Mercury A to Z: Jumping on Mercury

~ J.S. Pailly ~ 6 Comments

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:

JUMPING ON MERCURY

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.

WANT TO LEARN MORE?

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

~ J.S. Pailly ~ 6 Comments

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:

INTERNATIONAL ASTRONOMICAL UNION

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.

WANT TO LEARN MORE?

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

~ J.S. Pailly ~ 9 Comments

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:

HOT POLES

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.

WANT TO LEARN MORE?

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 SpaceRef.com, 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.

Mercury A to Z: Graphite

~ J.S. Pailly ~ 15 Comments

Hello, friends!  For this year’s A to Z Challenge, I’m giving you a guided tour of the planet Mercury, perhaps the Solar System’s most under-appreciated planet.  In today’s post, G is for:

GRAPHITE

I did not know this prior to doing my research for this A to Z series, but apparently Mercury is the least reflective, darkest colored planet in the Solar System.  For a long time, scientists assumed Mercury’s dark color must have something to do with iron.  Thanks to the planet’s unusually high density, we know that Mercury is an iron-rich planet, after all.  However, NASA’s MESSENGER spacecraft was unable to detect significant amounts of iron on the planet’s surface.

After rethinking their assumptions and reanalyzing MESSENGER’s data, scientists now believe that Mercury’s crust might be covered in carbon, specifically carbon in the form of graphite.  The same material used in pencils.  So how did this happen?  How did Mercury get covered in graphite?

Let’s go back in time.  Billions of years ago, when the Solar System was still forming, Mercury would have been just a giant ball of liquid magma.  During that time, heavier elements, like iron, would have sunk down toward the center of the planet; meanwhile, lighter elements, like carbon, would have floated up toward the planet’s surface.  As a result, when the planet started to cool off and solidify, a significant amount of carbon would have been incorporated into the planet’s crust.

Something similar must have happened on Venus, Earth, and Mars; however, Venus, Earth, and Mars continued to be geologically active for a long time after they formed (fun fact: Earth is still geologically active today!).  Mercury didn’t.  So while volcanic eruptions, plate tectonics, and the like allowed Venus, Earth, and Mars to transform their carbon rich surfaces into more mineralogically mixed planetary crusts, Mercury’s crust stayed more or less the same.

I don’t want to make it sound like Mercury didn’t try.  Some amount of volcanic activity did happen on Mercury, long ago.  And in some cases, asteroid impacts punched through the planet’s crust, causing lava to spill out onto the planet’s surface (remember my post on Caloris Basin?).  But even in regions where Mercury’s original crust has been resurfaced by lava, small asteroid impacts have re-exposed the graphite layer underneath.  Those asteroid impacts have also scattered graphite dust over the surfaces around craters.

Based on what I read, it seems that NASA’s MESSENGER spacecraft was not well equipped to study the graphite on Mercury.  And why would it be?  When MESSENGER was launched, no one knew the graphite was there, and given how expensive space exploration is, you don’t want to load up a spacecraft with equipment that you don’t think you’ll need.  And honestly, who would have expected to go to another planet and find the place is covered in pencil lead?

Anyway, hopefully ESA/JAXA’s BepiColombo Mission will be able to follow up on this when it arrives in Mercury orbit in 2025.

WANT TO LEARN MORE?

Here’s an article from The Conversation about the discovery of graphite on Mercury.

And here’s the original research paper reporting on the discovery.

Also, if it’s true that Mercury is covered in graphite, then the force of asteroid impacts might have turned some of that graphite into diamonds.  Click here to learn more about that.

Mercury A to Z: Five

~ J.S. Pailly ~ 19 Comments

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

FIVE

Today’s post is really an important life lesson: you can’t always trust your own eyes.  Your eyes will play tricks on you, and they may cause you to make some pretty embarrassing mistakes.  Back in the late 1800’s, Italian astronomer Giovanni Schiaparelli’s eyes played a trick on him, causing him to miscalculate Mercury’s rotation rate.

