Planetary science

The Fireball That Killed the Dinosaurs Could Help Us Find Life on Other Planets

When David Kring of the University of Arizona gave a presentation at the Lunar and Planetary Science Conference in 1991, he didn’t expect a packed crowd for his talk on the petrology of the Chicxulub Structure in the Yucatan, Mexico. Normally, Kring knew, impact-cratering sessions were presented in the smallest room—the miserable Room D, a shoebox on the second floor. But the magnitude of his announcement attracted scientists across fields and disciplines, so he was bumped up to the main room.

Kring had been investigating a place called the Yucatán-6 borehole, and he and his team had discovered shock quartz and impact melt fragments in two thumb-sized bits of rock that were over half a mile beneath the surface of the Earth. This was evidence that the hole, thought for a very long time to be a volcanic center, was actually an impact structure. And not just any “impact structure,” and not just any crater―but the crater of all craters on Earth. The one behind the death of the dinosaurs 66 million years ago.

Last year, Kring was part of an expedition in which scientists drilled into Chicxulub to investigate how the disastrous collision of fireball and Earth that killed the dinosaurs also created the conditions for life to begin anew. Last month, Kring and his colleagues returned to the Lunar and Planetary Science Conference to present their findings from the new core samples they took on that expedition. The results provide new clues about how life may have begun on Earth about 4 billion years ago—and point us towards how and where we can look for life across the universe.

THE SMOKING CANNON

Back in the early 1990s, Kring knew what he was looking for—a crater of the size and magnitude that would provide evidence of catastrophic extinction—but he didn’t know where to look. “It was a race to find the impact site,” Kring tells mental_floss, “and we had made a discovery of this very thick impact ejecta deposit in Haiti, which pointed us to [the Yucatan].”

Impact ejecta is what’s blasted from the Earth or other body when a meteor crashes into it. In this case, a giant chunk of the Earth was blown a thousand miles away. Until the Haiti discovery, people were looking all over the planet for the crater. But now they had a target region. Meanwhile, Petroleos Mexicanos, an oil company, had drilled down into what they thought was a “geophysical anomaly” in the Yucatan―a salt dome, maybe, where there might be oil. That’s when Kring and his colleagues re-examined samples collected from the site and realized there were features consistent with an impact.

That the Yucatan site was still intact to be found wasn’t a given. In the last 65 million years, half of the seafloor has…

How Earth got its moon

full moon
full moon

The story of our moon’s origin does not add up. Most scientists think that that the moon formed in the earliest days of our solar system. That would have been back around 4.5 billion years ago. At that time, some scientists suspect, a Mars-sized rocky object — what they call a protoplanet — smacked into the young Earth. This collision would have sent debris from both worlds hurling into orbit. Some of the rubble eventually would have stuck together, creating our moon.

Or maybe not.

Astronomers refer to that protoplanet as Theia (THAY-ah). Named for the Greek goddess of sight, no one knows if this big rock ever existed — because if it did, it would have died in that violent collision with Earth.

moon Theia
Early Earth and a smaller protoplanet called Theia may have collided long ago, many scientists think. That would have hurled debris from both into space. In this simulation, red particles escaped the system, yellow formed the moon and blue fell to Earth.

And here’s why some astronomers have come to doubt Theia was real: If it smashed into Earth and helped form the moon, then the moon should look like a hybrid of Earth and Theia. Yet studies of lunar rocks show that the chemical composition of Earth and its moon are exactly the same. So that planet-on-planet impact story appears to have some holes in it.

That has prompted some researchers to look for other moon-forming scenarios. One proposal: A string of impacts created mini moons largely from Earth material. Over time, they might have merged to form one big moon.

“Multiple impacts just make more sense,” says Raluca Rufu. She’s a planetary scientist at the Weizmann Institute of Science in Rehovot, Israel. “You don’t need this one special impactor to form the moon.”

But Theia shouldn’t be left on the cutting room floor — at least not yet. Earth and Theia could have been built largely from the same type of material, new research suggests. Then they would have had a similar chemical recipe. There is no sign of “other” material on the moon, this explanation argues, because nothing about Theia was different.

“I’m absolutely on the fence between these two opposing ideas,” says Edward Young. He studies cosmochemistry — the chemistry of the universe — at the University of California, Los Angeles. Determining which story is correct is going to take more research. But the answer could offer profound insights into the evolution of the early solar system, Young says.

