Mercury’s a bit of a wild card, but we shouldn’t judge it too harshly. After all, it must be hard to figure yourself out with the sun breathing down your neck all the time.
Mercurial: changeable, volatile, fickle, flighty, erratic. It’s true that this definition originated from the properties of the element mercury, not the closest planet to the sun, but there’s a lot about Mercury that fits this fanciful characterization.
Despite its proximity to a colossal molten ball of plasma, some parts of Mercury are cold enough to harbor ice. Surface temperatures can fluctuate from 801 to -279 degrees Fahrenheit for a variety of reasons. First of all, Mercury barely has an atmosphere at all, so there’s nothing to shield it from the sun’s scorching radiation. On the other hand, the depths of some craters never even see any sunlight, which allows ice to form.
Another factor is the planet’s strange rotation rate. It used to spin much more rapidly, but the sun’s gravitational effect locked it into a slower rotation soon after Mercury’s formation. As a result of this “tidal locking,” Mercury rotates three times for every two revolutions around the sun.
This does weird things to the Mercurian day: if you were to stand on certain points at the planet’s equator, the sun would appear to stop at its apex at midday. That’s when the surface reaches its highest temperatures. But then the sun reverses its path and starts moving back toward the horizon from which it rose. Finally, it will progress across the sky in its original direction and set… only to rise again from that side. Needless to say, setting up a daily routine would be a bit tricky.
Mercury also has the most eccentric orbit out of all the planets. Its distance from the sun varies from 29 to 43 million miles as it progresses through a highly elliptical path. But that’s not the whole story: its orbit also differs from what mathematical theories would predict, in an effect called precession. Astronomers tried in vain to explain the oddities in Mercury’s orbit, suggesting that there might be another hidden planet called “Vulcan” that was disturbing its orbit.
The solution was finally uncovered when Einstein developed his theory of general relativity and tested it on the planet. He confirmed that gravity can warp space and time by beaming radar signals at Mercury. When the planet moved closer to the sun, the star’s immense gravitational field distorted the signals on their way back to Earth.
In addition to the planet’s movement, its very shape and composition are still undergoing constant change. Mercury’s surface is covered with geological features called “scarps,” which are basically huge wrinkles that form when the planetary crust buckles. It turns out that the core is actively shrinking, which makes the surface pucker like a raisin. This is likely because the liquid iron within its core is slowly cooling down, solidifying, and contracting.
The planet is also extremely vulnerable to solar winds containing radioactive particles. Unlike Earth, Mercury’s magnetic field is too weak to protect it from radiation. The magnetic field is particularly delicate at the south pole, where charged particles from the sun slip in and wreak havoc on the integrity of the surface. But despite its weakness, the magnetic field can also deflect these charged particles, whipping them into a tornado-like plume of plasma.
It would be unwise to try to rely on Mercury, seeing as it can’t seem to make up its mind about its orbit, temperatures, or size. We still have a lot to learn about the planet, although we might have trouble pinning down any solid facts if it continues to behave so unpredictably.
If you’re curious about the other planets in the solar system, make sure to read our article about Venus: 10 Facts About Venus We Earthlings Take For Granted