Module 18: Solar System Debris
Activity 2: Solar System Debris and its
Effects on Earth
Summary:
In this Activity we will investigate
(a) dust and the solar wind;
(b) meteoroids, meteors & meteor showers;
(c) meteorites - composition and possible origin;
(d) lunar and martian meteorites;
(e) meteorite craters, explosions, and global extinctions; and
(f) tracking space debris, risks of major Earth impacts,
and taking evasive action?
(a) Dust and the Solar Wind
As we saw in the Activity Comets, comets’ dust tails always
point away from the Sun. This lead Kepler to suggest in the
early 1600s that cometary tails were effected by the pressure
of sunlight.
Ion tail
The ion tails of comets, however,
point in a slightly different direction
(though still away from the Sun).
In 1951 Ludwig Biermann studied
Dust tail
the deviation between the ion and
dust tails of comets and suggested that a “solar corpuscular
radiation” must be sweeping away the ions at high speeds.
He predicted that the Sun released a steady stream of
charged particles with speeds of a few 100 km/s.
The solar wind, a stream of ionised gas emanating from
the Sun, was discovered ten years later in 1962. At the
location of the Earth (1 AU from the Sun) the solar wind
flows at a speed of 450 km/s and “sweeps” dust particles
along its path.
The solar wind is not steady, however, but varies in both
time and space. The solar wind will be examined in detail
in the Module Solar Activity and its Effects on Earth.
(b) Meteors & Meteor Showers
A meteoroid is a piece of rocky space debris
orbiting the Sun at typically 20 - 40 km/s,
some of which are on a collision
course with the Earth.
meteoroid
A meteor is the fireball seen in the sky
when a meteoroid collides with the
Earth’s atmosphere.
A meteorite is
the remnant
of a meteoroid
which has
survived the
fireball to
impact
on the Earth’s
surface.
If you stare up at the night sky for more than a few minutes, you’ll
probably see a “shooting star”, which is not actually stars at all, but
a meteor, which is a light phenomenon that occurs when
meteoroids burn up as they enter the Earth’s atmosphere.
Meteors are seen when meteoroids larger than a few millimetres in
size collide with the atmosphere at speeds greater than ~10 km/s.
As they stream through the Earth’s upper atmosphere, the friction
heats the grain, and atoms on the outer surface are ablated.
These ablated atoms continue to collide with atmospheric atoms,
which become excited and ionized. A train several kilometres long
of ionised and glowing atmospheric gas is seen in the sky as a
“shooting star” or meteor.
The brightness and the length of meteors depends on how fast the
meteoroid is travelling and the angle at which it strikes the upper
atmosphere.
So where do meteoroids come from? Some are leftover bits of
junk from the formation of the Solar System, others are bits of rock
that are ejected into space during collisions between asteroids and
meteorites, and others still are grains from evaporated comets.
As comets burn out*, dust and rock
fragments form a compact meteoric
swarm which for many years
continues to circle the Sun in the
Orbit of
orbit of the parent comet.
meteoroid
swarm
When the Earth’s orbit crosses the
path of a meteoritic storm of an
existing or burnt out comet, we
experience a meteor shower, when
Sun
hundreds or even thousands of
meteors per hour can be seen.
Earth
* refer to Activity Comets
and the Dirty Snowball Model
Meteor showers are more intense when the comet’s nucleus has
recently passed near the point where the two orbits cross.
The Leonids meteors are dust particles from the Temple-Tuttle
comet, which orbits the Sun every 33 years. The Earth’s orbit
carries it through the Temple-Tuttle meteoritic swarm on
November 17and 18 each year.
The highest concentration of
meteoroids is found just behind
the comet itself, so when comet
Temple-Tuttle passes closest to
the Sun - which it last did in
1998 - the dust supply of the
Leonids is replenished, giving
rise to stronger than usual
meteor showers.
Leonid meteor shower
(c) Rocks that Fall from the Sky...
If a chunk of rock from space survives the fiery passage through
the Earth’s atmosphere and actually lands on the surface, we
call it a meteorite. To survive the passage through the
atmosphere, the rock must be quite large to begin with.
Meteorite falls at any one location on the earth are rare, but
over the entire Earth, hundreds of meteorites fall each year.
