NAGGING doubts about a family of Martian rocks found on Earth have been laid to rest after 20 years of investigation. We can now explain why the meteorites are so young and so common.
In the 1980s, scientists found convincing evidence that several meteorites found on Earth came from Mars. Asteroid impacts must have chipped away the rocks before they crash-landed on Earth.
But the 26 Martian meteorites known today aren鈥檛 the chips off the old block you鈥檇 expect. In fact, most are relatively young, about 200 million years old. Which is strange, since over 90 per cent of the Martian surface is much older. 鈥淚f we had a random sampling of the Martian surface, most of these meteorites should be ancient, about 3.9 to 4.5 billion years old,鈥� says James Head, a planetary scientist at the University of Arizona in Tucson.
Advertisement
Another problem is that studies of the meteorites suggest they left Mars during 6 or 7 different impacts. Computer simulations imply the impacts must have been enormous, producing craters at least 12 kilometres wide. But such huge impacts are so rare we shouldn鈥檛 find any Martian rocks on Earth at all. So why are they here?
Head鈥檚 team say they鈥檝e resolved these contradictions by simulating impacts on Mars in much more detail than before. They have found that smaller impacts, which are far more likely, and would leave craters just 3 kilometres across, could throw millions of small, young rocks into space. This is because young exposed rock is much easier to chip off Mars than older rock, which is usually covered in debris that absorbs the energy of small impacts (Science, DOI 10.1126/science.1077483).
This explains the many young Martian rocks we鈥檝e found, says Head. It also suggests it will be hard to find another ALH84001, the ancient meteorite some scientists believe contains the remains of Martian bacteria. At 4.5 billion years old, it was probably knocked off Mars by a huge, rare impact.