by Lynn Yarris
from
BerkeleyLab Website
"Heavy snows are driven
and fall from the world’s four corners; the murder frost
prevails. The Sun is darkened at noon; it sheds no
gladness; devouring tempests bellow and never end. In
vain do men await the coming of summer. Thrice winter
follows winter over a world which is snow-smitten,
frost-fettered, and chained in ice."
—"Fimbul Winter"
from Norse saga, Twilight
of the Gods |
THE THEORIZED
COMPANION STAR, THROUGH ITS GRAVITATIONAL PULL,
UNLEASHES A FURIOUS
STORM OF COMETS IN THE INNER SOLAR SYSTEM LASTING FROM 100,000 TO
TWO MILLION YEARS.
SEVERAL OF THESE
COMETS STRIKE THE EARTH.
Our species,
Homo sapiens, arose
approximately 250,000 years ago. In the beginning, we used tools of
stone and sought shelter in caves. Today, our shelters scrape clouds
and our tools allow us to see galaxies far beyond our own, or peer
deep into the heart of matter itself. So much progress in such a
short time, for in geological terms, the reign of our species has
been but the proverbial blink of an eye. Imagine, however, what our
record of achievement would be had our history been disrupted no
less than five times by titanic nuclear wars, each delivering a
destructive blast 10,000 times more powerful than the combined yield
of all existing nuclear weapons in our world today.
Such upheaval is what many other species, including the dinosaurs,
may have faced during the history of our planet, according to a
theory set forth by a Lawrence Berkeley Laboratory (LBL) scientist
and his colleagues. The theory postulates that every 26 to 30
million years, life on Earth is severely jeopardized by the arrival
of a small companion star to the sun.
Dubbed "Nemesis" (after the Greek
goddess of retribution), the companion star—through its
gravitational pull—unleashes a furious storm of comets into the
inner solar system that lasts anywhere from 100,000 years to two
million years. Of the billions of comets sent swarming toward the
sun, several strike the Earth, triggering a nightmarish sequence of
ecological catastrophes.
"We expect that in a typical comet
storm, there would be perhaps 10 impacts spread out over two
million years, with intervals averaging 50,000 years between
impacts," says LBL astrophysicist Richard Muller.
In 1984, Muller, along with UC Berkeley
astronomer Marc Davis and Piet Hut, an astronomer with the
Institute
for Advanced Study at Princeton University, announced the Nemesis
theory in Nature magazine. As could be expected, it was and remains
controversial. However, although the evidence for the existence of
Nemesis is still circumstantial, this evidence continues to mount,
and the theory has so far withstood all challenges.
Nemesis was the culmination of a chain of events that began in 1977,
in Gubbio, Italy, a tiny village halfway between Rome and Florence.
Walter Alvarez, a UC Berkeley geologist, was collecting samples of
the limestone rock there for a study on paleomagnetism. The
limestone rock outside of Gubbio is a big attraction for geologists
and paleontologists because it provides a complete geological record
of the end of the Cretaceous period and the beginning of the
Tertiary period.
This transition took place 65 million
years ago, and is of special significance to our species, for it
marked the close of the "Age of Reptiles," when dinosaurs ruled the
Earth. Sometimes referred to as "the Great Dying," the massive
extinction that engulfed the dinosaurs claimed nearly 75 percent of
all the species of life on our planet, including most types of
plants and many types of microscopic organisms. As much as 95
percent of all living creatures might have perished at the peak of
destruction.
Sandwiched between the limestone of the two periods, forming a clear
line of demarcation, is a thin—maybe one-half-inch thick—layer of
red clay. Immediately below this clay layer, the Cretaceous
limestone is heavily populated with a wide mix of the tiny fossils
of marine creatures called forams. Above the clay layer, in the
Tertiary limestone, however, the fossils of but a single species of
foram can be seen. The clay layer itself contains no foram fossils
at all.
When Walter Alvarez brought his samples back to Berkeley, his
father, LBL Nobel laureate physicist Luis Alvarez, suggested that
subjecting them to neutron activation analysis could help determine
how long it took for the clay layer to form. The analysis, performed
by LBL nuclear chemists Frank Asaro and Helen Michel, revealed—to
the surprise of everyone involved—that the clay was about 600 times
richer in iridium than the surrounding limestone.
