Transcript (I) Dr Conor Nixon Fall 2006 ASTR 330: The Solar System

ASTR 330: The Solar System
Lecture 6 : Review Quiz
1. What is meant by differentiation?
2. What is the reason for the existence of the main belt of asteroids?
3. What is the Edgeworth-Kuiper Belt?
4. What is the Oort Cloud composed of?
5. How did the Oort Cloud come into being?
6. What evolutionary processes are continuing in the solar system
today?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Announcements
• HW assignment #2 due back on Tuesday, Sept 26th.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Pair Exercise
• Consider the following question: how would you go
out and look for a meteorite?
• Things to consider:
• look in the sky, or on Earth?
• how to tell an object is not terrestrial?
• where might meteorites accumulate?
• what types of terrain will make it easier or harder to
spot meteorite falls?
• how would you test a meteorite in a lab if you
weren’t sure it was one?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Lecture 6:
Meteorites (I)
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Meteorites
• In the last class we discussed the formation of the
planetary system.
• We noted that primitive rocks are not found on Earth, but
may exist elsewhere in the solar system, e.g. in asteroids.
• In this class we will learn about meteorites, serendipitous
objects which (literally) may fall in our lap.
• These objects come from diverse parts of the solar
system.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
L’Aigle Meteorite Fall
• Until the 19th century, the idea of rocks falling from the sky was
treated with skepticism by most people, including scientists. (See the
anecdote about Jefferson in the textbook).
• However, that all changed on April 26th 1803, when literally thousands
of rocks rained down on the town of L’Aigle, France.
• This fall was so large and witnessed by so many people that
scientists were forced thereafter to treat the idea of meteorites
seriously.
• From that time on, reports of stones falling from
the sky were seriously investigated, and museums
began to collect and categorize meteorites.
Picture: space.com
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Major Classifications
• Meteorites are classified into three major mineralogical categories:
1. STONY: similar to terrestrial rocks, and may be difficult to separate
from terrestrial origins unless the fall is witnessed.
2. IRON: composed of nearly pure nickel-iron, and easily identified by
high density: 7 g/cm3. Do you think it is easy to tell whether one is
extra-terrestrial or not?
3. STONY-IRON: a mixture of stone and metallic iron.
• Note that stony meteorites may contain iron compounds: however
an iron meteorite is an actual piece of metal.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Other Categories
• As well as the compositional divisions, we may also categorize
meteorites according to their history/evolution:
1. Primitive: primitive meteorites represent example of the original
material from which the solar system was made, almost unaltered by
the passage of time. These are all stony (but conversely, not all stony
meteorites are primitive!).
2. Differentiated or igneous: these meteorites have been melted and resolidified. All of the irons and stony-irons, and many stony meteorites
are differentiated.
3. Breccias: can be primitive or differentiated, but has been fragmented
into pieces and subsequently re-cemented back together again.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Characteristics And Composition
• The table below summarizes the characteristics of the main meteorite
types (after Table 4.3 of Morrison and Owen)
Type
Composition
Primitive Meteorites (chondrites)
Carbonaceous
silicates, carbon compounds, water
Other primitive stones
silicates, iron
Differentiated meteorites (achondrites)
Differentiated stones
igneous silicates
Stony-irons
igneous silicates, iron, nickel
Irons
iron, nickel
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Examples
• CHONDRITE: a primitive meteorite
which formed at the same time as the
planets, 4.55 billion years ago. (From the
Allan Hills, Antarctica).
• ACHONDRITE: an example of a
differentiated meteorite, an igneous rock
formed when an asteroid melted 4.5
billion years ago.
(From Reckling Peak,
Antarctica).
• IRON: another differentiated meteorite,
probably from the core of a large asteroid
that broke apart. (Found at Derrick Peak,
Antarctica).
• Now identify the samples in the room!
Images: NASA/JPL, with additional info from Calvin J Hamilton, solarviews.com
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Falls and Finds
• A third way of describing meteorites is based on the way it
is identified on Earth:
• Falls: A fall is a meteorite which has been witnessed to
fall and land on the Earth.
• Finds: A find is a meteorite whose fall is not witnessed,
but is later found lying on the ground.
• We already learned about the L’Aigle fall. Now let’s look at
some more famous falls and finds.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Allende Meteorite Fall
• A loud explosion accompanied a massive meteorite fall which
occurred in the early morning of February 8th, 1969 in Chihuahua,
Mexico.
• Thousands of meteorites were scattered over 130 sq miles, and over
3 tons of fragments were eventually recovered.
• Fortuitously, this occurred just before
the Moon landings, and facilities were
being built across the country to store
and study extra-terrestrial rocks.
• The Allende meteorite, dated at 4.56
billion years old, is the oldest known
remnant we have from the original
nebula.
Picture: space.com
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Murchison Meteorite
• Shortly after the first man walked on the Moon, on September 28th
1969, another huge meteorite fall landed on Murchison, Australia.
