Mercury: Crash Course Astronomy #13

Summary

This script explores Mercury, the solar system's innermost and hottest planet, known for its rapid movement and elliptical orbit. Despite its proximity to the Sun, it has a surprisingly long and complex day-night cycle due to a 2:3 spin-orbit resonance. The planet, largely unobservable from Earth, has been studied through space probes revealing a cratered surface, a large iron core, and the unexpected presence of water ice in permanently shadowed polar craters. Its unique characteristics challenge our understanding of planetary dynamics.

Takeaways

• 🌞 Mercury is the closest planet to the Sun and experiences extreme temperatures.
• 🏃 Mercury was named after the Roman messenger god, symbolizing its fast movement in the sky.
• 📡 In 1639, Giovanni Zupi observed Mercury's phases, supporting the heliocentric model of the solar system.
• 🌍 Mercury is the innermost planet and orbits the Sun at an average distance of about 58 million kilometers.
• ⏱️ Mercury completes an orbit around the Sun every 88 Earth days.
• 🌓 The planet shows phases similar to the Moon due to its position relative to the Sun and Earth.
• 🔭 Mercury's observation is challenging due to its proximity to the Sun and small size.
• 🛰️ Space probes like Mariner 10 and MESSENGER have provided significant data about Mercury's surface and composition.
• 🌗 Mercury has a highly elliptical orbit, affecting the intensity of sunlight it receives.
• 🌅 Mercury's day is 59 Earth days long, with a unique 2:3 spin-orbit resonance.
• ⏳ A day on Mercury is very long and peculiar, with the Sun appearing to move backwards in the sky at certain times.
• 💧 Despite its high surface temperatures, there is evidence of water ice in permanently shadowed craters near the poles.

Q & A

What is the closest planet to the Sun and why is it considered 'pretty cool'?

The closest planet to the Sun is Mercury. It is considered 'pretty cool' because, despite its proximity to the Sun and high temperatures, it has a unique set of characteristics such as its quick movement across the sky, phases similar to the Moon, and the presence of water ice in its deep polar craters.

Why does Mercury never appear far from the Sun in the sky?

Mercury never appears far from the Sun in the sky because its orbit around the Sun is much smaller than Earth's, keeping it close to our star. This results in it always being near the Sun's position from our viewpoint.

What significant discovery did Giovanni Zupi make about Mercury in 1639?

In 1639, Giovanni Zupi discovered that Mercury undergoes a complete cycle of phases over time, similar to the Moon. This observation supported the heliocentric model of the solar system, indicating that Mercury orbits the Sun and not the Earth.

What is the average distance of Mercury from the Sun and how does it compare to Earth's distance?

Mercury orbits the Sun at an average distance of about 58 million kilometers, which is roughly a third of the distance between Earth and the Sun.

How long does it take for Mercury to complete one orbit around the Sun?

Mercury completes one orbit around the Sun in approximately 88 days.

Why is it difficult to observe Mercury's surface features from Earth?

Observing Mercury's surface features from Earth is difficult due to its proximity to the Sun, causing it to be low to the horizon and observed through atmospheric turbulence. Additionally, Mercury's small size of about 4900 kilometers in diameter makes it a challenging target for telescopes.

What is unique about Mercury's orbit compared to other planets in our solar system?

Mercury has the most elliptical orbit of any planet in our solar system, ranging from 46 to nearly 70 million kilometers from the Sun. This results in significant variations in the amount of light and heat it receives at different points in its orbit.

How long is a day on Mercury and how does it relate to its year?

A day on Mercury, which is the time it takes for one rotation, is 58.65 Earth days long. Its year, or one orbit around the Sun, is 87.97 Earth days. Interestingly, the ratio of Mercury's day to its year is almost exactly 2/3.

What is the phenomenon where the Sun appears to move backwards in the sky on Mercury?

The phenomenon where the Sun appears to move backwards in the sky on Mercury is due to the combination of its elliptical orbit and the 2:3 spin to orbit ratio. At perihelion, when Mercury is closest to the Sun, its orbital speed is faster than its spin, causing the Sun to appear to move backwards for a few days.

What are the 'rupes' on Mercury's surface and how were they formed?

The 'rupes' on Mercury's surface are cracks that resemble wrinkles on a dried fruit rind. They were formed as the planet's interior cooled and the planet shrank, causing the crust to crack due to compression.

Why does Mercury have a magnetic field despite its slow rotation?

Mercury has a magnetic field due to its large molten iron core, which constitutes a significant portion of its interior. The large core allows for a stronger magnetic field despite the planet's slow rotation.

How is it possible for water ice to exist on Mercury, which is so close to the Sun?

Water ice exists on Mercury in the bottoms of deep craters near its poles, where sunlight never reaches, creating 'cold traps' with temperatures below -170°C. The ice likely comes from comets and asteroids that impacted the planet, and it persists in these craters due to the lack of sunlight.

Outlines

00:00

🌞 Mercury: The Swift Messenger of the Gods

This paragraph introduces Mercury as the closest planet to the Sun, highlighting its extreme temperatures and its association with the Roman god of the same name, known for his speed. It explains Mercury's visibility and rapid movement across the sky, its phases similar to the Moon's, and its unique orbit that never allows it to stray far from the Sun. The paragraph also touches on the planet's elliptical orbit, its fast orbital velocity due to stronger gravitational pull, and the difficulty of observing its surface due to atmospheric conditions and its small size. The discovery of Mercury's day being 59 Earth days long, rather than synchronized with its year, is also mentioned.

