ASTR 330: The Solar System

ASTR 330: The Solar System

Lecture 6 : Review Quiz

Dr Conor Nixon Fall 2006

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?


Announcements


• HW assignment #2 due back on Tuesday, Sept 26th.


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?


Lecture 6:

Meteorites (I)



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.


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


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.


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 re-solidified. 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.


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


Examples


• Now identify the samples in the room!

Images: NASA/JPL, with additional info from Calvin J Hamilton, solarviews.com

• 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).

• CHONDRITE: a primitive meteorite which formed at the same time as the planets, 4.55 billion years ago. (From the Allan Hills, Antarctica).


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.


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.

Picture: space.com

• 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.


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.

Picture: space.com

• 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.


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.


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


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.

Picture: Dr Ursula Marvin, Smithsonian Astrophysical Observatory, Cambridge, MA

• 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.


Allan Hills


• The Allan Hills in Eastern Antarctica forms a natural barrier to the flow of ice into the Ross Sea.

Picture: Dr Ursula Marvin, Smithsonian Astrophysical Observatory, Cambridge, MA

• American parties started searching there in 1976.

• In 1984 it produced one of the most famous meteorites of all: ALH84001.


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.


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?


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.


Meteorite Orbits


• Now lets examine the various meteorite types in more detail.

• 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.

Picture: University of Western Ontario/NASA JSC


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.

Picture: J.M. Derochette

• 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.


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.


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?


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.


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


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 re-deposited 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


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.



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?

• 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.

(below: Murchison Meteorite)



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


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

Picture: UCAR/ U Michigan

• 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.


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?


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?

Documents

ASTR 330: The Solar System