• Fragments were again scattered over a wide area, and about 100
kilograms of debris was collected. Again, the Apollo facilities proved
invaluable to decoding the history of the rocks.
• The Murchison pieces belonged to the
category of primitive meteorites called
carbonaceous chondrites, a carbon and
water rich rock.
• Analysis revealed the presence of amino
acids, organic molecules which are also
the building blocks of life. This was the
first identification of these chemicals
outside the Earth.
Picture: space.com
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Barringer Crater
• The Barringer crater, also called the “Meteorite Crater” in Arizona was
formed by the impact 50,000 years ago of a 300,000 ton iron
meteorite.
• The crater is 550 feet deep and nearly a mile wide. About 30 tons of
meteoritic iron have been recovered from the surrounding plains.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Happy Hunting Ground:
The Antarctic (Finds)
• Antarctic ice sheets near to topographical features (mountain ranges)
have proved to be very fruitful grounds for hunting meteorites.
• Antarctic ice sheets are particularly well suited to identifying falls: the
fragments stand out clearly against the blue ice.
Picture: (left) NASA/JSC (right) Dr Ursula Marvin, Smithsonian Astrophysical Observatory
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Antarctic Meteorites: Mechanism
• The figure below shows the mechanism of meteorite concentration
proposed by Kezio Yanai of the Japanese Nat. Inst. Of Polar
Research, in Tokyo.
• Meteorites fall on the
ice and are buried by
accumulation. The ice
moves: most of the falls
end up in the sea.
• But some ice comes
against a natural
barrier: mountains
ranges, and is forced to
the surface, where it is
exposed by winds.
Picture: Dr Ursula Marvin, Smithsonian Astrophysical Observatory, Cambridge, MA
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Allan Hills
• The Allan Hills in Eastern Antarctica forms a natural barrier to the flow
of ice into the Ross Sea.
• American
parties started
searching
there in 1976.
• In 1984 it
produced one
of the most
famous
meteorites of
all:
ALH84001.
Picture: Dr Ursula Marvin, Smithsonian Astrophysical Observatory, Cambridge, MA
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Nomenclature
• Meteorites are usually named for a town or region where they fell or
were recovered:
e.g. the ‘Allende Meteorite’ refers to the collection of Allende stones.
• However, in the Antarctic, where the number of objects is very large, a
three-part designator is used: e.g. ALH84001.
 The initial letters refer to the region: ALH=Allan Hills.
 The next two digits give the year: 84 = ‘1984’.
 The last three digits represent the order in which the objects
were found, so: 001 = first find in 1984, Allan Hills.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Falls vs Finds
• The table below summarizes the statistics of all 4660 meteorites
recovered between 1740 and 1990, excluding those found in
Antarctica. (Table credit: Vagn. F. Buchwald)
TYPE
FALL %
FIND %
FALL
WEIGHT (kg)
FIND
WEIGHT (kg)
Stony
95
79.8
15200
8300
Stony-Iron
1
1.6
525
8600
Iron
4
18.6
27000
435000
• What do you think is the reason for the difference in the numbers
between column 2 and column 3?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Where Do Meteorites Come From?
• We have talked loosely about some meteorites being primitive
remnants of the formation of the solar system, possibly from
asteroids, but how do we know this?
• You might expect that we could tell from composition. But until
recently, we had never visited an asteroid, and our knowledge of their
composition came from meteorites, not the other way round!
• In several rare cases, we have actually been able to photograph the
path of a meteorite (not meteor) through the sky, and then find the
object.
• The Peekskill meteorite of October 9th 1992 is notable for several
reasons, not the least because a piece landed on a car! But more
importantly, its fall was video recorded by 16 different people.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Meteorite Orbits
• When the trajectories for
well-recorded falls were
re-constructed, they
showed that the
meteorites had traveled
on highly elliptical orbits
from the asteroid belt to
cross the Earth’s path.
• A total of seven original
meteorite orbits have
now been determined.
• Now lets examine the various meteorite types in more detail.
Picture: University of Western Ontario/NASA JSC
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Primitive Meteorites:
Chondrites and Chondrules
• Primitive meteorites are also called chondrites, after the small round
inclusions called chondrules which they often contain.
• Chondrules appear to be frozen droplets of melted material, and are
about 1 mm in size. Not all primitive meteorites contain chondrules.
• Chondrules can be rock or
metal. This rock shows a
chondrule of olivine, the rim of
which is stained by iron oxide
from terrestrial weathering.
Picture: J.M. Derochette
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Primitive Meteorites (i)
• Composition:
Apart from some volatile gases (H, He, Ar etc) their inventory is very
similar to the Sun. Almost unchanged from the beginning of the solar
system.
• Appearance:
Primitive meteorites are dark to light grey rocks, perhaps with a
darker crust produced by ‘baking’ during the descent through the
atmosphere. Many are breccias.