A silvery-white metal, related to
platinum, iridium is quite scarce in the Earth’s crust, found
usually in concentrations of only 20 parts per trillion. When the
Earth was formed, most of the iridium sank into the planet’s core,
3,000 miles below the surface, where the concentration of the metal
is 10,000 times that in the crust. Other sources of high iridium
concentrations are extraterrestrial objects, such as meteorites or
comets.
Subsequent samples collected from clay layers found at locations in
Denmark and New Zealand, where the geological record of the
Cretaceous-Tertiary boundaries are also complete, revealed the same
iridium anomaly, plus an abundance of soot. This iridium anomaly has
now been identified at more than 75 sites worldwide, by scientists
from 11 different laboratories. Iridium is generally found in
combination with platinum, gold, and several other elements.
Measuring the concentrations of these elements and comparing their
ratio to iridium indicated that the widely scattered iridium all
came from the same source.
Putting all of the data together, Luis Alvarez concluded that the
iridium anomaly was the result of a collision between the Earth and
an extraterrestrial object approximately six miles in diameter. He
speculated further that it was this collision that led to the death
of the dinosaurs and all of the other species that perished during
the Great Dying.
When a rock the size of San Francisco, traveling at approximately
45,000 miles per hour, hits the Earth, there is an instantaneous
release of approximately 100 million megatons of kinetic energy—six
billion times the force of the Hiroshima bomb. Luis and Walter
Alvarez predicted the effects of such an explosion, based on the
aftermath of the volcanic eruption of Krakatoa in 1883, the biggest
eruption ever recorded.
If the impact takes place on land, a heavy shroud of fine dust
particles from the shattered planetary crust and the pulverized
meteorite or comet would be swept high into the stratosphere by the
mushrooming fireball, where it would slowly spread, wrapping the
entire globe in a dense cocoon. The fireball’s blazing heat would
ignite enormous wildfires, the soot and debris from which would rise
up and add to the sky-blackening dust, creating an extended period
of endless night.
Said Walter Alvarez in a report for the American Geophysical Union,
"For a few months, it would be so
dark you literally could not see your hand in front of your
face."
The darkness would shut down the
photosynthetic process, killing all but the hardiest of plant
species and driving the food chain into a state of collapse.
Worldwide starvation would ensue as animals that feed on the plants
die and the predators in turn follow. Extremely cold temperatures
brought on by the darkness might usher in an ice age. Even if the
impact takes place in the ocean, dust (from the crushed ocean floor)
would still be shot above the atmosphere, only accompanying the dust
would be tremendous volumes of vaporized water. After the dust
finally settled, the water vapor would still remain. Solar heat
reflected off the Earth’s surface would be prevented from escaping
into outer space by this thick moisture, and the consequence would
be an oppressive greenhouse effect.
"The bitter cold would be followed
by a sweltering heat," said Walter Alvarez in his AGU report.
To make matters worse, the energy
released by the impact could serve as a catalyst to combine
atmospheric nitrogen and oxygen into nitric acid that would fall
back on the surface as corrosive precipitation.
Singular event or an event that has
recurred
A PLOT OF DATA ON
LIFE EXTINCTIONS, COLLECTED BY DAVID RAUP AND JOHN SEPKOSKI AT THE
UNIVERSITY OF CHICAGO,
SHOWS PEAKS IN THE
EXTINCTION RATE OCCURRING AT 26- TO 30-MILLION-YEAR INTERVALS, AS
INDICATED BY ARROWS
As originally proposed, the Alvarezes saw the Great Dying and the
iridium anomaly as a singular event—a fluke in Earth’s history.
A second iridium anomaly was discovered in samples taken from
sediment that had been deposited on the floors of the Caribbean Sea
and the Gulf of Mexico about 35 million years ago, when a less
severe extinction occurred, but no one proposed a link between the
two events. Then, in 1984, came a report from two University of
Chicago paleontologists, David Raup and John Sepkoski, who had put
together a detailed list of sea life that had become extinct during
the past 250 million years. Containing more than 3,500 different
species, it was the most complete extinction list ever compiled.
