Mercury A to Z: q

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


Today, I’m going to expand a little on something we already talked about in a previous post.  Back in the late 1800’s, Italian astronomer Giovanni Schiaparelli made a bold effort to observe and characterize the planet Mercury.  He saw several prominent surface features (or at least he thought he saw them), and he determined that Mercury has a rotation period that is approximately 88 Earth days long (we now know this is incorrect).  So what happened?  Where did Schiaparelli go wrong?

In a previous post, I told you about Schiaparelli’s five.  When he looked in his telescope, he kept seeing a surface feature on Mercury that looked like a gigantic numeral five.  Looking at photographs of Mercury today, most people can’t find Schiaparelli’s five, and it’s really unclear what the heck Schiaparelli was looking at.

Specifically, Schiaparelli saw (or thought he saw) this gigantic five whenever Mercury happened to be east of the Sun, as seen in Earth’s sky.  And whenever Mercury appeared west of the Sun, as viewed from Earth, Schiaparelli saw (or thought he saw) a different large surface feature.  On his hand drawn maps on Mercury, Schiaparelli labeled this other large surface feature “q” (always lower case).

Unlike Schiaparelli’s five, which supposedly looked like the number five, q did not look like the letter q.  In Schiaparelli’s drawings of q, it reminds me a little of that Æ symbol (the combination of A and E) that you sometimes see in fantasy novels, very old English literature, and a few modern languages like Icelandic or Norwegian.  I’m not sure why this Æ feature got named q, but Schiaparelli labeled several other surface features on Mercury with lower case letters, so there must have been some method to his madness.

The important thing is that the five and q appeared, consistently, when Mercury reached certain points in his orbital path—either east of the Sun for the five or west of the Sun for q, as viewed from Earth.  Or at least they appeared consistently whenever Schiaparelli went looking for them.

The surface of Mercury is covered in light and dark splotches, making it a bit like a Rorschach test.  You see whatever your brain wants to see in those light and dark patterns.  I have tried my best to match Schiaparelli’s hand drawn maps to actual photos of Mercury.  I can kind of see the five, some of the time, but it takes a little squinting and a lot of imagination to make things line up right. I cannot find q, no matter how hard I try.

But Schiaparelli wasn’t too far off to believe he was seeing the same surface features time and again, whenever Mercury reached specific points in his orbital path.  Schiaparelli was only half wrong about that.  Exactly half wrong, in a sense.  I will try to explain what I mean by that in tomorrow’s post.


For this third time this month, I’d like to recommend Mercury, by William Sheehan.  It’s part of the Kosmos series, published by Reaktion Books, and it includes a lengthy and fascinating discussion of Schiaparelli and his sightings of five and q on Mercury.

Mercury A to Z: Prokofiev Crater

Hello, friends!  My theme for this year’s A to Z Challenge is the planet Mercury, a planet that just never seems to get the same love and attention as all the other planets of the Solar System.  In today’s A to Z post, P is for:


We haven’t talked about this as much as I expected to, but there is ice on Mercury.  The frozen water kind of ice.  Craters near Mercury’s north and south poles are shielded from direct sunlight throughout the Mercurian year.  As a result, despite the proximity of the Sun, the bottoms of these craters are dark enough and cold enough to allow ice to remain frozen.  One of the largest and, perhaps, most well studied of Mercury’s icy craters is Prokofiev Crater, near Mercury’s north pole.

Prokofiev Crater is named after Russian classical composer Sergei Prokofiev.  The crater is slightly removed from the north pole, and so part of the crater floor does get exposed to direct sunlight during part of the Mercurian year.  However, there is still a large region inside the crater that remains in permanent shadow.  That same region also happens to be a radar bright spot, meaning that radar beams directed at Mercury reflect off that region in an especially bright and brilliant way.

These super bright radar reflections could be caused by water, but they could also be caused by large deposits of metal.  In the early 2010’s, NASA’s MESSENGER space probe took a much closer look, using multiple scientific instruments, and confirmed that the radar reflections in Prokofiev (and other neighboring craters) are indeed caused by frozen water.

Now ever since I first heard about ice on Mercury, there was one thing I wanted to know: is it possible to go iceskating on Mercury?  I’ve done research on this in the past, and I couldn’t find a clear answer.  Is the ice exposed on the surface, or is it covered by a layer of dust and rock?  Most sources I looked at were vague about that.  I got the impression that nobody really knew for certain.  But it seems there’s been some new research since the last time I looked into this.

