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  • Mercury is the closest planet to the Sun. As you might expect, that makes it pretty hot.

  • But also, it’s pretty cool.

  • There are seven naked-eye solar system objects in the sky: Mercury, Venus, Mars, Jupiter,

  • Saturn, the Sun, and the Moon. Seven. Each of them was associated with a god in ancient

  • times. Mercury was the Roman messenger of the gods, fleet of footliterally, he had

  • wings on his shoesand a rapid traveler.

  • To anyone who’s seen Mercury in the sky, this affiliation with the swift god is no

  • surprise. Mercury the planet moves pretty quickly, visibly changing its position relative

  • to the background stars even after a single night.

  • Despite its speed, the planet never gets very far from the Sun. At best, it can reach a

  • separation of about 28°. That’s about three times the apparent size of your fist held at arms length.

  • In 1639 the Italian astronomer Giovanni Zupi used a telescope to observe Mercury, and he

  • discovered it undergoes a complete cycle of phases over time, just like the Moon does.

  • The only way that can happen is if Mercury orbits the Sun, and not the Earthanother checkmark in the

  • column for heliocentrism, which was starting to look better and better all the time.

  • And of course that’s the way things really are. Mercury is the innermost of the planets

  • in the solar system. It orbits the Sun at an average distance of about 58 million kilometers,

  • roughly a third the distance of the Earth from the Sun. That’s why we never see it

  • stray far from the Sun. From our viewpoint, its smaller orbit keeps it huddled closer to our star.

  • That’s why we see it move so rapidly, too; it’s closer to the Sun, so the gravity it

  • feels from the Sun is stronger, and therefore its orbital velocity is faster than Earth’s.

  • It orbits the Sun once every 88 days.

  • And that’s also why we see undergo phases. When it’s between us and the Sun were

  • looking at its dark side, and when it’s on the other side of the Sun were looking

  • at its fully illuminated half. In between it goes through the same phases as the Moon:

  • crescent, half full, gibbous, and so on.

  • Not that this is such an easy observation to make. Because it never gets far from the

  • Sun, it’s always low to the horizon after sunset or before sunrise. When we observe

  • it were looking through all the muck and turbulence in our air, so it’s usually pretty

  • fuzzy. Making matters worse, it’s a dinky planet, only about 4900 kilometers in diameter,

  • about a third the Earth’s width.

  • One upside to all this is that because it’s close to the Sun, it’s illuminated fiercely,

  • and can be pretty bright even near the horizon. If you ever get a chance to see it, you really

  • should. It’s pretty cool.

  • Mercury’s orbit is weird. It has the most elliptical orbit of any planet, ranging from

  • 46 to nearly 70 million kilometers from the Sun. When it’s closest to the Sun it receives

  • more than twice as much light and heat as when it’s furthest!

  • Mercury is too small and difficult to observe to see surface features on it, which for a

  • long time made it impossible to figure out how long its day is. Astronomers assumed that

  • the tides from the Sun had locked Mercury’s spin so that its day was equal to its year,

  • just like our Moon spins once for every time it goes around the Earth. However, in 1965,

  • astronomers used Doppler radar to observe Mercury and directly measure its spin and

  • they got a surprise: Its day was only 59 Earth days long, not 88.

  • But that’s a significant number as well. To be more exact, the actual length of Mercury’s

  • year is 87.97 days, and the actual length of its day is 58.65 Earth days. If you divide

  • those two numbers, you see their ratio is almost exactly 2/3!

  • It turns out there’s more than one way to tidally lock the rotation of a planet to its

  • orbit. Remember earlier, when I said Mercury’s orbit is highly elliptical? The tides from

  • the Sun are far stronger on Mercury when it’s at perihelion, the closest point in its orbit

  • to the Sun, than when it’s at aphelion, the farthest point in its orbit. After Mercury

  • first formed, tides from the Sun slowed its rotation just like the Earth’s tides on

  • the Moon slowed the Moon down as well.

  • But at some point, Mercury’s spin slowed to where it was 2/3 of its orbital period.

  • So, at one perihelion pass, one side of Mercury faces the Sun. Then, 88 or so days later,

  • it approaches perihelion again. But it’s spun 1.5 times, and this means the exact opposite

  • side of Mercury faces the Sun at this closest approach. 88 days later, Mercury has spun

  • 1.5 times again, and the whole thing repeats.

  • It turns out that’s a perfectly legitimate stable configuration, just like the one-to-one

  • spin/orbit setup. The way the physics works out, tides like simple multiples. Once the

  • day became 2/3 the period of the year, forced by Mercury’s elliptical orbit, the tides

  • stopped slowing it, and things have been that way ever since.

  • Mercury’s elliptical orbit, together with the 2:3 spin to orbit ratio, make for a very,

  • very weird day on Mercury. If you stay in one spot, it takes the Sun two Mercury years,

  • 176 days, for the Sun to go around the sky once! That’s because if youre on the

  • side of Mercury facing the Sun at one perihelion, the other side will face it one year later.

  • Itll only be after the second year ends that youre facing the Sun again.

  • But it gets weirder. Mercury’s spin is constant; it doesn’t speed up or slow down. However,

  • its motion around the Sun is faster at perihelion than aphelion. At aphelion, Mercury’s spin

  • is a bit faster than its orbital speed, so the Sun moves rapidly westward across the

  • sky. But at perihelion Mercury’s motion around the Sun actually more than compensates

  • for its spin, so the Sun appears to stop in the sky and actually move backwards for a

  • few days! Then, as Mercury pulls away from the Sun, its orbital velocity slows down,

  • and the Sun starts to move west once again as the planet’s rotation dominates.

