Characteristics of Mercury

by Fraser Cain on April 1, 2012

MESSENGER's first image from Mercury orbit, with the bright Debussy crater visible at upper right.  Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

MESSENGER's first image from Mercury orbit, with the bright Debussy crater visible at upper right. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington


Quick Mercury Stats
Mass: 0.3302 x 1024 kg
Volume: 6.083 x 1010 km3
Average radius: 2439.7 km
Average diameter: 4879.4 km
Density: 5.427 g/cm3
Escape velocity: 4.3 km/s
Surface gravity: 3.7 m/s2
Visual magnitude: -0.42
Natural satellites: 0
Rings? – No
Semimajor axis: 57,910,000 km
Orbit period: 87.969 days
Perihelion: 46,000,000 km
Aphelion: 69,820,000 km
Mean orbital velocity: 47.87 km/s
Maximum orbital velocity: 58.98 km/s
Minimum orbital velocity: 38.86 km/s
Orbit inclination: 7.00°
Orbit eccentricity: 0.2056
Sidereal rotation period: 1407.6 hours
Length of day: 4222.6 hours
Discovery: Known since prehistoric times
Minimum distance from Earth: 77,300,000 km
Maximum distance from Earth: 221,900,000 km
Maximum apparent diameter from Earth: 13 arc seconds
Minimum apparent diameter from Earth: 4.5 arc seconds
Maximum visual magnitude: -1.9

Size of Mercury
How big is Mercury? Mercury is the smallest planet in the Solar System by surface area, volume, and equatorial diameter. Surprisingly, it is also one of the most dense. It gained its ‘smallest’ title after Pluto was demoted. That is why older material refers to Mercury as the second smallest planet. The aforementioned are the three criteria that we will use to show the size of Mercury in relation to Earth.

Some scientists think that Mercury is actually shrinking. The liquid core of the planet occupies about 42% of the planet’s volume. The spin of the planet allows a small portion of the core to cool. This cooling and shrinking is thought to be evidenced by the fracturing of the planet’s surface.

The surface of Mercury is heavily cratered, much like the Moon, and the continued presence of those craters indicates that the planet has not been geologically active for billions of years. That knowledge is based on partial mapping of the planet(55%). It is unlikely to change even after NASA’s MESSENGER spacecraft maps the entire surface. The planet was most likely bombarded heavily by asteroids and comets during the Late Heavy Bombardment about 3.8 billion years ago. Some regions would have been filled by magma eruptions from within the planet. These created smooth plains similar to those found on the Moon. As the planet cooled and contracted cracks and ridges formed. These features can be seen on top of other features, which is a clear indication that they are more recent. Volcanic eruptions ceased on Mercury about 700-800 million years ago when the planet’s mantle had contracted enough to prevent lava flow.

This WAC image showing a never-before-imaged area of Mercury’s surface was taken from an altitude of ~450 km (280 miles) above Mercury. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington


Diameter of Mercury (and the Radius)
The diameter of Mercury is 4,879.4 km.

Need some way to compare that to something more familiar? The diameter of Mercury is only 38% the Earth’s diameter. In other words, you could put almost 3 Mercurys side to side to match the diameter of Earth.

In fact, there are two moons in the Solar System which actually have a larger diameter than Mercury. The largest moon in the Solar System is Jupiter’s moon Ganymede, with a diameter of 5,268 km and the second largest moon is Saturn’s moon Titan, with a diameter of 5,152 km.

The Earth’s moon is only 3,474 km, so Mercury isn’t much bigger.

If you want to calculate the radius of Mercury, you need to divide the diameter of Mercury in half. While the diameter is 4,879.4 km, the radius of Mercury is only 2,439.7 km.

Diameter of Mercury in kilometers: 4,879.4 km
Diameter of Mercury in miles: 3,031.9 miles
Radius of Mercury in kilometers: 2,439.7 km
Radius of Mercury in miles: 1,516.0 miles

Circumference of Mercury
The circumference of Mercury is 15,329 km. In other words, if Mercury’s equator was perfectly flat, and you could drive around it in your car, your odomotor would add 15,329 km from the trip.

Most planets are oblate spheroids, so their equatorial circumference is larger than their pole to pole. The more rapidly they spin, the more the planet flattens out, so the distance from the center of the planet to its poles is shorter than the distance from the center to the equator. But Mercury rotates so slowly that its circumference is the same no matter where you measure it.

You can calculate the circumference of Mercury all by yourself, using the classic mathematical formulae to get the circumference of a circle.

