Editor’s Note: This article is part of the series “What Is the Best Planet?”
Wu-Tang Clan rapper Method Man once said the following about fellow member Inspectah Deck: “He’s like that dude thatta sit back and watch you play yourself … and see you sit and know you lyin’, and he’ll take you to court after that.”
The same can probably be said of Mercury, the best planet in the solar system (other than Earth).
Mercury puts up with more crap than anyone else, so I stayed quiet while others incorrectly suggested that Venus, Mars, Jupiter, Saturn, or Uranus were any better. Few pay any attention to it: Astronomers researching the tiny rock often see their results smothered by the hype surrounding far lamer bodies like Pluto and Europa. And Mercury fans have to put up with shade like this quote from Ross Andersen: “Tiny thing, Sun-blasted and crater-pocked, more moon than planet.” But Mercury’s been the best planet all along. You just haven’t been paying any attention.
I get it. Mercury looks straight-up ragged. It lacks a real atmosphere, so there’s nothing preventing asteroids from hitting the surface, and the planet has billions of years worth of craters to show for it. Its only shield is a so-called exosphere, a thin layer of atoms kicked up by the constant onslaught of radiation from the nearby Sun. Mercury was raised in the solar system’s toughest neighborhood. Its temperature swings 600 degrees Celsius from day to night, negative-170 to positive-430 degrees.
Mercury once received the respect it deserved. The ancient Babylonians called it Nabou, ruler of the universe who woke the Sun up each morning, according to Robert G. Strom’s Mercury, The Elusive Planet. The Scandinavians and Teutonic people called it Odin and Woden—god of war, father of Thor. To them, it looked like a bright star that appeared sometimes just before dawn, sometimes just after sunset, heralding or retiring the Sun. That’s right, they named the planet after their best gods. The Greeks originally thought the planet was two stars and gave it two names, musical god Apollo for the morning appearance and his brother, messenger god Hermes in the evening. They eventually figured out the two stars were one, and stuck with Hermes. “Mercury” is the Roman version of Hermes.
The planet took the messenger god’s name likely for the way it heralded the Sun. But Hermes was also a mischief maker, a trickster, and kind of a badass (he killed the hundred-eyed monster, Argos). As it turns out, the name is way more appropriate than the Greeks and Romans probably thought. If you look at Mercury the wrong way, it could tear apart the solar system.
Konstantin Batygin, the Caltech professor of Planet Nine fame and fellow Mercury fan, explains that as far back as the 1600s, Isaac Newton pondered a question that astrophysicists still wonder about—whether our solar system is immutable, whether the planets will orbit the Sun forever or fly away eventually. “If the solar system really interacts with the universal law of gravitation where planets pull on each other, intuitively such a system can’t be indefinitely stable. It must fall apart,” said Batygin. He entered the centuries-old argument with calculations showing there’s a one-percent chance that eventually, the other planets’ gravitational influence will send Mercury shooting out of the solar system, or maybe even crashing into Earth. Which other planet has the distinction of having a small, but measurable probability that it will literally destroy us?
“It’s like a gangsta that’s chillin’ next to the Sun,” said Batygin. “It looks harmless because it’s kind of small… but it has some bullets up its sleeve.”
Mercury taketh away, but it can also giveth: Mercury helped confirm Albert Einstein’s general relativity, the modern theory of gravity. Its eccentric orbit processes 43 arcseconds per century, meaning that rather than returning to the same spot every year, its orbit traces out a Spirograph around the Sun. Before general relativity, the only thing that could have explained this eccentric behavior would have been another planet’s gravity, an imaginary planet that Neptune predictor Urbain Le Verrier called Vulcan. Mercury, forever the Inspectah, took Le Verrier to court and Vulcan got the death penalty. Einstein’s theory of general relativity perfectly explained Mercury’s 43 arcsecond procession. I don’t see any other planets playing such a pivotal role in a theory as important as relativity.
Speaking of gravity, Mercury’s elongated orbit locks it into a unique trip around the Sun—one Mercury year equals one-and-a-half Mercury days. There’s only one New Year’s Eve every two Mercury years, but who can blame it? I’d also need a break if the night of New Year’s eve lasted two-thirds of a year.