In most cases, Mercury’s ice is covered by some sort of insulating layer.  But in the largest, deepest, and coldest craters on Mercury—craters like Prokofiev Crater, as well as nearby Kandinsky Crater, Tolkien Crater, and Chesterton Crater—the ice is most likely out in the open, exposed or partially exposed, just waiting for somebody with a spacesuit and a pair of ice skates to show up.

Someday, if humans decide (for some reason) to settle on Mercury, places like Prokofiev Crater will be prime real estate.  Not just for iceskating but for basic survival reasons. Humans need water, after all.

Personally, I wouldn’t mind living in nearby Tolkien Crater, because that’s the kind of nerd that I am.  However, I’d strongly advise that nobody should live in Lovecraft Crater, a large, icy crater near Mercury’s south pole.  If you’re familiar with H.P. Lovecraft’s work, you’ll know why.


Here are maps of Mercury’s north and south poles, showing the locations of radar bright spots, regions in permanent shadow, and where the two overlap.

Here’s an article from NASA explaining the different experiments MESSENGER used to confirm the presence of water in Mercury’s polar craters.

And here is a 2022 paper published in The Planetary Science Journal confirming that water ice lies exposed on the surface inside Prokofiev Crater.

Mercury A to Z: Orbiting Mercury

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


On April 1, 2012 (note that date), NASA announced the discovery of a moon orbiting Mercury.  NASA went on to propose naming this newly discovered moon Caduceus, after the coiled-snake-shaped staff that Mercury carried in ancient Roman mythology.  This would have been a very exciting discovery except, of course, this was announced on April 1st.  Maintaining orbit around Mercury is hard… so hard it’s basically impossible.  The idea of a moon maintaining orbit around Mercury is so absurdly impossible that NASA thought it would make a good April Fool’s Day joke.

But for the sake of argument, let’s pretend that Caduceus is real.  Let’s pretend that Mercury does have a little, tiny moon, similar to the asteroid-like moons of Mars.  What would happen to Mercury’s moon?  Well, very rapidly, she’d find herself caught in a gravitational tug-of-war between Mercury and the Sun—and sadly, this is a tug-of-war that Mercury could never, ever hope to win.

With each successive orbit around Mercury, Caduceus would be feel the increasing and decreasing gravitational force of the Sun.  When she circles around to the dayside of Mercury, the Sun’s pull would be stronger; when Caduceus circles around the Mercury’s nightside, the Sun’s pull would be weaker.  A little stronger, a little weaker, a little stronger, a little weaker, over and over again.  If Caduceus’s orbit started off as near circular, that orbit would gradually stretch into a wider and and wider oval shape.  Eventually, inevitably, that oval would become so stretched out that it would extend beyond the reach of Mercury’s gravity.

Caduceus would not necessarily crash into the Sun after that.  Remember that every action has an equal and opposite reaction.  Every time the Sun’s gravity pulled Caduceus hard one way, she would then swing just as hard in the opposite direction.  So when the moment came and Caduceus finally broke free of Mercury’s gravity, there’s a very good chance that she would launch herself off into space like a child leaping from a swing set.

But regardless of Caduceus’s ultimate fate (crashing into the Sun or flinging herself off into space), the outcome for Mercury is the same.  He loses his moon.  Mercury will always lose his moon, no matter what.  Even artificial satellites, like MESSENGER or BepiColombo, cannot maintain orbit around Mercury for long without their thrusters.  Orbiting Mercury is really, really hard work for a spacecraft, and for a small, asteroid-like moon?  It’s basically impossible.

So if you have ever wondered why Mercury doesn’t have a moon, now you know why Mercury doesn’t have a moon.


Here’s NASA’s April Fool’s Day announcement about the discovery of Caduceus.

And here’s an article from Universe Today entitled “How Many Moons Does Mercury Have?” written by a good friend of this blog, Matt Williams.

Mercury A to Z: NASA Missions to Mercury

Hello, friends!  We are halfway through this year’s A to Z Challenge.  I have to admit when I picked the planet Mercury as my theme for this year’s challenge, I was a little worried I wouldn’t be able to find enough material for a full alphabet worth of posts.  But Mercury has not disappointed me.  There are more than enough Mercury facts to cover!  In today’s post, N is for:


Which planet is closest to the Sun?  More often than not, the answer is probably Mercury.  That may seem counterintuitive, since the orbital path of Venus (the 2nd planet) lies between the orbital paths of Mercury (the 1st planet) and Earth (the 3rd planet).  But consider it this way: every time Venus and Earth happen to be on opposite sides of the Sun, Mercury is somewhere in between.  So on average, Mercury ends up being the closest planet to Earth more often than Venus, Mars, or any other planet.

