I’m about to show you what might be the most important rock in human history.

Wait, let me say that again. I’m about to show you what might be the most important rock in human history: Voila!

Doesn’t look like much, does it? Well, this rock is a piece of Mars, and it might contain the very  first evidence of alien life. Or at least that’s the controversial claim that some scientists have been making since 1996.

And three decades later, researchers are still arguing over whether this grapefruit-sized  rock is our first clue that we're not alone in the universe. [ INTRO ] Our story begins in December 1984, when scientists from NASA’s  Antarctic Meteorite Laboratory were combing the icy landscape  of Allan Hills for meteorites. Over the course of their weeks-long mission, they collected over a hundred samples. But one of those chunks of space rock was particularly special.

It was the lab manager, Roberta “Robbie” Score, who found it. The rock was an unusual gray-green color, and covered in a black crust. It looked so interesting, in fact, that someone added the note “Yowza-yowza” to the field notes.

Score eventually labeled the rock ALH84001. ALH for Allan Hills, 84 for the year of the mission, and 001 to indicate she wanted this specimen to be studied first. But to perhaps everyone’s disappointment, that initial study turned up nothing unusual.

Sure, it was a rock that fell from space, but nothing more. And with that lack of revelation, it was tucked away into the Johnson Space Center’s massive collection, with all the other meteorites. Then in 1988, the Meteorite  Lab received a request.

Another researcher, David Mittlefehldt, wanted to study asteroids to learn more about what the early solar system was like. In response to his perfectly ordinary request, the Meteorite Lab sent him what seemed like some  perfectly ordinary meteorites… including ALH84001. But when Mittlefehldt studied those samples, he noticed something odd.

While the rest of his space rocks seemed to be made of the same stuff, chemically speaking, ALH84001 was different. Initially, researchers had thought the meteorite had come from some kind of asteroid, hence its inclusion among the samples sent to Mittlefehldt. But based on the specific elements inside of it, ALH84001 wasn’t from an asteroid at all.

Instead, it matched the composition of a group of meteorites called SNCs, named after three rocks that were found in Shergotty, India, El Nakhla, Egypt, and Chassigny, France. And back in 1983, researchers had proven that little pockets of air trapped within some SNC meteorites had a similar makeup to the Martian atmosphere. It took Mittlefehldt until 1993 to be sure enough to publish his findings, but the conclusion was clear.

ALH84001 had come from Mars. Now, at this point in time, scientists had no evidence that life had ever developed on Mars. And as far as most scientists are concerned, they still don’t.

But it wasn’t for lack of trying. NASA had launched nine Mars-bound robots by then to gather as much information as they could about the Red Planet. In the 60s and early 70s, the Mariner missions performed flybys and orbits, collecting atmospheric data and taking the very first close-up photographs of the planet’s surface.

The topography was stunning: Mars had huge volcanoes, craters from prehistoric asteroid impacts, and great canyons where liquid water had once flown across its terrain. Sure, the vision of a Martian surface covered in alien-crafted canals, proposed over half a century earlier by the famous astronomer Percival Lowell, was dashed. But all life on Earth needs water to exist.

And if life were to develop  anywhere else in the universe, it would almost certainly need water too. If there had been water flowing on the surface of Mars at some point in history, maybe there could have been life as well. This excitement peaked in the mid-70s with the Viking missions.

NASA sent two probes to touch down on the planet’s surface, and search specifically for any signs of life. They collected soil samples, took more photos, and found even more signs of liquid water that had existed eons ago. But alas, the Viking missions  never found any signs of life.

Not only were no living organisms detected, but there weren’t even clear signs that such organisms might have ever existed. It was a total dead end. Literally.

Thanks to missions like Mariner and Viking, Mars was cemented in scientific circles as a dead and desolate world. The fervor around finding alien life died down. And given how expensive it was to go to Mars, general exploration of the planet did, too..

So when it turned out we had several Martian rocks already on Earth, hanging out in our storage, it was great news! Instead of having to spend  hundreds of millions of dollars getting a probe to Mars, we could examine the bits of Mars that had already found their way to us! And further research revealed that not only had ALH84001 come from Mars, but it had come from the  unimaginably distant past.

The rock itself was four billion years old, and finally chipped off due  to some kind of cosmic impact around 16 million years ago. It orbited the Sun just like all the planets. And finally, somewhere around 11 thousand BCE, it smashed into the Antarctic ice.

What rocky bits survived the fall then slept… as much as an inanimate object can sleep… until Robbie Score wrapped one up and shipped it to Houston. It’s quite an adventure for such a little rock. And it was also vastly older than any other Martian meteorite we’d discovered, so it gave us a rare chance to find out what Mars had been like in the early days of our solar system.

