Mass Extinctions of Life: An Update on Astrophysical Causes
Charles A. Breiterman
January, 2004
This project could not have been accomplished without the generous help of Brian F. Rauch, Tobias Dürkop, Derek Richardson and Kevin Walsh in translating very challenging mathematics to a more accessible level.
Abstract:
Rampino and Haggerty (1996) have proposed the "Shiva Hypothesis," which relates mass extinctions of life to asteroid and comet impacts and thus to astrophysical causes.[1] They concluded that "major events in the history of life (and possibly geophysical changes) on Earth may be tied to the dynamics of the Galaxy." However, they covered only briefly the mechanics of the astrophysical processes behind the impacts.
The underlying causes behind asteroid and comet collisions with the planet Earth include: the Yarkovsky effect, the solar companion hypothesis, passing stars and the galactic tide. These very long-term, subtle and exquisite processes demonstrate that life and its fate are linked to quantum phenomena such as photons radiated from an asteroid (Yarkovsky effect), as well as immensely large-scale phenomena such as the gravity of all the matter in the galaxy interior to our solar system (galactic tide).
1. Introduction
The probable asteroid impact that triggered the extinction of the dinosaurs some 65 million years ago has generated tremendous attention. Researchers have plausibly implicated asteroid or comet impacts in at least three other mass extinctions of life on Earth.[2] Explanatory hypotheses from the 1980s such as ‘Nemesis’ and ‘Planet X’ have been covered in the science press, introductory textbooks and David M. Raup’s The Nemesis Affair.
There is a relatively new body research into the astrophysical mechanisms behind the asteroid and comet impacts that may cause mass extinctions. These findings have a firmer basis in theoretical development, observational verification and computer modeling than most of the earlier hypotheses. These recently elucidated mechansims include: Yarkovsky effect, the solar companion hypothesis, passing stars and the galactic tide.
These very long-term, subtle and exquisite astrophysical processes arguably have had a decisive impact on the evolution of life on Earth. Mainstream theory holds that mass extinctions cripple the dominant species, allowing new or previously unexceptional species to multiply into empty ecological niches. For example, the mass extinction of the dinosaurs cleared the way for the rise of mammals and in turn the evolution of humans. Therefore, without mass extinctions, humans probably would not exist.[3] Since some mass extinctions of life appear to have been caused by asteroid impacts while others may have been caused by comet impacts, this article covers research on the causes of both types of impact. The evolution and fate of life on Earth may be causally dependent upon astrophysical phenomena on the quantum scale such as photons radiated from an asteroid (Yarkovsky effect), as well as to astrophysical phenomena on an immensely large-scale such as the gravity of all the matter in the galaxy interior to our solar system (the galactic tide).
The articles on this material in professional astrophysics journals generally
require immense mathematical ability. This webpage endeavors to simplify yet
still convey the grace of the mechanisms at work. Therefore it requires about
one semester each of physics and calculus.
2.
Asteroids: The Yarkovksy Effect
In the current epoch, asteroids are the majority of impactors. A combination of asteroid-to-asteroid collisions, gravitational effects such as the pull of Jupiter and Mars, and the Yarkovsky effect constitute a process that can put an asteroid on a collision course with Earth. The Yarkovsky effect can be operative on asteroids up to 20 kilometers in diameter,[4] while the Cretaceous-Tertiary object, which was probably an asteroid, is estimated to have been 12-15 km in diameter. The Yarkovsky effect is relevant to mass extinctions of life on Earth because without it, computer simulations are not able to reproduce the observed abundance and characteristics of near-Earth asteroids.
The Yarkovsky effect is the movement of an asteroid caused by the recoil force of photons emitted as heat from that asteroid. Photons are electromagnetic radiation: packets of energy with properties of both waves and particles. In certain wavelengths, photons are visible light. In the next longer set of wavelengths, the infrared range, photons are heat.
When the asteroid emits (radiates) an infrared photon, the situation is analogous to that of a recoiling cannon. The photon is akin to the projectile which is fired, while the asteroid is the cannon which recoils backward. The photon pushes off the asteroid, exerting a recoil force opposite to the direction of travel. Remember from first year physics that over a short period of time, a force acting on a particle (think of an asteroid as a big, heavy particle) imparts a momentum to that particle. This is Newton’s second law and can be expressed by the formula (F)(Δt)=(m)(Δv).[5] Since the mass of the asteroid does not change, the velocity must change: the asteroid moves. Over a short period of time with just one photon pushing off the asteroid, the motion of the asteroid is extremely insignificant, of course. But over tens of millions of years, trillions of photons each exert their minuscule force upon the rock. The sum of the force exerted upon the rock over time becomes appreciable and the asteroid can be moved a significant distance.
