Mars meteorites are rare and scientifically significant rocks that originated on the Red Planet. These fragments of Mars’s surface or crust were ejected into space by powerful impact events. After traveling through the solar system, they eventually landed on Earth, providing scientists with the only physical samples of Mars available for direct study. As of September 2020, approximately 277 meteorites have been classified as Martian, representing less than half a percent of all classified meteorites found on Earth.
The Journey to Earth
The journey of a rock from Mars to Earth begins with a forceful asteroid or comet impact on the Martian surface. These impacts must be powerful enough to launch material with sufficient velocity to escape Mars’s gravitational pull, which is about one-third as strong as Earth’s. The thin Martian atmosphere, about 125 times less dense than Earth’s, also makes it easier for ejected material to escape.
Once ejected, these Martian fragments embark on an interplanetary voyage, orbiting the Sun for millions of years. Their trajectories are influenced by the gravitational forces of various planets. Eventually, some of these rocks intersect Earth’s orbit, entering our atmosphere as meteorites. They must then survive the intense heat and pressure of atmospheric entry before landing on Earth’s surface.
Unmasking Martian Identity
Scientists employ several methods to confirm a meteorite’s Martian origin. One technique involves analyzing gases trapped within the meteorite, often in tiny bubbles within shock-formed glass. The composition of these trapped gases, particularly noble gases like neon, argon, and xenon, provides a unique “fingerprint” that matches Mars’s known atmospheric composition, as measured by NASA’s Viking landers in the 1970s. This isotopic signature confirms their Martian origin.
Beyond trapped gases, mineralogical and geochemical signatures offer additional clues. Martian meteorites exhibit mineral compositions and isotopic ratios characteristic of Martian geology. For example, some contain minerals like augite and olivine crystals, formed from basaltic magma. Scientists also analyze oxygen isotope ratios, which differ significantly between terrestrial, lunar, and Martian rocks, helping to group meteorites from the same parent body.
Age dating techniques, such as radiometric dating, further support the Martian classification. Unlike most meteorites from the asteroid belt, which date back to the early solar system around 4.56 billion years ago, many Martian meteorites are geologically much younger. For instance, nakhlites formed from basaltic magma between 1.416 billion and 1.322 billion years ago, while shergottites show crystallization ages as recent as 150 to 475 million years ago. These young ages imply they originated from a geologically active planet like Mars, capable of sustaining volcanic activity more recently.
Insights into the Red Planet
Studying Mars meteorites provides direct evidence about the Red Planet’s geological history. These samples, predominantly igneous rocks from volcanic regions, offer insights into past volcanic activity, crustal formation, and internal processes. Shergottites, which account for about 90% of all Martian meteorites, show Mars was geologically active between 150 and 475 million years ago. Analysis of individual meteorites, like Governador Valadares, reveals details about slow crystallization of lava underground.
Many Martian meteorites show evidence of past water activity on Mars. For example, the Lafayette Meteorite contains minerals that formed through interaction with liquid water about 742 million years ago. This suggests that while abundant surface water may not have been present, melting subsurface ice, or permafrost, likely occurred due to magmatic activity. The “Black Beauty” meteorite (NWA 7034), discovered in the Sahara Desert, contains ten times more water than other Martian meteorites, and its composition matches the Martian crust. Water-rich fluids from 4.45 billion years ago have also been identified in a zircon grain within Black Beauty, suggesting water was present during early Martian magmatic activity.
The findings from some Martian meteorites, such as ALH84001, have fueled discussions about Mars’s potential for life. This meteorite, found in Antarctica, contained what some initially interpreted as fossilized Martian life forms and organic compounds like polycyclic aromatic hydrocarbons (PAHs). While initial claims of biosignatures in ALH84001 have largely been refuted, the presence of organic nitrogen in ancient Martian groundwater, as indicated by some meteorites, suggests environments with ingredients for life could have existed.
Finding Mars Meteorites
Mars meteorites are primarily discovered in two environments: the ice sheets of Antarctica and hot deserts, such as the Sahara Desert. These locations are ideal for meteorite preservation and discovery. The dry, stable conditions of deserts limit weathering and erosion, allowing meteorites to remain well-preserved.
In Antarctica, continuous ice accumulation acts as a natural conveyor belt, concentrating meteorites in specific “stranding zones” where ice flows encounter obstacles like mountains. The dark color of meteorites contrasts sharply against the white ice, making them easier for meteorite hunters to spot. Programs like the Antarctic Search for Meteorites (ANSMET) have been instrumental in recovering many Martian meteorites from these icy landscapes.