What a Misplaced Meteorite Told Us About Mars

11 million years ago, Mars was a frigid, dry, dead world, just like it is now. Something slammed into the unfortunate planet, sending debris into space. A piece of that debris made it to Earth, found its way into a drawer at Purdue University, and then was subsequently forgotten about.

Until 1931, when scientists studied and realized it came directly from Mars. What has it told them about the red planet?

11 million years ago, the Himalayas were rising on a warmer, more humid Earth. Early ape species made their home in an Africa covered by tropical forests. Diverse mammal species roamed the continents.

At the same time, on Mars, the frigid wind blew across a desiccated, forlorn world. The planet’s thin atmosphere is a weak barrier to meteorites, and the planet’s cratered surface bears witness to its nakedness. Some impacts were powerful enough to launch debris into space beyond the planet’s gravitational pull. The meteorite in the drawer is one such piece of debris.

The meteorite was long forgotten in its storage place until 1931. Scientists identified it as a piece of Mars, and now new research is uncovering clues about Mars’ past hidden in the 800-gram piece of rock.

11 million years ago is not a long time in geological and planetary terms, and the number fits neatly into most people’s imaginations. But rock has deep temporal roots, and the meteorite that reached Earth is an igneous rock that dates back 1.4 billion years. That much time is more difficult to understand, but science is at its best when it opens human minds to a more fulsome understanding of nature.

The meteorite, named “Lafayette” after the city in Indiana that’s home to Purdue University, is the subject of new research published in Geochemical Perspectives Letters. It’s titled “Dating recent aqueous activity on Mars,” and the lead author is Marissa Tremblay. Tremblay is an assistant professor with the Department of Earth, Atmospheric, and Planetary Sciences (EAPS) at Purdue University.

There’s ample evidence that some minerals on Mars formed in the presence of water. Though Lafayette itself is an igneous rock 1.4 billion years old, some of the minerals it contains are younger.

“Dating these minerals can therefore tell us when there was liquid water at or near the surface of Mars in the planet’s geologic past,” Tremblay said. “We dated these minerals in the Martian meteorite Lafayette and found that they formed 742 million years ago. We do not think there was abundant liquid water on the surface of Mars at this time. Instead, we think the water came from the melting of nearby subsurface ice called permafrost, and that the permafrost melting was caused by magmatic activity that still occurs periodically on Mars to the present day.”

Lafayette is one of the Nakhlite meteorites, an igneous rock that formed from basaltic lava around 1.4 billion years ago. Scientists think these rocks formed in one of Mars’ large volcanic regions: Elysium, Syrtis Major Planum, or the largest one, Tharsis, which is home to the three shield volcanoes, Tharsis Montes.

Ancient rocks and their embedded minerals contain information about Mars’ ancient past. The history of Mars’ hydrological cycle is a key objective in our ongoing study of Mars. This research is focused on a particular mineral in Lafayette called iddingsite. It forms when basalt is weathered in the presence of water.

The difficulty with meteorites and the clues they contain about ancient Mars is that they’ve been exposed to and potentially altered by the heat of the initial impact and the heat of entry into Earth’s atmosphere. The chemical signals inherent in rock can become muddied. But Lafayette is different. It’s clear that it was blasted off of Mars 11 million years ago.

“We know this because once it was ejected from Mars, the meteorite experienced bombardment by cosmic ray particles in outer space that caused certain isotopes to be produced in Lafayette,” Tremblay says. “Many meteoroids are produced by impacts on Mars and other planetary bodies, but only a handful will eventually fall to Earth.”

“The age could have been affected by the impact that ejected the Lafayette Meteorite from Mars, the heating Lafayette experienced during the 11 million years it was floating out in space, or the heating Lafayette experienced when it fell to Earth and burned up a little bit in Earth’s atmosphere,” Tremblay said. “But we were able to demonstrate that none of these things affected the age of aqueous alteration in Lafayette.”

Study co-author Ryan Ickert is a senior research scientist in Purdue’s EAPS. Ickert uses heavy radioactive and stable isotopes to study geological processes over time. He showed how isotope data used to date water-rock interactions on Mars were problematic and that the data had likely been polluted by other processes. According to Ickert, he and his colleagues got it right this time.

“This meteorite uniquely has evidence that it has reacted with water. The exact date of this was controversial, and our publication dates when water was present,” he says.

The researchers used a novel technique involving the isotopes Argon 40 and Argon 39 to date Lafayette’s exposure to water and its formation of Iddingsite. That showed them that the exposure occurred 742 million years ago. Their explanation is that magmatic activity melted subsurface ice, and the water subsequently found its way into cracks in the igneous rock, altering some of the olivine into Iddingsite.

All this from a meteorite that was lost in a drawer.

The Solar System is a puzzle. It’s an artifact of Nature’s ordered complexity, but at the same time, it’s shaped by Nature’s steadfast chaos. Each molecule, each tiny piece of rock, including the Lafayette meteorite, is a part of it. Each piece holds a clue to the puzzle.

“We can identify meteorites by studying what minerals are present in them and the relationships between these minerals inside the meteorite,” said Tremblay. “Meteorites are often denser than Earth rocks, contain metal, and are magnetic. We can also look for things like a fusion crust that forms during entry into Earth’s atmosphere. Finally, we can use the chemistry of meteorites (specifically their oxygen isotope composition) to fingerprint which planetary body they came from or which type of meteorite it belongs to.”

Dating these rocks, these pieces of the puzzle, is difficult. However, this research has made progress by developing a novel way to date minerals in the Lafayette meteorite.

“We have demonstrated a robust way to date alteration minerals in meteorites that can be applied to other meteorites and planetary bodies to understand when liquid water might have been present,” Tremblay concluded.

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