When they subjected their list to computer analysis, Raup and
Sepkoski discovered that mass extinctions occur periodically,
approximately every 26 to 30 million years.
Scientists immediately scrambled to find
an explanation that could account for a persistent, recurring cycle
of planet-wide species die-outs. Volcanic eruptions were the most
obvious suspects, but volcanoes fail to account for the clay layer,
the high soot content and, most significantly, the high iridium
concentrations. Casting further doubt on the culpability of
volcanoes was the discovery of shock quartz and microtektites along
with the iridium and soot in the clay layer samples taken from
around the world.
Shock quartz silt-sized grains of quartz, which, under a microscope,
show cracks and strains, is formed in the heat and pressure of a
powerful explosion. It showed up routinely in rocks brought back
from the moon by the lunar astronauts, but on Earth it has been
found only in meteorite craters and at nuclear weapon test sites.
Microtektites are tiny pieces of glass, believed to be droplets of
rock that were melted in the heat of an impact and hurled up beyond
the atmosphere where they cooled. Upon reentry, the droplets were
reheated.
The heating-cooling-reheating sequence
gave the microtektites in the clay layer a unique spherule shape.
Violent volcanic eruptions, such as took place on Mt. St. Helens,
Washington, in 1980, can produce glassy material, but always in
angular shapes because the melted rock is never ejected beyond the
atmosphere. The quiet eruptions of the gentle basaltic volcanoes,
prominent in Hawaii, will cough up spherule-shaped glass, called
"Pele’s tears," but distribute the material only in the immediate
vicinity.
Ruling out other terrestrial causes, many scientists turned to the
heavens. One possibility was meteorites, which are chips of
asteroids or planets moving randomly through space. However, a
mechanism to explain the periodicity of the extinctions has yet to
be found. A second possibility was comets, "dirty snowballs" of ice
with a rocky center. Looping the solar system, beyond the orbit of
Pluto and extending out to more than eight trillion miles, is a vast
bracelet of comets known as the "Oort cloud," after its discoverer,
Dutch astronomer Jan Oort.
The trillions of comets in the Oort
cloud generally maintain a slow but steady orbit around the sun.
Occasionally, the gravitational field of a passing star will jar
some comets loose, but few of these ever reach the inner solar
system (Mercury, Venus, Earth, and Mars), as the gravitational pulls
of Jupiter and Saturn—acting somewhat like giant vacuum
cleaners—keep this part of the system relatively clean of comets and
other space debris.
However, a strong enough gravitational force could dislodge so many
comets that, through sheer numbers, the inner solar system’s
protective cleaning mechanism would be overwhelmed. One of the first
possible sources of this gravitational force to be considered was
the molecular dust clouds in the central plane of the Milky Way.
As the solar system revolves around the
center of the galaxy, it bobs up and down, periodically crossing
through the star-crowded central plane that is foggy with molecular
dust—star stuff that never coalesced. One of the many problems with
this suggestion is that measurements have shown the molecular dust
clouds to be far too thinly dispersed to exert sufficient tidal
gravitational force. Also, the bobbing of the sun does not match the
times of extinction—in fact, the sun is close to the central plane
right now.
Another source of gravitational pull that has been proposed is the
existence of a tenth planet in the solar system. Called "Planet X,"
this planet would be a gas ball as much as five times the size of
Earth, occupying a peculiar shifting orbit that is tilted at an
angle to the solar plane of the nine known planets. This theory also
calls for the existence of an as yet undetected inner disk of the
Oort cloud, between the orbits of Neptune and Pluto.
Every 26 to 30 million years, the orbit
of Planet X would be shifted so that it would scrape the edge of the
inner disk, sending a host of comets towards the sun. The major
problem with this proposal is that the hypothetical inner disk of
the Oort cloud would be unstable and could not remain a disk.
Consequently, comets would be shaken loose in a steady shower over
the 26 to 30 million year time periods, rather than torn loose in a
concentrated storm.
A mechanism to explain the periodicity
of the extinctions
The Nemesis theory fulfills all the requirements prescribed by the
Raup and Sepkoski mass extinction timetable.
