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This chapter covers all of the solar system that is not the planets or the Sun:
meteorites,
asteroids, and comets. I group them all together as ``solar system fluff'' because
the objects are much smaller than planets and most moons. The ``fluff'' may be
small in size but certainly not in importance. We get clues of the origin of the
solar system from these small objects.
There are thousands, even millions, of small rocks that orbit the Sun, most of them
between the orbits of Mars and Jupiter.
A plot of the known
asteroids is available at the
Minor Planet Center.
About one million of them are larger than
1 kilometer across. Those smaller than about 300 kilometers across have irregular
shapes because their internal gravity is not strong enough to compress the rock
into a spherical shape. The largest asteroid is Ceres with a diameter of 1000
kilometers.
Pallas and Vesta have diameters of about 500 kilometers and about 15 others have diameters
larger than 250 kilometers. The number of asteroids shoots up with decreasing size. The
combined mass of all of the asteroids is less than the Moon's mass.
Very likely the asteroids are pieces that would have formed a planet if Jupiter's
strong gravity had not stirred up the material between Mars and Jupiter.
The rocky chunks collided at speeds too high to stick together and grow into a
planet.
Though there are over a million asteroids, the volume of space they inhabit is very
large, so they are far apart from one another. Unlike the movie The Empire
Strikes Back, where the spacecrafts flying through an asteroid belt could not
avoid crashing into them, real asteroids are at least tens of thousands of kilometers
apart from each other. Several spacecraft sent to the outer planets have travelled
through the asteroid belt with no problems.
Two of the three types of asteroids are represented by
the asteroids that have been explored up close with spacecraft. Mathilde
(left) is a dark C-type (brightness enhanced several times to match the
other two). Gaspra and Ida are S-type asteroids.
There are three basic types of asteroids:
- C: they are carbonaceous---made of silicate materials with a lot of
carbon compounds so they appear very dark. They reflect only 3 to 4% of the sunlight
hitting them. You can tell what they are made of by analyzing the spectra of sunlight
reflecting off of them. This reflectance spectra shows that they are
primitive, unchanged since they first solidified about 4.6 billion years
ago. A sizable fraction of the asteroids are of this type. The asteroid
called Mathilde, recently explored by the NEAR spacecraft is an example of
this type (see picture above).
- S: they are made of silicate materials without the dark carbon compounds
so they appear brighter than the C types. They reflect about 15 to 20% of the sunlight
hitting them. Most of them appear to be primitive and they make up a smaller fraction
of the asteroids than the C types. Gaspra and Ida, explored by the Galileo
spacecraft on its way to Jupiter, are examples of this type (see picture
above).
- M: they are made of metals like iron and nickel. These rare type of
asteroids are brighter than the S and C types. We think they are the remains of the
cores of differentiated objects. Large objects were hot enough in the
early solar system so that they were liquid. This allowed the dense materials like
iron and nickel to sink to the center while the lighter material like ordinary
silicate rock floated up to the top. Smaller objects cooled off quicker than
larger objects, so they underwent less differentiation. In the early solar system,
collisions were
much more common and some of the smaller differentiated large asteroids collided
with one another, breaking them apart and exposing their metallic cores.
A few other asteroids have surfaces made of basalt from volcanic lava flows. When
asteroids collide with one another, they can chip off pieces from each other. Some
of those pieces, called meteoroids, can get close to the Earth and be pulled
toward the Earth by its gravity.
The quick flashes of light in the sky most people call ``shooting stars'' are
meteor---pieces of the rock glowing from friction with the atmosphere as
they plunge toward the surface. Most of the meteors you see are about the size of a
grain of sand.
More meteors are seen after midnight because your local part of the
Earth is facing the direction of its orbital motion around the Sun. Meteoroids
moving at any speed can hit the atmosphere. Before midnight your local part of
the Earth is facing away from the direction of orbital motion, so only the fastest
moving meteoroids can catch up to the Earth and hit the atmosphere. The same
sort of effect explains why an automobile's front windshield will get plastered
with insects while the rear windshield stays clean.
