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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.
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- 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.
- 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.
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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 unproductive.
biology Noun study of living things.
chassignite Noun Martian meteorite composed largely of the mineral olivine. Also called olivine achondrite.
collision Noun crash.
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 exact.
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.
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Writer
Caryl-Sue, National Geographic Society
Editor
Melissa MacPhee, National Geographic Society
Producer
Caryl-Sue, National Geographic Society
Last Updated
Oct. 16, 2015
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