And yet, despite the fact that Mercury is so close to Earth so much of the time, Mercury is still one of the absolute hardest places for Earth-launched spacecraft to reach.  The problem is the Sun.  The Sun is very big, and the gravitational pull of the Sun is very strong.  For our purposes, imagine that the Sun is “down,” and you’ll start to see what the problem is.  Flying to Mercury is an awful lot like falling toward the Sun.

Now I do want to acknowledge that I’m glossing over a whole lot of technical details here.  The purpose of this blog post is not to teach you the science and mathematics behind orbital mechanics.  All I want is to give you a small taste of what makes flying to Mercury so very challenging, so that you can better appreciate the amazing accomplishments of NASA’s Mariner 10 and MESSENGER Missions.


NASA’s original plan for Mariner 10 was to aim carefully and fly by Mercury one time.  A certain Italian astronomer had a better idea, involving a never-before-attempted gravity assist maneuver near Venus.  This tricky maneuver allowed Mariner 10 to perform three flybys of Mercury for the price of one.

Gravity assist maneuvers, where a spacecraft uses a planet’s gravity to make a “for free” course adjustment, are standard practice in spaceflight today, but Mariner 10 was the first to ever attempt such a thing.  Mariner 10 was also the first spacecraft to visit two planets, collecting some data about Venus before continuing on its way to Mercury (Mariner 10 was also lucky enough to collect data from a nearby comet—another first in space exploration).

Mariner 10 flew by Mercury in March of 1974, September of 1974, and March of 1975.  During those three encounters, Mariner 10 discovered Mercury’s magnetic field and Van Allen radiation belt.  Mariner 10 also discovered Caloris Basin, Kuiper Crater, and many other important surface features.  Unfortunately, only half of the planet was in daylight during Mariner 10’s three flybys, and it was always the same half of the planet, so the other half of Mercury remained unseen and mostly unknown for decades thereafter.

Shortly after Mariner 10’s third flyby of Mercury, the spacecraft ran out of fuel for attitude control.  Without attitude control, the spacecraft couldn’t keep its communications system pointed toward Earth.  So before contact was lost, mission control ordered the spacecraft to shut down.  The now defunct spacecraft is still, presumably, orbiting the Sun somewhere near the orbit of Mercury.


MESSENGER is an acronym for MErcury Surface, Space Environment, Geochemistry, and Ranging.  The name is also a reference to Mercury’s role in Roman mythology as the messenger of the gods.  The MESSENGER Mission was funded through NASA’s Discovery Program, a highly competitive program for space missions that can be done on a tight and highly-restrictive budget.

MESSENGER launched on August 3, 2004.  Unlike Mariner 10’s series of flybys, the plan for MESSENGER was to enter orbit of Mercury.  This required a much longer and more intricate flight trajectory, with one gravity assist maneuver at Earth, two at Venus, and a series of three maneuvers at Mercury to help match Mercury’s orbital velocity.  MESSENGER achieved Mercury orbit on March 18, 2011, after seven-plus years of travel.

Over the next four years, MESSENGER photographed the entire surface of Mercury (including the half of the planet Mariner 10 couldn’t see), continued to study Mercury’s magnetic field, and revealed Mercury’s internal structure through a process called gravity mapping, which involved measuring subtle variations in a planet’s gravitational field.  Oh, and who could forget this?  MESSENGER also discovered water on Mercury.  Believe it or not, there is water (frozen as ice) inside craters around the north and south poles of Mercury.

In early 2015, MESSENGER ran out of fuel, and the spacecraft’s orbit around Mercury began to deteriorate.  On April 30, 2015, MESSENGER finally crashed into the planet’s surface, giving the most heavily cratered planet in the Solar System one additional crater.


The work of NASA’s Mariner 10 and MESSENGER Missions will be continued by BepiColombo, a collaborative mission by ESA (the European Space Agency) and JAXA (the Japanese Aerospace eXplotation Agency).  I wrote about BepiColombo in a previous post.

Now I want to correct something I’ve been saying about BepiColombo in previous posts.  I’ve said that BepiColombo will arrive at Mercury in 2025; that’s not quite right.  BepiColombo will enter Mercury orbit in 2025, but much like MESSENGER, BepiColombo needs to perform several gravity assist maneuvers near Mercury first.  Two of those gravity assists have already happened, and during those maneuvers, BepiColombo already started snapping photos and gathering science data.

