Mercury’s Spin‑Orbit Resonance, Core, and Polar Ice Explained

Name: Mercury: Crash Course Astronomy #13
Uploaded: 2015-04-16T21:00:00+00:00
Duration: 10 min 17 s
Channel: CrashCourse
Description: Summary and key takeaways on Mercury: Crash Course Astronomy #13: Summary & Key Takeaways, covering Historical Context Mercury is one of the seven objects

CrashCourse

Apr 16, 2015

•

10 min video

•

2 min read

YouTube video ID: P3GkZe3nRQ0

Source: YouTube video by CrashCourse — Watch original video

PDF

Mercury is one of the seven objects visible to the naked eye, and ancient cultures linked its swift motion to the Roman messenger god. In 1639 Giovanni Zupi observed that Mercury shows phases, a discovery that bolstered the heliocentric model of the solar system.

Orbital Dynamics

Mercury circles the Sun every 87.97 days at an average distance of 58 million km. Its rotation period of 58.65 Earth days creates a 2:3 spin‑orbit resonance, meaning the planet rotates three times for every two revolutions around the Sun. The orbit is highly elliptical, ranging from 46 to 70 million km. Because orbital speed peaks at perihelion while the rotation rate stays constant, the Sun can appear to pause, reverse, and rise again for an observer at the right location.

Surface and Interior

Data from Mariner 10 in the 1970s and the MESSENGER orbiter in 2011 reveal a heavily cratered landscape. The 1,600 km‑wide Caloris Basin dominates the surface, and compression folds called “rupes” mark the planet’s cooling and contraction. Craters bear the names of artists such as Botticelli, Vivaldi, and Tolkien. Mercury’s density indicates a massive iron core that potentially extends 75 % of the way to the surface. A thin atmosphere composed of sodium, calcium, and other elements forms a comet‑like tail, sustained by the magnetic field and impact‑ejected material.

Environmental Extremes

Surface temperatures soar to 430 °C (800 °F) on the sun‑facing side, yet permanently shadowed polar regions act as “cold traps” where temperatures stay below –170 °C. Water ice resides in these traps, likely delivered by comet and asteroid impacts, demonstrating a stark contrast between the planet’s hottest and coldest environments.

Mechanisms Behind the Phenomena

The 2:3 spin‑orbit resonance arises because solar tides slow Mercury’s rotation until a stable configuration is reached; the stronger tidal forces at perihelion lock the planet into this ratio. The apparent reversal of the Sun’s motion results from the mismatch between rapid orbital speed at perihelion and the planet’s steady rotation. Mercury’s trace atmosphere persists as the magnetic field captures solar wind particles while high‑velocity impacts continuously release surface material. Impacts are especially energetic because Mercury’s high orbital velocity adds to the relative speed of incoming bodies, producing larger craters despite the planet’s weaker gravity.

Takeaways

Mercury’s 2:3 spin‑orbit resonance means it rotates three times for every two orbits around the Sun, a state locked by strong solar tides at perihelion.
The planet’s dense iron core may extend three‑quarters of the way to the surface, accounting for its unusually high density.
Mariner 10 and MESSENGER missions revealed a heavily cratered surface, the massive Caloris Basin, and compression folds called rupes.
Temperatures on Mercury range from a scorching 430 °C on the day side to below –170 °C in permanently shadowed polar cold traps that hold water ice.
A thin atmosphere of sodium and calcium forms a comet‑like tail, sustained by the magnetic field and material ejected during high‑energy impacts.

Frequently Asked Questions

Why does Mercury have a 2:3 spin‑orbit resonance?

Solar tides gradually slowed Mercury’s rotation until the planet settled into a stable 2:3 ratio, where it rotates three times for every two solar orbits. The elliptical orbit intensifies tidal forces at perihelion, locking the resonance.

How can water ice exist on Mercury’s extremely hot surface?

Ice persists in permanently shadowed polar craters called cold traps, where temperatures remain below –170 °C despite the planet’s overall heat. Impacts from comets and asteroids likely delivered the ice that now resides in these permanently dark regions.

Who is CrashCourse on YouTube?

CrashCourse is a YouTube channel that publishes videos on a range of topics. Browse more summaries from this channel below.

Does this page include the full transcript of the video?

Yes, the full transcript for this video is available on this page. Click 'Show transcript' in the sidebar to read it.

Helpful resources related to this video

If you want to practice or explore the concepts discussed in the video, these commonly used tools may help.

High Quality Astronomical Telescope For Beginners Recommended

Allows the user to observe Mercury and other planets directly, helping them visualize the phases and orbital movements discussed in the lecture.

Amazon →

Solar System Planetary Model Kit

Provides a physical representation of orbital mechanics, helping the user visualize the 2:3 spin-orbit resonance and elliptical paths.

Amazon →

Book About The History Of Astronomy

Deepens the user's knowledge of the observational history mentioned, such as the work of early astronomers like Giovanni Zupi.

Amazon →

Coffee Table Book Solar System Photography

Contains high-resolution imagery of planetary surfaces and craters, providing visual context for the geological features like the Caloris Basin.

Amazon →

Links may be affiliate links. We only include resources that are genuinely relevant to the topic.

Summarize another video

Full Transcript YouTube

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.

Help & FAQ

Venus: Crash Course Astronomy #14

CrashCourse

Apr 18, 2026

Orbital Dynamics

Surface and Interior

Environmental Extremes

Mechanisms Behind the Phenomena

Takeaways

Frequently Asked Questions

Why does Mercury have a 2:3 spin‑orbit resonance?

How can water ice exist on Mercury’s extremely hot surface?

Who is CrashCourse on YouTube?

Does this page include the full transcript of the video?

Helpful resources related to this video

Share This Summary

Embed This Summary