How Does Planet Mercury Work?

Mercury is the closest planet to the Sun, but it's often also the closest planet to Earth. Learn why -- and lots of other strange-but-true facts about Mercury -- in this episode of BrainStuff.

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Episode Transcript

Speaker 1 (00:01):
Welcome to brain Stuff production of I Heart Radio. Hey
brain Stuff, Lauren Bogelbaum. Here. You'd like to think you
know your neighbors, but sometimes they'll catch you off guard.
That diminutive world called Mercury is the first planet from
our Sun, Venus's second, Earth is the third, and dear
old Mars is number four common knowledge, but that numbering

(00:25):
obscures certain truths. With its tight orbit around the Sun,
Mercury can't ventures far away from Earth as Venus and
Mars can during their larger orbits, so most of the time,
Mercury is closer to Earth than any other planet, despite
its place in the planetary lineup. And that's far from
the weirdest thing about Mercury's travel patterns. Of all the

(00:47):
planets in the Solar System, Mercury orbits our Sun at
the fastest rate. It zips around our star with an
average speed of around one hundred and five thousand, nine
hundred miles per hour or a hundred and seventy thousand,
five hundred kilometers per hour. The planet's namesake was a
real speedster too. In Roman mythology, Mercury was the fleet
footed god of commerce and communication, among other things. Recognizable

(01:10):
by his winged sandals. Earth takes a little more than
three hundred and sixty five days to loop around the sun.
Mercury does the same thing in a fraction of the time.
One Mercury in year is equal to a brisk eighty
eight earth days vacationing there would be a disorienting experience
on Mercury. The years may be quick, but the days

(01:31):
are very long. The planet finishes a new rotation around
its own axis once every fifty nine earth days, which
means that every day on Mercury takes up almost exactly
two thirds of every year. But due to some further strangeness,
each day on Mercury isn't guaranteed to have a sunrise
and a sunset. If we define a day not as

(01:54):
a complete rotation of a planet around its axis, but
rather as a solar day, a one complete day night cycle,
the amount of time it takes our son to return
to a specific point in the sky, then a single
day on Mercury is a hundred and seventy six earth
days long. That's the rough equivalent of two full Mercury
in years. That's because Mercury's orbit is what we refer

(02:17):
to as eccentric. No, not eccentric like Willy Wonka is eccentric.
Eccentricity is a term astronomers used to describe the shape
of a planet or moon's orbit. No planet orbits its
star in a perfect circle. The extent to which had
given orbit deviates from being circular is cold. It's eccentricity.
If an orbit was one circular, would say it had

(02:39):
an eccentricity of zero. Let The records show that Earth's
orbit boasts an eccentricity of just zero point zero one seven,
so it's almost a circle, but not quite. Mercury's orbit
looks more squashed or elliptical. By comparison of all the
planets in our beloved Solar system, Cury has the most

(03:01):
eccentric orbit. The amount of space between this world and
our Sun ranges from just twenty nine million miles to
thirty six million miles during the course of its elliptical
trip around the Sun. That's a forty six million to
fifty eight million kilometers. Consequently, Mercury's travel speed increases as
it gets closer to the star. Those changes in speed

(03:22):
and distance from the Sun account for the strange solar days.
When you view the Sun from some places on Mercury,
it appears to freeze in place part way through its
journey across the sky, then move backwards before going forward
again in its route to the opposite horizon. Depending on
where you are on the planet, this might mean that
you get to sunrises in a row as the Sun

(03:42):
dips back below the skyline before fully rising, or two
sunsets in a row as it pops back up before
fully setting, And of course all of this reeks havoc
on the planet's temperatures on Mercury, they can range as
high as eight hundred degrees fahrenheit that's four thirty celsius,
or as low as negative two nine degrees fahrenheit or

(04:03):
negative one eight celsius. As you could expect, the side
of the planet that's facing the Sun at any time
will be a good deal hotter, and on hot days
we could all use some ice in NASA's Messenger spacecraft
observed deposits of frozen water on Mercury. The ice was
found at the bottoms of deep craters around the northern pole,

(04:24):
which are protected from the sun all year long due
to Mercury's lack of tilt around its axis of rotation.
Earth tilts twenty three point five degrees on its axis,
giving us seasons and exposing the poles to the Sun.
But Mercury only tilts two degrees, meaning it spins almost
completely upright, protecting its poles and preventing seasons from occurring.

(04:45):
But back to that ice. Paradoxically, Mercury's hottest temperatures could
be responsible for some of the ice that it possesses.
Our Sun is constantly releasing charged protons and electrons in
streams known as solar winds. Earth is shielded by a
powerful magnetic field, which deflects many of these. Mercury has
had a magnetic field of its own four around three

(05:07):
point nine billion years. However, it's significantly weaker than the
one we enjoy here on Earth. If a recent scientific
model is correct, The intense heat of the planet's sun
facing side triggers chemical reactions between solar wind particles and
minerals buried in the soil. These are thought to create
traveling water molecules that later become ice. Of all the

(05:30):
frozen water on Mercury may have been generated this way,
Mercury doesn't have any moons to call its own. As
a matter of fact, With a diameter of just three thousand,
thirty miles or four thousand kilometers, Mercury isn't much bigger
than Earth's Moon easily. Mercury is the smallest planet in
our Sun's orbit, yet evidence tells us that it used

(05:53):
to be larger. Mercury is interesting to look at from
a topographic standpoint. The thin atmosphere for his very little
protection from asteroids, so impact craters are abundant. A single
photo taken by the Messenger Probe in two thousand eight
shows seven hundred and sixty three identifiable craters within a
region of the planet's surface just a hundred and seventy

(06:13):
two miles wide. That's two hundred and seventy six kilometers.
It's traditional to name these craters after writers, given that
eloquence is one of the realms of the mythological god Mercury.
Truman Capote, Batteline la Engel, Betty Alver, and John Lennon
are just some of the visionaries with their own namesake craters.
The planet's face is also marked by fault scarps. These

(06:36):
are cliff ridges that were first sighted on Mercury in
the nineteen seventies. Some of the smaller ones appeared to
be around just fifty million years old, making them geologically
pretty young. Mercury itself formed around four point five billion
years ago. Since then, its metallic core has been cooling
down at a rapid pace, and as a result, the

(06:56):
whole planet is currently shrinking. The diameter of mercury may
have contracted by as much as eight and a half
miles or fourteen kilometers over the eons. This has had
a tremendous effect on the planet's rocky outer surface. Those
previously mentioned fault scarps are born when crust materials break
apart and press into each other, forcing some terrain upward.

(07:18):
And because it's tectonically active in its own peculiar way, Mercury,
like Earth, may experience surface level earthquakes. Mercury quakes will
definitely be a topic worth investigating as we plot out
future pro missions to our curious little neighbor. Today's episode

(07:39):
was written by Mark Mancini and produced by Tyler clang Or.
More on this and lots of other curious topics, visit
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