So every time this month that I said only two spacecraft have ever visited Mercury, that was incorrect.  BepiColombo has already become Mercury’s third visitor.


NASA has posted some nice articles about Mariner 10, MESSENGER, and BepiColombo on one of their educational websites.  Click these links to check them out:

Mercury A to Z: Magnetosphere

Hello, friends!  Welcome back to the A to Z Challenge, a month long blogging event.  For this year’s challenge, my theme is the planet Mercury, and in today’s post M is for:


Back in the 1960’s and 70’s, Earth’s magnetosphere seemed like something special.  Both the American and Soviet space programs had sent missions to the Moon, Venus, and Mars.  None of those places had protective magnetic fields, like Earth does.  It was assumed that Mercury would be the same.  But in 1974, as NASA’s Mariner 10 space probe approached Mercury, a charged particle experiment got some unexpected readings, and Mariner 10’s magnetometer picked up a weak magnetic field.

Mercury’s magnetic field is said to be about 1% as strong as Earth’s.  That’s very weak, but 1% is not 0%.  And Mercury even has his own Van Allen belt, a region encircling the planet where the magnetic field gathers and concentrates radiation.  Again, Mercury’s Van Allen belt is not as powerful as Earth’s, but it is there.

For scientists in the 1970’s, the discovery of Mercury’s magnetosphere was difficult to explain.  To generate a magnetic field, a planet needs two things:

  • A molten metal core
  • A rapid rotation rate

Mars is a rather small planet, and small planets lose their internal heat very quickly.  Since Mars doesn’t have enough internal heat left to maintain a molten metal core, Mars can’t generate any meaningful magnetic field.  Venus, meanwhile, is a very large planet—almost as large as Earth.  She probably does have a molten metal core, BUT her rotation rate is extremely slow.  It takes Venus over 200 Earth days to rotate once.  That’s way too slow to generate a magnetic field.

As we’ve discussed previously, Mercury has a very slow rotation rate.  Mercury is also very small.  That should be a double whammy for Mercury’s magnetosphere, and yet the magnetosphere persists anyway.  Somehow, Mercury retained enough internal heat to have a molten metal core.  And somehow, Mercury overcame his own slow rotation rate to keep a weak magnetosphere alive.  Science accepts that Mercury has done these things.  But how?  How did Mercury do these things?

I don’t think anyone can answer that yet.  Mercury’s magnetic field is still something of a mystery.  Hopefully the upcoming BepiColombo Mission will help find some answers.


Here’s a brief article from Scientific American about the magnetic fields of all the planets in the Solar System (well, all the planets that have magnetic fields, at least—sorry, Venus and Mars).

And here’s an article from about how Mercury’s magnetic field may have changed over time—at some point in the past, it may even have been as strong as Earth’s.

Mercury A to Z: Lobate Scarps

Hello, friends!  Welcome back to this year’s A to Z Challenge.  My theme for this year’s challenge is the planet Mercury, an often under-appreciated but still fascinating little world.  In today’s post, L is for:


The planet Mercury is shrinking!!!  How do scientists know this?  Well, the story begins with photos taken by NASA’s Mariner 10 space probe, back in the 1970’s.  Mariner 10 found these long, serpentine features on Mercury, winding their way across the planet’s surface.  Scientists decided to call these strange features “lobate scarps” (a sciency way of saying “curvy cliffs”).

Mariner 10 only photographed part of Mercury, but in the 2010’s, NASA’s MESSENGER space probe was able to map almost all of the planet’s surface, confirming that these lobate scarps are everywhere.  Rough, heavily cratered terrain?  Covered in scarps.  Smoother, flatter terrain?  Interrupted by scarps.  Young terrain, old terrain, middle-aged terrain?  Doesn’t matter.  The lobate scarps are all over the place (though one source I looked at suggested that some parts of Mercury are more scarp-y than others—especially parts of the southern hemisphere).

Images from both Mariner 10 and MESSENGER show scarps that are hundreds of kilometers long, and by looking at the scarps’ shadows, scientists are able to determine how tall they are—up to three kilometers in height, in some cases!  Just imagine that: a cliff three kilometers tall!  The tallest cliff on Earth isn’t even half that high.

The most likely explanation for all this is that Mercury is shrinking.  The planet’s core is cooling off, plus gasses trapped beneath Mercury’s crust are slowly leaking to the surface and escaping into space.  This ongoing loss of internal heat and mass puts stress on the planet’s crust, causing thrust faults and earthquakes Mercury-quakes.  I’ve read several sources that said Mercury is shriveling up like a raisin, but it sounds to me more like Mercury is very slowly crumpling like a tin can.

