Planetary Profile: The Weirdness of Mercury

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In the most recent edition of Spacing Out newsletter I made a note that Mercury is weird. Some of that weirdness was specifically covered in the newsletter, but the rest was just vaguely referenced, which doesn’t feel fair. All of Mercury’s little oddities deserve attention.

So for this, the 50^th post in our Spacing Out blog, let’s lavish some attention on the planet that tends to get the least and get to know our littlest planet (don’t come at me, Pluto fiends) and all the ways in which it prefers to march to the beat of its own drummer.

To the Core of the Matter

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As the newsletter readers now know (if you didn’t already), Mercury has a bizarre internal makeup compared to any of the other rocky worlds. Most of us probably have, during the course of our education, seen cutaway drawings of the Earth showcasing its basic layers: crust, mantle, outer core, inner core.

The crust, in comparison to the other layers, is barely there. And although the core layers take up a big chunk of real estate in Earth’s center, it’s the mantle that makes up most of its volume. About 84% of Earth’s internal volume is mantle. Basically the entirety of Earth’s radius that isn’t core is mantle. Point is, Earth has a whole lot of mantle topped by an intimidatingly thin layer of crust. You know who else has thick mantle layers? Venus and Mars. 

You know who doesn’t? Mercury of course! Mercury’s radius is 85% core. There is some mantle, but not much. No way it originally formed that way—that’s just now how planetary formation works. It’s looking increasingly like a much larger proto-Mercury suffered a big collision early in its history which knocked almost all of the outer layers into the void, leaving Mercury the tiny world with the intense core we see today.

Tilt-a-Whirl

Mercury also moves weirdly for a planet. It has the most eccentric orbit of all the classical planets (don’t come at me, Pluto fiends), with an eccentricity of 0.21. That means that Mercury’s distance from the Sun varies by 14 million miles over the 88 Earth days it takes it to complete an orbit. As a comparison, Earth’s distance varies by 3 million miles. So, you know, that’s weird!

It also has the greatest orbital tilt (or inclination) of the planets. It’s still not a large inclination, only about seven degrees from the ecliptic. But that’s over twice as high as the next highest planetary inclination (Venus, 3.4 degrees), though not nearly as high as other objects like the asteroid Pallas (nearly 35 degrees) or Pluto (over 17 degrees, and seriously it’s not a planet).

Despite having the highest orbital tilt among the planets, Mercury has the smallest axial tilt. It’s so small as to functionally be zero. I find this especially funny given the impact theory for Mercury’s internal structure. Earth, of course, also underwent a massive impact early in its history (that’s why we have the Moon), and that impact left us, we think, with the 23.5 degree tilt that gives us our seasons. But not so for Mercury. No seasons for Mercury. That would be too conventional.

Twirling

Mercury’s rotation deserves its own section. It’s…ugh, it’s very weird. Mercury has a 3:2 spin-orbit resonance. This means that it spins exactly three times for every two times it orbits. Combined with the high eccentricity of Mercury’s orbit, this means saying what the length of a day is on Mercury is not at all straightforward.

If you want to say about how long it would take for a specific star to get back to a specific spot in Mercury’s sky due to the planet’s rotation (known as a sidereal day), the answer is about 59 Earth days. This is usually what is quoted for how long a Mercurian day is. But if you want to know how long it takes Mercury to go through a full day-night cycle (known as a solar day) from sunrise to sunrise…ugh.The relative sizes of inner solar system objects. From left to right: Mercury, Venus, Earth, the Moon, Mars, Ceres. Credit: Wikipedia Commons

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So we’ve already said that Mercury has an eccentric orbit. That also means it’s moving faster around the Sun at some points than others, because you move faster when you’re closer to what you orbit. But it rotates at a steady rate. And it turns out there is a point when Mercury is closest to the Sun when you can think of it as orbiting as quickly as it spins.

For this period of its orbit Mercury is essentially tidally locked, like our Moon, with one face constantly facing the Sun. If you were standing on that side of Mercury at that point in the orbit, the Sun would appear to hold still in the sky for weeks. Some spots on the planet during this time will see the Sun rise a little, set immediately, then rise again, or set briefly, rise again, and set again.