Mother of the moon

Earth’s moon is an oddball. Most other moons in our solar system live way out among the gas giants, such as Saturn and Jupiter. The only other terrestrial planet with orbiting moons is Mars. Its moons, Phobos and Deimos, are small. The leading explanation for them is that likely were once asteroids. At some point, they were captured by the Red Planet’s gravity. Earth’s moon is too big for that scenario. If the moon had come in from elsewhere, it probably would have crashed into Earth or escaped and fled into space.

An alternate explanation dates from the 1800s. It suggests that moon-forming material flew off of a fast-spinning young Earth. (Imagine children tossed from an out-of-control merry-go-round.) That idea fell out of favor, though, when scientists calculated the spin speeds required. They were impossibly fast.

In the mid-1970s, planetary scientists proposed the giant-impact hypothesis. (Later, in 2000, they named that mysterious planet-sized body as Theia.) The notion of a big rocky collision made sense. After all, the early solar system was like a game of cosmic billiards. Giant space-rock smash-ups were common.

But a 2001 study of rocks collected during NASA’s Apollo missions to the moon cast doubt on the giant-impact hypothesis. Research showed that Earth and its moon were surprisingly alike. To figure out a rock’s origin, scientists measure the relative abundance of different forms of oxygen. Called isotopes (EYE-so-toaps), they are forms of an element with different masses. (The reason they differ: Although each has the same number of protons in its nuclei, they have different numbers of neutrons.)

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isotope moon
The mix of oxygen isotopes inside moon material and meteorites called enstatite chondrites (shaded yellow) is surprisingly similar to that of Earth rocks (blue line). Other solar system materials are largely composed of different isotopic mixes.

Scientists can use amounts of various isotopes as something like fingerprints at a crime scene. Rocks from Earth and its moon, the scientists found, had seemingly identical mixes of oxygen isotopes. That didn’t make sense if much of the moon’s material came from Theia, not Earth. Rufu and her colleagues modeled the impact on a computer. From that they calculated the chance of a Theia collision yielding a moon with an Earthlike composition. And it was very slim.

Studies have been done of other elements in moon rocks, such as titanium and zirconium. They, too, suggest that Earth and its moon…

It’s time to redefine what qualifies as a planet

Pluto
PLANET OR NOT? A group of planetary scientists label Pluto and many other orbs in the solar system as planets, despite the definition set down by the International Astronomical Union in 2006.

Pluto is a planet. It always has been, and it always will be, says Will Grundy of Lowell Observatory in Flagstaff, Arizona. Now he just has to convince the world of that.

For centuries, the word planet meant “wanderer” and included the sun, the moon, Mercury, Venus, Mars, Jupiter and Saturn. Eventually the moon and sun were dropped from the definition, but Pluto was included, after its discovery in 1930. That idea of a planet as a rocky or gaseous body that orbited the sun stuck, all the way up until 2006.

Then, the International Astronomical Union narrowed the definition, describing a planet as any round object that orbits the sun and has moved any pesky neighbors out of its way, either by consuming them or flinging them off into space. Pluto failed to meet the last criterion (SN: 9/2/06, p. 149), so it was demoted to a dwarf planet.

Almost overnight, the solar system was down to eight planets. “The public took notice,” Grundy says. It latched onto the IAU’s definition — perhaps a bit prematurely. The definition has flaws, he and other planetary scientists argue. First, it discounts the thousands of…

It Turns Out Cosmic Dust Is Everywhere

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Who among us doesn’t thrill to catch a glimpse of a meteorite streaking across the night sky? Except for a few colorful cases — a living room in Connecticut, an explosion in the sky over Chelyabinsk, Russia — these beauties disappear into our atmosphere. They’re just a small fraction of the objects that hit the earth — scientists estimate that some 4,000 tons of them arrive yearly. Some are so tiny they don’t even fall: They just float down. So where is all this stuff? Researchers have found micrometeorites — which are typically smaller than width of a human hair — in Antarctica and other remote locations. Now a new picture book, In Search of Stardust: Amazing Micro-Meteorites and Their Terrestrial Imposters, reveals that, really, it’s everywhere.

The author of the book, amateur scientist Jon Larsen, has coauthored an article in Geology with Imperial College earth and planetary science lecturer Matthew J. Genge and two of his students, Martin D. Suttle of Imperial College and Matthias Van Ginneken of the Université Libre in Brussels. In the article, they reveal that this “cosmic dust” can be found most anywhere — most of the photos in the book come from flecks collected from house gutters in Norway. The cosmic stuff is so ubiquitous that…