We touched on this briefly in a previous post.  Based on telescopic observations of Mercury, Schiaparelli determined that Mercury has a rotation rate of approximately 88 Earth days.  This matches nicely with Mercury’s orbital period, which is also about 88 Earth days long.  If Schiaparelli’s calculations were correct, this would mean that Mercury is tidally locked to the Sun.  The same thing happened to Earth’s Moon.  The Moon’s rotation rate and orbital period are both approximately 27 Earth days long, which is why the same side of the Moon always faces toward the Earth.

But Schiaparelli’s calculations were not correct.  We now know that Mercury’s true rotation rate is about 59 Earth days, not 88.  So how did Schiaparelli, an otherwise highly competent and highly accomplished astronomer, get this so wrong?  It’s because when he started his observing campaign of Mercury, he noticed a pattern of splotches on Mercury’s surface that kind of looked like the number five.  And as he continued his observations, he kept seeing this splotchy five shape on Mercury’s surface.

The thing is, if you stare long enough and hard enough at the surface of Mercury, you can probably find the number five in several different places.  I’d normally include one of my own drawings here, but in this case I think you really need to see an actual map of Mercury.

Map of the surface of Mercury, reproduced three times, with outlines showing locations where Giovanni Schiaparelli's figure of five might be.
Original map from NASA, public domain image.

A bit of confirmation bias was probably at work.  After seeing a five on Mercury the first few times he looked, Schiaparelli had an expectation.  He expected to see the five again, and every time he did find a five on Mercury, Schiaparelli assumed it was the same five.  To make matters worse, Schiaparelli also thought he could see clouds on Mercury, so whenever he saw only part of a five, he could easily deceive himself into assuming the rest of the five must be hidden under cloud cover.

As a result, Schiaparelli calculated Mercury’s rotation rate based on faulty observations, and he got a result that triggered a second case of confirmation bias.  Just as the Moon is very close to the Earth, Mercury is very close to the Sun, so it made sense—it fit well with Schiaparelli’s expectations—that Mercury rotation rate would match its orbital period.  It made sense, in Schiaparelli’s mind, for Mercury to be tidally locked to the Sun.

To be fair to Schiaparelli, another astronomer had previously tried to calculate Mercury’s rotation rate and gotten an answer of 24 hours (the same as Earth’s rotation rate).  So while Schiaparelli was wrong, he was, at least, less wrong than the last guy.  And that’s often the way science advances.  Science isn’t always right, but it keeps becoming less and less wrong than it was before.

WANT TO LEARN MORE?

Here’s an article from Astronomy.com about Schiaparelli’s five, and some of the other shapes he thought he saw on Mercury’s surface.

And regarding that point I made at the end, about science being less and less wrong than it was before, here’s a famous article by Isaac Asimov called “The Relativity of Wrong.”  It’s a must read for anyone who has even a passing interest in how science works.

Mercury A to Z: Exosphere

~ J.S. Pailly ~ 4 Comments

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:

EXOSPHERE

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.

WANT TO LEARN MORE?

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.

#IWSG: Putting Facts on the Internet

~ J.S. Pailly ~ 15 Comments

Hello, friends!  Welcome to this month’s meeting of the Insecure Writer’s Support Group, a blog hop created by Alex J. Cavanaugh and co-hosted this month by Jemima Pett, Nancy Gideon, and Natalie Aguirre.  If you’re a writer and if you feel in any way insecure about your writing life, then this is the support group for you!  Click here to learn more!

I’m sorry, but I’m participating in the A to Z Challenge this month, and I just don’t have enough time to write an IWSG post, too.  Fortunately my muse has gotten used to writing these IWSG posts without me, so I’m going to turn the floor over to her.  My muse has something to say.  Perhaps it is something you and your muse would like to hear.

When my writer first started writing science fiction, he thought he wouldn’t have to do any research.  He thought he could just make stuff up.  As his muse, I quickly disabused him of that assumption.  No, science fiction need not be factual about everything all the time, but mixing a generous helping of facts into the fiction will add a sense of credibility to a Sci-Fi story, making it that much easier for readers to suspend their disbelief.

This blog exists because my writer thought that writing a blog about science would force him to do the research he needed to do to become a better science fiction writer.  And that worked.  My writer does his research now, and his Sci-Fi writing has improved as a result.  Now one of his favorite ploys, when writing Sci-Fi, is to try and make real science sound made up while making the stuff he made up sound like real science.

