Martian meteorites are rare rocks that formed on Mars, were launched into space by a powerful event, and eventually journeyed to Earth. These stones are our only tangible pieces of the Red Planet, offering a direct way to study its composition in terrestrial laboratories. Their scarcity, with just over 350 identified out of tens of thousands of meteorites, provides a unique window into the geological and potentially biological history of our planetary neighbor.
Identification of Martian Origin
Confirming a meteorite’s Martian origin relies on atmospheric science. During the impact that ejects rocks from Mars, heat and pressure can create glassy inclusions that trap small pockets of the Martian atmosphere. The definitive link was established when analysis of these trapped gases found a precise match to the atmospheric data collected by NASA’s Viking landers in the 1970s.
This atmospheric fingerprint is the primary line of evidence, but it is supported by other chemical clues. The specific ratios of oxygen isotopes in these meteorites are distinct from those found on Earth, the Moon, or in other meteorite types. Martian meteorites are also geologically young compared to most other meteorites, which are ancient remnants from the solar system’s formation. Their younger ages are consistent with a planet like Mars that has experienced more recent volcanic activity.
The Journey from the Red Planet to Earth
The journey of a Martian rock to Earth begins when a large asteroid or comet strikes the planet with enough force to blast surface material into space. This material must exceed Mars’s escape velocity to break free from its gravity. The planet’s thinner atmosphere and lower gravity make it easier to launch rocks into space compared to Earth.
Once in orbit around the Sun, these rocks can journey for millions of years. Drifting through interplanetary space, they are exposed to cosmic radiation before their trajectory intersects with Earth’s. Upon entering our atmosphere, the rock endures a fiery descent, which creates a characteristic dark, glossy fusion crust. The most successful recovery sites for these meteorites are desolate landscapes like the deserts of Northwest Africa and the Antarctic ice sheets, where the dark stones stand out and are well-preserved.
Geological and Atmospheric Clues
The composition of Martian meteorites offers insights into the planet’s geological evolution. The vast majority are igneous rocks, specifically basalts formed from cooled lava, providing direct evidence that Mars was once volcanically active. Scientists categorize these samples into groups known as Shergottites, Nakhlites, and Chassignites, each representing different volcanic events and source regions.
Some meteorites also carry evidence of past water. Certain minerals within the rocks show signs of chemical alteration that could only happen in the presence of liquid water. This discovery confirms that water once flowed or pooled on the Martian surface. By studying the different rock types and their ages, researchers can piece together a history of the Martian crust and mantle, revealing a dynamic planet that has changed significantly over billions of years.
The Famous Hunt for Martian Fossils
The search for life in Martian meteorites gained attention with the study of Allan Hills 84001 (ALH84001), discovered in Antarctica in 1984. In 1996, NASA scientists announced they had found evidence of ancient microbial life within this meteorite. This included organic molecules known as polycyclic aromatic hydrocarbons (PAHs) and tiny magnetite crystals, similar to those produced by some terrestrial bacteria.
The most discussed evidence was electron microscope images of what appeared to be segmented, worm-like structures, interpreted as fossilized Martian microorganisms. These structures, along with small carbonate globules where the other signs were found, formed the basis of the claim. The announcement ignited a scientific debate and re-energized the field of astrobiology.
The scientific community later scrutinized each piece of evidence. It was shown that PAHs can form through non-biological processes and that the magnetite crystals could be formed by the shock of an impact. The nanofossils were met with skepticism, with many arguing they were too small to be life and were likely just mineral formations. While the claims about ALH84001 are now largely attributed to non-biological processes, the controversy spurred a new era of Martian exploration.