Topics covered on this page:

What is a meteorite?

What are the different types of meteorites?

Why are meteorites important to study?

What scientific tools are used to study meteorites?

How do we find meteorites?

What is a meteorite?

Meteorite, an extraterrestrial rock found on the Earth.

Meteorite, an extraterrestrial rock found on the Earth. Photo courtesy: Svend Buhl, meteorite-recon.com

A meteorite is an extraterrestrial rock that survives an impact on the Earth.  Most meteorites are from the asteroid belt, but some originate from the Moon and Mars. The light that is observed as the rock goes across the sky is called a meteor (see Chelyabinsk meteor video).  The light is from the frictional heating of the surface of the rock as it passes through the atmosphere. The heating produces a thin black layer of melted material around the rock called a fusion crust, but if you cut through the rock, you will see the original rock (see image below). Here are two characteristics that help us quickly distinguish meteorites from other rocks on the Earth: (1) The inside of a meteorite looks really different from the outside because of the fusion crust. (2) Meteorites contain varying  amounts of pure metal, which terrestrial rocks do not have. So, since most meteorites are metallic a simple hand magnet can help distinguish whether a rock is a meteorite or a meteor-wrong.  Check out this website for more information on identifying meteorites.

FusionCrust

Credit: photograph (left) by R. Pelisson, SaharaMet; (right) K.-C. Lyncker

Recent News:  In 2005, Mars Exploration Rover-Opportunity found an extramartian rock on the surface of Mars very similar to the ones we find on Earth. This was the first time meteorites have been observed on the surface of a planet other than our own (see image below)!

This is an iron meteorite about the size of a basketball on the surface of Mars.

This is an iron meteorite about the size of a basketball on the surface of Mars. Image credit: NASA/JPL/Cornell

What are the different types of meteorites?

Meteorites are classified into two main groups: (1) primitive and (2) differentiated meteorites.  Primitive meteorites are called chondrites. They are the most abundant type of meteorite.  Chondrites are stony meteorites that have experience very mild heating.  They retain the original structure and composition of the dust that first formed in our Solar System over 4.6 billion years ago.  They contain circular features called chondrules and calcium- and aluminum-rich inclusion (CAIs). Chondrules are made up of olivine, pyroxene, glass and small nuggets of metal and sulfide.  CAIs are high temperature condensates that formed around the Sun early in Solar System history.  They are the oldest know material in the Solar System.  The Sun’s age of 4.6 billion years come from studying isotopes of radioactive elements in CAIs.  Chondrites also consist of matrix, which is the fine-grained material that is between all of the chondrules and CAIs.  The matrix contains presolar grains, which are nano- to submicron-sized particles that condensed around other stars in the vicinity to where the Sun originally formed. Chondrites are separated into various groups depending on their oxygen-isotope composition.  They come from asteroids that did not experience enough heating to destroy  these components.

Chelyabinsk meteorite (left) and Allende meteorite (right)

Chelyabinsk meteorite (left; image credit: http://www.meteorites.ru) and Allende meteorite (right; image credit: Dr. Gary R. Huss)

Optical image (reflected light) of chondrules

This is an optical microscope image ( in transmitted light) of chondrules (whitish, circular features). The matrix is the fine-grained black material around the chondrules. Presolar grains can be found within the matrix of some of primitive chondrites.

These are presolar silicon carbide grains from the matrix of Murchison meteorite.

These are presolar silicon carbide grains from the matrix of Murchison meteorite. Photo credit: Andrew Davis

Differentiated meteorites have experience significant heating that caused melting and separations of the elements (differentiation).   When a planetary body undergoes widespread melting, heavy elements like Fe and Ni sink to form a Fe-Ni core, while the lighter elements form the crust or mantle of the body.  Achondrites are stony meteorites that have experienced melting and recrystallization. They do not have chondrules or CAIs.   Without a fusion crust, a chondrites would be difficult to distinguish from terrestrial rocks.