I’ve seen many different estimates for how much Mercury has shrunk, from as little as 2 kilometers in radius to as much as 20.  Since lobate scarps are found on young and old terrain alike, this process of global shrinkage must have been happening for billions of years, and it’s likely continuing to happen to this day.


Here’s an article from The Atlantic, quoting one of those larger estimates for how much Mercury has shrunk.

And here’s an article from, quoting a much lower estimate.

And here’s another article from, which talks about how some parts of Mercury are more scarp-y than others.

Lastly, if you want to get a better sense of what a lobate scarp looks like, click here to see a picture of one cutting across one of Mercury’s craters.

Mercury A to Z: Kuiper Crater

Hello, friends, and welcome back to the A to Z Challenge.  For this year’s challenge, my theme is the planet Mercury, and in today’s post K is for:


Mercury is a big, grey rock covered in craters.  In fact, Mercury is the most heavily cratered object in the whole Solar System.  So what’s so special about Kuiper Crater?  Why am I devoting an entire blog post to this one crater in particular?  Well, because Kuiper Crater is a surprisingly young and bright-looking crater among all the darker, older-looking craters of Mercury.

Kuiper Crater was officially discovered in 1974 by NASA’s Mariner 10 space probe.  Earth-based astronomers had seen it before (it is, as I said, very bright-looking), but they didn’t realize what it was.  Giovanni Schiaparelli, for example, apparently thought it was a cloud.  The crater was named after famed planetary scientist Gerard Kuiper, who was highly involved in the Mariner 10 mission but who, unfortunately, died only a few months before Mariner 10 reached Mercury (this was, by the way, before the I.A.U. established the rule that craters on Mercury should be named after artists, writers, and musicians).

Now you may be wondering how scientists can look at a crater and tell how old it is.  Unlike with people, lines and wrinkles are a clear sign of youth for a crater.  Fresh, recently formed craters have tall crater walls, sharply defined crater rims.  They have deep crater basins, and ejecta scattering away from a crater after impact leave obvious trails that radiate away from the crater across the planetary surface.  In the case of Mercury, newer craters also tend to be brighter in color.

Time wears all these signs of youth away.  Crater walls slowly crumble.  Crater basins get filled in with debris.  Those lines radiating away from newer craters gradually start to disappear.  And for craters on Mercury, solar and cosmic radiation causes the bright color to slowly fade away.

Looking at Kuiper Crater, it is very line-y, very wrinkly.  It’s also very bright, as I said before.  Kuiper Crater is, in fact, the single brightest spot on all of Mercury.  There does seem to be some scientific debate over Kuiper Crater’s exact age, but everyone seems to agree that it must be very young, that it formed very recently—within the last few hundred million years, perhaps.  That’s not a long time when compared to the age of the Solar System.

Much like Earth, Mercury’s geologic history is divided up into different eras.  Kuiper Crater is young enough and prominent enough that it lends its name to the current era of Mercury’s history: the Kuiperian Period.


This was not an easy topic to research.  I got most of my information for today’s post from this paper, titled “Revised Constraints on Absolute Age Limits for Mercury’s Kuiperian and Mansurian Stratigraphic Systems.”

Mercury A to Z: Jumping on Mercury

Hello, friends!  It always seems like Mercury doesn’t get the same love and attention as the other planets, which is why I chose Mercury as my theme for this year’s A to Z Challenge.  In today’s post, J is for:


If you’re anything like me, you probably lie awake at night wondering what it would feel like to walk on another world.  With each step, what would feel different, and what would feel the same?  It’s the kind of thing you can read about, or you can watch videos from the Apollo era to see what walking on another world looks like.  But to get the actual sensory experience of moving about in low gravity?  I doubt I’ll ever get to experience that for myself.

But while I may never have the first hand physical experience of walking in low gravity, a few years back I read a paper that clarified some things for me, at least intellectually.  The key thing to understand is that gravity helps you walk, more so than you probably realize.

When you take a step, you first lift one foot off the ground.  This requires your muscles to do work.  This takes energy.  But when you put your foot down again, gravity helps you get your foot back down to the ground.  Gravity makes it so your muscles don’t have to do quite as much work during your foot’s downward motion.  Gravity saves you from expending just a little bit of extra energy as you finish taking a step.  But if you’re on the Moon or Mars (or Mercury), there’s less gravity, and so your muscles get less help.  It takes a little more energy than you might expect to put your foot back down to the ground.