You might be thinking that if you rather lightly push upon your computer monitor with your pinky for 10 million years, the sum of the force exerted upon the monitor over time would be quite large, yet still your monitor would never move. So why would the photons move the asteroid? The difference is that your monitor is restrained by the static friction force exerted on it by the desk, which exactly balances out the force you have applied. With no net force applied, the computer monitor is subject to no acceleration, and hence there is no change in velocity or position. An asteroid does not have any such force restraining it. Thus, from the little push of a photon the asteroid will receive a slight net force with the attendant change in velocity and position.
In which direction does the asteroid move and how far? Asteroids rotate just as do planets. The sunward side of the asteroid is the “day” side and is both being heated by and emitting solar photons, while the other side of the asteroid is in the “night” side and is cooling by emitting (radiating) infrared photons. Both sides are emitting photons, although the day side is preponderant because it is directly receiving and re-emitting solar photons. The asteroid surface’s temperature cools very quickly once it is out of the sun because bare rock and regolith have very little thermal inertia. On Earth, the atmosphere and oceans retain heat and delay the onset of the daily minimum temperature until the early morning hours, but these do not exist on an asteroid. It is the unequal (synonyms are anisotropic or asymetric) emission of photons by the asteroid that causes the Yarkovsky effect—the day, especially the afternoon, surfaces of the asteroid are emitting with greater intensity and exerting more recoil force than are the night and morning (not yet heated) surfaces.
Figure 1: Illustrates an asteroid’s day and night sides and shows the direction of rotation.
The temperature data is for the near-Earth asteroid 6489 Golevka at ~4 AU from the sun. Golevka has a diameter of 530 meters.
The asteroid 6489 Golevka will be used as our sample asteroid.[6] Golevka is an Earth-crossing asteroid. Its orbit has a perihelion (closest approach to sun) of about .98 AU and an aphelion (farthest distance from sun) of about 4.02 AU. At perihelion, the maximum day temperature is 390 K and the minimum night temperature is 150 K. At aphelion, the maximum/minimum are 180/115 K. We are interested in what makes asteroids become Earth-crossing and therefore possible Earth-impactors. For this reason we are interested in what happens while it is in the asteroid belt that causes it to become Earth-crossing. The asteroid belt is between the orbit of Mars at 1.5 AU and Jupiter at 5.2 AU. This is why we will use the temperature data for aphelion.
Knowing that the day temperature is 180K and the night is 115K, we can get a rough measurement of the net force the emitted photons are exerting on the asteroid.[7] In this situation, an estimation of the force on either side of the asteroid is given by the formula F=(σT4πr2)/(c).[8] σ is the Stefan-Boltzmann constant, πr2 is the cross-sectional area of the object, and c is the speed of light. Since we are looking at opposite sides of the same asteroid, the only term that is not a constant is the temperature. Therefore, for our representative asteroid, Golevka, the difference in forces is (1804)/(1154)=6. This is illustrated in the figure below.
Figure 2. Yarkovsky effect. Estimate of the location, direction and magnitude of the force vectors acting on the asteroid.
Each side has an arrow representing the estimated summation of force vectors on that side.
On the day side, the asteroid is hottest in the afternoon, so the force arrow is skewed to the afternoon. On the night side,
the asteroid loses heat quickly so the nightly minimum temperature is reached soon after nightfall.
Therefore the entire night side is roughly approximated as a straight line aimed at the center of the asteroid.
The formula for net force of emitted photons acting on the asteroid (day side force minus night side force is: [(8/3)( σπr2T4)/(c)][ΔT/T]. With this formula, we can use some numbers from a real asteroid of 12 km in diameter (big enough to cause a mass extinction) to yield an approximate net acceleration of 6.5 x 10-15 (m/sec2).[9] At any given time the acceleration is very slight, but over millions of years, the constant acceleration provided by the Yarkovsky effect builds the velocity and the asteroid is moved an appreciable distance.