The total amount of time for the Sun to return to the same spot in the sky on Mercury is 176 Earth days, over two Mercurian years, which is way more than 59 Earth days. I told you it was weird.

Sheer Magnetism

Mercury has a magnetic field. We don’t know why. It really shouldn’t. Planets get magnetic fields by spinning relatively quickly and having liquid iron cores. Earth does and has a nice magnetic field. Mars spins fine but doesn’t have a liquid iron core and so doesn’t have a magnetic field. Venus is thought to have at least a partially liquid core, but it spins painfully slowly. No magnetic field.

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Mercury doesn’t spin fast, and it shouldn’t have any liquid in its core. Earth still has a liquid core because it’s still cooling off from its formation 4.5 billion years ago. Mars, being a lot smaller, cooled off faster and its once-liquid core solidified, as liquids do if you cool them off enough. Mercury, being waaay smaller than Mars, should also have a totally solid core at this point. And yet.

It’s not a strong field, only 1% as strong as Earth’s, and for some reason it’s stronger in the northern hemisphere than in the southern. How it’s happening is still a mystery. Some part of Mercury’s core clearly is still molten for…reasons?? But that doesn’t explain how such a slowly-rotating planet is making a magnetic field.

Also sometimes that field interacts with the solar wind from the Sun and makes magnetic tornadoes of solar plasma. That’s a fun fact.

Airless…Except Kind of Not?

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Mercury is the only planet to have no atmosphere. That’s not to say it doesn’t have a layer of gas above its surface though! As the solar wind bombards Mercury’s pitted surface, it knocks atoms loose. These atoms, a tasty mix of oxygen, sodium, hydrogen, helium, and potassium, hover above the surface for a while before being dispersed and replaced by new ones.

It’s not enough of a layer to call an atmosphere, so it is referred to as an exosphere. Our Moon has one too, if you want to get technical. But such a thin layer of barely-there gas atoms can’t hold in heat from the Sun the way a proper atmosphere can, giving rise to another of Mercury’s oddities.

Despite being the closest planet to the Sun, it’s not the hottest. That honor goes to Venus, with its thick, smothering atmosphere of carbon dioxide serving as an incredibly heavy blanket. Don’t get me wrong, the daytime side of Mercury certainly gets hot, over 800F. But the nighttime side has no atmosphere to keep the heat in so it radiates away, dropping nighttime temps down to nearly -300F, which is close to the average temperature on Neptune. The closest planet to the Sun can get nearly as cold as the farthest…how’s that for irony?

Enter Bepi!

Obviously there’s a lot going on at Mercury that we want to know more about. It’s never very far from the Sun in Earth’s sky, so observing it from Earth can be tricky. It would be much better to observe it up close with a space mission anyway. But there’s a problem: it’s really hard to get to.

Thanks to the way gravity works, and thanks to the Sun having so darned much of it, any spacecraft that wants to get to Mercury can’t just go there. It has to take a very long, looping path to spill off the speed it keeps gaining thanks to the pull of the Sun. It can take many years to cover, all of which is spent being bathed in strong solar radiation.

That’s a major reason why Mercury hasn’t had many spacecraft visit it. It got flown past by Mariner 10 in 1974, and the MESSENGER spacecraft arrived in 2011 after a seven-year trip. That mission ended in 2015.

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But now there’s a new mission on its way! And it has the best name ever: BepiColombo! In the grand tradition of Mercurian weirdness, it’s the first spacecraft titled after a person’s nickname rather than an actual name (Giuseppe “Bepi” Colombo is the one who figured out how to get Mariner 10 to Mercury back in the 70s).

What will BepiColombo find when it gets there? Will it be able to figure out some of our little planet’s, shall we say, unique qualities? Maybe, but my bet is that it’s just gonna find even more weird stuff to replace the things it figures out. Space science tends to happen that way.

We’ll have plenty of time to speculate though—despite having started its journey in 2018, BepiColombo isn’t going to achieve Mercury orbit until 2027. Fly, Bepi, fly! Mercury awaits!