Stony Irons are differentiated meteorites that consists of a roughly equal amount of rock and metallic iron.  These meteorites are separated into two groups: Pallasites and Mesosiderites.  These meteorites are a result of mixing between the rocky material and iron-nickel metal during the break up of an asteroid from a collision (likely a collision with another asteroid or a larger body).

Stony Irons: mesosiderite (left) and pallasite (right)

Stony Irons: mesosiderite (left; image from http://www.meteorite.com.au) and pallasite (right)

Iron meteorites are differentiated meteorites that are made up of mostly iron-nickel metal.  Iron meteorites come from the cores of asteroids.  When etched with acid, most iron meteorites revealed a cross-hatched pattern called Widmanstatten Patterns.  The width of the Widmanstatten patterns correlate with the cooling rate at the core of the asteroid: the thicker the patterns, the slower the cooling at the core of the asteroid.

Gibeon-iron

Photo courtesy: Svend Buhl, meteorite-recon.com

Why are meteorites important to study?

Chronology of Early Solar System Events

        • CAIs (Calcium- and aluminum-rich inclusions) found within chondrites (primitive meteorites) are the oldest known objects in the Solar System.  CAIs condensed from the hot gas around the Sun. Based the results from several radioisotope systems, CAIs formed about 4.6 billion years ago.  This is how we obtain the age of the age of the Solar System.

Composition of the early Solar System, how it was originally and how it has changed over time

        • Chondrites are similar in composition to the Sun.  So they allow us to study the composition of the Sun without actually going to the Sun.  They also tell us about the original composition of the dust from the disk around the Sun where planets, moons, asteroids and comets formed.  So chondrites have been used to estimate the original composition of all the planets and model what processes changed their compositions over time (e.g., impacts, core formation, weathering, melting, etc).
        • Some chondrites contain presolar grains, which are particles that condensed around other stars and were later incorporated into the Solar System as it formed.
        • Meteorites contain evidence for extinct radionuclides, which are radioactive nuclides (isotopes) that have very rapid decay times.  Some radionulides also formed in another star (or stars) and were later incorporated into the Solar System as it formed.
        • Iron meteorites give us a very good estimate of the composition of the Earth’s core and the cores of other planets. This has been confirmed by seismographs that measure vibrations from off of the Earth’s core.

Meteor impacts impact life on Earth

        • Meteorites give us information on the chemical and physical properties of all asteroids, especially Near-Earth asteroids, which potentially pose a threat to human life on Earth.

What scientific tools are used to study meteorites?

There is a wide variety of tools that are used to study meteorites.  Some are listed here:

        • An optical microscope (also called a petrographic microscope) uses visible light to study the mineralogy of a rock.  It provides a simple, nondestructive and affordable way to characterize meteorites, but a thin section of the meteorite is generally required.  Thin sections are 30 microns thick slices of the rock.  This allows light to pass through the rock.  The light reacts differently to the structure of different minerals, which allows us to distinguish between the various minerals that make up a meteorite.  The microscope can be used with the light underneath the thin section (transmitted light mode).  In transmitted light mode, polarizers can be added to bend the light, producing colors called birefrengence or interference colors.   The light can be used above the thin section  in reflected light mode (See examples images below).  These modes provide different information about the minerals in the meteorite.  With an optical microscope, you can zoom in on a feature up to 100X. Finally, the optical microscope can be equipped with a camera to take pictures (see below).
A chondrule in transmitted-plain light.

A chondrule in transmitted-plain light.

Same chondrule, but in transmitted polarized light

Same chondrule, but in transmitted polarized light

Same chondrule, but in reflected light

Same chondrule, but in reflected light

        • A Scanning Electron Microprobe (SEM) is an electron microscope that uses an electron beam to illuminate the rock.  It is also a nondestructive technique.  Electrons behave similar to light in that they reacts in a particular way depending on mineral structure.  The SEM can be used in scanning electron mode, which is used to study topographical features, or it can be used in backscatter electron mode, which is used to study mineralogy.  For backscatter mode, light elements absorbed more electron than heavy elements.  So in a meteorite with metal and silicates, the metal will appear brighter than the silicates because more electrons are reflected off of the metal than off of the silicates.  SEMs provide much better spatial resolution than optical microscopes and you can zoom in on feature up to 10,000X or more.  Finally, SEMs are commonly equipped with other tools including an energy dispersive spectrometer, which is used to estimate the chemical composition of a mineral, an electron backscatter diffraction detector, which is used to analyze crystal structure of a mineral.  Thin sections are not necessary for SEM analyses.
SEM at NASA Johnson Space Center

