Mercury A to Z: Hot Poles

Hello, friends!  Welcome back to this year’s A to Z Challenge.  My theme this year is the planet Mercury, and in today’s post H is for:


I remember a certain cartoon that I saw as a kid.  The main character wanted to go exploring the world, to discover lands that were totally new.  This character knew that somebody had already reached the North Pole and that somebody else had already been to the South Pole.  But what about the East Pole?  What about the West Pole?  Surely the East and West Poles had yet to be discovered!

Of course, Earth doesn’t have an East or West Pole.  But Mercury does… sort of.  There are two points on Mercury’s equator, on exactly opposite sides of the planet, that reach maximum temperatures higher than anywhere else on the planet.  These two points are called Mercury’s hot poles.

Now you may be wondering why would only two specific points on Mercury’s equator get extra hot?  Shouldn’t all points along Mercury’s equator get equally hot?  To answer those questions, I first need to explain two key things: Mercury’s orbit is really eccentric, and Mercury’s day is really long.

Mercury’s Eccentric Orbit

Planetary orbits are never perfectly circular.  They are always at least a little bit oval-shaped.  Eccentricity (in the context of astrophysics) is a measure of just how non-circular a planet’s orbit is, and Mercury has the most eccentric orbit of any planet in the Solar System.

As you know, Mercury is the planet closest to the Sun, but thanks to that highly eccentric orbit, sometimes Mercury gets a little extra close to the Sun.

Mercury’s closest approach to the Sun is called perihelion.  As you can see in the highly technical diagram above, whenever Mercury is at perihelion, that’s when things get extra hot.

Mercury’s Super Long Day

A year on Mercury is about 88 Earth days long.  A day on Mercury (by which I mean a solar day, not a sidereal day) is about 176 Earth days long.  That makes a day on Mercury twice as long as a Mercurian year.  In fact, a day on Mercury is exactly twice as long as a Mercurian year.

It’s really important for you to understand that, so I’m going to repeat it: a day on Mercury is exactly and precisely twice as long as a year on Mercury.  So if it’s noon (local time) on Mercury, you’ll have to wait exactly one Mercurian year (one full orbit around the Sun) before it’ll be midnight.  And once it’s midnight, you’ll have to wait another full Mercurian year (another full orbit around the Sun) before it’ll be noon again.

The Hot Poles of Mercury

Now, with those two facts about Mercury in mind, let’s imagine that Mercury is at perihelion.  Mercury is extra close to the Sun, and the dayside of Mercury is getting extra hot.  Now let’s fast forward.  Mercury has orbited all the way around the Sun and returned to perihelion.  It is one full Mercurian year later, but it has only been half of a Mercurian day.  Exactly half.  What was the daylight side of Mercury is now in darkness, and what was the nighttime side of Mercury is now in full daylight.  Where it was noon, one Mercury year ago, it is now midnight, and where it was midnight, it is now noon.

Fast forward another Mercurian year.  Mercury is at perihelion again, and the two sides of the planet have once again swapped places.  It is always like this.  Every time Mercury reaches perihelion, either one side of the planet is facing toward the Sun, or it’s the exact opposite side facing the Sun.  It’s always one way, or the other.  Never anything in between.

And so the two points along Mercury’s equator which always end up being the bullseye center of the planet (from the Sun’s point of view) during perihelion keep reaching maximum temperatures higher than anywhere else on Mercury.  Scientists call these two points the hot poles of Mercury, and they have been officially designated as zero degrees and 180-degrees longitude, for the purposes of mapping Mercury’s surface.

So in a way, these hot poles are kind of like the east and west poles of Mercury.


I’m a little disappointed that there isn’t more info on the Internet about Mercury’s hot poles.  I did find this article from, from when the MESSENGER Mission photographed one of the hot poles.

I also found this heat map of Mercury, which basically just shows how maximum temperatures are not even distributed around the planet’s equator.

9 thoughts on “Mercury A to Z: Hot Poles

    1. There is a reason! I was worried about over complicated this post, so I’m saving that for later.

      Basically, it’s similar to what Earth’s gravity does to the Moon. Earth’s gravity slows down the Moon’s rotation, to the point that the Moon’s rotation stops relative to the Earth. The Sun’s gravity has a slightly looser hold on Mercury, though, so Mercury’s rotation (relative to the Sun) slows down to only half a rotation per orbit, but it doesn’t stop entirely like the rotation of the Moon (relative to Earth).

      Liked by 1 person

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