Mercury A to Z: Zero Degrees Longitude

Hello, friends!  Oh my goodness, we made it!  We made it to the end of the A to Z Challenge!  For this year’s challenge, my theme is the the planet Mercury, and in today’s post Z is for:


Zero degrees longitude.  The prime meridian.  It’s an imaginary line that helps define the latitude-longitude coordinate system for mapping the surface of a planet.  On Earth, the prime meridian runs through the very English Royal Observatory in Greenwich, England.  On Mars, the prime meridian runs through Airy-0, a crater named after Sir George Airy, the very English scientist who decided where Earth’s prime meridian should be.  So where is the prime meridian on Mercury?

Actually, we talked about this in a previous post.  Mercury’s 0 and 180 degree longitude lines are supposed to run through the planet’s “hot poles,” the two points along Mercury’s equator where the temperature gets highest.  But the hot poles aren’t visible surface features, like Airy-0 or the Greenwich Royal Observatory.  So in the 1970’s, when NASA’s Mariner 10 space probe arrived at Mercury, scientists were hoping they could find an obvious surface feature to serve as an official prime meridian marker.

Mariner 10 visited Mercury three times.  It flew by Mercury, looped around the Sun, then flew by Mercury again, and then again one more time, before the space probe ran out of fuel.  During each of those three visits, only half of Mercury was visible to Mariner 10’s cameras, and unfortunately it was always the same half of the planet.  As a result, Mariner 10 never saw Mercury’s prime meridian, nor could it see any surface features on or near that imaginary line.

So Mercury’s prime meridian ended up being defined in a rather awkward way.  Scientists picked a tiny crater 20 degrees west of where the prime meridian was supposed to be.  They named the crater Hun Kal, which means twenty in an ancient Mayan language (this is one of the rare craters on Mercury not named after an artist, writer, or musician).  Scientists then officially defined Mercury’s prime meridian as a line of longitude exactly 20 degrees east of the center of Hun Kal Crater.

Thanks to NASA’s MESSENGER Mission, we now have photos of the entire surface of Mercury.  Presumably this means scientists could redefine Mercury’s prime meridian, if they wanted to, but nobody seems interested in doing that.  Using Hun Kal Crater to define the prime meridian may not be ideal, but it seems to work well enough.  And if it works, why fix it?


Here’s an article from, featuring an image of Hun Kal Crater as seen by MESSENGER.

Mercury A to Z: Year of Mercury Watching

Hello, friends!  For this year’s A to Z Challenge, I decided to talk about the planet Mercury.  I wasn’t sure at first if I’d be able to do a whole alphabet’s worth of posts about this one planet, but at this point, I think I just might pull it off!  In today’s post, Y is for:


My original plan for this post was to talk about Mercury’s year.  It’s 88 Earth days long, which (oddly enough) is only half the length of Mercury’s solar day.  That’s because of a spin-orbit resonance, sidereal rotation, yada yada… we’ve already talked about this stuff.  So instead, today I want to talk about the rest of this year here on Earth and tell you when the best opportunities to see Mercury will be.

The best time to see Mercury is during an “elongation,” which is when Mercury (as viewed from Earth) is as far away from the Sun as he can get.  To say that another way, if you drew an imaginary line between Mercury and the Sun, elongation is when that line would be at its longest.

Some elongations end up being a little higher (or should I say longer?) than others.  This is because of Mercury’s highly elliptical (non-circular) orbit.  The highest elongation of Mercury this year occurred on April 11.  I wish I’d known that earlier this month, because I definitely would have mentioned it.  Anyway, the next elongation will occur in the morning on May 29, 2023.  After that, there will be an elongation in the evening on August 10.  Another elongation will occur on the morning of September 22, and another will be on December 4.

Don’t worry too much about the specific dates.  You’ll still get a pretty good view of Mercury a few days before and after an elongation occurs.

I have only seen Mercury two times in my life that I know of.  I recently learned, however, that Mercury is known to twinkle like a star, so I may have seen him many times without recognizing him as a planet.  After all the Mercury research, Mercury writing, and Mercury artwork I’ve done for this year’s A to Z Challenge, I am very eager to get out there and see Mercury again (even if it means getting up before sunrise on or around May 29).


I found the dates for upcoming elongations of Mercury in this article from  The article also goes into a little more detail about how elongations work.

Mercury A to Z: X-Class Solar Flares

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


X was the hardest letter to find for this A to Z series.  For a while, I thought I was going to have to do something like “eXtreme temperatures on Mercury,” or maybe “eXoplanets like Mercury.”  There is a crater on Mercury named Xiao Zhao, which could have worked, but we’ve talked about so many craters already.  That seemed boring.  Then, just as I was wrapping up my research, I stumbled upon a paper titled “Modeling the Impact of a Strong X-Class Solar Flare on the Planetary Ion Composition in Mercury’s Magnetosphere.”  I have never in my life been so excited by the title of a scientific paper.