Despite these revelations, Mercury has maintained an air of mystery, especially compared to our rocky neighbors. Scientists have successfully sent two dozen probes to fly by, orbit or land on Venus, another two dozen to Mars and just two, Mariner 10 and MESSENGER, the MErcury Surface, Space ENvironment, GEochemistry and Ranging mission, to Mercury. Trying to stick a ship into Mercury’s orbit is difficult: MESSENGER needed to fly by Earth once, Venus twice, and Mercury three times so it wouldn’t fly into the Sun, said Sean Solomon, a professor at Columbia University and MESSENGER’s principal investigator. No one has ever dropped a lander on the planet, since the lack of a true atmosphere provides little cushioning to slow an approaching craft. Even orbiting the planet is a challenge, thanks to the Sun’s gravitational tug.
But when scientists do manage to send spacecraft to Mercury, they find a stranger planet than they could ever have imagined.
When Mariner 10 arrived at Mercury 40 years ago, it found that the little rock, unlike Mars and Venus, generates its own internal magnetic field. Mercury also has plate tectonics like the Earth, but rather than many plates, it has one massive plate cracking and contracting above its liquid outer core. “That puts Mercury in a special place with the Earth,” said Tom Watters, senior scientist at the Center for Earth and Planetary Studies of the Smithsonian National Air and Space Museum. “The two bodies are tectonically active today with active magnetic fields ... there’s no evidence that either Mars or Venus have active tectonics.”
There’s a reason for Mercury’s magnetic field. Its iron innards take up over 80 percent of its radius, or more than 60 percent of its volume. The Earth’s core, for comparison, only takes up a little over half its radius, less than a third of its volume. That means Mercury is a dense metal sphere—a lot more metal than rock, said Solomon. So scientists wondered: How do planet formation processes create a mostly metal ball? They had a few theories: Maybe Mercury formed in a metal-rich region around the Sun, or maybe the early Sun blasted some outer layer of rock away. Or, the most badass theory, maybe Mercury was once the size of Mars and took a major wallop from some giant unknown visitor, stripping away part of its diameter. None of these theories turned out to be correct. When MESSENGER arrived, the planet revealed way more volatile elements, those with low boiling-points like sulfur or potassium, than scientists expected to see. Any of the above scenarios would have vaporized these materials off of the planet, and yet they remained.
“We were expecting to distinguish among the current hypothesis for how the planet ended up so metal rich,” said Solomon, “and basically discarded all of them.”
Mercury’s the gangsta planet. It’s gonna take more than a couple of probes to reveal its deepest secrets.
And yet, Mercury has been generous with the secrets of the solar system’s other planets, including our own, thanks to deposits of water ice hiding at its poles. Despite sitting a mere 36-million miles from the Sun on average, it has some valleys hiding in the shadows forever, staying frigid without radiation exposure, or an atmosphere to distribute the heat. That’s where the water ice hides, covered in dark splotches that scientists like David Paige at the University of California, Los Angeles haven’t confirmed, but have reason to believe, is carbon.
If these dark splotches are carbon, then the dark craters might even help explain how we ended up with life here on Earth. Scientists believe that much of our own carbon and water came from the outer reaches of the solar system, said Solomon. Those craters could contain the oldest samples of some of the carbon and water that eventually let you, a carbon-based organism, sit down and read this piece today. As some of the oldest undisturbed carbon samples in the solar system, they’d allow us to look at the earliest carbon and water looked like on our own planet. “It’s a little bit bizarre in a way,” said Solomon. “We might have to go to Mercury to address the question of how life might have started on our own planet.”
So understanding one of the most important secrets of our own planet relies on sending a lander to a tiny, atmosphere-less, scorching metal ball, sending the craft circling the inner solar system for several years just to stick it into orbit. Mess anything up and the craft shoots directly into the Sun.
The Japanese Aerospace Exploration and the European Space Agencies plan to send their BepiColombo mission, a pair of orbiters, to Mercury next year. And who can blame them? Sure, our possibly habitable neighbors and the big, colorful gas giants shine in the spotlight, studied by dozens of missions, satellites and telescopes. But the fact of the matter is that Mercury doesn’t care if you like it or not, because it’s just trying to survive out in the toughest part of town. It doesn’t have time to pose for the cameras.
So next time one of your friends complains that things aren’t going well because the gangsta planet is minding its own business, traveling in retrograde relative to the Earth, remind them it’s actually the best planet. Because if they don’t watch out, it will mess them up real good.
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