The Allan Hills meteorite, named for the site where it was found in Antarctica, was once thought to contain fossil traces of life. That idea has been mostly dismissed, and now the rock also appears to be not quite as old as previously thought.
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.
The Allan Hills meteorite has been a lightning rod for controversy since scientists announced in 1996 that it might hold fossils of Martian bacteria. The scientific community has since mostly abandoned that idea, as one by one every line of evidence for life has been given a non-biological explanation.
“People usually ask me about the life aspect, and I’m so sick to death of that,” says Allan Treiman of the Lunar and Planetary Institute in Houston, who was not involved in the new work. Treiman and others now believe that what once looked like fossils is actually rock that was shaped by ordinary geological activity.
The 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.
“To understand how the Martian mantle has evolved, it’s critical to get samples that are old, to see what the mantle sources were early in the planet’s history,” says Thomas Lapen of the University of Houston, a coauthor of the new study. “This is the only sample in that age range.”
Lapen and his colleagues used radioactive isotope dating to calculate the age of the meteorite, using different elements than the 1995 analysis did. Lapen says that the elements used back then were mostly found in minerals called phosphates, which succumb relatively quickly to weathering and geological processes. Like hair dye or a fake ID, weathering could disguise the rock’s age in some ways, but not so thoroughly that more reliable indicators are obscured.
“If it’s subject to weathering, the phosphate would be the first to be disturbed,” Lapen says. “Then ages dependent on the phosphates are altered.”
Instead of elements found in phosphates, Lapen’s group used lutetium and hafnium, elements that are mostly found in more change-resistant components of the rock. This method showed that the meteorite is just 4.091 billion years old.
Surprisingly, the researchers also found that several younger meteorites have essentially the same composition as the Allan Hills meteorite, meaning some of the same basic geologic processes have been at work on Mars for almost its entire history.
“That connection is perhaps the most amazing outcome of this research,” Lapen says. “Mars is a very steady state planet. Igneous processes were happening the same way four billion years ago as they are happening right now.”
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.
“That had been a bit of a problem,” Treiman says. “You’d have to do whatever mantle processing, whatever happened on the planet, before this rock came to be formed. There’s not a lot of time for that.”