The oldest known Martian meteorite isn’t so old after all. Though it’s still the oldest chunk of Mars scientists have ever found, new research suggests the Allan Hills meteorite — officially known as ALH84001 — is about 400 million years younger than previously estimated.
A new analysis published in the April 15 Science pegs the meteorite’s age at a mere 4.091 billion years. Previously the meteorite was commonly accepted to have formed 4.51 billion years ago, when the planet’s surface was still solidifying out of its primordial magma ocean. But the new age indicates the rock would have formed during a later, chaotic period when Mars was being pummeled by meteorites that fractured and shocked the planet’s solid surface.
he previously accepted age of 4.51 billion years old was calculated in 1995 by measuring radioactive isotopes of samarium and neodymium. Radioactive elements decay from a “parent” isotope (in this case, samarium) to a “daughter” isotope (neodymium) at a set rate. By comparing the amount of the parent element to the daughter element, scientists can infer how long a rock has been around.
The new age places the rock’s birth date right at a period in the solar system’s history when all of the inner planets were being bombarded with meteorites. That could clear up some confusion about the meteorite, Treiman says. Parts of the rock show signs of having been melted and reformed a second time since its birth, which would have been tough to explain if the rock were all original Martian crust.