Finding and analyzing Martian meteorites on Earth is just one piece of the puzzle. Explaining how they got here is another. Today, scientists think they know how a typical Martian meteorite journeys to Earth. Here's what may (emphasis on the "may") have happened to ALH84001:
- About 4.5 billion years ago, just when a cataclysmic space impact was forming the Earth-moon system, the rock crystallized from magma below the surface of Mars.
- Half a billion years later, a period of heavy bombardment fractured the Martian surface, allowing water to seep underground and stimulate chemical reactions that led to the formation of carbonate minerals.
- There the Martian rock remained, safe and happy, until a comet or asteroid struck the planet about 16 million years ago. The blast sent a piece of the rock hurtling into space.
- After traveling in its own lonely orbit around the sun for 15 million years, the rock entered Earth's atmosphere and fell onto the ice of Antarctica.
It doesn't require a lot of imagination to embellish this story. If water did indeed flow on Mars, perhaps the planet had an environment suitable to life. And if life developed on Mars, perhaps a few cells or spores hitched a ride on a blast-ejected rock. The question then becomes: Could living matter survive a long interplanetary journey, where cosmic rays wreak havoc on biological molecules such as proteins and nucleic acids?
A voyage of 15 million years, similar to the one ALH84001 took, most certainly would devastate any life clinging to the surface of a space rock. Even microbes nestled deep in the core of a meteorite would feel the penetrating effects of X-rays and gamma rays. But not all Martian meteorites take the slow boat to China (or Africa or Antarctica). Many arrive within a few years of being ejected from their original home. One out of every 10 million reaches Earth in less than a year [source: Warmflash].
So, it's not so far-fetched to think that life evolved first on Mars and then, by way of interplanetary seeding, on Earth. And recent evidence from other NASA missions is making the idea even more tantalizing. In 2008, the Phoenix Mars Lander surprised astronomers when it detected perchlorate in Martian soil. This prompted a team of scientists to add perchlorate to desert soil containing organic compounds and then analyze the sample using quick-heating techniques borrowed from the Viking missions. They learned that perchlorate, in the Viking tests, may have destroyed or masked important organic compounds linked to processes carried out by living things.
All of the questions remain, but one thing is clear: For astronomers studying Martian meteorites, it really is a wonderful life.