Mercurial personality

(NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)

Right now, Mercury is at its easiest to see of the entire year. It’s truly worth a minute to take a look. Starting Thursday evening, February 6, and extending through the weekend and beyond, simply look towards sunset at 6 p.m., or maybe ten minutes before that. Make sure tall hills or trees don’t block that direction from where you’re standing, because you need to look lowish, though not super-low.

The fading light of dusk hugs the western sky. Dazzling Venus pops out at you as the brightest thing in the sky. Now look far to the lower right of Venus. Any “star” you see down there is Mercury. Easy!

The innermost planet is unique and odd in so many ways, it’s hard to find aspects that aren’t strange. The way it moves, the way it looks, its features and newly discovered oddities all conspire to create a carnival of curiosities.


For centuries, observers fighting the solar glare caught fleeting glimpses of blotchy surface markings, which led them to conclude that one of its hemispheres always faces sunward – the way the Moon forever aims one side to Earth. This made sense. With the nearby Sun pulling on it with ten times the gravitational force we experience here, why shouldn’t its spin be locked in sync to match its 88-day orbital period?

But radar pulses in the 1960s showed that it actually spins in 58.6 days. This means that three rotations, three Mercury days (58.6 times 3) happen in the same interval as two of its years (88 times 2). The consequences are dramatic. This 3:2 resonance between its day and its year lets us see the same face of Mercury every second time it orbits the Sun. So those venerable observers weren’t quite wrong. They did observe repeating patterns – but on alternate orbits. No doubt they shrugged off the observations where the markings didn’t fit.

Then, too, Mercury has the most lopsided, out-of-round orbit of any planet in the solar system. Its Sun distance mutates from 30 to 40 million miles. This is huge: Around Mercury’s perihelion, you want to be sure to use SPF 2 billion sunblock instead of your usual 1 billion. The eccentric orbit also makes that crater-covered planet speed up and slow down more than any other: a variation that would sometimes make a sunrise on Mercury stop in its tracks and reverse itself. The Sun comes up, goes back down, then rises a second time.

Mercury alters its brightness more than any other planet, varying by three hundredfold. Each year its light goes from fainter than the “Seven Sisters” to more than double the brilliance of the Dog Star Sirius, the brightest star. These nights it’s near its brightest, but it’s fading rapidly.

As if jealously to resent Venus’ greater dazzle, Mercury may smash it to pieces sometime in the next five billion years. Thanks to perturbations caused by the Sun, and especially Jupiter, the Mercury orbit wildly changes shape. It goes from being a perfect circle to being twice as lopsided as it is at present – squashed enough actually to reach innocent Venus, the planet with the most perfectly circular orbit of all.

As Mercury spins, it displays not the least axial tilt. Earth, Mars and Saturn are all tilted 20-something degrees, but Mercury alone rotates straight up and down: not even one-tenth of a degree offset from perfectly vertical. This means that at its poles, half the solar disc is always below the horizon. Standing within the slightest polar depression or crater, you never see the Sun at all. This results in permanently dark places, filled with ice. Strangely enough, then, the Sun’s nearest planet has ice deposits extensive enough to be detectable from Earth. They offer winter sports on a world badly needing them.

And even that isn’t the end of Mercurian strangeness. Its largest impact feature is the enormous Caloris Basin. At its antipodal point – the precise opposite location on Mercury to Caloris – is the so-called Weird Terrain. This hilly region is unlike anything else. Apparently, shock waves or else debris from the colossal meteor impact that formed Caloris traveled around the planet and then collided with themselves at the antipodal point to wreak havoc there.

But above all else, don’t you want another planet notch on your belt? Don’t you want to join the ranks of the minority of humans through the centuries who have laid eyes on the speediest and strangest of all worlds? This is your chance, the next clear evening.

Want to know more? To read Bob’s previous columns, click here. Check out Bob’s podcast, Astounding Universe, co-hosted by Pulse of the Planet’s Jim Metzner.