Circumference = 2 x pi x radius

We know the radius of Mercury is 2,439.7 km. So if you put these numbers in: 2 x 3.1415926 x 2439.7, you get 15,329 km.

Circumference of Mercury in kilometers: 15,329 km
Circumference of Mercury in miles: 9,525 miles

Crescent Mercury

Crescent Mercury


Volume of Mercury
The volume of Mercury is 6.083 x 1010km3. That seems to be a huge number on the face of it, but Mercury is the smallest planet in the Solar System by volume (since the demotion of Pluto). It is even smaller than some of the moons in our Solar System. The Mercurian volume is only 5.4% of Earth’s and the Sun has 240.5 million times the volume of Mercury.

Over 40% of Mercury’s volume is occupied by its core, 42% to be exact. The core is about 3,600 km in diameter. That makes Mercury the second most dense planet amongst our eight. The core is molten and mainly consists of iron. The molten core is able to produce a magnetic field which helps to deflect the solar wind. The magnetic field and slight gravity of the planet allow it to hold onto a tenuous atmosphere.

It is thought that Mercury was at one time a larger planet and; therefore, had a higher volume. There is one theory to explain its current size that many scientists accept on several levels. The theory explains Mercury’s density and the high percentage of core material. The theory states that Mercury originally had a metal-silicate ratio similar to common meteorites, as is typical of rocky matter in our Solar System. At that time, the planet is thought to have had a mass approximately 2.25 times its current mass, but, early in the Solar System’s history, it was struck by a planetesimal that was about 1/6 its mass and several hundred kilometers in diameter. The impact would have stripped away much of the original crust and mantle, leaving the core as a large percentage of the planet and greatly reducing the planet’s volume as well.

Volume of Mercury in cubic kilometers: 6.083 x 1010km3

Mass of Mercury
The mass of Mercury is only 5.5% of the Earth’s; the actual value is 3.30 x 1023 kg. Since Mercury is the smallest planet in the Solar System, you would expect this relatively small mass. On the other hand, Mercury is the second most dense planet in our Solar System (after Earth). Given its size, the density comes largely from its core, estimated at almost half the planet’s volume.

The planet’s mass is comprised of materials that are 70% metallic and 30% silicate. There are several theories to explain why the planet is so dense and the abundance of metallic material. The most widely held theory holds that the high core percentage is the result of an impact. In this theory the planet originally had a metal-silicate ratio similar to the chondrite meteorites common in the Universe and around 2.25 times its current mass. Early in the history of our Solar System, Mercury was struck by a planetesimal sized impactor that was about 1/6 of its hypothesized mass and hundreds of km in diameter. An impact of that magnitude would strip away much of the crust and mantle, leaving behind a large core. Scientists believe that a similar incident created our moon. An additional theory says that the planet formed before the Sun’s energy had stabilized. The planet would have had much more mass in this theory as well, but the temperatures created by the protosun would have been as high as 10,000 K and the majority of the surface rock could have been vaporized. The rock vapor could have then been carried away by the solar wind.

Mass of Mercury in kg: 0.3302 x 1024 kg
Mass of Mercury in pounds: 7.2796639 x 1023 pounds
Mass of Mercury in tonnes: 3.30200 x 1020 tonnes
Mass of Mercury in tons: 3.63983195 x 1020

Artist's concept of MESSENGER in orbit around Mercury. Courtesy of NASA

Artist's concept of MESSENGER in orbit around Mercury. Courtesy of NASA


Gravity on Mercury
Gravity on Mercury is 38% of the gravity here on Earth. A man weighing 980 Newtons on Earth (about 220 pounds), would only weigh about 372 Newtons (83.6 pounds) landing on the planet’s surface. Mercury is only slightly bigger than our moon, so you might expect its gravity to be similar to the Moon’s at 16% of Earth’s. The big difference Mercury’s higher density – it’s the second densest planet in the Solar System. In fact, if Mercury were the same size as Earth, it would be even more dense than our own planet.

It’s important to clarify the difference between mass and weight. Mass measures how much stuff something contains. So if you have 100 kg of mass on Earth, you will have the same amount on Mars, or intergalactic space. Weight, however, is the force of gravity you feel. While bathroom scales measure pounds or kilograms, they should really be measuring newtons, which is a measure of weight.

Take your current weight in either pounds or kilograms and then multiply it by 0.38 with a calculator. For example, if you weigh 150 pounds, you’d weigh 57 pounds on Mercury. If you weigh 68 kilograms on the bathroom scale, your weight on Mercury would be 25.8 kg.

