Arthur Ross Hall of Meteorites American Museum of Natural History



Origins Section
NOTE: Site is under construction.

Meteorites: Clues to our Origins
Origins: The history of the solar system is told in meteorites.
The science of this Hall begins with the context and scale provided by the Hall of the Universe. Our galaxy is about 10 billion years old. Our solar system formed about 4.6 billion years ago. Here we display some of the first rocks formed in the solar system.
This section is about the History of the Solar System, because these 4568 million year old meteorites are the only samples we have left over from that time. The Museum is a leader in the science of reading the record in these rocks.
Our solar system formed from a disk of dust and gas. We have some of that very ancient dust in our Hall, in a vial of microscopic diamonds taken out of the Allende meteorite. These grains formed in supernova explosions, and are older than our sun. They are the oldest objects in New York City, and in this Museum.
Also in this section, we discuss the chemistry of the solar system. Most meteorites come from asteroids. Here we will display meteorites which might be pieces not of asteroids but of comets.

Allende (feature)
"Drops of Fiery Rain"
Chondrules... It was once thought that meteorites came from the sun. Some meteorites, the chondrites, formed almost at the same time as the sun, from the 'leftover' material which did not enter the sun or planets.
Free-floating Objects in Space
Chondrules give "chondrites" their name, even though the chondrites with compositions most like the sun do not contain chondrules! The minerals in chondrules are rich in magnesium, silicon, and iron.
Most chondrules were once partially or totally molten, and cooled to spherical shapes, like glassy beads, while they were freely floating in space. Then they...
The Oldest Rock
Some very old rocks which can still be found on the Earth's surface are about 4.2 billion years old.
Rocks recovered from the moon are about 4.4 billion years old. The refractory white inclusions found in chondrites date to 4.56 billion years ago, making them the oldest rocks formed in our solar system.

How do we know?
A very very small portion of the elements in all these rocks are radioactive.
These are 'unstable isotopes'. Atoms of these "parent" isotopes release part of their nuclear material, to 'decay' into stable "daughter" elements. Radioactive decay of isotopes of uranium and other elements occurs slowly, over very long time periods.
By measuring the amounts of both the unstable and stable isotopes, the time over which the decay occurred can be known.
(This text is very, very incomplete)
Other topics:
Condensation of solids from the vapor phase.
Refractory inclusions are rich in calcium, aluminum, and other elements which melt at very high temperatures, so they are also called "CAIs". They appear as white-colored objects in the Allende meteorite. Some are large, like the one shown at left.
CAIs are the first objects which formed in our solar system. Not all CAIs were once molten. Many condensed directly from the gas to solid state.

Stardust
Matrix dust is the carbon-rich, extremely fine-grained material between chondrules and refractory inclusions in carbonaceous chondrites. The matrix contains nearly all the water molecules, carbon, and other volatile elements in chondrites. Many large organic molecules, such as amino acids, have been isolated from the matrix of carbon-rich chondrite meteorites. The minerals in matrix are thought to be dust which was accumulated in the meteorites as they grew into small, early planets.

Mineral grains, which formed before our solar system, have been isolated from chondritic meteorites.
How do we know?
The chondrites are the oldest rocks formed in the solar system.
The pre-solar grains contain isotopes of many elements. Isotopes of elements are formed in nuclear reactions, which occur in stars. The ratios of abundances of isotopes in these grains are so unlike those in our sun, Earth, or any other solar system materials, that they could only have formed in other stars, with isotopic abundances very different from our own.
Other topics:
How were these presolar grains found?
How did the grains get into the meteorites in the first place?
What do the grains tell us about other stars? about the formation of our solar system?
Other types of dust exist between the planets, dropped by comets and asteroids. This is the 'zodiacal dust' seen by astronomers. A great deal of this dust falls to Earth every year.

Element Abundances


Abundance of the Elements
Carbonaceous chondrites have elemental abundances nearly identical to those observed in the Sun, except for elements that occur primarily as gases.
The elements were not separated, one from another, before the chondrites were formed.

Carbon-rich (carbonaceous) chondrites are made of dust and small objects left over from the formation of the solar system. Most of the solar system is in the sun. The rest is in planets, moons, asteroids, and comets.
Material in the early solar system accreted into larger and larger objects, called planetesimals. The carbonaceous chondrites are from planetesimals which were not large enough to ever melt.
The carbonaceous chondrites most like the sun in composition do NOT contain chondrules, or high-temperature inclusions. They are made only of matrix, and their oxygen isotope signatures are like that of the Earth.
Accretion: What are chondrites made of?

Element Abundances

Chondrule from the ordinary chondrite Bjurbole.
Large colorful grains are the mineral olivine Mg2SiO4, and dark material in between is silicate liquid cooled quickly to make a glass.

Free-floating Objects in Space
Chondrules give "chondrites" their name, even though the chondrites with compositions most like the sun do not contain chondrules. The minerals in chondrules are rich in magnesium, silicon, and iron.
Most chondrules were once partially or totally molten, and cooled to spherical shapes, like glassy beads, while they were freely floating in space. Then they...
Refractory inclusions are rich in calcium, aluminum, and other elements which melt at very high temperatures, so they are also called "CAIs". They appear as white-colored objects in the Allende meteorite. Some are large, like the one shown at left.
CAIs are the first objects which formed in our solar system. Not all CAIs were once molten. Many condensed directly from the gas to solid state.
Matrix dust is the carbon-rich, extremely fine-grained material between chondrules and refractory inclusions in carbonaceous chondrites. The matrix contains nearly all the water molecules, carbon, and other volatile elements in chondrites. Many large organic molecules, such as amino acids, have been isolated from the matrix of carbon-rich chondrite meteorites. The minerals in matrix are thought to be dust which was accumulated in the meteorites as they grew into small, early planets.

