Oldest Mars Meteorite Younger Than Thought
Tom Lapen and his colleagues' data showed that the true age of the Martian meteorite ALH84001 is about 400 million years younger than earlier age estimates.
Credit: Thomas Campbell

The oldest known Martian meteorite ? a space rock that fell to Earth ? is some 400 million years younger than originally thought. It formed about 4.091 billion years ago, a time when the red planet was wet and had a magnetic field, a new study suggests.

Studying this chunk of ancient Mars, which reflects the volcanic processes and bombardment by space debris, could help scientists better understand Mars' early evolution, as well as Earth's.

The meteorite, dubbed ALH84001, was found during a snowmobile ride on Dec. 27, 1984 in the Far Western Icefield of Allan Hills in Antarctica.

Many other Martian meteorites have been found in Antarctica, as well as in remote regions of Africa, but ALH84001 is the "oldest by billions of years," said Tom Lapen of the University of Houston, who conducted the new study of the meteorite. It's about 2.5 billion years older than other Mars meteorites, which get kicked up from the surface of Mars by space rock impacts, flung into space, and then eventually hit Earth.

Not primordial crust

Previously, ALH84001 had been given an age of about 4.51 billion years, which would make it part of the primordial crust of Mars ? the first rock to solidify from the planet's cooling magma ocean. But "there was some question about the generally accepted age," Lapen told SPACE.com.

Lapen and his colleagues used a relatively new technique to evaluate the age of the meteorite, one which has been applied to other Martian meteorites. This method looked at the variations in the isotopes of two minerals, lutetium and hafnium, which formed as the rock crystallized from magma.

The team's analysis, detailed in the April 16 issue of the journal Science, shows that the age of the meteorite is actually 4.091 billion years old, about 400 million years younger than the earlier estimate.

The new age rules out the rock being a fragment of Mars' primordial crust. Instead, it likely formed from more recent volcanic activity in very active regions such as the Tharsis and Elysium volcanic regions, which are home to the largest volcanoes in the solar system. The age also supports the long-lived volcanic activity of these regions.

"We uncovered evidence that the volcanic system in Mars were likely active more than 4 billion years ago," Lapen said. "This connection allows the possibility that regions with the largest volcanoes in the solar system perhaps host some of the longest-lived volcanic systems in the solar system."

Shocked!

The meteorite also shows signs of being shocked by impacts to the Martian surface. Estimates of the periods of bombardment of Mars made by examining the chronology of craters on the planet's surface suggest that a period of heavy bombardment occurred just before this rock formed, or perhaps while it was still forming.

The age of the rock also puts it at a time when liquid water was present on parts of the Martian surface. Mars also had a magnetic field at the time that the rock was formed. The presence of water and a magnetic field, which can shield the surface of a planet, are both conditions favorable to the formation of basic life forms, though whether or not Mars ever hosted life is still an unanswered question.

ALH84001 has been at the center of the Martian life debate before, as some argue it holds fossilized bacteria, though that claim is highly contentious.

Having a piece of Mars' early crust is helpful to scientists who want to better understand the conditions on the planet early in its history. While most Mars meteorites are much younger, with few older rocks, "I think we'll eventually find more," Lapen said.

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