The extraterrestrial nature of meteorites was not widely
believed until the late 1700s, when German physicist Ernst
Chladni suggested that the strange looking rocks fell from the
sky. This was confirmed in 1803 by French physicist Jean
Baptiste Biot, who studied freshly fallen meteorites from a well
observed fall. Most meteorites are picked up off the ground in
finds rather than actually observed in falls .
There are 3 main classes of meteorites: the irons, the stones,
and the stony-irons.
As their names imply, meteorites are classified by their
composition. The irons are composed of nearly pure
metallic iron-nickel; the stones are made of rocky mostly
silicate material; and the much rarer stony-irons are a mix of
stone and iron.
The irons and stony-irons are more obviously
extraterrestrial, as ‘pure’ iron is very rare on Earth (it is
usually found in oxides). The stones are the more common
type, but are difficult to recognise as they look much like
terrestrial rocks.
Meteorite Composition
The iron meteorites are composed
of about 90% iron and 10% nickel.
They make up about 4% of the
Earth’s meteorites.
The stony-irons are composed
of about 50% silicates and 50%
iron, and make up only about 1%
the of meteorite falls.
Iron meteorite
The stones make up the remaining 95% of meteorite falls,
and are divided into three types: chondrites, achondrites &
carbonaceous.
(1) chondrites are composed of
silicate rock & rounded bits of glassy
rock called chondrules. They are the
most common form of stony
Chondrite
meteorite.
(2) achondrites are stony
meteorites which do not
contain chondrules or volatiles
Achondrite
and have far less iron than
chondrites.
(3) carbonaceous meteorites, as their name suggests,
contain carbon and some complex organic material. They
can also contain as much as 20% water and other volatile
compounds.
Meteorite Origins
The real significance of meteorites was not fully appreciated
until their ages were determined, the oldest being dated at
4.6 billion years old! This means that meteorites not only
sample planetary bodies (mostly asteroids) that we otherwise
couldn’t study, but they also give us an insight into the early
Solar System and its formation.
A more fundamental classification scheme for meteorites than
their composition is the distinction between the differentiated
and primitive meteorites, as this tells of their thermal history.
Some meteorites - including all of the iron, all the stony-irons
and some of the stones - have clearly experienced a lot of
heating in their past and are derived from broken up
differentiated bodies that have been chemically sorted. This
means that the parent bodies have been heated to a molten
state, allowing the more dense material to fall to their centre
and the less dense material to float to their surface. The irons
would have come from the cores of these bodies, which were
most probably asteroids, while the stony-irons come from the
region between the core and stony mantle.
The primitives, on the other hand, have not experienced
high temperatures or pressures and are thought to record the
earliest history of the Solar System. They are believed to
have condensed directly out of the solar nebula, making them
the most ‘pristine’ material in the Solar System.
Most of the meteorites found on Earth are primitive stones,
including the chondrites and carbonaeous stones - both of
which contain volatiles, which indicate little or no heating.
(The achondrites, on the other hand, have clearly been
heated and are in the differentiated group.)
The following relationship
between meteorites and
asteroids, while not strictly
true, can be used as a guide:
Meteorites
Irons
Stony-Irons
Stones*
Asteroids
M class
S class
C class
* carbonaceous stones
Meteorites and the Early Solar System
To learn more about the conditions in the early Solar System, we
need to look more closely at the primitive meteorites, which contain
a complex but primitive mixture of pre-planetary material. In fact,
some primitive meteorites actually contain traces of interstellar gas
that has survived the planet formation process!
Chondrules are small spherical pebbles, generally 0.1
mm to 10 mm in size, found in the primitive chondrites.
They are thought to have solidified from melted
droplets directly out of the solar nebula, making them
some of the oldest solid material in the Solar System.
0.1mm
Chondrules formed in very rapid heating events
(maybe via a shock front or lightning discharge), and the glassy
nature of many chondrules also indicates that they cooled extremely
rapidly. Their exact formation mechanism is still strongly debated.
(d) Lunar and Martian Meteorites
While most meteorites are thought to have derived from
asteroids, there are a small number that have been identified
as originating from Mars and the Moon.
When a large piece of space debris (such as an asteroid)
impacts with a planetary body and forms impact craters 100km
in size or more, some of the surface rock can be ejected with
enough speed to escape the gravity of the planet. The ejected
material may later* collide with the Earth.
clongg!