[Scientists have proposed a great many theories for how chondrules and CAIs formed. No single theory explains all the many things we know about them.]


Anatomy of a Chondrite

Anatomy of a Chondrite


Reading the Rocks
Geologists cut rocks from Earth to discover what they are made of and how they formed. Meteoriticists do the same with meteorites.
Slicing
Meteoriticists cut meteorites into sections, then make polished, translucent thin sections which can be viewed through a microscope.
Microbeam Analysis
Using ion beam instruments, the chemical and isotopic compositions of tiny mineral grains can be learned. From these results, scientists deduce how the solar system formed.
(THIS IS TOO COMPLICATED)
Age Dating
The ages of chondrules and refractory inclusions can be learned by chemical analysis, after they are separated from the rest of the rock. Their ages tell the sequence of events which occurred in the early solar system.

"Fly Through" or Model

Parent Bodies

Carbonaceous Chondrites


Oxygen Isotopes


This diagram shows a fundamental difference between the types of carbonaceous chondrites. It is central to all ideas about the origin of the solar system. What does it say?
There are three stable, naturally occurring isotopes of oxygen: oxygen 16, 17, and 18. Since 16 is the most abundant, the ratios of 17 and 18 to 16 in meteorite samples are compared with the same ratios in ocean water, plotted where the dashed lines cross at zero.
Oxygen isotopes are the key to distinguishing between meteorite types. All material on the Earth and moon falls along a single line (blue). Many carbonaceous chondrites, in contrast, retain minerals which have excesses of oxygen 16.
Why? Scientists do not know. The GENESIS mission will sample the solar wind in 200_. Comparing the oxygen isotopes in the solar wind to those in meteorites will answer many questions.
Although a number of theories have been proposed to explain these chemical differences, no one theory fits all the available facts.

Solar System Chemistry
Chondrites are pieces of larger bodies, primarily asteroids in orbit around the sun. Two themes of the history of the solar system is the accumulation of small things to make big things, and the chemical changes with distance from the sun.
The solar system has a chemical structure.
The inner planets, and the asteroids are rocky, like the Earth. The comets, and the outer planets such as Jupiter, are rich in frozen gases like methane, carbon dioxide, and water.

[There is not enough known about the asteroids for scientists to definitively associate particulare types of chondrite with specific asteroids. This has only been done for asteroids studied by spacecraft, such as 433-Eros, target of the highly successful US near-earth asteroid renezvous (NEAR) mission of 2001.]

Building Planetesimals and Planets
Exactly how did the chondrules, refractory inclusions, dusty matrix material, and icy material accumulate into larger bodies, and how did these accumulate into small planets (planetesimals)?
Astrophysicists can model how the process might have occurred, but we can only look to meteorites and their parent bodies, the asteroids, for hard evidence.

Asteroidal Evidence
We have only begun to collect evidence from the asteroid belt. Many asteroids appear to be loosely bound "rubble piles" with low density. Others appear to be the fragmented cores, mantles, and crusts of small planets that were once completely melted. Together, the asteroids record the process of planet formation , frozen in time.
Heating Planetesimals
When the solar system was very young, 4.5 billion years ago, many radioactive isotopes had not decayed to the stable isotopes we see today. The decay of short-lived isotopes such as Aluminum 26 produced heat. The decay products of these isotopes, such as 26 Magnesium, can be found in refractory inclusions today. When two objects collide in space, and one is larger than the other, the smaller one usually becomes part of the larger one. Part of the kinetic energy of impact becomes heat.
Big Bodies Melt
When planetesimals are large, they trap much of the heat produced by impacts and by radioactive decay of radioactive isotopes. The parent asteroids of the chondrites did not melt. The differentiated meteorites come from planetesimals which were, at one time, completely melted. When planetesimals melt, heavy elements like iron sink to the core, while light elements like silicon rise to the surface.


Ordinary and Enstatite Chondrites
Other meteorites have chondrules in them, but the abundances of elements in them is less like the abundances in the sun. These meteorites are the ordinary and enstatite chondrites. Scientists think that these meteorites are related to the carbonaceous chondrites by fractionation of elements such as oxygen, sulfur, iron, and carbon, during processes which may include condensation, evaporation, mechanical separation, and/or crystallization.

Enstatite & Ordinary Chondrites


NOTE: Site is under construction.

Oxide & Reduced Iron


What does this diagram say?
Iron is a very important element in the cosmos. Iron can occur in an oxidized form, like rust, where it combines with oxygen. Iron can also occur in a reduced form, as metal or combined with sulfur. This diagram compares these two forms of iron, reduced and oxidized, and also shows the total amount of iron in each type of chondritic meteorite.
Total iron: The diagonal dotted line corresponds to the total amount of iron in the sun, where iron occurs as vapor. Any pair of meteorites having the same ratio of total iron, compared to silicon, will fall on a diagonal line parallel to the one shown. Notice that the H (high iron) chondrites, and the carbon-rich chondrites, all fall on the line corresponding to the solar iron content. The L (low iron) and LL (very low iron) chondrites do not.
Reduction of iron: This plot illustrates another important fact. The ordinary chondrites, and the enstatite chondrites, differ from the carbon-rich (carbonaceous) chondrites because their iron is less oxidized. The carbonaceous chondrites are most like the sun in their over-all compositions. The ordinary and the enstatite chondrites are differentiated. They are lower in oxygen content, and the L and LL types are deficient in iron, measured relative to silicon.
Although a number of theories have been proposed to explain these chemical differences, no one theory fits all the available facts.

Planetary Breakup

Planetary Breakup





Last revised: 10-Sep-03 (DSE)