Scientists have come up with something like a Richter scale for solar flares.  The smallest, least energetic flares are called A-class solar flares.  B-class flares are ten times stronger than A-class.  C-class flares are ten times stronger than B-class.  Then, confusingly, we get M-class flares (ten times stronger than C-class) followed by X-class flares (at least ten times stronger than M-class).

Here’s how I rationalize this A-B-C-M-X system.  A, B, and C-class flares are low level flares that are too weak to affect us here on Earth.  We Earthlings can basically ignore them.  The M in M-class flare probably stands for medium. We do need to worry about these medium level flares.  If an M-class flare were aimed directly at Earth, it could damage our satellites, endanger astronauts on the International Space Station, and scramble some forms of radio communications here on Earth.  On th upside, these medium-level flares can also trigger geomagnetic storms around Earth’s poles (a.k.a. auroras).

And X-class flares are eXtreme!  If an X-class flare hit Earth, it could overwhelm our planet’s magnetic field.  Potentially, it could overload our power grids, cause worldwide communications blackouts, and basically wreck the global economy, at least for a few days.  The auroras would be truly impressive, though, possibly extending all the way to Earth’s equator.  So we’d at least be able to enjoy that while waiting for the world’s banking computers to reboot.

Scientists have a pretty good understanding of how solar flares affect Earth.  They’ve also had opportunities to see up close what powerful solar flares do to Mars, Jupiter, and Saturn.  But what about Mercury?  You’d think solar flares would be a pretty big deal on the planet closest to the Sun, but according to the paper I read, we don’t know much about how solar flares affect Mercury.  That’s surprising at first, but it makes sense when you consider how much time and energy (and money) we’ve spent exploring those other planets I mentioned compared to how little we’ve spent thus far exploring Mercury.

Solar flares don’t happen all at once; they occur in phases.  According to the paper I read, when an X-class solar flare hits Mercury, we can expect different elements of Mercury’s exosphere to ionize during different phases of the flare.  Magnesium would ionize right away, during the impulsive phase.  Other elements, like oxygen and helium, would ionize later, during what’s called the gradual phase (also known as the decay phase).  And some elements, most notably sodium, might not be affected at all.

Hopefully more research on this will come soon.  Maybe the BepiColombo Mission will be lucky enough to observe Mercury up close during an M-class or X-class solar flare (I presume BepiColombo is designed to protect itself from that kind of thing).


Here’s the paper I referenced in today’s post.

And here’s a brief article from one of NASA’s education/outreach websites explaining the solar flare classification system.

And here’s another article from NASA that briefly discusses the different phases of a solar flare.

Mercury A to Z: Solar vs. Sidereal Days

Hello, friends!  Welcome to another posting of the A to Z Challenge.  My theme for this year’s challenge is the planet Mercury, a planet that often gets overlooked by space geeks like me.  In today’s post, S is for:


We’ve been talking about Mercury all month long, and we’ve been talking a lot about Mercury’s rotation rate.  In some posts, I’ve told you that Mercury has a rotation rate equal to approximately 59 Earth days.  In other posts, I said a day on Mercury is about 176 Earth days long.  That seems like a contradiction, but both of those numbers are correct.  It all depends on whether we’re talking about a solar day or a sidereal day.

A solar day can be defined as the time it takes a planet to rotate once relative to the Sun, while a sidereal day is the time it takes a planet to rotate once on its own axis.  A solar day is what we Earthlings usually mean by the word “day.”  It’s the 12 a.m. on Monday to 12 a.m. on Tuesday kind of day.  A sidereal day (pronounced si-der-e-al) is more like a planet’s true rotation period, relative to the rest of the universe.  Why are these things different?  Because planets orbit the Sun.  Because as they orbit, they change position relative to the Sun.

In Earth’s case, it takes about 23 hours and 56 minutes to rotate once on its own axis, but because Earth moves through space during that time (changing positions relative to the Sun), it takes an extra 4 minutes to rotate once in reference to the Sun. And according to my math, 23 hours and 56 minutes, plus an extra 4 minutes, makes Earth’s solar day 24 hours long.

The situation is similar for Mars.  A Martian sidereal day is about 24 hours and 37 minutes, while a Martian solar day is more like 24 hours and 40 minutes.  For both Earth and Mars, the difference is small.  Most of the time, it’s not worth mentioning.  But on Mercury, a sidereal day is 59 Earth days long, while a solar day ends up being 176 Earth days in length.  You see, Mercury rotates very, very slowly.  Over the course of one sidereal day, Mercury travels two-thirds of the way around the Sun.  That puts Mercury in a very different position, relative to the Sun, at the end of a sidereal day.  As a result, Mercury’s solar day ends up being longer—a whole lot longer—than Mercury’s sidereal day.