*sometimes millions of years later!
Lunar meteorites
How do we know that lunar meteorites
are actually from the Moon? The
composition and isotope ratios of these
meteorites have been compared with the
samples brought back from the Moon
during the Apollo mission.
Lunar meteorites are extremely rare - less that 0.1% of all meteorites
are of lunar origin. There have been about 60 lunar meteorites found
on Earth, some of these are “paired” (meaning they fell to Earth in a
group). Overall they represent just over 20 impact sites on the
Moon. The lunar meteorites help us understand the geological
history of the Moon in greater detail.
For more information, visit the Lunar Meteorites website at:
http://epsc.wustl.edu/admin/resources/moon_meteorites.html
Martian Meteorites
There are also 30 meteorites that are know to originate from
Mars. We know these meteorites are Martian from trace amounts
of gas that resembles the atmosphere of Mars, which is trapped
within microscopic pockets of the rocks.
The most interesting (or at least the
most famous!) of the Martian meteorites
is ALH84001. This Martian rock (which
is 4.5 billion years old) was ejected from
Mars 16 million years ago, landed in the
Allen Hill Ice Field of Antarctica 13,000
years ago, and was finally picked up in
1984.
ALH84001 made world headlines in 1996 when it was found to
contain evidence for ancient life on Mars...
ALH84001 contains tiny traces of organic compounds called
Polycyclic Aromatic Hydrocarbons (PAHs), which can be
produced by decaying organisms!*
These tubular structures in ALH84001,
found clinging to carbonate grains,
resembles terrestrial bacteria.
Whether or not this is really evidence of
ancient life of Mars is still unclear...
Magnified 100,000 times. Note
the tubular structure which may
be fossils of microorganisms.
For details of the
Martian meteorites, visit:
http://www.jpl.nasa.gov/snc/
* but also produced chemically and
commonly found in space
(e) Meteorite Craters, Explosions and
Mass Extinctions
Meteoroids (and asteroids) generally travel with velocities in the
range of 10 to 70 km/s, which means that they have
considerable kinetic energy. Upon impact, the kinetic energy is
transferred to the surface of the target body and the meteoroid
will generally dig 2 or 3 times its diameter into the surface. What
results is an impact crater.
The exact dimensions of the crater
also depend on the target body.
The resulting crater diameter is
The more massive the target body,
usually several times its depth.
the faster the meteoroid will be
pulled towards it. An atmosphere
can help slow down the impacter.
Craters
The diameter of craters formed when large meteorites strike
the Earth are typically 10-20 times as large as the diameter
of the meteorite which causes it.
Wolf Creek Crater, Western Australia
Using this rule of thumb, the meteoroid which created the
Barringer Crater would have had a diameter between 50
and 100 meters.
About 1000m
200m
Barringer Meteor Crater, Arizona USA
The Tunguska Explosion
Not all meteoroids make it to the Earth’s surface. Obviously
small meteoroids burn up in the Earth’s atmosphere, but some
larger meteoroids can actually explode in the air before they hit
the ground. Such an event is often called a fireball.
One example was when a stony asteroid struck the Earth’s
atmosphere above the Tunguska region of Siberia in 1908.
When it exploded before reaching the surface, the mass of
about 100 megatonnes released the energy equivalent to a
nuclear detonation of several hundred kilotons. The Tunguska
explosion was felt hundreds of kilometers away and felled trees
over an area of 2000 square kilometers.
Obviously the effects of an impact can be devastating. A large
meteorite impacting with the Earth would vaporise itself as well
as a large volume of terrestrial rock. Solid and molten rock
would be sprayed several thousand kilometres.
As well as localised effects, giant impacts can lead to prolonged
darkness due to dust particles obscuring the Sun. Intense acid
rain and heating of the atmosphere are also possible
consequences of such an event.
If we look at the cratering rate with time, we can see that there
are far less impacts now than in the past (which is good for us!).
impacts
per year
time
It is thought that an asteroid 1 km in diameter hits the Earth
once every 100,000 years or so, and an asteroid 5 km in
diameter once every 5 million years...
However, recent simulations from scientists at the Imperial
College London and the Russian Academy of Sciences have
found that more asteroids are destroyed upon entry into the
Earth’s atmosphere than previously estimated. The study found
that asteroids with diameters greater than 200 metres will hit the
Earth’s surface every ~160,000 years – compared to previous
estimates of impacts every ~2,500 years.