But that’s a game for science fiction writing only.  When my writer is writing this blog, he has developed a new anxiety, a new insecurity.  He is terrified that he might unwittingly spread misinformation about science on the Internet.  It is not unusual for my writer to get emails from science students—or even science teachers—asking him questions about the topics he blogs about.  It’s flattering, of course, but also a little bit scary, because he very much does not want to lead anyone astray in their science education.

This year’s A to Z Challenge is, as usual, a science heavy project.  So my writer tries to word things carefully, to ensure that he won’t mislead anyone.  He debates with himself which details must be included in a blog post and which can be safely glossed over or ignored.  He double checks his sources, and if he’s still not sure he’s got his science facts straight, he’ll either state that uncertainty explicitly, or he’ll cut that section entirely out of the post.  And despite all of that, he knows that he will still make mistakes.

All he can do is promise himself (and his readers) that he will correct his mistakes as soon as he finds out about them, because there is far too much misinformation about science on the Internet already.  My writer very much does not wish to make that problem worse.

Mercury A to Z: Density

~ J.S. Pailly ~ 10 Comments

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:

DENSITY

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.

WANT TO LEARN MORE?

I’m going to recommend this article from EarthSky.org, 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 ScienceNews.org, 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!).

Mercury A to Z: Caloris Basin

~ J.S. Pailly ~ 8 Comments

Hello, friends!  For this year’s A to Z Challenge, we’re exploring the planet Mercury.  In today’s post, C is for:

CALORIS BASIN

It seems like just about every planet has its thing.  Saturn has her rings.  Jupiter has his Great Red Spot.  Mars has both Olympus Mons and Valles Marineris, the largest volcano and the largest canyon, respectively, in the entire Solar System.  And as for Mercury, Mercury has Caloris Basin, an absurdly large crater in Mercury’s northern hemisphere.

So how did Mercury end up with such a big crater?

Based on what science currently knows about the history of the Solar System in general, and the history of Mercury in particular, Caloris Basin most likely formed during an event known as the Late Heavy Bombardment.

About four billion years ago, the Solar System looked a little different than it does today.  The gas giants (Jupiter, Saturn, Uranus, and Neptune) were engaged in these gravitational tug-of-wars with each other, pulling each other into new orbits, swapping places with each other, and generally causing chaos in the outer Solar System—and generally making a mess of the inner Solar System, too.  All those gravitational tug-of-wars in the outer Solar System sent tons and tons and tons of stray asteroids hurtling toward the inner Solar System.  All the inner planets (Mercury, Venus, Earth, and Mars) took a beating.  Earth’s Moon took a beating, too.

A particularly large asteroid must have slammed into Mercury near the end of the Late Heavy Bombardment.  We know this must have been near the end of the Late Heavy Bombardment because Caloris Basin has only a few smaller, younger-looking craters inside it, while the surrounding terrain is thoroughly peppered with older-looking craters.  That impact must have been a truly Earth-shattering Mercury-shattering event, sending ripples and shockwaves all the way around the planet, leaving geological marks that can still be seen to this day.

Caloris Basin was discovered in 1974 by NASA’s Mariner 10 space probe.  At the time of the discovery, Caloris Basin was only half in daylight, so the full size of the crater was unknown.  You may recall from yesterday’s post that Mariner 10 visited Mercury three times, but due to an unfortunate coincidence of orbital mechanics, Caloris Basin was only half in daylight every single time Mariner 10 showed up.  And it was always the same half of Caloris Basin, too.  So the full size of the crater remained uncertain until the 2010’s, when the MESSENGER Mission entered orbit of Mercury and finally imaged the entire crater in full daylight.

Based on Mariner 10’s data, scientists originally guessed that Caloris Basin was 1300 km (810 miles) in diameter, making it larger than Texas.  MESSENGER revealed that its actually 1550 km (960 miles) in diameter, making it even more larger than Texas.

WANT TO LEARN MORE?

Here’s an article from Astronomy.com, going into a little more detail about how Caloris Basin formed and what we currently know about it.