The computer model of Farinella and Vokrouhlicky (1999) showed that an asteroid 20 km in diameter would see a semi-major axis shift of .01 AU per billion years.[10] Even such a small change is significant because it can be sufficient to push the asteroid into a Kirkwood gap. In the late 19th century an astronomer, Daniel Kirkwood, noted that there are certain orbits in the asteroid belt that are curiously devoid of material. These turned out to be a sub-group of the orbits that are in integer ratios with the orbit of Jupiter, such as 3:1, 5:2, 7:3, 2:1. These ratios mean, for example, that the object orbits the sun 3 times for every 1 time that Jupiter does so. An asteroid in these particular regions will be subject to a resonance: a periodic disturbance from the gravity of Jupiter that causes changes in the orbit of the asteroid. The resonance can force the asteroid towards the inner solar system. Further interactions with the gravity of Mars may eventually lead to a collision with Earth. So the Yarkovsky effect is relevant to mass extinctions on Earth because it can nudge asteroids into a Kirkwood gap from which the asteroid is ejected by Jupiter’s gravity leading to a potential collision with Earth.
J.N. Spitale has proposed that the Yarkovsky effect could be exploited to avert the collision of an asteroid with the Earth. Painting an asteroid white or otherwise changing its reflection and/or absorbtion properties vis-a-vis photons could slowly move it off a collision course. The technique would work only with relatively small asteroids, but could still save a city from becoming a crater. The reason it would work on only smaller asteroids is that as the radius the object increases, its volume (4/3πr3), and therefore its mass, increases much more rapidly than its surface area (4πr2). For a large asteroid, there is very much more mass to move, but not so much more surface area to radiate the photons.
3. Comets: Solar companion hypotheses, Passing Stars
Other explanations of the causes of Earth impacts focus on comets. Comets may be the cause of the majority of the Earth’s impact craters that are over 50 km in diameter.[11] These large craters are most likely to have caused a mass extinction.
A comet is said to be “perturbed” when it is disturbed from its dormant state and nudged towards the inner solar system or out into interstellar space. When the comet enters the inner solar system and is heated by the sun sufficiently, it begins to have a tail (actually up to 3 separate tails: gas, ion and dust) and is then said to be “observable.” If it is determined that it is the first time the comet has journeyed to the inner solar system, it is said to be a “new” comet.
The question is, what causes comet perturbations? Two hypotheses, ‘Planet X’ and ‘Nemesis’ were prominent the 1980s. ‘Planet X’ proposes that a yet unobserved Planet X disturbs the Kuiper belt of comets,[12] dislodging some into an intercept course with Earth. It is these comets which cause mass extinctions when they collide with the planet. Nemesis proposes that an unobserved companion to the sun, probably a dwarf star with a highly elliptical orbit, periodically passes near enough to the Oort cloud of comets[13] to yank some from their orbits and send them towards Earth. Eminent Paleontologist David M. Raup has charged that these hypotheses are ad hoc.[14]
In fact, these are not ad hoc hypotheses; these are hypotheses designed to explain a legitimate set of geological data and associated findings. That dataset was Earth’s known impact craters for approximately the past 600 million years. The findings were statistical analyses reporting that there was a 28 or 31 million year regularity (periodicity) in the cratering record.
One problem with ‘Planet X’ and ‘Nemesis’ is that they were explaining an incomplete geological data set. Since 1984, we have discovered many more impact craters (see footnote 2-3). Second, the dating of the craters was and is (to a lesser extent with improved technology and more research) uncertain. Third, even after 20 years of additional statistical analysis, there is still vigorous debate over whether there is a periodicity to the terrestrial cratering record. However, whether or not the impacts occur on any regular cycle, as long as it is established by specific investigation of each crater and the global biological and geological evidence in that layer of sediment that at least 4 impact events are linked to mass extinctions, then it is worth enquiring into the causes of these impacts. If there is no periodicity to the impacts, it may not be useful to look for one sole cause of bolide impacts on Earth. That is the reason this review article looks at several possible causes of the impacts that contribute to mass extinctions.