This is why the Apollo astronauts ended up “loping” or “bunny hopping” all over the surface of the Moon.  In interviews, the astronauts often said it just felt more natural and comfortable to move about that way.  Scientifically speaking, it’s a matter of metabolic efficiency.  Walking is a metabolically efficient way to get around on Earth, but without Earth-like gravity to help bring your foot back down to the ground, the metabolic efficiency of walking is diminished.  The lower the gravity gets, the less efficient walking becomes, and if the gravity gets low enough, then skipping, hopping, and jumping start to feel, by comparison, a whole lot easier.

Mercury is about the same size as the Moon, but due to Mercury’s ginormous iron core, Mercury is a whole lot denser than the Moon.  Higher density means higher gravity, and the surface gravity on Mercury is roughly twice the surface gravity on the Moon (or roughly the same as the surface gravity on Mars, even though Mars is a much larger planet).  But Mercury-like (or Mars-like) gravity is still only one-third of the gravity we’re accustomed to here on Earth.

So if you ever want to go for a stroll on the surface of Mercury, first: remember to wear a spacesuit that can handle the extreme temperatures.  And second, don’t feel embarrassed if you end up jumping, hopping, or skipping all over the place.  It’s all for the sake of metabolic efficiency.


Here’s a short video from the Apollo era, showing astronaut Gene Cernan bunny hopping down a slope on the Moon while talking about how it is “the best way” to travel.

And here’s a short compilation of videos, also from the Apollo era, showing various astronauts tripping and falling all over themselves in lunar gravity.

And lastly, here’s the paper I mentioned, titled “Human Locomotion in Hypogravity: From Basic Research to Clinical Applications.”  It’s not an easy read, but if you really want to understand what “human locomotion” would feel like on other worlds, this paper is the absolute best resource I’ve ever found.

Mercury A to Z: International Astronomical Union

Hello, friends!  The planet Mercury doesn’t get nearly as much love and attention as he deserves, which is why I picked Mercury as my theme for this year’s A to Z Challenge.  In today’s post, I is for:


The International Astronomical Union (I.A.U.) has the awesome responsibility of naming astronomical objects and defining important astronomical terminology.  The I.A.U. is probably most famous (or infamous) for their 2006 decision to change the definition of the word planet in such a way as to exclude Pluto from the planet club.  But we’re not going to talk about that today.  Instead, I want to tell you about one of my favorite Mercury fun facts.  According to I.A.U. rules, all craters on Mercury are supposed to be named after artists, musicians, and writers.

Apparently the I.A.U. was originally planning to name Mercury’s craters either after birds or cities.  It was Carl Sagan, always one for blending the sciences with the humanities, who lobbied for naming the craters on Mercury after poets and authors.  The I.A.U. ultimately went with Sagan’s idea, expanding it to include musicians, painters, sculptors, etc.

Some craters did have well established names before the I.A.U. made that rule, so Caloris Basin (named after the Latin word for heat) and Kuiper Crater (which we’ll visit in just a few days) are exceptions to the rule.  But otherwise, the I.A.U. has been consistent about following their naming convention.  There’s a Shakespeare Crater, a Beethoven Crater, a Mark Twain Crater… there are craters named after John Lennon and Chuck Berry… craters named after J.R.R. Tolkien and H.P. Lovecraft….  Maya Angelou has a crater.  Dr. Seuss has a crater.  There’s even a crater named after Walt Disney (and if you’ve never seen a picture of Disney Crater, you need to see a picture of Disney Crater).

So if you would like to have a crater on Mercury named after you, the I.A.U. says you only have to meet two criteria:

  • Be famous or be considered historically significant as a writer, artist, or musician for more than fifty years, and…
  • Be dead for at least three years.

The I.A.U. also tries to avoid using names that are too heavily associated with politics, the military, or religion.  So try to avoid getting too tangled up in those things during your lifetime.


Curious to see if your favorite artist, writer, or musician has a crater on Mercury?  Here’s a list of named craters on Mercury.

Wondering how the I.A.U. names things other than craters on Mercury?  Here’s a list of official themes the I.A.U. uses for naming surface features on the various planets and moons of the Solar System.

Lastly, I know I recommended this book before, but I’m going to recommend Mercury, by William Sheehan, once again.  It’s part of the Kosmos series.  That’s where I learned about Carl Sagan’s role in coming up with the I.A.U.’s Mercurian crater naming convention.

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.