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  • [♪ INTRO]

  • Our solar system is full of rocky debris from its formation 4.6 billion years ago,

  • including comets, meteors, and millions of asteroids.

  • Most of those asteroids hang out in the Asteroid Belt between Mars and Jupiter.

  • But plenty of them follow less typical orbits.

  • And earlier this month, astronomers found the first asteroid orbiting closer to the Sun than Venus,

  • giving it the tightest orbit of any asteroid we've ever seen.

  • Astronomers discovered this rock at the Palomar Observatory in southern California.

  • The camera that captured it had been scanning the sky for short-lived signals

  • like the kind you get from supernovas,

  • but it picked up something interesting inside our solar system.

  • It was an asteroid, which astronomers dubbed 2020 AV2.

  • The entire thing is about the size of a few city blocks,

  • and it zips around the Sun every 151 days.

  • That's the shortest year of any asteroid we've found so far.

  • And it's really exciting to find something this close to the Sun because it is not easy.

  • See, the closer something orbits the Sun, the closer to the Sun it'll appear in the sky.

  • And the Sun has a way of outshining things.

  • Like, we only ever see Mercury and Venus right after sunset or right before sunrise,

  • because that's the only time sunlight isn't overpowering them.

  • So no asteroid that orbits at a similar distance is going to appear in a pitch-black sky,

  • which is where telescopes do their best work.

  • These asteroids are only visible for a short time during dusk and dawn,

  • which doesn't leave a lot of time to actually find them.

  • In fact, we've only ever found 21 asteroids orbiting the Sun closer than Earth,

  • and they make up a small population called Atiras.

  • Since this is the first Atira to live inside Venus's orbit, astronomers have crowned it

  • as the first member of the so-called Vatira class of asteroids.

  • Yep! They just slapped a V for Venus on the front of it.

  • In any case, scientists aren't sure if it's part of a larger population of Vatira asteroids

  • or if it's special, but astronomers will be looking for more like it.

  • Because these tightly-orbiting asteroids

  • may hold some clues about the evolution of the solar system.

  • Right now, astronomers don't know exactly how 2020 AV2

  • ended up with such a small orbit, but it probably didn't start out that way.

  • It's more likely that it migrated from somewhere farther out.

  • That means that it and any other Vatiras could help astronomers

  • figure out what was happening in our solar system in the past

  • that sent these space rocks plunging toward the Sun.

  • It can be hard to think about a time when our solar system was different than the way it is today,

  • but there's a ton of evidence showing how it has evolved over the last few billion years.

  • And recently, some scientists have been looking specifically at Earth,

  • trying to understand the origins of our protective magnetic field.

  • This week in the journal PNAS, scientists published the latest evidence

  • that the Earth's early magnetic field was way stronger than we used to think.

  • And what's even more incredible is that the history of our magnetic field

  • can help us understand what was going on under the surface

  • way back when our planet was just a baby.

  • But to figure out what Earth's magnetic field was like in the past,

  • scientists had to do some pretty amazing sleuthing.

  • In this study, they started by measuring the magnetic flecks inside minerals.

  • Because all across time, as minerals have solidified,

  • they've locked in details about the magnetic field at the time, including its strength.

  • So if scientists know the age of a rock,

  • they can figure out what the magnetic field was like back when the rock formed.

  • To look back at when Earth was super young,

  • the authors of this study turned to tiny crystals called zircons,

  • which are the oldest solid bits of Earth we've ever found.

  • And when I say these crystals were tiny, I mean tiny.

  • The crystals themselves are just a fraction of a millimeter across.

  • And the magnetic flecks inside them, made of magnetite, are even smaller.

  • The zircon crystals this team tested are between 3.3 and 4.2 billion years old.

  • And their measurements of the magnetite showed that right around 4 billion years ago,

  • the strength of the magnetic field more than doubled

  • compared to a hundred million years or so earlier.

  • Then, around half a billion years later, it faded to its previous strength.

  • That unusual spike was a bit of a mystery because

  • the whole reason Earth has a magnetic field today

  • is because of the really intense heat coming from its solid inner core,

  • which churns the liquid in its outer core.

  • The thing is, back in those early days, Earth's core was completely molten.

  • It hadn't cooled down enough to form the inner core yet.

  • But the team has an idea about what might have happened.

  • They suggest that a mineral called magnesium oxide,

  • which was dissolved in liquid core, began crystallizing as things cooled down.

  • These chunks of magnesium oxide would have been less dense than the surrounding matter,

  • so they'd likely have shot upward, stirring the liquid metal around them

  • in a way that could generate a planetary magnetic field.

  • Then, around half a billion years ago,

  • all that magnesium oxide finished floating out of the core.

  • The team suggests that this nearly left Earth without a protective magnetic field,

  • completely vulnerable to radiation from the Sun.

  • But we may have gotten really lucky.

  • Because the team's previous research shows that right around the same time,

  • the inner core likely solidified.

  • And it started producing enough heat to churn the liquid in the outer core.

  • Scientists don't completely agree on how old Earth's inner core is, but if this team is right,

  • that timing may have worked out almost perfectly to protect life on Earth.

  • It seems like, just like the asteroids in tight orbits around the Sun,

  • our magnetic field is another sign that the stability we seem to enjoy on Earth now

  • is the result of a much wilder past than we often imagine.

  • Thanks for watching this episode of SciShow Space News!

  • While you're here, we wanted to let you know that

  • our January pin of the month is available now.

  • This month's pin is of Explorer 1, the first satellite the U.S. ever launched into space!

  • And it's only available during the month of January, so if you want one, grab yours now.

  • You can check it out at DFTBA.com/SciShow or find the pin in the merch shelf below.

  • [♪ OUTRO]

[♪ INTRO]

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水星と金星の間に小惑星があるらしい|宇宙ニュース (There's Apparently an Asteroid Between Mercury and Venus | Space News)

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    林宜悉 に公開 2021 年 01 月 14 日
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