If the little piece of rock makes it to the surface
without burning up, it is called a meteorite.
There are three basic types of meteorites.
- Stones: they are made of silicate material with
a density around 3 (relative to the density of water) and look
like ordinary Earth rocks. This makes them hard to distinguish from Earth rocks so
they don't stand out. About 95 to 97% of the meteorites are of these type. About 85%
of the stones are primitive, unchanged since they first solidified about
4.6 billion years ago. Most of the primitive stones have
chondrules---round glassy structures 0.5 to 5 millimeters across embedded in
the meteorites. They are solified droplets of matter from the
early solar nebula and are the oldest part of a primitive meteorite.
A
carbonaceous meteorite containing chondrules (courtesy of the
Planetary Data Center).
The oldest of the stone meteorites are the carbonaceous meteorites. They
contain silicates, carbon compounds (giving them their dark color), and a
surprisingly large amount of water (about 22%). They are probably chips of C-type
asteroids. Some of the carbonaceous meteorites
have organic molecules called amino acids. Amino acids can be connected together
to form proteins that are used in the biological processes of life. There is the
possibility that meteorites like these may have been the seeds of life on the Earth.
In addition, these type of meteorites may have provided the inner planets with a lot
of water. The terrestrial planets may have been so hot when they formed that most of
the water in them at formation evaporated away to space. The impact of millions to
billions of carbonaceous meteorites in the early solar system may have replenished
the water supply on the terrestrial planets.
About 10 to 12% of the stones are from the crust of differentiated parent
objects. Therefore, they are younger (only 4.4 billion years old). The
lighter-colored stones are chips from the S-type of asteroids.
- Stoney-Irons: only 1% of the meteorites are of this
type. They have a variable mixture of metal (iron and nickel) and rock (silicates)
and have densities ranging from 4 to 6 (relative to the density of water).
They come from a differentiated object at the boundary between the metal core
and the rock crust. They are 4.4 billion years old.
- Irons: although they make up about 40% of the
meteorites found worldwide, only 2 to 3% of the meteorites are these type. They make
up so many of the ones found because they are easily distinguished from Earth rocks.
They are noticeably denser than Earth rocks, they have a density around 7.
They come from the core of a differentiated body and are made of iron and nickel.
They are 4.4 billion years old. Irons
sometimes have large, coarse-grained crystalline patterns
(``Widmanstatten patterns'') that is evidence that they cooled slowly.
An
iron meteorite (courtesy of the
Planetary Data Center).
The primitive meteorites are probably the most important ones because they hold
clues to the composition and temperature in various parts of the early solar nebula.
Most stoneys look like Earth rocks and so they are hard to spot. The rare irons are
easy to distinguish from Earth rocks and make up most of the ones found worldwide.
Usually the meteorites science museums show off are iron meteorites. Not only does
their high density and metal composition set them apart from ordinary rocks, the
iron meteorites are stronger. This means they will more likely survive the passage
through the atmosphere in one piece to make impressive museum displays. The stone
meteorites are more fragile and will break up into several pieces (less impressive
for museum displays).
To get an accurate number for the proportion of meteorites that fall to the Earth
(an unbiased sample),
meteorite searchers go to a place where all types of rocks will stand out. The best
place to go is Antarctica where the stable, white ice pack makes darker meteorites
easy to find. Meteorites that fell thousands of years ago can still be found in
Antarctica without significant weathering. Since the 1980's thousands of meteorites
have come from here. For further exploration, check out the
Antarctica
Meteorite web site at the
Planetary Materials
Curation office of NASA and the ANSMET site at Case
Western Reserve University.
Meteorites stand out against the snow and ice background of
Antarctica.
