nuclei will remain suspended unless some gravitational nudge, perhaps from a proximate star, perturbs them out of their stations toward the inner solar system. A binary star companion could exert such an influence and its orbital period could readily explain the periodicity. It reaches periastron, a comet armada moves inexorably toward the inner planets, Earth experiences a sudden bombardment and mass extinctions ensue. The primary objection the Nemesis theory is that such a distant star wouldn't likely remain in orbit around the Sun. The gravitational attraction binding them would be tenuous at best and Nemesis would have most likely become dislodged from Sol long ago. Also, astronomers haven't yet found Nemesis. At such a close distance, Nemesis should have been discovered already, even if it were a brown dwarf: a low mass gaseous sphere that never became hot enough to attain stellar status. Recently, two Harvard theoretical physicists, Lisa Randall and Matthew Reece, proposed that the highly mysterious dark matter might have been involved in these mass extinctions, including the one which killed the dinosaurs. Dark matter is so named because scientists know little of its nature, yet. Through its gravitational influence on surrounding material, dark matter is believed to comprise about 21% of the Universe and perhaps even ninety percent of the galaxy. Recent observations made through the Fermi Gamma Ray Telescope suggest that dark matter might be concentrated within the center and along the main band of the Milky Way Galaxy. Some theorists believe that a hitherto unknown matter type called WIMPS (Weakly Interacting Massive Particles) might account for most, if not all, of the cosmic dark matter. Models suggest that the population of WIMPS includes a significant amount of their own anti-particles. Anti-particles are exactly the same as regular particles, except they have opposite charges. The collision of a particle and its anti-particle counterpart would annihilate both in an outburst of gamma rays. These annihilations, if they occur, would necessarily be most frequent within the galactic plane, where the WIMP density is greatest. The observation, though still disputed, of unusually high gamma ray emissions within the band seems to confirm this assumption. A thick dark matter spine might extend along the galactic plane. Our solar system, like all of the Milky Way's star systems, revolve around the galaxy in wide orbits. Our Sun requires nearly 220 million years to complete one orbit. However, not only is the Sun moving along this orbital path, it is also traveling along an undulating curve that brings it alternately above and below the galactic plane. The solar system's oscillation period through this plane is nearly 70 million years. This means we pass through that plane every 35 million years: a time period nearly equal to the extinction periodicity. The theory posits that the more intense gravitational fields around the galactic plane might perturb the Oort cloud nuclei, precipitating the cometary barrage. Now, we are destined to pass through the plane again in a few hundred thousand years, meaning that our previous pass through was almost 34 million years ago: around the time of the Eocene-Oligocene extinction. A coincidence? It very well might have been, or, perhaps, our incursions through the galactic plane might have more dire ramifications for our planet and, by extension, life on the planet. These assumptions are just that, assumptions. Within this theory once encounters great uncertainty: the extinction periodicity is not well established; the existence of dark matter reserves within the galactic plane hasn't been established; and the role of Oort cloud comets in those extinctions is undetermined. We won't have any definite answers straight away. Again, this is merely a theory and we'll never prove it correct. However, we'll never stop wondering what truly killed the dinosaurs. Nor we will ever stop being curiously disappointed that these formidable predators never met us.