How Did Scientists Determine the Age of Lunar Rocks Brought Back to Earth?

December 18, 2021 Post a Comment

How Apollo moon rocks reveal the epic history of the cosmos

Lunar samples are a time capsule. Scientists say we should go back for more.

By Updated Nov 19, 2019, 10:40am EST

Additional reporting by Byrd Pinkerton; graphics by Javier Zarracina/Vox

In a brilliant white room at the Naval Research Laboratory in Washington, DC, lies a clear plastic chest filled with bits of the heavens. Inside are meteorites recovered from Antarctic ice and grains of material believed to predate the formation of our solar system. These are treasures, helping us humans understand our place among the stars.

From the chest, geologist Kate Burgess pulls out another treasure: a tiny Teflon vial, double-wrapped in Teflon bags. It contains soil from the moon, collected by the astronauts of Apollo 17 in 1972.

Geologist Kate Burgess stands near an electron microscope that can resolve images on the scale of atoms.

The amount of lunar dirt in this vial is tiny. But its scientific value is immense. — The amount of lunar soil in this vial is tiny. But its scientific value is immense.

For a very long time, that soil rested undisturbed on the moon, exposed only to the immense radiation of space. When Burgess peers at the specimen with an electron microscope so powerful it can see down to the scale of atoms, she's looking for evidence of how exposure to that radiation changed the soil color. This sounds like small-bore science. But it's in service of a grand, even beautiful, idea.

Burgess is working to make moon rocks a reference guide to the greater cosmos. She's investigating how much of the soil's color comes from its composition (what it's made out of) and how much comes from space weathering. She says figuring that out will help identify the composition of objects — like asteroids — spotted by telescopes.

In this way, the lunar samples are a link between us and the heavens, helping us see deeper into them and understand what we find. For planetary scientists, research on lunar samples is invaluable. Unlike Earth, the moon hasn't changed much since it formed. That makes it a time capsule, a Book of Genesis for the geologically inclined.

In other words: Moon rocks rock.

Scientists are still studying the lunar samples from the Apollo moon landings. But there is now renewed interest in sending humans back to the moon for more.

President Trump wants them to get there by 2024. (We'll see about that.) And planetary scientists are salivating over the chance to study rocks from the lunar south pole and the side of the moon that never faces Earth. Whether a lunar return is worth the cost, at this point in time, is debatable. But the planetary scientists I spoke with all said, at least, that it would lead to important scientific gains.

That's because the moon rocks we have tell an incredible story about our place in the universe. The more we can collect, the more we'll learn.

Why the moon is so darned important for planetary science

The moon landings — the second of which, Apollo 12, happened 50 years ago this week — were about a lot of things: beating the Soviets in the space race, the engineering puzzle of sending humans to the moon's surface, the challenge for the sake of a challenge. But they were also about geology. Over the course of the six moon landings, astronauts brought back 842 pounds of lunar rocks, pebbles, and soil.

It's not an exaggeration to say those rocks changed our understanding of our solar system and rewrote its history. "Before Apollo, we really did not know how the moon formed," says Juliane Gross, a planetary scientist at Rutgers University.

To study geology is to study history. But Earth is constantly erasing its old geologic record.

"The Earth is a gigantic recycling machine," Gross says. "We have wind, we have rain, we have ice and weather, and so all the rocks weather away." The crust of our planet is dynamic; our continents float, move, and change. Through the ages, rocks are recycled, remelted, and reformed as continents smash into one another.

The moon, on the other hand, doesn't erase its history. Aside from asteroid impacts, Gross says, "the moon hasn't changed much since its formation." That makes it a time capsule, a ledger for the history of our solar system.

In a moon rock, "you have this tiny treasure trove in your hands," Gross says. Growing up, she had a dream of becoming an astronaut, which was eventually quashed by her susceptibility to motion sickness. Working with these rocks, she says, "that's as close as I can get to be[ing] an astronaut." But instead of exploring space, she and her colleagues are exploring time.

"The [lunar] crust is basically an archive," Gross says. "And we need to learn how to interpret and how to read that archive." One of its most important lessons is about how the Earth and moon were formed in the first place.

Moon rocks tell the story of creation

The picture below shows a 4-pound moon rock recovered in 1972 from Apollo 16. It's mostly made of plagioclase, a rock formed out of molten magma. Rocks like this one make up most of the moon's crust. And that tells scientists the moon had a very violent beginning.

Around 4.5 billion years ago, when the solar system was still in its infancy, it was a much more chaotic place.

Not long before that (cosmically speaking), the sun had burst into being, fusing together hydrogen atoms from an immense ball of gas, setting alight a fire that burns to this day. And that young star was still surrounded by bits of debris clumping together, smashing into one another, forming the planets.

It's believed that around this time, the Earth (or more like an Earth predecessor) was hit by another planet maybe the size of Mars.

The resulting cataclysm fused the two worlds together, forming our Earth. The power of the collision ejected material from both bodies, and that material melted together to form our moon. The early moon was covered in an ocean of magma, which settled and cooled into the form we know today.

In this way, the Earth and the moon were a (fraternal) twin birth.

But wait, how do we suspect all this from a boring old white rock?

The answer is kind of simple. Plagioclase is not very dense; it's the type of mineral you'd expect to arise on the surface of a magma ocean as it cools. When the moon was formed, the plagioclase "actually rose to the surface of the moon and started creating a crust," says Darby Dyar, a senior scientist at the Planetary Science Institute who has been studying lunar samples for decades.

