• Scientists have identified more than 52,000 meteorites—space rocks that have crashed into Earth. 
    Of these rocks, fewer than 100 have been traced to the planet Mars, one of our closest neighbors in the solar system
    Scientists didn’t know these strange space rocks came from Mars until NASA’s Viking spacecraft successfully landed on the Red Planet in the 1970s. The Viking space probes measured chemicals in the Martian atmosphere and surface. Scientists realized many meteorites found on Earth contained the same, precise concentrations of rocks, minerals, and even trapped gases. These rocks could only have come from Mars.
    Martian meteorites were formed as asteroids and other space rocks crashed into the Martian surface millions of years ago. These collisions formed craters and sent tons of dust and rocks into the Martian atmosphere. Some collisions were so violent that debris was forced out of Mars’ gravitational field altogether in a process called spallation.
    Ejected Martian rocks drifted in space for millions of years before being pulled into Earth’s gravitational field. Astronomers know this because they measure the cosmic ray exposure (CRE) of Martian meteorites. In outer space, asteroids and other space rocks are exposed to high-energy nuclear particles called cosmic rays. Cosmic rays turn some elements found in space rocks into unstable isotopes of those elements, which decay at very predictable rates. This radioactive decay allows scientists to accurately estimate the amount of time space rocks spent between their residences in the atmospheres of Mars and Earth.
    Major collisions that create meteorites are much more rare now than they were in the early solar system. But even today, Martian meteorites are still blazing into Earth’s atmosphere as “shooting stars”—and dozens, and maybe thousands, of meteorites are probably unidentified or undiscovered in the barren deserts of Antarctica and the Sahara, where most meteorites are found.
    • Rare, ungrouped “other” Martian meteorites have unusual characteristics. The most famous Martian meteorite in the world, ALH 84001, for instance, is classified as an “orthopyroxenite” for its unique crystal silicate structure. ALH 84001 is less famous for its geology, however, than its possible biology—it may preserve fossils of primitive bacteria, providing evidence for life on Mars more than 4 billion years ago.
    • Chassignites are named after the town where the first sample was found, Chassigny, Haute Marne, France. Very few chassignites have been identified, but all are rich in the mineral olivine. The olivine chemical structure leads scientists to think chassignites formed in the Martian mantle, not its crust. Chassignites are about 1.4 billion years old.
    • Shergottites are named after the village where the first sample was found, Sherghati, Bihar, India. Shergottites are mafic rocks, meaning they are rich in magnesium and iron. They are the most abundant type of Martian meteorite, and also among the youngest. Scientists think shergottites may have formed as recently as 165 million years ago.

    Astronomers and geologists have classified Martian meteorites into four major categories: shergottites, nakhlites, chassignites, and an oddball “other” or “unclassified” group.

    • Nakhlites are named after the suburb where the first sample was found, Nakhla, Alexandria, Egypt. Nakhlites are volcanic rocks rich in the silicate mineral augite. They also provide evidence of abundant liquid water on Mars at some point in the planet’s history. Nakhlites are about 1.3 billion years old.
  • Term Part of Speech Definition Encyclopedic Entry
    accurately Adverb

    exactly or perfectly.

    Antarctic Desert Noun

    dry, barren rocks covered by an ice sheet that makes up most of the continent of Antarctica.

    asteroid Noun

    irregularly shaped planetary body, ranging from 6 meters (20 feet) to 933 kilometers (580 miles) in diameter, orbiting the sun between Mars and Jupiter.

    astronomer Noun

    person who studies space and the universe beyond Earth's atmosphere.

    atmosphere Noun

    layers of gases surrounding a planet or other celestial body.

    Encyclopedic Entry: atmosphere
    bacteria Plural Noun

    (singular: bacterium) single-celled organisms found in every ecosystem on Earth.

    barren Adjective


    biology Noun

    study of living things.

    chassignite Noun

    Martian meteorite composed largely of the mineral olivine. Also called olivine achondrite.

    collision Noun


    concentration Noun

    measure of the amount of a substance or grouping in a specific place.

    cosmic ray Noun

    radiation originating in outer space and consisting mostly of high-energy atomic nuclei.

    crater Noun

    bowl-shaped depression formed by a volcanic eruption or impact of a meteorite.

    Encyclopedic Entry: crater
    crust Noun

    rocky outermost layer of Earth or other planet.

    Encyclopedic Entry: crust
    crystal Noun

    type of mineral that is clear and, when viewed under a microscope, has a repeating pattern of atoms and molecules.

    debris Noun

    remains of something broken or destroyed; waste, or garbage.

    desert Noun

    area of land that receives no more than 25 centimeters (10 inches) of precipitation a year.

    Encyclopedic Entry: desert
    dust Noun

    tiny, dry particles of material solid enough for wind to carry.

    Encyclopedic Entry: dust
    eject Verb

    to get rid of or throw out.

    element Noun

    chemical that cannot be separated into simpler substances.

    estimate Verb

    to guess based on knowledge of the situation or object.

    gas Noun

    state of matter with no fixed shape that will fill any container uniformly. Gas molecules are in constant, random motion.

    gravitational field Noun

    influence that a massive object extends around itself, in which another massive object would experience an attractive force.

    mafic Adjective

    having to do with igneous rocks that contain large amounts of iron and magnesium.

    mantle Noun

    middle layer of the Earth, made of mostly solid rock.

    Encyclopedic Entry: mantle
    measure Verb

    to determine the numeric value of something, often in comparison with something else, such as a determined standard value.

    meteorite Noun

    type of rock that has crashed into Earth from outside the atmosphere.

    Encyclopedic Entry: meteorite
    mineral Noun

    inorganic material that has a characteristic chemical composition and specific crystal structure.

    nakhlite Noun

    Martian meteorite composed largely of the silicate mineral augite.

    olivine Noun

    type of silicate mineral.

    orbit Noun

    path of one object around a more massive object.

    outer space Noun

    space beyond Earth's atmosphere.

    particle Noun

    small piece of material.

    planet Noun

    large, spherical celestial body that regularly rotates around a star.

    Encyclopedic Entry: planet
    precise Adjective


    predictable Adjective

    regular or able to be forecasted.

    primitive Adjective

    simple or crude.

    radioactive decay Noun

    transformation of an unstable atomic nucleus into a lighter one, in which radiation is released in the form of alpha particles, beta particles, gamma rays, and other particles. Also called radioactivity.

    residence Noun

    home or place where a person lives.

    rock Noun

    natural substance composed of solid mineral matter.

    Sahara Desert Noun

    world's largest desert, in north Africa.

    shergottite Noun

    Martian meteorite composed mostly of mafic and ultramafic rocks.

    shooting star Noun

    rocky debris from space that enters Earth's atmosphere. Also called a meteor.

    silicate Noun

    most common group of minerals, all of which include the elements silicon (Si) and oxygen (O).

    solar system Noun

    the sun and the planets, asteroids, comets, and other bodies that orbit around it.

    spacecraft Noun

    vehicle designed for travel outside Earth's atmosphere.

    space probe Noun

    set of scientific instruments and tools launched from Earth to study the atmosphere and composition of space and other planets, moons, or celestial bodies.

    spallation Noun

    process in which fragments of material (spall) are ejected from a larger body due to impact or stress.

    unstable isotope Noun

    atom with an unbalanced number of neutrons in its nucleus (isotope) that is radioactive, or decays by emitting particles from its nucleus. Also called a radionuclide.

    volcanic Adjective

    having to do with volcanoes.