I have to admit I had a hard time understanding the distinction between solar and sidereal days the first time I heard about it.  I hope that I have done a decent job explaining it to you.  But if anyone was wondering why I quoted different numbers in different posts for Mercury’s day/rotation period, this is the reason I did that.  For some topics, the sidereal day matters; for others, it’s the solar day that’s important.  And for the purposes of the A to Z Challenge, I wanted to save this discussion for S-day, so I just glossed over it until now.


I’m going to recommend an article from Universe Today called “How Long is a Day on Mercury?”

I also want to recommend another article from Universe Today called “How Long is a Day on Venus?” because if you think Mercury’s day is confusing, wait until you hear how messed up a day on Venus is.

Mercury A to Z: Resonance

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 R is for:


There are many weird coincidences in the Solar System, most of which really aren’t coincidences.  It’s usually just the force of gravity doing its thing.  Sometimes objects in space keep tugging on each other, gravitationally, pulling each other into these oddly specific mathematical relationships which astrophysicists call “resonances.”

As an example, for every three complete orbits of Neptune, Pluto completes exactly two (a 3:2 orbital resonance).  Another example: for every thirteen complete orbits of Venus, Earth completes exactly eight (a 13:8 orbital resonance).  And then there are the resonating moons of Jupiter.  For every four complete orbits of Io, Europa completes exactly two orbits, and Ganymede completes exactly one (a 4:2:1 orbital resonance).

Earth’s Moon is locked into a different kind of resonance, called a spin-orbit resonance.  For every orbit the Moon completes around the Earth, the Moon rotates (spins) exactly one time (a 1:1 spin-orbit resonance).  This happens because the Moon is not perfectly spherical.  The Moon bulges a little at the equator, kind of like a football (or a rugby ball, for readers outside the U.S.).  As the Moon circles around the Earth, Earth’s gravity keeps pulling one of the pointy ends of this football-shaped Moon towards the Earth.  This is why you always see the same side of the Moon whenever the Moon appears in the sky.

Mercury is also locked into a spin-orbit resonance, specifically a 3:2 spin-orbit resonance.  So for every two orbits around the Sun, Mercury rotates (spins) exactly three times.  This happens for two reasons:

  • Like the Moon, Mercury is slightly football shaped.
  • Unlike the Moon, Mercury’s orbit is highly eccentric (non-circular).

Since Mercury is slightly football-shaped, gravity tries to keep one of Mercury’s pointy ends pointed toward the Sun.  However, Mercury’s highly eccentric orbit means that Mercury’s orientation, with respect to the Sun and with respect to the Sun’s gravity, changes.  This changing orientation creates torque, which causes Mercury to spin.

So every time Mercury reaches perihelion (the point when Mercury is closest to the Sun) one end of the football-shaped planet points directly toward the Sun.  As Mercury moves away from perihelion, gravitational torque sets the planet spinning.  Mercury spins very slowly, only managing to turn 180° before reaching perihelion again, at which time gravity once again forces the football-shaped planet to point directly toward the Sun.  This has probably been happening for billions of years now, and it will probably keep happening for billions of years to come.


I want to recommend this very short video I found on YouTube.  I’ve read a lot about this topic.  Most sources I looked at get very technical very fast.  This video was, by far, the clearest and most straightforward explanation of Mercury’s spin-orbit resonance that I could find.

I also want to recommend this article from  Some exoplanets may be locked into Mercury-like spin-orbit resonances, and for exoplanets orbiting red dwarf stars, a spin-orbit resonance has implications for potential alien life.

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: 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: Exosphere

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


When I was preparing for this A to Z series on Mercury, a friend and I were joking that I should do “atmosphere” for the letter A.  The body of the post would simply say: “There isn’t one.”  And that would be the end of it.  But that wouldn’t be 100% true, and it wouldn’t be fair to poor, little Mercury.  Mercury does, in fact, have an atmosphere.  An extremely thin atmosphere, so thin it’s almost nonexistent.  But it is not entirely nonexistent.

Scientists usually refer to Mercury’s atmosphere as an “exosphere” to help distinguish it the thicker, heavier air layer that the word atmosphere traditionally implies.  Mercury’s exosphere is made of a little hydrogen, a little helium… there’s a little oxygen and a little sodium… a little potassium… a little calcium… there’s a little of a lot of different things, which adds up to not very much.

The hydrogen and helium presumably come from the Sun.  As the solar wind washes over the planet, hydrogen and helium atoms get tangled up in Mercury’s magnetic field and end up being incorporated (temporarily) into Mercury’s exosphere.  Some of the helium may also come from the radioactive decay of elements like uranium in Mercury’s crust.  As for the oxygen, sodium, and everything else, that stuff probably outgasses from the planet’s interior.  When Mercury formed, certain gases were trapped inside, and those gases have been very slowly leaking out of the planet ever since.  This outgassing process may help explain why Mercury appears to be shrinking (but we’ll talk about that in a future post).