05:03

🌌 The Eccentricities of Mercury's Orbit and Time

The second paragraph delves into the peculiarities of Mercury's orbit and the implications for timekeeping on its surface. It discusses the planet's highly elliptical orbit and the 2:3 spin-orbit resonance, which results in a very long and complex day on Mercury. The description includes the phenomenon of the Sun appearing to move backwards in the sky at perihelion due to Mercury's faster orbital speed at that point. The paragraph also covers the exploration of Mercury by space probes like Mariner 10 and MESSENGER, revealing its cratered surface, Caloris Basin, and the presence of rupes. It mentions the planet's large iron core, its measurable magnetic field, and the surprising discovery of water ice in the permanently shadowed craters near its poles.

Keywords

💡Mercury

Mercury is the closest planet to the Sun and the smallest planet in our solar system. It is a central theme of the video, as it discusses the planet's unique characteristics and phenomena. The script mentions its proximity to the Sun, which influences its high temperatures and rapid movement across the sky, as well as its elliptical orbit and the resulting unusual day-night cycle.

💡Heliocentric

Heliocentric refers to the model of the solar system where the Sun is at the center and the planets, including Mercury, orbit around it. This concept is important in the script as it explains the observation of Mercury's phases and its orbit around the Sun, which supports the heliocentric model over the geocentric model.

💡Orbital Velocity

Orbital velocity is the speed at which an object moves in its orbit around another object. In the context of the video, Mercury's high orbital velocity is a result of its strong gravitational pull from the Sun due to its close proximity, which causes it to orbit the Sun once every 88 days.

💡Phases

Phases, in the script, refer to the varying degrees of illumination of a celestial body as seen from another. Mercury undergoes phases similar to the Moon because of its orbit around the Sun, with the script noting that when it is between the Earth and the Sun, we see its dark side, and when it is on the opposite side, we see its fully illuminated side.

💡Elliptical Orbit

An elliptical orbit is a type of orbital path where the object moves in an elongated circle around another body. Mercury's elliptical orbit is highlighted in the script as the most pronounced of any planet, causing it to receive more than twice as much light and heat when it is closest to the Sun compared to when it is furthest.

💡Tidally Locked

Tidally locked refers to a celestial body that has a rotation period equal to its orbital period around another body, causing one side to always face the other body. The script explains that Mercury has a unique 2:3 spin-orbit resonance, meaning its day is 59 Earth days long, and its year is 87.97 Earth days, resulting in a complex relationship between its rotation and orbit.

💡Perihelion

Perihelion is the point in a celestial body's orbit when it is closest to the Sun. The script mentions that Mercury's orbit is highly elliptical, so when it is at perihelion, the tides from the Sun are stronger, affecting its spin and the appearance of the Sun in the sky.

💡Aphelion

Aphelion is the point in an orbit when a celestial body is furthest from the Sun. In the script, it is mentioned in contrast to perihelion, explaining how Mercury's spin and the Sun's position in the sky change at different points in its orbit.

💡Craters

Craters are depressions formed on a planet's surface by impacts from space objects. The script describes Mercury as being covered in craters, which is a result of its lack of atmosphere to protect it from impacts, and it mentions the largest crater, Caloris Basin.

💡Magnetic Field

A magnetic field is a region around a body within which the force of magnetism acts. The script notes that Mercury has a measurable magnetic field, which is surprising due to its slow rotation, but it is likely due to its large molten iron core.

💡Water Ice

Water ice is frozen water found in solid form. The script reveals a surprising fact about Mercury, stating that despite its high surface temperatures, there is water ice in the bottoms of deep craters near its poles where sunlight never reaches, indicating the existence of extreme temperature variations on the planet.

Highlights

Mercury is the closest planet to the Sun, which influences its temperature and visibility.

Mercury is associated with the Roman god of messengers, symbolizing its quick movement across the sky.

Giovanni Zupi's observations of Mercury's phases supported the heliocentric model of the solar system.

Mercury's proximity to the Sun results in a fast orbital velocity and never strays far from it in the sky.

Mercury's elliptical orbit causes significant variations in the amount of sunlight and heat it receives.

Mercury's day is surprisingly only 59 Earth days long, not equal to its year.

Mercury's rotation is tidally locked in a unique 2:3 spin-to-orbit ratio.

A day on Mercury lasts two of its years, with the Sun taking 176 days to complete a sky circuit.

The Sun appears to move backwards in the sky at Mercury's perihelion due to the planet's orbital speed.

Mariner 10 and MESSENGER probes have provided significant data on Mercury's surface and composition.

Mercury's surface is covered in craters and smooth plains with compression folds called rupes.

Mercury has a large iron core, possibly due to a past massive impact or solar vaporization.

Mercury's magnetic field is related to its large molten iron core, despite its slow rotation.

Mercury has a trace atmosphere, primarily composed of solar wind trapped by its magnetic field.

High-velocity impacts on Mercury result in more violent and larger craters compared to Earth.

Astronomers have discovered water ice in the permanently shadowed craters near Mercury's poles.

Craters on Mercury are named after artists, reflecting a unique aspect of its surface mapping.

Transcripts

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 foot—literally, he had

wings on his shoes—and 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 Earth — another 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 we’re

looking at its dark side, and when it’s on the other side of the Sun we’re 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 we’re 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 you’re on the

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

It’ll only be after the second year ends that you’re 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 you’re 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 slowly—rotation

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°

Fahrenheit — astronomers have found water ice on Mercury!

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

These are called “cold 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 we’ve 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.