Most meteorites are pieces of asteroids, but
a few are from the Moon. A select few, the Shergotty-Nakhla-Chassigny (SNC)
meteorites, may be from Mars. The relative abundances of
magnesium and heavy nitrogen (N-15) gases trapped inside the SNC meteorites is
similar to the
martian atmosphere as measured by the Viking landers and unlike any meteorites
from the asteroids or Moon. Also, the isotope ratios of argon and xenon gas trapped
in the meteorites most closely resemble the martian atmosphere and are
different than the typical meteorite. The analysis of the soil and rocks by the
Mars Pathfinder confirm this.
A
meteorite blasted from Mars.
Most SNC
meteorites are about 1.4 Gyr old, but the one with suggestions of extinct
Martian life is about 4.5 Gyr old. The discovery was published in the August 16, 1996
issue of the journal
Science.
Non-subscribers can find a copy of the article
here.
Recent studies of the meteorite have cast considerable doubt on the
initial claims for fossil microbes in the rock. There is strong evidence of
contamination by organic molecules from Earth, so this meteorite does not
provide the conclusive proof hoped for.
A detailed description of SNC meteorites is given at
http://www.jpl.nasa.gov/snc/.
There are several ways to figure out relative ages, that is, if one thing
is older than
another. For example, looking at a series of layers in the side of a cliff, the
younger layers will be on top of the older layers. Or you can tell that certain
parts of the Moon's surface are older than other parts by counting the number of
craters per unit area. The old surface will have many craters per area because it
has been exposed to space for a long time. But how old is ``old''? If you assume
that the impact rate has
been constant for the past several billion years, then the number of craters will
be proportional to how long the surface is exposed. However, the crater number
relation must be calibrated against something with a known age.
To measure the passage of long periods of time, scientists take advantage of a regularity
in certain unstable atoms. In radioactive atoms the nucleus will spontaneously
change into another type of nucleus. When looking at a large number of atoms,
you
see that a certain fraction of them will change or decay in a certain amount
of time that depends on the type of atom---more specifically, the type of
nucleus. Radioactive dating is an
absolute dating system because you can determine accurate ages from the
number of remaining radioactive atoms in a rock sample. Most of the radioactive
isotopes used for radioactive dating of rock samples have too many neutrons
in the nucleus to be stable.
An isotope is a particular form of an element. All atoms of an element
have the same number of protons in their nucleus and behave the same way in
chemical reactions. The atoms of an isotope of a given element have
same number of protons AND neutrons in their nucleus. Different isotopes of a given
element will have the same chemistry but behave differently in nuclear
reactions. In a radioactive decay, the original radioactive isotope is called a
parent isotope and the resulting isotope after the decay is called a
daughter isotope. For example, Uranium-238 is the parent isotope that
breaks apart to form the daughter isotope Lead-204.
Radioactive isotopes will decay in a regular exponential way such that one-half
of a given amount of parent material will decay to form daughter material in a
time period called a half-life. A half-life is NOT one-half the
age of the rock! When the material is liquid or gaseous, the
parent and daughter isotopes can escape, but when the material solifies, they
cannot so the ratio of parent to daughter isotopes is frozen in. The parent
isotope can only decay, increasing the amount of daughter isotopes. Radioactive
dating gives the solidification age.
There are two simple steps for radioactive dating:
- Find out how many times you need to multiply (1/2) by itself to get the observed
fraction of remaining parent material. Let the number of the times be n. For
example 1/8 = (1/2) × (1/2) × (1/2), so n = 3.
The number n is the number of half-lives the sample has been
decaying. If some material has been decaying long enough so that only 1/4 of the
radioactive material is left, the sample is 2 half-lives old: 1/4 = (1/2) × (1/2),
n =2.
- The age of the sample in years = n × (one half-life in years).
How do you do that?