• Density:
About 3 g/cm3, similar to terrestrial rocks.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Primitive Meteorites (ii)
• Iron content:
Many contain metallic iron grains, 10-30% by weight. This may also
lead to a classification scheme: H=high, L=low, LL=very low iron.
• Also:
Silicon, Oxygen, Magnesium and Sulfur are next most abundant
elements after iron.
• Age:
Primitive meteorites show a uniform age of 4.55 to 4.56 billion years:
so all formed at the same time. None from outside the solar system!
Is that a likely possibility in your opinion?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Carbonaceous Chondrites (i)
• These are a special class of primitive meteorites, which contain very
little metallic iron and high carbon content, several percent by weight.
• Dark grey to black in color.
• Density is about 2.5 g/cm3, less than other typical meteorites.
• Low density due to high volatile content, e.g. water.
• Question: what does this tell us about their place of formation in the
nebula, relative to iron meteorites?

Answer: they formed in a cooler part of the nebula, further from
the Sun, than the other denser types.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Tagish Lake Fall
• A famous fall which occurred in the Yukon Territory of Canada on
January 18th 2000.
• The meteorite was a carbonaceous chondrite, very fragile and
crumbly to the touch, with a charred exterior.
• Around 500 pieces were recovered
without skin contact, made easy by
their contrast sitting on the snow of a
frozen lake. The piece (right) has
been sealed in nitrogen.
• Why might a lack of skin contact be
important?
Picture: Planetary Society
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Carbonaceous Chondrites (ii)
• A specialty of some carbonaceous meteorites is the evidence that
they have been processed by liquid water.
• We see minerals which have been dissolved from the rock and redeposited in mineral grains.
• But how did liquid water flow in
these small cold rocks?
• We believe the parent meteorite
bodies must have been sufficiently
warm and pressured at some
period in history for liquid water
to flow.
(right: Fusion crusted fragment: Orgueil, France).
Picture: NEMS/Meteorlab.com
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Organics Molecules From Space
• Many carbonaceous meteorites contain complex hydrocarbon
molecules, called organics, because once they were thought to be
produced only by living organisms.
• Coal and oil are common hydrocarbons on the Earth, but these are
truly ‘organic’ in origin. Limestone (calcium carbonate) is the most
common non-biological ‘organic’ example.
• But are the organics in meteorites biological in origin?
• No: we can show that certain non-biological chemistry can produce
them. But we still have the intriguing idea of whether organics from
space gave a kick-start to the rise of life on Earth.
Picture: NEMS/Meteorlab.com
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Amino Acids
• The Murchison meteorite yielded the first signs of amino acids outside
the Earth. Amino acids are the building blocks of nucleic acids (DNA
and RNA) and life on Earth. 74 separate amino acids were identified.
• How do we know that the amino
acids are not contaminants from
Earth biology?
(below: Murchison Meteorite)
• Firstly, the many of the amino
acids found are rare on the Earth.
• Secondly, amino acids produced
biologically occur in only one
chiral form (a kind of mirror-image
symmetry), the left, whereas the
meteorite showed equal numbers
of both left and right forms.
Picture: NEMS/Meteorlab.com
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Chirality
• Chiral molecules occur in two forms: left-handed and right-handed.
• This is a spatial property of a molecule that has no plane of symmetry.
• A good example is a glove: you can take a left-handed glove, and no
matter how much you rotate it, the left glove will never fit a right hand!
• The two chiral forms of 2-butanol
are shown here.
• If you had a model, you could try
turning it yourself to prove that the
two forms are indeed different.
• Only left-handed forms will ‘work’
in biological reactions. Can you
see a problem for drug makers?
Graphics: Steven P Wathen, Siena Heights
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Interplanetary Dust
• Interplanetary dust particles (or Brownlee particles) are small pieces
of space dust which ‘rain’ down on the Earth as it orbits the Sun.
• These are typically no larger than the width of a human hair
• These are not true
‘meteorites’, being
cometary in origin.
• Their unusual deuterium
signature indicates that
they are interstellar in
origin, dust grains of the
origin primordial nebula.
Picture: UCAR/ U Michigan
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Quiz - Summary
1. What is the difference between a meteor and a meteorite?
2. What are the three main classes of meteorites, based on
composition?
3. What are the three main classes of meteorite, based on history/
evolution?
4. What other categories do we use?
5. Name three famous meteorites, and say what they are famous for.
6. What does the designation ALH85010 mean?
7. Why are the Allan Hills in Antarctica a good place to hunt for
meteorites?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Quiz - Summary
8. For some meteorites, the orbits have been determined. Describe
these orbits in general.
9. What visual identifying feature is normally present in primitive
meteorites?
10. What are these features?
11. What is a Breccia?
12. Why are carbonaceous meteorites especially interesting?
13. What is a chiral molecule, and what is the chirality of organics
derived from carbonaceous meteorites?
14. What is a Brownlee particle?
Dr Conor Nixon Fall 2006