As envisioned by Muller, Davis, and Hut, Nemesis is probably a
red
dwarf, the most common type of star in the galaxy (three-fourths of
all the stars in the Milky Way are believed to be red dwarfs). Less
than a third the size of the sun and about one one-thousandth as
bright, Nemesis might travel in an elliptical orbit that at its
perihelion (closest point) brings it within a half light year of the
sun (one light year is about six trillion miles) and into the midst
of the Oort Cloud. Right now, Nemesis may be at its aphelion (most
distant point), nearly three light years away. The sun’s closest
known neighbor, Proxima Centauri, is about 4.25 light years distant.
Another group of scientists, led by Daniel Whitmire, an
astrophysicist with the University of Southwestern Louisiana, and
Al
Jackson, of the Computer Science Corporation, announced their own
theory of a companion star to the sun in the same issue of Nature as
Muller and his colleagues. Although the means of triggering massive
extinctions are essentially the same, this second group believes the
companion star is invisible: either a brown dwarf, a star so tiny
that it never ignited, or a black hole, a shrunken star so dense
that its gravity prevents any light from escaping.
"We see no reason to assume the star
is invisible," says Muller, "since most of the stars in the sky
have never had their distance from us measured. If the companion
has a magnitude between 8 and 12, it would be dim enough to have
been missed in full sky parallax surveys."
That the sun would have a companion star
is by no means farfetched. More than 50 percent of the stars in the
galaxy are partners in a binary system. The elliptical orbit of
Nemesis would carry it farther away from the sun than the distance
separating companions in any known binary system. Some scientists
have protested that this orbit is too elliptical to be maintained
and that Nemesis would have long since left the system. However, the
calculations of Hut show Nemesis' orbit being stable for about a
billion years.
Says Muller,
"The stability of the orbit is
sufficiently long to account for the regularity in the
extinctions, but it also implies that the companion star could
not have been in this orbit since the formation of the Earth.
Since gravitational capture is very improbable, the most likely
scenario is that the companion star was once more tightly bound
to the sun and its orbit is slowly being dissipated by passing
stars."
It is even possible, Muller suggests,
that the gravitational shoving of Nemesis out into a more distant
orbit coincided with an event referred to by astronomers as "the
late great bombardment." Approximately four billion years ago, a
celestial version of saturation bombing left the surface of the moon
badly scarred with craters, which, because of the absence of
atmospheric erosion, can still be seen. Voyager has shown the moons
of Mars, Jupiter, and Saturn to be similarly pocked.
The first fossil records on Earth also date back four billion years
ago. Mysteriously enough, the division between the Earth’s two eons,
the Cryptozoic eon ("hidden life") and the Phanerozoic eon ("visible
life") is sharply etched. Rather than a gradual appearance of
increasingly complex fossils, the records show that the Cryptozoic
eon ends with no fossils at all above the microscopic level, then
the Phanerozoic eon begins and suddenly a dozen different types of
elaborate organisms materialize.
Testing the theory
When Muller told Walter Alvarez about the Nemesis theory, the
younger Alvarez saw that one means of testing it would be an
examination of impact craters on Earth. If the theory is correct,
craters should be clumped together in periodic segments of time
corresponding to the times that mass extinctions took place. Unlike
on the airless moon, where craters are preserved in near perpetuity,
on the Earth, most craters are erased by water and wind erosion, as
well as continental drift. However, some do survive, about a hundred
of which are known.
Examining 13 of the largest, most
accurately dated of these craters, spanning the 250 million years of
the mass extinctions studied by Raup and Sepkoski, Muller and
Alvarez found the same 26 to 30 million year periodicity.
"Our analysis has proven to be
rather robust against changes in the data set," says Muller,
"including the addition or elimination of a few craters, or
changes in the minimum crater diameter examined."
Recently, Muller and LBL physicist
Saul Perlmutter used cosmic ray exposure ages to show that meteorites
created by comets fell on Earth in flurries at approximately the
dates of the last three major extinctions.
"Exposure to cosmic rays begins when
a meteorite is broken out of the parent body that had previously
shielded it, usually an asteroid or the planet Mars, and ends
when the meteorite lands on Earth," says Muller.
