Planet Mercury: Fast, Pockmarked and Shrinking

By: Mark Mancini  | 
Mercury Planet
Mercury is the smallest planet in our solar system. 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, which means it's one of the four terrestrial planets. 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 order of the planets. And that's not even the weirdest thing about Mercury's travel patterns.


Mercury's Orbit Is Fast

Mercury's orbit
Mercury's orbit is elongated, taking an almost egg-shaped course around the sun. Wikipedia (CC BY-SA 4.0)

Of all the planets in the solar system, Mercury's orbital period around our sun is fast. 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 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 rocky 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 number 59 is almost 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, the sun appears 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.

In other words, the sun appears to rise briefly and then set before rising again. How is that for a unique sunset?


Mercury's Orbit Is Also Egg-shaped

4 Planets Closest to the Sun
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

Mercury is eccentric, and not 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 solar system, Mercury has the most eccentric orbit. The gap between Mercury and the sun ranges from 28.5 to 35.9 million miles (46 million to 57.9 million kilometers) in length making it almost egg shaped.

Consequently, Mercury's travel speed increases as it gets closer to the sun. 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.


Sun, Water Vapor and Ice

Mercury Planet Surface
The solar system's smallest planet doesn't have a substantial atmosphere to protect it from space debris so Mercury has a cratered surface. No crater is more spectacular than 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

With its runaway greenhouse effect, Venus is hotter than Mercury. But Mercury's surface experiences high solar radiation and extreme temperatures. Temperatures on the tiny planet can be as high as 800 degrees Fahrenheit (430 degrees Celsius) or as low as minus 290 degrees Fahrenheit (minus 180 degrees Celsius).

Just as you'd expect, the side of the planet 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 (Mercury Surface, Space Environment, Geochemistry and Ranging) spacecraft one of only three spacecraft to orbit Mercury so far, observed deposits of frozen water on Mercury. The ice was found at the bottoms of deep craters around the North 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 wind." Earth is shielded by a powerful magnetic field that deflects many of these.

Mercury's magnetic field has been around for 3.9 billion years. However, it's significantly weaker than the magnetic field here on Earth.

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


Mercury's Surface Features

Mercury topographical map
NASA’s MESSENGER mission unveiled the first digital elevation model of Mercury, revealing the topography across the entire planet. NASA/U.S. Geological Survey/Arizona State University/Carnegie Institution of Washington/JHUAPL

Neither Venus nor Mercury has 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.

But Mercury is very 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 Caloris Basin; this depression is 950 miles wide (1,525 kilometers), making it larger than the state of Texas. The lowest point of the planet is in the Rachmaninoff basin.

Mercury's surface also is marked by fault scarps; these are cliff ridges that were first seen on Mercury in the 1970s by the MESSENGER spacecraft. 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 Is Shrinking

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

Mercury itself formed about 4.5 billion years ago. Since then, its molten, iron 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 rocky planet. 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.


Mercury Planet FAQ

How many moons does Mercury have?
Mercury has no moons.
Is Mercury the hottest planet?
No. Venus is the hottest planet in our solar system.
Can humans live on Mercury?
Mercury's extreme temperatures can be as high as 800 degrees Fahrenheit (430 degrees Celsius) or as low as minus 290 degrees Fahrenheit (minus 180 degrees Celsius), which makes it a planet that humans would likely never live on.
What is special about planet Mercury?
Of all the planets in this solar system, Mercury orbits our sun at the fastest rate. Also, most of the time, Mercury is closer to Earth than any other planet.