If 1/8 of the original amount of parent isotope is left
in a radioactive sample, how old is the sample? Answer: After 1 half-life,
there is 1/2 of the original amount of the parent left. After
another half-life, there is 1/2 of that 1/2 left = 1/2 × 1/2 = 1/4 of
original amount of the parent left. After yet another half-life, there is 1/2 of
that 1/4 left = 1/2 × 1/2 × 1/2 = 1/8 of the original amount of the
parent left (which is the fraction asked for). So the rock is 1 half-life +
1 half-life + 1 half-life = 3 half-lives old (to get the age in years, simply
multiply 3 by the half-life in years).
|
If you have a fraction that is
not a multiple of 1/2, then it is more complicated. The age =
[ln(original amount of parent material / current amount of parent
material) / ln(2)] × (half-life in
years), where ln() is the ``natural logarithm'' (it is the ``ln'' key on a
scientific calculator).
There are always a few astronomy students who ask me the good question (and many
others who are too
shy to ask), ``what if you don't know the original amount of parent material?'' or
``what if the rock had some daughter material at the very beginning?'' The
age can still be determined but you have to be more clever in determining it.
One common sense rule to remember is that the number of parent isotope atoms + the
number of daughter isotope atoms = an unchanging number throughout time. The
number of parent isotopes decreases while the number of daughter isotopes increases
but the total of the two added together is a constant.
You need to find how much of the daughter isotopes in the rock (call that isotope
``A'' for below) are not the result of a radioactive decay of
parent atoms. You then subtract this amount from the total amount of daughter
atoms in the rock to get the number of decays that have occurred since the rock
solified. Here are the steps:
- Find another isotope of the same element as the daughter that is never a
result of radioactive decay (call that isotope ``B'' for below).
Isotopes of a given element have the same chemical properties, so a radioactive
rock will incorporate the NONradioactively derived proportions of the two
isotopes in the same proportion as any nonradioactive rock.
- Measure the ratio of isotopes A and B in a nonradioactive rock.
This ratio, R, will be the primitive (initial) proportion of the two
isotopes.
- Multiply the amount of the non-daughter isotope (isotope B) in the radioactive
rock by the ratio of the previous step: (isotope B) × R = initial amount
of daughter isotope A that was not the result of decay.
- Subtract the initial amount of daughter isotope A from the rock sample to get
the amount of daughter isotope A that IS due to radioactive decay. That number is
also the amount of parent that has decayed (remember the rule #parent + #daughter =
constant). Now you can determine the age as you did before.
The long ages (billions of years) given by radioactive dating of rocks seems an
impossibly long time for some people. Since
radioactive rocks have been observed for only a few decades, how do you know
you can trust these long half-lifes and the long ages derived? Here are some points
to consider:
- The rate of decay should follow a simple exponential decline based on the
simple theory of probability in statistics. This same probability theory is used to
figure the odds of winning by gamblers.
- An exponential decay is seen for short-lived isotopes with half-lives of only a
few days.
- For the decades they have been observed, the long-lived isotopes also follow
an exponential decay.
- The decay probability should not depend on time because:
- An exponential decay IS observed for short-lived isotopes.
- Decays are nuclear reactions. Nuclear reactions only care about size
scales of 10-13 centimeters (100 million times smaller than the
wavelength of visible light). The composition and state of the surrounding
material will not affect the rate of decay.
- The laws of nature or physics at the
nuclear level should not change with time.
- Astronomical observations show that the laws of physics are the same
everywhere in the universe and have been unchanged for the past 15 billion
years.
Some asteroids have
orbits that cross the orbit of the Earth. That means that the
Earth will be hit sometime. Recent studies have shown that the
Earth has been hit an alarmingly large number of times in the past.
One large impact is now thought to have contributed to the quick demise of
the dinosaurs about 65 million years ago. What would be the effects of an asteroid
hitting the Earth?
Known impact
sites on the Earth's continents.
What follows is a condensation of an excellent article by
Sydney van den Bergh called ``Life and Death in the Inner Solar System'' in the
May 1989 issue of the Publications of the Astronomical Society of the Pacific
(vol. 101, pages 500-509). He considers a typical impact scenario of
a 10-kilometer object
with density = 2.5 times that of water, impacting at a speed of 20
kilometers/second. Its mass =
1.31 trillion tons (1.31 × 1015 kilograms). A 1-kilometer object
has a mass = 1.31 billion tons.