Scientists are still debating the details of this hypothesis. But it seems reasonable because the Earth and moon are made out of similar base materials (suggesting they were created from the same source material) and because that material was molten at the time they formed (due to the great power of the impact).

But that's just the beginning of the story moon rocks tell.

What moon craters can tell us about the history of the solar system

A huge part of the "archive" of the lunar crust is its craters. And scientists have been able to use the Apollo samples to accurately date those craters.

The moon has changed far less than the Earth, but that doesn't mean it hasn't changed at all. Asteroids have hit it over and over again, leading to the pockmarked surface we can see in the night sky. Those craters tell the story of what happened in the solar system after the Earth and the moon were formed.

By age-dating the moon's craters, we can age-date craters elsewhere. The bigger the craters, the longer ago they were made (because bigger chunks of debris were more common farther back in time). "And now ... we have a beautiful impact history of the solar system," Dyar says. There are craters on other planets, like Mercury, for example. We now know the age of Mercury's craters "because we have a reference set of information from the moon."

Learning how old the moon's craters are then led to another stunning hypothesis: that the outermost planets — Jupiter, Saturn, Uranus, Neptune — have changed their orbits over their lifetimes.

The craters show that around 600 million years after the planets formed, there was a period of heavy bombardment, meaning that the moon got smacked with a lot of asteroids. This was weird. The frantic pace of asteroid collisions ought to have settled down by then.

So what explains the impacts during this time? One possible idea is that if those big gas giant planets moved closer to the sun and then farther away, "they would have disturbed asteroids and they would have flung the asteroids around," creating the collisions, Gross says.

Scientists still aren't sure if this is the case. But without moon rocks, they might not have considered the case at all.

Why scientists want more lunar samples

We've learned a ton from less than a ton of moon rocks. But these planetary geologists are hungry for more. One reason is that all the Apollo missions landed near the moon's equator.

Would the scientists like to study samples from other areas? "Oh, hell yeah," Gross says. "Absolutely."

"To try to interpret something about the history of the moon from a few hundred kilograms of rocks is very frustrating," Dyar says, adding that we don't have any samples from the far side of the moon at all. "We don't know what other interesting science we're gonna find."

The White House is currently pushing NASA to send humans to the moon again by 2024. For now, the plan is for those astronauts to visit the lunar south pole at a crater called the South Pole–Aitken basin — one of the biggest, deepest, and therefore oldest of the moon's craters. It's possible the impact that created the basin was so powerful that it exposed the mantle, or interior, of the moon.

Scientists can't directly study the Earth's mantle. The moon's would be the next best thing. "If we can get some of that back, that would be absolutely spectacular," Gross says. It could help us understand why the Earth has such active geology and the moon does not.

Burgess hopes that if humans get to the moon, they can bring home some samples from areas that have not been exposed to as much space radiation so she can see a more pristine example of an unweathered space rock. Again, that's in service of understanding what other objects — ones we don't have pieces of — are made out of.

And that knowledge could have a lot of practical implications. For instance, in the future, if humans want to start mining asteroids for metals and minerals, it will be enormously helpful to know the exact geologic makeup of a particular asteroid before we arrive.

There are a lot of reasons to return humans to the moon and establish a more permanent presence there. The moon would be a good laboratory to teach astronauts how to better survive long, lonely missions in deep space. It would be a good launching ground for missions to Mars, or beyond. And it would potentially be a spot to mine for natural resources.

One of Burgess's favorite discoveries is bits of helium she found stuck into teeny pits on the lunar sample dirt. The helium "is some of the sun trapped in the moon," she says. The sun blasts off gases and particles in every direction, and our moon soaked up some of them like a sponge. The finding is as poetic as it is practical: Helium is an increasingly scarce resource on Earth. Perhaps we can learn to harvest it from the moon.

Moon rocks represent what happens when human curiosity is allowed to flourish

To study the moon is to study the Earth and wonder: How special is our world?

"I always think that the most important question for human beings to answer is the issue of, are we alone?" Dyar says. "Is Earth unique?" And in a small way, studying a pile of moon rocks helps us answer that question.

Figuring out how our solar system formed, how our planet formed, helps us understand how rare we are and how special a place this truly is. What if a Mars-sized body never collided with an Earth-sized one? Was that cataclysm somehow necessary for the chain of events that led to life, to you and me, to pizza?

If the moon never existed, Earth would be very different (there wouldn't be ocean tides, for example). But we would be different too. And we'd possibly be less curious about our place in the universe.

Astronauts on Apollo 17, the last moon landing, look out on moon dust and rock. What secrets are still hidden in this rubble?

Without the moon, "I think humanity would have probably never looked up into the sky [and thought], 'Oh, this object is fairly close, let's try and get there,'" Gross says. "So we would never have had the curiosity to develop our technology and tools to leave our own planet."

For so many reasons, the moon is our first stepping stone to the greater reaches of space and the mysteries that lie within. I don't know if we need to get more moon rocks by the year 2024 specifically. But sometime, someday, we ought to go back.

Additional reporting by Byrd Pinkerton; graphics by Javier Zarracina/Vox

How Did Scientists Determine the Age of Lunar Rocks Brought Back to Earth?

Source: https://www.vox.com/the-highlight/2019/6/21/18677691/apollo-anniversary-moon-rock-lunar-sample-geology

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