"The cosmic ray exposure age of a
meteorite can be determined by the amount of certain isotopes,
such as neon 21, which are produced at a known rate by energetic
cosmic rays hitting the meteorite. This exposure age tells us
the time the meteor spent orbiting in the solar system since its
creation."
There are two types of meteorites,
high-iron and low-iron. The high-iron meteorites (28 percent by
weight), called "H chondrites," are formed when material from the
iron-rich core of an asteroid or planet is blown out into space by a
violent collision with a speeding comet. Low-iron meteorites, or "L
chondrites," are formed from surface material tossed out by
low-velocity collisions between asteroids. During their periodic
flurries, high-iron meteorites fall on Earth in much greater numbers
than low-iron meteorites, but in between these periods, the number
of high- and low-iron meteorites striking Earth is about the same.
"The distribution of the H chondrite
cosmic ray ages provides new evidence confirming the claim of
comet storm theory that a large fraction of the impacts on the
Earth occur during relatively brief periods," says Muller. "This
is the first evidence for comet storms not based on terrestrial
effects."
The evidence for Nemesis-triggered
periodic comet storms based on cosmic ray exposure ages was drawn
primarily from reviews of existing data.
"In these days of tight budgets,"
observes Muller wryly, "the cheapest way to do research is in
the library."
Another review of existing data, this
time by Muller and LBL physicist Donald Morris, uncovered evidence
for periodic comet storms in the Earth’s magnetic field.
Volcanic rock, as it cools from the lava state, aligns itself with
the Earth’s magnetic field. In 1906, French physicist Bernard Brunhes discovered volcanic rock magnetized in the opposite
direction of today's field. It is now known that the Earth's
magnetic field has reversed itself many times throughout the
planet’s history, and at times has even been switched off. Muller
and Morris felt that at least some of these geomagnetic flips were
caused by comet impacts, and they developed a model to explain how
it happened.
The Earth’s magnetic field is generated by slow eddies in its molten
nickel-iron core that are the product of the heat flow out of the
core, modified by the planet’s rotation. When a crashing comet
plunges the world into darkness, temperatures on the land drop much
faster than those in the sea because water retains heat much longer
than soil.
According to the model of Muller and Morris, water near
the equator evaporates and is redistributed as ice and snow on the
polar caps. The result is a sudden (within a few hundred years) drop
in the level of the oceans. In accordance with the conservation of
angular momentum, the redistribution of mass alters the rotation
rate of the Earth’s crust and mantle with respect to the liquid core
and leads to a disruption of the magnetic field.
"It is the same as when figure
skaters go into a spin with their arms extended, then draw their
arms in to increase their rotational speed," says Muller. "The
Earth’s magnetism is so sensitive to the motions of the liquid
core that it doesn’t take much of a change in rotational rate to
affect the field."
Prior to the work of Muller and Morris,
Chicago’s Raup had examined the frequency of 296 geomagnetic
reversals that took place during the last 170 million years and
found peaks in the rate of reversals occurring approximately every
30 million years. Deposits of microtektites were also found in
volcanic and seabed rocks from times when reversals took place.
There was a sudden drop in sea level during the die-out of the
dinosaurs, but there is no evidence of a geomagnetic reversal. This
does not blemish the model of Muller and Morris, however, for it
predicts that magnetic excursions, during which the field is turned
off, would result from half of the impacts.
Magnetic excursions are difficult to
detect in volcanic rock.
"Our model readily explains observed
geophysical correlations, and accounts for the behavior of the
Earth’s magnetic field during a reversal," says Morris.
"Although somewhat speculative, it
is based on assumptions that are considered plausible by experts
in the relevant scientific fields."
A geomagnetic reversal could also take
place should the polar caps melt and cause a sudden swelling of the
seas. This, too, would alter the rotation of the Earth's crust and
mantle with respect to the core and disrupt the dynamo.
The Nemesis scenario
When Luis and Walter Alvarez first presented their idea that the
impact of an extraterrestrial object sparked the death of the
dinosaurs, many paleontologists were quick to protest that the
extinction of the dinosaurs did not transpire within a year or two,
but was a gradual decline that dragged on for several hundred
thousand years. Nemesis-launched comet storms reconcile this
apparent contradiction.