Obviously, something this big hitting the Earth is going to hit with a lot of energy!
Let's use the energy unit of 1 megaton of TNT (=4.2× 1015
Joules) to
describe the energy of the impact. This is the energy one million tons of
dynamite would release if it was exploded and is the energy unit used for nuclear
explosions. The largest yield of a thermonuclear
warhead is around 50--100 megatons. The kinetic energy of the falling object is
converted to the explosion when it hits. The 10-kilometer object produces an explosion of
6 × 107 megatons of TNT (equivalent to an earthquake of magnitude 12.4 on
the Richter scale). The 1-kilometer object produces a milder explosion of ``only''
6 × 104 megatons (equivalent to an earthquake of magnitude 9.4 on the
Richter scale).
On its way to the impact, the asteroid pushes aside the air in front of it creating
a hole in the atmosphere. The atmosphere above the impact site is removed for several
tens of seconds. Before the surrounding air can rush back in to fill the gap,
material from the impact: vaporized asteroid, crustal material, and ocean water (if
it lands in the ocean), escapes through the hole and follows a ballistic flight
back down. Within two minutes after impact, about 105 cubic kilometers of
ejecta (1013 tons) is lofted to about 100 kilometers. If the asteroid
hits the ocean, the
surrounding water returning over the the hot crater floor is vaporized, sending
more water vapor into the air.
There will be a crater regardless of where it lands. The diameter of the
crater in kilometers is = (energy of impact)(1/3.4)/106.77.
Plugging in the typical impact values, you get a 150-kilometer diameter crater for the
10-kilometer asteroid and a 20-kilometer diameter crater for the 1-kilometer asteroid.
Meteor (Barringer) Crater in
northern Arizona (about 1 kilometer across). |
Chicxulub Crater in Yucatan,
Mexico (from the one that may have killed off the dinosaurs). |
The oceans cover about 75% of the Earth's surface, so it is likely the asteroid will
hit an ocean. The amount of water in the ocean is nowhere near large enough to
``cushion'' the asteroid. The asteroid will push the water aside and hit the ocean
floor to create a large crater. The water pushed aside will form a huge tidal wave,
a tsunami. The tidal wave height in meters = (distance
from impact)-0.717 × (energy of impact)0.495/
(1010.17). What this means is that a 10-km asteroid hitting any deep
point in the Pacific (the largest ocean) produces a megatsunami along the entire
Pacific Rim.
Some
values for the height of the tsunami at different distances from the impact site
are given in the following table. The heights are given for the two typical
asteroids, a 10-kilometer and a 1-kilometer asteroid.
| Distance (in km) | 10 km | 1 km |
| 300 | 1.3 km | 43 m |
| 1000 | 540 m | 18 m |
| 3000 | 250 m | 3 m |
| 10000 | 100 m | 3 m |
The material ejected from the impact through the hole in the atmosphere will
re-enter all over the globe and heat up from the friction with the atmosphere. The
chunks of material will be hot enough to produce a lot of infrared light.
The heat from the glowing material will start fires around the globe. Global
fires will put
about 7 × 1010 tons of soot into the air. This would ``aggravate
environmental stresses associated with the ... impact.''
The heat from the shock wave of the entering asteroid and reprocessing of the air
close to the impact produces nitric and nitrous acids over the next few months to
one year. The chemical reaction chain is:
- N2 + O2 ‚> NO (molecular nitrogen combined
with molecular oxygen produces nitrogen monoxide)
- 2NO + O2 ‚> 2NO2 (two nitrogen monoxide
molecules combined with one oxygen molecule produces two nitrogen dioxide
molecules)
- NO2 is converted to nitric and nitrous acids when it is mixed
with water.