There is a recent update to the solar companion genre of hypotheses, this time with a firm basis in astronomical observation. In the late 1990s, several researchers proposed the existence of a solar companion in an orbit within the Oort cloud. One group (Matese, Whitman and Whitmire) looked at about 30 comets with anomalous orbital characteristics. They projected the path of the comets back to aphelion (the point at which a comet is farthest from the sun). They proposed that the pattern of the comets’ aphelia traces the near-circular orbital path of a large planet. The gravitational field of the planet perturbs comets along its orbital path. This planet would have a mass about 3 times that of Jupiter. It would be orbiting in the middle of the Oort cloud, about .4 light years (25,000 AU) away from the sun. It should be detectable by the Space Infrared Telescope Facility, to be launched in 2003. The Matese group does not mention whether there may be a 28 or 31 million year periodicity to the proposed planet’s orbit. Horner and Evans (2001) critically examine the Matese group’s proposal and write, “We conclude that Matese et al.’s planet is a possible, perhaps even likely, explanation of the unusual pattern in the data ... Matese et al.’s planet warrants further and serious consideration.”
A thorough article has considered whether passing stars perturb the Oort cloud. Sanchez et al. (1999) look at the paths of nearby stars and project that in 1.5 million years the star Gliese 710 (a dwarf star of .6 solar masses) will pass within 1 light year of Earth and actually enter the Oort cloud. Its gravitational influence will eject about 2 million comets into an earth crossing orbit. The arrival of these comets will be spread over about 2 million years, meaning there will be only one extra comet per year. So, the Earth would probably not experience a perceptible increase in comet activity. The authors do not claim any connection to a mass extinction on Earth. Yet one of these 2 million comets might actually impact the Earth rather than merely pass close by. The probable reason the authors do not address this the impossibility of predicting specifically which comets will be ejected from the Oort cloud and sent towards Earth, and which of those will actually collide with the planet. The article looked 10 million years backward and 10 million years forward, and Gliese 710 is the best candidate they found for an Oort cloud perturber.
We are focusing on “live” comets that still display a tail when they pass near the sun. A significant portion of the Earth-crossing asteroids may actually be asteroid-like “extinct” comets. These are rocky/metallic objects whose orbits have comet-like characteristics. They are thought to be comets that have lost all their gas, dust and water during repeated close passages around the sun. We are not considering extinct comets here, because generally these are only 1 km in diameter- not large enough to cause a mass extinction.
4. Comets: The Galactic Tide
Another cause of comet impacts is the galactic tide. The galactic tide is responsible for as much as 90% of the observable Oort cloud comets.[15] About 30% of the large (over 20 km in diameter) impact craters on Earth are ascribed to Oort cloud comets.[16] Galactic tide research describes how each comet in the Oort cloud is subject to a differential (net) gravitational force at each point in its orbit. To see how this is a tide, consider that the ocean tides on the Earth are caused by net gravitational forces in the sun-Earth-moon system. Meanwhile, the galactic tide is caused by the net gravitational forces in a sun-comet-galaxy system. The galactic tide is shifts in the orbits of comets, the earth tide is the rise and fall in sea level; both are caused by net gravitational force.
There are two components of the tide: the radial tide and the disk tide (or z-tide). While the disk tide[17] is dominant, here I explain the radial tide because it occurs in a simple flat x-y plane.[18] The radial tide works as follows: the gravitational force of the entire mass of the galaxy interior to our solar system acts as a point-source which gently alters the orbits of dormant comets, setting in motion events that can lead to a comet collision with the Earth. Understanding the galactic radial tide requires an overview certain aspects of solar system dynamics. The sun is about 2/3 of the way to the outer edge of the Milky Way galaxy, as shown in figure 3. The location is about 27,000 light years from the galactic center. The solar system oscillates vertically with amplitude of about 114 light years and a period of 60-90 million years.
Figure 3. The Milky Way viewed from the side. The bright, dense core is the galactic center. “ly” stands for “light years.”
The arrows indicating the vertical oscillation are exaggerated due to the difficulty of
rendering such a small fraction of the horizontal radius (114 compared to 27,700).
After figure 12-6, Michael A. Seeds,Horizons, 6th edition, Pacific Grove: Books/Cole, 2000). Illustration credit: Nguyet Mai Vuong.
