5 Cratered Facts About Mercury

An enhanced-color composite image of Mercury's Caloris basin. Lavas appear orange, and blue areas are likely the original basin floor. Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

by Alex Carter

Mercury is the closest planet to the sun, and you would be forgiven for thinking it’s just a boring lump of rock. After all, there’s nothing interesting on it like aliens or places to drink. Still, Mercury is probably a more interesting place than you think...

1. IT HAS WATER ICE.

Although you would be forgiven for thinking Mercury, being the closest planet to the Sun, is one of the hottest places in the solar system, it in fact is subject to enormous temperature fluctuations. Some regions reach 800°F, but without an atmosphere to retain heat, Mercury’s poles and nightside of the planet plunge well below even the coldest temperatures recorded on Earth. With some regions never getting above -279°F, conditions are perfect for ice to form. Mercury’s regolith is home to possibly a trillion tons of ice, which would make it one of the wettest places in the solar system.

Fitting, then, that the ancient Chinese called it the Water Star.

2. IT ONCE HAD AN IMAGINARY FRIEND CALLED VULCAN.

In the 19th century, scientists were smug in the knowledge that they knew everything there was to know. There were of course a couple of curiosities that they could not understand, one being the precession in the orbit of Mercury. That is to say, Mercury goes around the Sun in an ellipse rather than in a circle, and the ellipse changed the direction it was pointing from time to time. It was thought that there must be a new planet between Mercury and the sun that was changing its orbit—the planet Vulcan. But despite trying, the planet could not be observed.

Albert Einstein eventually disproved the theory of Vulcan via the general theory of relativity. Rather than looking for an external cause, Einstein showed that Mercury was doing exactly what it should do, and that gravity was just acting in ways no one had known it could. Mercury is so close to the Sun that not only is it pulled around the Sun, space itself is too. Were it not for Mercury demonstrating this effect, Einstein probably would not have been so readily believed.

Incidentally, the other curiosity was the photoelectric effect, which needed quantum mechanics to explain. That, not relativity, was what Einstein won his Nobel Prize for.

3. IT CONTAINS THE BIGGEST HUMANMADE CRATER IN SPACE.

After six years orbiting the planet, NASA’s MESSENGER probe ran out of fuel in 2014 and could no longer correct its course. As its orbit decayed, it got closer to the planet, and much faster. The resulting crash into Mercury a year later occurred at more than 8000 mph and left a crater more than 50 feet wide. That makes it easily the biggest humanmade crater anywhere in the universe … apart from those on Earth, obviously.

4. ITS DAY IS LONGER THAN ITS YEAR.

Mercury orbits so close to the Sun that the conventional ideas of days and years don’t really make much sense. Mercury’s rotation (remember, rotation on a planet's axis causes its days) and orbit (which causes its years) are linked through gravity more tightly than Earth. In fact, Mercury rotates three times on its axis every two years. This makes each Mercury day—as in sunrise to sunset and back to sunrise—last two of its years. It’s a weird day, too: The Sun rises, goes backwards in the sky to set, rises again, then finally sets a year later. The next year, from the perspective of Mercury, the Sun would appear to move in the opposite direction.

5. IT MAY SMELL AWFUL.

Mercury’s small size means it has no permanent atmosphere, just a thin layer called an exosphere; its gravity is too weak to hold onto any gas in the wake of the strong solar wind. However, it does have a remarkably strong magnetic field, which means it keeps a hold of whatever ions come its way. There are a lot of ions in that part of space.

So while planets such as Earth are nice and oxygen filled, Mercury’s atmosphere contains the kind of things planets normally don’t hang on to, including ions of magnesium, calcium, sodium (you may remember from chemistry class that those are the ones that explode in water), as well as unusual ions of water. Mercury may smell like wet, metallic burps.