But any gas you might find in Mercury’s exosphere is only there temporarily.  Mercury’s low gravity, plus the intense heat of the Sun, plus the constant pressure of the solar wind “blowing” on the planet, mean that Mercury’s exosphere is constantly blowing off into space.  Just as quickly as Mercury can gain a few atoms worth of atmosphere, he’ll lose them again.  In fact, as you can see in the totally legit Hubble image below, Mercury has a very faint comet-like tail of atmospheric gases, billowing off into space.

Cartoon image of Mercury, singing "You Take My Breath Away" to the Sun, while Mercury's atmospheric gasses blow off into space as a comet-like tail.

Just kidding.  That’s not really a Hubble image.  The Hubble Space Telescope has never observed Mercury.  Due to Mercury’s proximity to the Sun, trying to image Mercury would run the risk of burning out Hubble’s optics.  Some other space telescope must have taken that picture.


Today, I want to recommend a book simply titled Mercury, by William Sheehan.  It’s part of a series of books on the Solar System called Kosmos.  I’ve read a few of these Kosmos books now, and they are all wonderful.  Finding a book about one specific planet can be difficult (unless that planet is Mars), so if there’s a specific planet you want to learn more about, I highly recommend checking out the Kosmos series.

Also, if you want to see a for real picture (a for real for real picture) of Mercury’s comet-like tail, click here.

Mercury A to Z: Density

Hello, friends!  For this year’s A to Z Challenge, my theme is the planet Mercury.  Mercury may not be the most exciting planet in the Solar System, but he’s interesting in his own way, and I think he deserves a little more love and attention than he usually gets.  In today’s post, D is for:


In recent years, astronomers have discovered literally thousands of exoplanets (planets orbiting stars other than our Sun).  Every once in a while, one of these newly discovered exoplanets will be described as “Mercury-like.”  Now what do you think makes a planet “Mercury-like” in the minds of exoplanet hunters?  Are Mercury-like exoplanets small?  No, not necessarily.  Are they very close to their suns?  Again, not necessarily.  The most Mercury-like quality of a Mercury-like exoplanet is its density.  Mercury is an abnormally dense planet, due to the fact that Mercury has an abnormally large core.

Mercury’s core takes up roughly 85% of the planet’s internal volume.  For the sake of comparison, Earth’s core constitutes only 17% of Earth’s total volume.  For this reason, I sometimes like to call Mercury the avocado planet, because much like the seed inside an avocado, the core of Mercury is shockingly large.

The most likely explanation is that Mercury started out as a much larger planet, perhaps even an Earth-sized planet.  But then, in the very early days of the Solar System, young Mercury collided with another planetary body (in case anyone’s wondering, this would have happened long before the collision that created Caloris Basin).  Most of Mercury was destroyed.  Most of the debris from the collision probably fell into the Sun.  All that’s left today is the planet’s original iron core, buried under a relatively thin skin of rocky material.

So modern day Mercury is almost entirely made of iron, an extremely dense metal—which explains why Mercury is such an extremely dense planet.  The second densest planet in the Solar System, after Earth.

Now I have to level with you: I thought this was going to be one of the easier blog posts to write for this A to Z series, because I thought I already knew basically everything I needed to know about this topic.  But apparently there’s been some new research since the last time I read up about Mercury’s density and internal structure.

Decades ago, scientists assumed that Mercury’s core would be solid.  A planet as small as Mercury surely would have lost all his internal heat by now.  However, Mercury does have a magnetic field.  Planetary magnetic fields are usually caused by liquid metal sloshing around in a planet’s interior; ergo, Mercury must have a liquid core after all.  Right?

But apparently a few years ago (and this is the part I only learned about a few days ago), scientists were looking over gravity data from NASA’s MESSENGER Mission and realized that Mercury’s core cannot be entirely liquid.  Mercury’s core must be part liquid, to explain the magnetic field, but also part solid to explain MESSENGER’s gravity measurements.  So scientists now believe Mercury has a solid inner core surrounded by a liquid outer core.

So that’s a new thing that I have learned, and now it is a thing that you have learned, too.


I’m going to recommend this article from, explaining (in layperson’s terms) how scientists determined that Mercury must have this part liquid/part solid core.

And for anyone interested in the original research, here’s a link to the original research paper about Mercury’s liquid/solid core (I haven’t had a chance to read that paper yet, but I’m looking forward to doing so soon).

I also want to mention this article from, which briefly discusses one of those Mercury-like exoplanets I was talking about in the beginning of this post.  In fact, the exoplanet K2-229b is so Mercury-like that scientists have nicknamed it “Freddy” (get it?—because of the singer Freddy Mercury!).