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  • Consider the spot where you're sitting.

  • Travel backwards in time

  • and it might've been submerged at the bottom of a shallow sea,

  • buried under miles of rock,

  • or floating through a molten, infernal landscape.

  • But go back far enough

  • about 4.6 billion years,

  • and you'd be in the middle of an enormous cloud of dust and gas

  • orbiting a newborn star.

  • This is the setting for some of the biggest, smallest mysteries of physics:

  • the mysteries of cosmic dust bunnies.

  • Seemingly empty regions of space between stars

  • actually contain clouds of gas and dust,

  • usually blown there by supernovas.

  • When a dense cloud reaches a certain threshold called the Jeans mass,

  • it collapses in on itself.

  • The shrinking cloud rotates faster and faster, and heats up,

  • eventually becoming hot enough to burn hydrogen in its core.

  • At this point a star is born.

  • As fusion begins in the new star,

  • it sends out jets of gas that blow off the top and bottom of the cloud,

  • leaving behind an orbiting ring of gas and dust called a protoplanetary disk.

  • This is a surprisingly windy place;

  • eddies of gas carry particles apart, and send them smashing into each other.

  • The dust consists of tiny metal fragments, bits of rock, and, further out, ices.

  • We've observed thousands of these disks in the sky,

  • at various stages of development

  • as dust clumps together into larger and larger masses.

  • Dust grains 100 times smaller than the width of a human hair stick to each other

  • through what's called the van der Waals force.

  • That's where a cloud of electrons shifts to one side of a molecule,

  • creating a negative charge on one end, and a positive charge on the other.

  • Opposites attract, but van der Waals can only hold tiny things together.

  • And there's a problem: once dust clusters grow to a certain size,

  • the windy atmosphere of a disk should constantly break them up

  • as they crash into each other.

  • The question of how they continue to grow is the first mystery of dust bunnies.

  • One theory looks to electrostatic charge to answer this.

  • Energetic gamma rays, x-rays, and UV photons

  • knock electrons off of gas atoms within the disk,

  • creating positive ions and negative electrons.

  • Electrons run into and stick to dust,

  • making it negatively charged.

  • Now, when the wind pushes clusters together,

  • like repels like and slows them down as they collide.

  • With gentle collisions they won't fragment,

  • but if the repulsion is too strong, they'll never grow.

  • One theory suggests that high energy particles

  • can knock more electrons off of some dust clumps,

  • leaving them positively charged.

  • Opposites again attract, and clusters grow rapidly.

  • But before long we reach another set of mysteries.

  • We know from evidence found in meteorites

  • that these fluffy dust bunnies eventually get heated, melted

  • and then cooled into solid pellets called chondrules.

  • And we have no idea how or why that happens.

  • Furthermore, once those pellets do form, how do they stick together?

  • The electrostatic forces from before are too weak,

  • and small rocks can't be held together by gravity either.

  • Gravity increases proportionally to the mass of the objects involved.

  • That's why you could effortlessly escape an asteroid the size of a small mountain

  • using just the force generated by your legs.

  • So if not gravity, then what?

  • Perhaps it's dust.

  • A fluffy dust rim collected around the outside of the pellets

  • could act like Velcro.

  • There's evidence for this in meteors,

  • where we find many chondrules surrounded by a thin rim of very fine material

  • possibly condensed dust.

  • Eventually the chondrule pellets get cemented together inside larger rocks,

  • which at about 1 kilometer across

  • are finally large enough to hold themselves together through gravity.

  • They continue to collide and grow into larger and larger bodies,

  • including the planets we know today.

  • Ultimately, the seeds of everything familiar

  • the size of our planet, its position within the solar system,

  • and its elemental composition

  • were determined by an uncountably large series of random collisions.

  • Change the dust cloud just a bit,

  • and perhaps the conditions wouldn't have been right

  • for the formation of life on our planet.

Consider the spot where you're sitting.

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私たちの惑星を築いたダストバニー - ロリン・スウィント・マシューズ (The dust bunnies that built our planet - Lorin Swint Matthews)

  • 23 1
    林宜悉 に公開 2021 年 01 月 14 日
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