Figure 3 allows an explanation of the galactic disk tide. The galaxy is structured like a disk with a spherical center (consider figures 3 and 6 together). The “galactic plane” is a flat line running horizontally through the center of the disk in figure 3. When an object is inside the comparatively flat disk, it is said to be in the galactic plane. The disk tide refers to the gravitational force of all the matter in the galactic disk influencing the orbit of a comet. As the solar system oscillates above and below the galactic plane, the solar system’s comets move with it, and the disk tide is operative. The gravity of the sun is so weak at Oort cloud distances that the gravity of the galactic disk can change the orbit of comets.
The mass of the entire Milky Way is 1011 solar masses, most of which is located interior to our solar system because the inner part of the galaxy has a much denser concentration of matter. Our solar system is surrounded by the Oort cloud [see footnote 11]. Because the sun’s gravity is so weak at Oort cloud distances, the gravity of other objects in the galaxy can have a significant effect on Oort cloud comets. In this instance we are considering the effect of the gravitational force of all the mass of the galaxy interior to our solar system on Oort cloud comets. The galaxy is essentially a collection of particle-like objects: stars, dust clouds and planets. This multitude of particles can be approximated as a sphere with all its gravitational force concentrated in a point at the center.[19] Thus we model the gravitational force of the mass of the galaxy interior to our solar system as emanating from the galactic center. This reduces the analysis to just three bodies: the sun, an Oort cloud comet, and the galactic center of gravity. The comet is orbiting the sun, and we are analyzing the effect of the sun’s and the galactic center’s gravity on this orbit.
Figure 4. Galactic tide all forces.
The sun exerts a gravitational force on the comet along the radius between the sun and comet. Taken alone, this force exerts no torque because the angle between r and F is 0.[20] Simultaneously, the galactic center of gravity exerts a gravitational force along the radius between it and the comet. Again, the angle between r and F is 0. However, the sum of these two forces produces a net force vector in the angle between the two forces (parallelogram rule of vector addition). This net force is at an angle with the sun-comet axis of rotation “r,” so Φ (phi) does not equal zero and we have a torque. It is a “torque” and not a “force” because torque is the rotational analogue of force: the net gravitational force on the comet is acting at an angle. The angle and magnitude of the net force varies continuously as the comet moves through its orbit, so the magnitude of the torque varies as well. At any given point, the situation can be represented in the manner of a torque diagram found in a typical university physics textbook:
Figure 5. Galactic tide net force and torque.
An Oort cloud comet is considered part of the solar system; it is (weakly) gravitationally bound to the sun.
Therefore, the sun is the axis of rotation and position vector “r” is rooted at the sun.
Φ is the angle between r and F when the two vectors are placed tail to tail (slide the F vector so that its origin is the sun).
The torque (symbolized by T) changes the angular momentum of the comet (L), since T=dL/dt.[21] The change in angular momentum changes the orbit of the comet according to the formula L=[GMa(1-e2)]1/2: where G=the universal gravitational constant; M=the mass of the sun; a=the length of the semi-major axis and e=the eccentricity of the orbit. Since M and G are constants, only a and e can change. Hence, the gravitational force changes changes the orbital elements a and e. The perihelion distance q is given by q=a(1-e). Simply following the equations demonstrates that the galactic radial tide can change the perihelion distance of an Oort cloud comet.
The torque we are describing is not an overwhelming force which yanks a comet out of its orbit. It is a subtle force which works on an Oort cloud comet over thousands of orbits. Since each orbit of an Oort cloud comet may take 2-3 million years, the galactic tidal process works over a period of billions of years.
Depending on the original position of the comet and the cumulative effect of the torque, a particular comet’s perihelion can move closer to the sun or farther away from the sun. This is why the phenomenon is called the radial tide. Radial motion is the motion of an object directed straight towards or away from a given vantage point, here the sun. When an Oort cloud comet is moved closer to the sun, it may enter the inner solar system, and then may collide with the Earth. Specifically, the comet may be caught by the gravity field of one of the gas giants and then may be forced towards the inner solar system, where it becomes observable. As noted above, about 10% of the large impact craters on Earth may be due to the galactic radial tide.
5. Conclusion
This article has sought to explain several areas of comparatively recent astrophysical research linking extraterrestrial phenomena to terrestrial mass extinctions. These include the Yarkovsky effect, the solar companion hypothesis, passing stars, and the galactic tide. These very long-term and exquisite astrophysical processes range in scale from the quantum to the immensely large. They appear to have caused asteroid and comet impacts on Earth and through these, they have led to the very existence of the human species.
