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Mercury: Fast, Pockmarked and Shrinking

Mercury
Mercury is the smallest planet in our sun's orbit. In fact it's not much bigger than Earth's moon. NASA

You'd like to think you know your neighbors, but sometimes they'll catch you off guard. That diminutive world called "Mercury" is the first planet from our sun. Venus is the second, Earth is the third and dear old Mars is No. 4. Common knowledge, right?

Well, the numbering obscures certain truths. With its tight little orbit around the sun, Mercury can't venture as far away from Earth as Venus and Mars can. So most of the time, Mercury is closer to Earth than any other planet — despite its place in the planetary lineup. And that's not even the weirdest thing about Mercury's travel patterns.

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The Sun Also Rises

Of all the planets in this solar system, Mercury orbits our sun at the fastest rate. It zips around the star with an average speed of 105,947 miles per hour (170,505 kilometers per hour). The planet's namesake was a real speedster, too. In Roman mythology, Mercury was the fleet-footed god of commerce recognizable by his winged sandals.

Earth takes a little more than 365 days to loop around the sun. Mercury does the same thing in a fraction of the time. One Mercurian year is equal to a brisk 87.97 Earth days. Vacationing there would be a disorienting experience: On Mercury, the years may be quick, but the days are long.

The planet finishes a new rotation around its own axis once every 58.65 Earth days. If you've got a head for numbers, you might've noticed something peculiar. When we round 58.65 and 87.97 up to the nearest whole numbers, we get 59 and 88.

Guess what? The No. 59 is exactly two-thirds of 88.

Here's why those figures matter. Pretend you're an astronaut camping out on Mercury's surface. While we're at it, let's say you get to watch the sun rise early in your stay. Be sure to have a camera handy. According to the European Space Agency, you'd have to wait 176 Earth days before the sun rises again.

See, if we define a "day" as the amount of time it takes our sun to return to a specific point in the sky, then a single day on Mercury is 176 Earth days long. That's the rough equivalent of two full Mercurian years!

Here's another thing that might take some getting used to. When you view the sun from Mercury, it actually seems to freeze in place partway through its journey across the sky. Then it moves backward for a spell before changing course en route to the opposite horizon.

Mercury
Mercury (far left) is the closest planet in the solar system to the sun. This artist's concept shows its approximate relative size compared to (from left) Venus, Earth and Mars.
NASA/Lunar and Planetary Institute

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Stay Eccentric

The planet's slow spin rate explains some of this craziness. Another contributing factor? Glorious eccentricity.

No, we don't mean Mercury is eccentric in the Willy Wonka sense of the word. "Eccentricity" is the term astronomers use to describe the shape of a planet or moon's orbit. No planet orbits its star in a perfect circle. And the extent to which a given orbit deviates from being circular is called its eccentricity.

If an orbit was 100 percent circular, we'd say it had an eccentricity of 0.0. Let the record show that Earth's orbit boasts an eccentricity of just 0.0167. So it's almost a circle — but not quite.

Mercury's orbit looks more "squashed" by comparison. Of all the planets in our beloved solar system, Mercury has the most eccentric orbit. The gap between this world and the sun ranges from 28.5 to 35.9 million miles (46 million to 57.9 million kilometers) in length.

Consequently, Mercury's travel speed increases as it gets closer to the star. The planet would have much shorter years — lasting a scant 56.6 Earth days apiece — if it could maintain this high velocity throughout its orbit. But hey, those are the breaks.

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You're Hot, Then You're Cold...

With her runaway greenhouse effect, Venus is hotter than Mercury. Temperatures on the latter are extremely variable; they can be as high as 800 degrees Fahrenheit (430 degrees Celsius) or as low as -290 degrees Fahrenheit (-180 degrees Celsius).

Just as you'd expect, the side of the planet that's facing the sun at any time will be a good deal hotter. And on hot days, we could all use some ice.

In 2012, NASA's Messenger spacecraft observed deposits of frozen water on Mercury. The ice was found at the bottoms of deep craters around the northern pole — which are protected from the sun all year long.

Paradoxically, Mercury's hottest temperatures could be responsible for some of that ice. Our sun is constantly releasing charged protons and electrons in streams known as "solar winds." Earth is shielded by a powerful magnetic field which deflects many of these.

Mercury's had a magnetic field of its own for around 3.9 billion years. However, it's significantly weaker than the one we enjoy here on Earth.

If a new scientific model is correct, the intense heat of the planet's sun-facing side triggers chemical reactions between solar wind particles and minerals buried in the soil. These are thought to create traveling water molecules that later become ice. Ten percent of all the frozen H2O on Mercury may have been generated this way.

Mercury
Mercury doesn't have a thick atmosphere to protect it from space debris so it's riddled with craters, but none as spectacular as the Caloris Basin seen here. The Caloris Basin is about 950 miles (1,525 kilometers) across and ringed by mile-high mountains.
NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

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Planetary Shrinkage

Neither Venus nor Mercury have any moons to call their own. As a matter of fact, with a diameter of 3,030 miles (4,878 kilometers), Mercury isn't much bigger than Earth's one and only moon.

Easily, Mercury is the smallest planet in our sun's orbit. Yet the evidence tells us it used to be larger.

Mercury is interesting to look at from a topographic standpoint. The thin atmosphere offers very little protection from asteroids, so impact craters are abundant. A single photo taken by the Messenger probe in 2008 shows 763 identifiable craters within a region of the planet's surface that's just 172 miles (276 kilometers) wide.

One of the better-known craters is the Carolis Basin; this depression is 950 miles wide (1,525 kilometers), making it larger than the state of Texas.

The planet's face is also marked by fault scarps; cliff ridges that were first sighted on Mercury in the 1970s. Some of the smaller ones appear to be around 50 million years old, making them geologically young. (For context, the last non-avian dinosaurs died out 65.5 million years ago.)

Mercury itself formed about 4.5 billion years ago. Since then, its metallic core has been cooling down at a rapid pace — and as a result, the whole planet is currently shrinking.

Indeed, the diameter of Mercury may have contracted by as much as 8.6 miles (14 kilometers) over the eons. This has had a tremendous effect on the planet's rocky outer surface. Those previously mentioned fault scarps are born when crust materials break apart and press into each other, forcing some terrain upward.

Being tectonically active (in its own peculiar way), Mercury — like our own planet — may experience surface-level earthquakes. "Mercuryquakes" will definitely be a topic worth investigating as we plot out future probe missions to our curious little neighbor.

Originally Published: Sep 9, 2005

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