SEM at NASA Johnson Space Center. image from http://ares.jsc.nasa.gov/ares/

        • An electron microprobe are nondestructive microscopes that can be used to measure the chemical composition of minerals within a rock by measuring the characteristic X-rays that are emitted from the mineral after it is bombarded with electrons.  It uses both energy-dispersive  and wavelength dispersive spectroscopy.  It has It has better spatial resolution than the SEM.
        • A transmission electron microscope is very similar to the SEM, but it allows the study on nano-scale features.
        • Mass spectrometers are important for studying the isotopic composition of meteorites.  A mass spectrometer separates ions by mass, which allows you to measure isotopes of the same or different elements.  They are destructive techniques, in that some or all of the meteorite is lost after the analysis. There are wide range of mass spectrometers, but the common ones include:
          • Secondary ion mass spectrometry (SIMS): For this technique, a high-energy ion (charged particle) beam (commonly oxygen or cesium) is generated and used to ablate material off the surface of the rock (sputtering). The secondary ion beam produced after sputtering the sample is then sent through a mass spectrometer. The sputtering produces a pit in the sample, but it does not completely consume the rock. So the SIMS is the preferred technique for analyzing precious samples and it is preferred if you want to study the petrology of the sample after you have measured it. The Resonant Ionization Mass Spectrometry is a similar technique.

Secondary ion spectrometer at the University of Hawaii at Manoa

          • Inductively-Coupled Plasma and Thermal Ionization mass spectrometry:  These techniques require chemical digestion of the sample to remove the elements of interested and then to analyze the elements in the form of some kind of solution. This is a completely destructive technique; however, it can provide a lot more precision that the in-situ techniques.
Multicollector Inductively Coupled Plasma Mass Spectrometer at University of Hawaii at Manoa

Multicollector Inductively Coupled Plasma Mass Spectrometer at University of Hawaii at Manoa

      • Synchrotron: These are massive instruments that accelerate electrons to the speed of light. These high energy particles produce light at a wide range of the electromagnetic spectrum (from infrared to x-rays). The Australian Synchrotron website provides a nice basic explanation of how this instrument works: http://www.synchrotron.org.au/index.php/synchrotron-science.  Cosmochemists use synchrotron light sources to study the composition, crystal structure and oxidation state of meteorite components.
Australian Synchrotron

Australian Synchrotron

 How do we find meteorites?

Once you know how to distinguish a meteorite from normal Earth-rocks, you can practically go looking for them in your back yard or on your roof or in the park nearby because meteorites technically fall all over the Earth, randomly. However, deserts are the best places to actually find meteorites because there is little/no vegetation and there is contrast between the meteorites and the landscape.  The two popular deserts for meteorite hunting are (1) the Antarctic and (2) the Saharan Deserts.

The Antarctic Search for Meteorites (ANSMET) organizes annual expeditions to Antarctica to search for meteorites on the ice.  Why is Antarctica such a great place for meteorite hunting? Antarctica is the largest desert on the Earth.  It is easier to spot meteorites on the ice because the meteorites are black rocks and the ice is white.  ANSMET has recovered over than 20,000 meteorites from Antarctica!  My advisor, Gary, went on an ANSMET expedition in the 80s and several other members of the Hawaii Institute of Geophysics and Planetology have participated in these expeditions.

Meteorite hunters in Antarctica

Meteorite hunters in Antarctica. Image from Dr. B. A. Cohen

The Saharan Desert is another great place to search for meteorites. The Sahara is the largest hot desert on Earth.  It covers most of North Africa.

Meteorite found in the Sahara Desert

Meteorite found in the Sahara Desert. Photo courtesy: Svend Buhl, meteorite-recon.com

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