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.