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.
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.

Noun

layers of gases surrounding a planet or other celestial body.

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.

Noun

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

Noun

rocky outermost layer of Earth or other planet.

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.

Noun

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

Noun

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

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.

Noun

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

measure
Verb

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

Noun

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

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.

Noun

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

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.