These are really nasty acids. They will wash out of the air when it rains---a
worldwide deluge of acid rain with damaging effects:
- destruction or damage of foliage;
- great amounts of weathering of continental rocks;
- the upper ocean organisms are killed. These organisms are responsible for
locking up carbon dioxide in their shells (calcium carbonate) that would eventually
become limestone. However, the shells will dissolve in the acid water. That along
with the ``impact winter'' (described below) kills off about 90% of all marine
nanoplankton species. A majority of the free oxygen from photosynthesis on the
Earth is made by nanoplankton.
- The ozone layer is destroyed by O3 reacting with NO.
The amount of ultraviolet light hitting the surface increases, killing small
organisms and plants (key parts of the food chain). The NO2 causes
respiratory damage in larger animals. Harmful elements like Beryllium, Mercury,
Thallium, etc. are let loose.
All of the dust in the air from the impact and soot from the fires will
block the Sun. For several months you cannot see your hand in front of your
face! The dramatic decrease of sunlight reaching the surface produces a drastic
short-term global reduction in temperature, called impact winter. Plant
photosynthesis stops and the food chain collapses.
The cooling is followed by a much more prolonged period of increased
temperature due to a large increase in the greenhouse
effect. The
greenhouse effect is increased because of the increase of the carbon dioxide and
water vapor in the air. The carbon dioxide level rises
because the plants are burned and most of the plankton are wiped out. Also,
water vapor in the air from the impact stays aloft for awhile. The temperatures
are too warm for comfort for awhile.
Astronomers have requested funding for an observing program called
Spaceguard to
catalog all of the near-Earth asteroids and short period comets. Of the
approximately 2000 asteroids that are thought to pose a threat of impacting
Earth, less than 10% have been
found so far. The international
program would take 10 years to create a comprehensive catalog of all
of the hazardous asteroids and comets. The cost for the entire program
(building six special purpose telescopes and operation costs for ten
years) would be less than what it costs to make a popular movie like Deep
Impact or Armageddon. Select the hot links in this paragraph to
find out more about Spaceguard.
Another nice site is the Asteroid and Comet Hazards
site at NASA.
Vocabulary
| carbonaceous meteorite | chondrules | differentiated |
| half-life | isotope | meteorite |
| primitive | radioactive dating |
- Where are most of the asteroids found?
- Why are spacecraft able to pass through the asteroid belt without getting hit?
- What are the three types of asteroids and what are they made of?
- Why are more meteors seen after midnight?
- What are the proportions of the three types of meteorites and what are they made
of?
- What type of asteroid does each type of meteorite correspond to?
- Which meteorites are primitive and why are they particularly important
for understanding the origin of the planets?
- What makes carbonaceous meteorites so special?
- Why are chondrules especially important for solar system formation
models? What type of meteorite are they likely to be found in?
- How old are the various types of meteorites and why are they used to
find the age of the solar system?
- Why do iron meteorites make up 40% of the ``finds'', but are only 2 to 3% of the
total meteorites? What causes the biasing of the ``finds''?
- Why is Antarctica a good place to get an unbiased sample of meteorites?
- What makes SNC meteorites so unique and how are they different than other types
of meteorites?
- Why are iron and stoney-iron meteorites thought to come from a
differentiated body?
- Does radioactive dating give us relative or absolute ages for rocks?
What type of ``age'' does it tell us?
- How do you use the half-life to find the age of radioactive rocks?
- If the half-life of a radioactive sample is 1 month, is a sample of it
completely decayed after 2 months? If not, how much is left?
- Uranium-235 has a half-life of 700 million years. How long will you have to
wait until a 1-kilogram chunk decays so that only 0.0625 kilograms (1/16 kg) is left?
- How are the very old ages derived for some radioactive rocks known to be correct?
- If a large asteroid were to hit Earth, how much of the Earth's surface would
be affected?
Go to
Earth-Venus-Mars section
Comets and
solar system formation sections
last update: 18 February 1999
Nick Strobel --
Email:
strobel@lightspeed.net
(661) 395-4526
Bakersfield College
Physical Science Dept.
1801 Panorama Drive
Bakersfield, CA 93305-1219