Mass Extinctions
One of the best studied large Earth impacts occurred 65 million
years ago , which separates the Cretaceous and Tertiary geologic
periods. This boundary, the K-T boundary, coincides with a mass
extinction that wiped out over half the species on Earth. There are
about a dozen mass extinctions in geological history, but the KT
boundary gets a lot of attention because it wiped out the dinosaurs.
A worldwide layer of sediment rich in iridium (a mineral rare on
Earth but common in meteoroids) at the KT boundary, from New
Zealand to Denmark, was discovered in 1980. It was suggested
that the entire Earth was enshrouded in a dust cloud from a giant
impact, which sent dust and ash into the atmosphere, turning day to
night and lowering the average temperature. This dark winter
probably lasted several years. Plants and then animals died leading to a mass extinction.
The search for the giant impact crater began,
and finally in 1990 the Chicxulub crater was
found off Mexico’s Yucatan Peninsula. The
crater is buried under several kilometres of
sediment, but geologic mapping allows us to
reconstruct the crater, which is about 180km
in diameter - making it the largest impact
crater on Earth.
The impacting body must have been about 20km in size. The
impact itself would have created a massive earthquake, a 100 m
tidal wave that raced across the Gulf of Mexico, and lifted about
100 trillion tons of dust into the atmosphere!
(f) Tracking Space Debris
The potential for Solar System debris to cause global
catastrophe makes it crucial to trace the trajectories of objects
whose paths will take them near the Earth.
To determine the orbit of a Near Earth
Object you need as many precise
measurements of its position in the sky
as possible.
The time spread of these observations
is important; better to have 10 positions over 3 months than
50 positions over one week. This means it is advantageous
to have teams working on detection in both hemispheres.
This plot shows the location of the
4 terrestrial planets and the minor
planets of the inner Solar System.
The green objects reside in the
asteroid belt, while the red objects
have their closest approach to the
Sun within 1.3AU, and are thus
considered Near Earth Objects.
It is estimated that there are about
1000 NEOs with diameters over
1 km, including over 550 “potentially
hazardous” asteroids.
Inner Solar System, 7 February 2003
from the Minor Planet Centre
For more information about NEO tracking, visit:
NASA’s Near Earth Asteroid Tracking
http://neat.jpl.nasa.gov/
NASA’s Near Earth Object Program
http://neo.jpl.nasa.gov/
In this Activity, we have examined the nature of
various types of Solar System debris, from meteors
to meteorites, and their effect on the Earth.
In the next Activities we will move to the centre of the
Solar System and examine the Sun; its structure,
energy and fuel, and its effects on the Earth.
Image Credits
Leonid meteor shower © Yan On Sheung
http://www.leonidslive.com/images/hkdawn_big.jpg
1966 Leonids meteor show, Scott Murrell
http://science.nasa.gov/newhome/headlines/ast10nov98_1.htm
The Earth, NASA
http://antwrp.gsfc.nasa.gov/apod/ap971026.html
Lunar craters: Images from Apollo 11 mission
http://www.nasm.edu/APOLLO/AS11/AS11-44-6611.t.gif
ALH84001
http://wwwcurator.jsc.nasa.gov/curator/antmet/marsmets/life.htm
Comet Hale Bopp, © Michael Brown Uni. Of Melb., used with permission
http://www.ph.unimelb.edu.au/~mbrown/astrogif/hale-bopp.html
Barringer Meteor Crater
http://antwrp.gsfc.nasa.gov/apod/ap971117.html
Wolf Creek meteorite crater, Camboon Primary School
http://iinet.net.au/~ramen/graphics/wolf.jpg
Image Credits
Meteorite chondrule, © Sasha Krot
http://www.higp.hawaii.edu/~sasha/caichd.html
Chicxulub crater, V.L.Sharpton, Lunar & Planetary Institute
http://antwrp.gsfc.nasa.gov/apod/ap000226.html
NEOs of the inner Solar System - Minor Planets Centre
(used with kind permission)
http://cfa-www.harvard.edu/iau/lists/InnerPlot2.html
Now return to the Module 18 home page, and
read more about Solar System debris in the
Textbook Readings.
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