Samples of ALH84001 soon made their way to a different research team, this time led by David McKay. And they quickly discovered even more strange and wonderful things. It was an igneous rock, having cooled from magma on the Martian surface in its initial formation.

They also found tiny grains of carbonate minerals sprinkled throughout it. This was strong evidence that the rock had once been around liquid water. But also, within the tiny mineral deposits, McKay found microscopic, worm-shaped structures.

They looked a lot like some  fossilized nanobacteria that have been found on Earth before. So the team proposed they were connected: some kind of life had made its mark on this Martian meteorite billions of years ago. If this hypothesis was right, it would be a total reversal of the conclusion that scientists had drawn based on all the other Mars research  that had been done so far.

But to be sure, McKay asked another researcher to join his team and check their work. He didn’t want to set off a false alarm. That researcher’s name was Kathie Thomas-Keprta, and she was a firm skeptic on  the question of life on Mars.

She actually thought the whole team was wrong, and wanted to set the record  straight by joining them. Instead, she became a believer. Thomas-Keprta discovered tiny crystals of an iron-based mineral called magnetite embedded within some of the  larger, carbonate structures.

Some of these crystals looked like they could have formed from lifeless, geologic processes… but some looked so strikingly similar to magnetite crystals made by real, aquatic bacteria that she could think of no other explanation. For McKay, Thomas-Keprta, and their team, this was enough evidence to  go public with their findings. If they were right, there had been life on Mars, even if it was on a microscopic scale.

In August 1996, the researchers put together a paper to announce their hypothesis … and it blew up. The news spread so fast that before the paper was even published, then-President Bill Clinton gave a speech to address the incredible possibility that the hypothesis was true. “If this discovery is confirmed, it will surely be one of  the most stunning insights into our universe that  science has ever uncovered,” he remarked. But of course, this was still just a hypothesis.

Clinton also acknowledged that this finding, and the meteorite itself, would need to be rigorously scrutinized in order to find out whether we truly had evidence of ancient Martian life on our hands. And scientists really took up the challenge. But from the moments of the initial announcement, there was disagreement throughout the scientific community.

Even McKay’s own brother, who had an office down the ha entered the debate… and his team’s research put him on the other side! Some scientists had been irrevocably convinced by the evidence. Others were adamant that there were plenty of inorganic explanations for ALH84001’s peculiarities.

Many more were undecided on whether or not they believed the evidence. Some argued that the worm-like “fossil shapes” found within the meteorite weren’t even big enough to be nanobacteria… only their remains. Others went so far as to suggest that the shapes were nothing more than the  result of sloppy lab work interfering with the examination.

On the other hand, some scientists agreed with the team’s findings, arguing that the evidence pointed in the direction of Martian microbes. Papers were published, and then responses to those papers, and then responses to those papers. And it’s still not over!

Researchers are still arguing over ALH84001, defending their interpretation of the data first uncovered almost 30 years ago. So, what have we learned from all this? Forty years after Robbie Score picked up that rocky Martian  grapefruit at Allan Hills, we still don’t know for sure whether it contains evidence of prehistoric, microscopic Martians.

But we do know that this explosion of excitement around alien life in our galaxy gave NASA the jumpstart it  desperately needed in 1996. Over the past decade, the agency had been contending with a series of very public, very expensive… and in the case of the Challenger disaster… very tragic failures. People weren’t exactly excited about pouring more time and money into space research.

NASA’s funding had been slashed in the early 90s, and things were looking grim for the future of America’s space exploration. But when there’s life on the line? The picture changes.

Space exploration became an exciting and worthwhile endeavor, again. NASA’s budget got a significant boost for 1997, and in 1998, an entire new branch of the Agency was created: the Astrobiology Institute. Since then, we’ve also continued to learn more about ALH84001.

In 2005, data from Mars orbiters found a spot on the planet’s surface that was a solid match for  the meteorite’s composition, giving us a pretty good guess at where the rock originally formed. Perhaps exploring the terrain there, to see if more signs of life might be found, will settle the debate once and for all. On the other hand, it’s possible that we’ll never know whether life existed on Mars, or whether ALH84001 contains any evidence of it.

We might never find evidence compelling enough to convince all the holdouts, in one direction or the other. But this little rock has helped us uncover so much fascinating information along the way. We’ve looked at a snapshot of our solar system’s earliest years.

We’ve learned more about mineral deposits and how they relate to the  presence of water and life. And we’ve even spurred forward the technology that it takes to explore outer space. Whether or not we unravel  the secrets of Martian life, NASA’s most controversial space rock has deepened our understanding of the universe, and our tiny place within it.

And that’s what science is all about. [ outro]