"We would not necessarily expect all
species to die out simultaneously during a storm," says Muller.
"Some species would be destroyed by an early impact, while
others make it through, only to be killed by a later and larger
impact."
Under the Nemesis scenario, what at
first glance might appear to be a single, gradual extinction, would,
upon closer scrutiny, turn out to be a series of individual, abrupt,
mass die-outs. This picture fits closely with the new school of
evolutionary thought, coined "punctuated equilibrium" by Harvard
paleontologists Steven Jay Gould and Niles Eldredge. In contrast to
Charles Darwin’s view of evolution being a steady process of smooth
transitions to ever higher forms of life, what fossil records
actually show are long stretches of inactivity, then a sudden jump
over a few hundred generations.
"It is as if evolution has its own
kind of death, giving new species a chance," says Muller. "The
species-versus-species competition that Darwin proclaimed
appears to take place only during the relatively quiet periods
between comet storms. Every 26 million years or so, instead of
survival of the fittest, we may be looking at survival of the
first, where the species that fills an open ecological niche
first has the advantage. Without this catastrophe mechanism,
Earth might still be a world ruled by trilobites."
The extinction of the dinosaurs is the
best illustration of the effect a Nemesis companion star could have
on our planet’s history. For years, school children were taught that
the dinosaurs died out because they were cold-blooded clods, too
big, too bulky, too slow, and too stupid to adapt to changing
environmental conditions and competition from swift, small, clever,
egg-eating mammals. This orthodoxy conveniently overlooked the fact
that dinosaurs co-existed and ruled over mammals for more than 100
million years, 400 times longer than the reign of Homo sapiens. At
the height of their glory, during the Cretaceous period, the
menagerie of different dinosaurs filled nearly every ecological
niche.
When they were toppled, the ecological
slate was wiped clean and mammals rapidly diversified to refill it.
"Why are we here?" Steven Jay Gould
has asked. "Because the dinosaurs disappeared, not because the
mammals out-competed them."
Search for Nemesis
For now, Nemesis is a tantalizing specter. The case for the
companion star is perhaps solid enough to score a victory in a court
of law, but in the court of science, the ultimate proof will be in
the finding. Joining Muller in the search for Nemesis at LBL are
Perlmutter and physicists Carl Pennypacker, Frank Crawford, and
Roger Williams.
Using the computer-controlled 30-inch
reflecting telescope at Leuschner Observatory, in Lafayette, Calif.,
the scientists are in the process of photographing 5,000 red stars
in the northern hemisphere and measuring the parallax of each—the
shift in its apparent position as the Earth rotates around the sun.
The telescope has been programmed to photograph each candidate, wait
two to six months, then photograph each star a second time. The two
positions can then be compared. A star far away will show little if
any change in position, but a star close enough to be Nemesis will
have moved perceptibly.
So far, the Nemesis search has eliminated 41 stars. Says Perlmutter,
"The system was difficult to start,
but we’ve got it down now and could soon have the data on 3,000
more stars."
It is Muller’s suspicion that Nemesis
might well be hiding in a constellation in the southern hemisphere
called Hydra,
"simply because," he muses, "It’s
the biggest."
Terrestrial-based testing of the Nemesis
theory also continues. The presence of an iridium anomaly in craters
that correspond to mass extinctions, and in volcanic rocks and sea
beds that correspond to geomagnetic reversals would be a strong
supporting argument for the occurrence of comet storms. Sediment
samples are now being collected from far-flung locales and send to LBL’s Asaro and Michel for analysis.
The analysis process should go much
faster than ever before with the use of a new detection device
called the "Iridium Coincidence Spectrometer." Conceived by
Luis
Alvarez and designed by Asaro, the ICS should do in three years what
previous equipment would have taken more than 100 years to do. Asaro
and Michel expect to be able to analyze 6,000 samples a year.
Humanity has never had to face the megablast of even one major comet
impact. Perhaps the most important aspect of the Nemesis theory, and
the one for which we as a species can be most grateful, is that the
deadly little companion star is not due to return until the year 15
million A.D.
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