You can also turn this number around to figure out how much stronger you would be. For example, how high you could jump, or how much weight you could lift. The current world record for the high jump is 2.43 meters. Divide 2.43 by 0.38, and you get the world’s high jump record if it were done on Mercury. In this case, it would be 6.4 meters.

In order to escape the gravity of Mercury, you would need to be traveling 4.3 kilometers/second, or about 15,480 kilometers per hour. Compare this to Earth, where the escape velocity of our planet is 11.2 kilometers per second. If you compare the ratio between our two planets, you get 38%.

Surface gravity of Mercury: 3.7 m/s2
Escape velocity of Mercury: 4.3 kilometers/second

Density of Mercury
The density of Mercury is the second highest in the Solar System. Earth is the only planet that is more dense. It is 5.427 g/cm3 compared to Earth’s 5.515 g/cm3. If gravitational compression were to be removed from the equation, Mercury would be more dense. The high density of the planet is attributed to its large percentage of core. The core constitutes 42% of Mercury’s overall volume.

Mercury is a terrestrial planet like Earth, one of only four in our Solar System. Mercury is about 70% metallic material and 30% silicates. Add the density of Mercury and scientists can infer details of its internal structure. While the Earth’s high density mainly results from gravitational compression at the core, Mercury is much smaller and is not so tightly compressed internally. These facts have allowed NASA scientists and others to surmise that its core must be large and contain overwhelming amounts of iron. Planetary geologists estimate that the planet’s molten core accounts for about 42% of its volume. On Earth that percentage is 17.

Interior of Mercury

Interior of Mercury


That leaves a silicate mantle that is only 500–700 km thick. Data from Mariner 10 led scientists to believe that the crust is even thinner, at a mere 100–300 km. This surrounds a core that has a higher iron content than any other planet in the Solar System. So, what caused this disproportionate amount of core material? Most scientists accept the theory that Mercury had a metal-silicate ratio similar to common chondrite meteorites several billion years ago. They also believe that it had a mass of about 2.25 times its current; however, Mercury may have been impacted by a planetesimal 1/6 that mass and hundreds of km in diameter. The impact would have stripped away much of the original crust and mantle, leaving the core as a major percentage of the planet.

While scientists have a few facts about the density of Mercury, there is still more to be discovered. Mariner 10 send back a great deal information, but was only able to study about 44% of the planet’s surface. The MESSENGER mission is filling in some of the blanks as you are reading this article and the BepiColumbo mission will go even farther in extending our knowledge of the planet. Soon, there mat be more than theories to explain the high density of the planet.

Density of Mercury in grams per cubic centimeter: 5.427 g/cm3

Axis of Mercury
Like all of the planets in the Solar System, the axis of Mercury is tilted away from the plane of the ecliptic. In this case, Mercury’s axial tilt is 2.11 degrees.

What exactly is a planet’s axial tilt? First imagine that the Sun is a ball in the middle of a flat disk, like a record or a CD. The planets orbit around the Sun within this disk (more or less). That disk is known as the plane of the ecliptic. Each planet is also spinning on its axis as it’s orbiting around the Sun. If planet was spinning perfectly straight up and down, so that a line running through the north and south poles of the planet was perfectly parallel with the Sun’s poles, the planet would have a 0-degree axial tilt. Of course, none of the planets are like this.

So if you drew a line between Mercury’s north and south poles and compared it to an imaginary line if the Mercury had no axial tilt at all, that angle would measure 2.11 degrees. You might be surprised to know that this Mercury tilt is actually the smallest of all the planets in the Solar System. For example, the Earth’s tilt is 23.4 degrees. And Uranus is actually flipped completely over on its axis, and rotates with an axial tilt of 97.8 degrees.

Here on Earth, the axial tilt of our planet causes the seasons. When it’s summer in the northern hemisphere, the Earth’s north pole is angled towards the Sun. and then in the winter, the north pole is angled away. We get more sunlight in the summer so it’s warmer, and less in the winter.

Mercury barely experiences any seasons at all. This is because it has almost no axial tilt. Of course, it also doesn’t have much of an atmosphere to hold the Sun’s heat. Whichever side is facing the Sun is heated to 700 degrees Kelvin, and the side facing away drops to less than 100 Kelvin.

Axial tilt of Mercury: 2.11°

References:
NASA StarChild: Mercury
Wikipedia
NASA: Mercury
European Space Agency
NASA: Mercury Exploration
NASA Solar System Exploration
JAXA: Mercury Quantities
NASA MESSENGER Mission
European Space Agency
NASA Solar System Exploration: Mercury

About 

Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay.

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