And here’s an article from Wondrium about the Late Heavy Bombardment, how it happened, and how we know about it.

Also, I thought I read somewhere that Caloris Basin was the largest impact basin in the Solar System, and an early draft of this blog post included that detail.  But that’s not true.  Apparently the largest impact basin in the Solar System is Utopia Planitia on Mars.  For anyone interested, here’s a Wikipedia page listing all the largest craters known to exist in the Solar System.

Mercury A to Z: BepiColombo

~ J.S. Pailly ~ 18 Comments

Hello, friends!  Welcome to the second posting of this year’s A to Z Challenge!  My theme this year is the planet Mercury, and in today’s A to Z post, the letter B is for:

BEPICOLOMBO

Mercury is a pretty lonely planet.  Only two spacecraft have ever come to visit: NASA’s Mariner 10 space probe, which conducted a series of flybys in the 1970’s, and NASA’s MESSENGER Mission, which orbited Mercury for several years in the 2010’s.  But don’t feel too bad.  Soon, Mercury will be welcoming not one but two new guests, thanks to a joint mission by the European and Japanese space agencies.  And that name of that mission?  BepiColombo.

But first, a little history lesson.  Back in the late 1800’s, Italian astronomer Giovanni Schiaparelli determined that Mercury’s rotation rate (the time it takes for Mercury to spin on its axis) equals approximately 88 Earth days, or exactly one Mercurian year.  Unfortunately, Schiaparelli’s calculations were way off (we’ll talk about that more in a future post), and it took another Italian scientist, named Giuseppe “Bepi” Colombo, to fix Schiaparelli’s mistake.

In the 1960’s, thanks to new RADAR observations of Mercury, astronomers discovered that Mercury’s true rotation rate is approximately 59 Earth days, or precisely two-thirds of a Mercurian year.  And I do mean precisely two-thirds of a Mercurian year.  Odd coincidence, right?  Don’t worry.  We’ll talk about that more in future posts, too.  For now, all you need to know is that Giuseppe Colombo was the lead author on a paper that explained how Mercury could have gotten itself into this rather curious predicament.

The history lesson’s not over yet!  In the 1970’s, NASA was planning their first mission to Mercury, a mission known as Mariner 10.  But getting to Mercury isn’t easy.  Mercury is very small, and the Sun is very big.  The orbital mechanics of approaching such a small object in space, so close to such a big object, are really complicated.  NASA scientists thought the best they could do was aim carefully and do one quick flyby of Mercury.  But NASA was wrong, and once again, Giuseppe Colombo stepped in to correct the mistake.

Colombo showed NASA an alternative flight trajectory, involving a never-tried-before gravity assist maneuver near Venus, which would cause Mariner 10 to fly past Mercury, circle around the Sun, then fly past Mercury again… and again!  Thanks to Colombo’s orbital calculations, Mariner 10 was able to do three flybys of Mercury for the price of one.

Fast forward to modern times.  When ESA (the European Space Agency) and JAXA (the Japanese Aerospace eXploration Agency) decided to team up for a Mercury mission, they had no trouble picking a name.  In honor of Colombo’s outstanding contributions to the study and exploration of Mercury, the mission was officially named BepiColombo (one word, no space or hyphen—I’m not sure why it’s like that, but it’s one word).

BepiColombo is already in space, on route to Mercury.  When it arrives in 2025, it will separate into two spacecraft: the Mercury Planetary Orbiter (M.P.O.), built by Europe, and the Mercury Magnetospheric Orbiter (M.M.O.), built by Japan.  Together, these two spacecraft will follow up on some of the lingering mysteries about Mercury (i.e., other stuff that we’ll talk about in future posts).

WANT TO LEARN MORE?

I’m going to recommend this article from Univere Today, entitled “Who was Giuseppe “Bepi” Colombo and Why Does He Have a Spacecraft Named After Him?”

I’m also going to recommend this article from the Planetary Society, entitled “BepiColombo, Studying How Mercury Formed.”

And for those of you who enjoy reading scientific papers for fun, like I do, here is Giuseppe Colombo’s original research paper from 1965, explaining how Mercury’s rotation rate ended up being precisely two-thirds of a Mercurian year.