  • If youre at just the right spot on the planet’s surface, this means you could actually

  • watch the Sun rise, slow, stop, set again, then rise again!

  • And you think time zones on the Earth are a pain.

  • Mercury’s hard to observe from Earth, and much of what we know about it is due to observations

  • from space probes sent there. Mariner 10 made three flybys of Mercury in the 1970s, and

  • mapped about half the surface. We learned that it had almost no atmosphere, and was

  • therefore unsurprisingly covered in craters.

  • In 2011, the MESSENGER probe entered orbit around Mercury after making a series of close

  • flybys. The pictures it returned were breathtaking, and revealed a world that has seen a lot of

  • pummeling over the eons. It’s covered in craters, pole to pole, some hundreds of kilometers in diameter.

  • The largest is called Caloris Basin, a whopping huge impact feature 1600 kilometers across.

  • There are some smoother plains on the planet’s surface too, which appear to be older than

  • the cratered regions. These plains are covered in cracks called rupes. These are compression

  • folds, like wrinkles on a fruit rind that’s dried out. Apparently, as Mercury’s interior

  • cooled after it formed, the planet shrank, and the crust cracked as it tried to shrink as well.

  • Several of the craters have extensive ray systems. Like on our Moon, these are formed

  • when impacts fling out long plumes of material that then settle down on the surface.

  • One of my favorite things of all about Mercury: Craters are named after artists. Musicians,

  • writers, painters, and more, so we have craters like Botticelli, Chekov, Debussy, Degas, Okyo,

  • Sibelius, Vivaldi, and Zola. There’s even one named Tolkien!

  • Dipping below the surface, we can only infer what Mercury’s internal structure is like.

  • But the planet’s dense, nearly as dense as Earth. We know the surface is rocky, so

  • to be as dense as it is it must have a large iron core, far larger in proportion to the

  • planet than Earth’s. Mercury’s core may reach ¾ of the way to the planet’s surface!

  • Why does it have such a high proportion of iron? Mercury may have formed as a larger

  • planet, then got blasted in a huge grazing impact that blew away the lighter materials

  • that had risen to the surface, leaving behind the denser part. Or maybe the heat of the

  • still-forming Sun vaporized the lighter materials off its surface.

  • Mercury has a measurable magnetic field, which is a bit surprising since it rotates so slowlyrotation

  • plays a big part in the Sun’s and Earth’s magnetic fields. But that fits with so much

  • of its interior being molten iron; the bigger core may allow for a stronger field despite its slow spin.

  • It doesn’t have much of an atmosphere, but there is a trace of one, mostly due to its

  • magnetic field trapping the solar wind, and to material flung up from the surface after

  • violent impacts from comets and asteroids. A lot of this material blown off the surface

  • escapes the planet and gets blown away by the solar wind and pressure from sunlight.

  • It forms a long comet-like tail that is tens of millions of kilometers long. This tail

  • is comprised of elements like sodium, calcium, and magnesium, material that’s known to

  • be abundant on the surface.

  • Speaking of which, here’s a fun fact: pound for pound, impacts on Mercury are more violent

  • than they are on Earth. Mercury has weaker gravity, so it doesn’t pull in impactors

  • as hard as the Earth does, but it orbits the Sun far faster than Earth does, so asteroids

  • and comets tend to hit at higher velocity. That makes the explosive energy higher, making

  • craters bigger.

  • And there’s one more surprise Mercury has, and it’s really surprising: Despite being

  • so close to the Sun, and having a surface temperature that can reach 430°C — 800°

  • Fahrenheitastronomers have found water ice on Mercury!

  • It exists in the bottoms of deep craters near Mercury’s poles, where sunlight never reaches.

  • These are calledcold traps,” and temperatures there don’t get above -170° C. It’s not

  • known for sure where the water comes from, but it’s likely to be from comets and asteroids

  • that impacted the planet, scattering the water across the surface. Of course, in the harsh

  • heat that water just goes Fffffft and goes away. But in those deep craters it can persist,

  • accumulating over the eons. There may be billions of tons of it there!

  • It’s bizarre to think that in one of the hottest places in the solar system there can

  • be conditions so cold ice can exist, but one thing weve learned about nature over and

  • again: It has a lot more imagination than we do.

  • Today you learned that Mercury is the closest planet to the Sun. It’s airless and dense,

  • and is covered with craters. Its rotation is locked to its orbit in a 2 to 3 ratio,

  • and together with its elliptical orbit makes a day on Mercury very long and very weird.

  • And despite being very hot, there’s actually water ice in deep craters at its poles.

  • Crash Course Astronomy is produced in association with PBS Digital Studios. Head to their channel

  • to discover more awesome videos. This episode was written by me, Phil Plait. The script

  • was edited by Blake de Pastino, and our consultant is Dr. Michelle Thaller. It was directed by

  • Nicholas Jenkins, edited by Nicole Sweeney, and the graphics team is Thought Café.

Mercury is the closest planet to the Sun. As you might expect, that makes it pretty hot.

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水星クラッシュコース天文学#13 (Mercury: Crash Course Astronomy #13)

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    羅紹桀 に公開 2021 年 01 月 14 日
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