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Lecture 13

ASTA01H3 Lecture Notes - Lecture 13: Star Formation, Carbonaceous Chondrite, Comet Nucleus

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Kristen Menou

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Lecture 13
Most meteoroids are specks of dust, grains of sand, or tiny pebbles.
Almost all the meteors you see in the sky are produced by meteoroids that weigh less
than 1 g.
Only rarely is one massive enough and strong enough to survive its plunge, and reach
Earths surfae.
Such a rock is called a meteorite
Meteorites can be divided into three broad categories.
Iron meteorites-are solid chunks of iron and nickel.
Stony meteorites-are silicate masses that resemble Earth rocks.
Stony-iron-meteorites are iron-stone mixtures.
One type of stony meteorite called carbonaceous chondrites has a chemical
composition that resembles a cooled lump of the Sun gas with the hydrogen and helium
These meteorites generally contain abundant volatile compounds including significant
amounts of carbon and water.
They may have similar composition to comet nuclei.
Heating would have modified and driven off these fragile compounds.
So, carbonaceous chondrites must not have been heated since they formed.
Astronomers conclude that carbonaceous chondrites, unlike the planets, have not
evolved and thus give direct information about the early solar system.
Meteor showers are seen when Earth passes near the orit of an ative or a dead
The meteors in meteor showers are produced by dust and debris released from the icy
head of the comet.
Orbits of many meteorites have been calculated to lead back into the asteroid belt.
Asteroid collisions are the 2nd major source of meteoroids and account for the
background of meteors: in a shower the number of meteors is typically only 4 times
higher than during other times.
Meteorites provide a specific clue concerning the solar nebula: meteorites can reveal
the age of the solar system and the relative ages of its components.
The most accurate way to find the age of a rocky body is to bring a sample into the
laboratory and determine its age by analyzing the radioactive elements it contains.
A few of those elements are radioactive and can decay into other elements, called
daughter elements or isotopes.
The half-life of a radioactive element is the time it takes for half of the radioactive atoms
to decay into the daughter elements.
Measuring the present abundances of the parent and daughter elements allows you to
find the age of the rock.
The oldest Earth rocks so far discovered and dated are tiny zircon crystals from
Australia, 4.4 billion years old
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